Workshop sponsors:

 

 

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International Workshop on the Geology and Metallogeny of Critical Metals (CM2013)

Welcome! Тавтай морилогтун! The  CM2013  Organizing  Committee  welcomes  you  to  Ulaanbaatar  for  the  Second International Workshop on the Geology and Metallogeny of Critical Metals. Critical metals are  metals  whose  availability  is  essential  for  high‐technology,  green  and  defense applications, but  that are vulnerable  to politically or economically driven  fluctuations  in supply. This concept shaped up relatively recently, but hardly a day goes by without some news  item  or  a  TV  report  highlighting  the  importance  of  these  elements  for  the advancement of  technology, or  reminding us  that new sources of critical metals beyond the  already  existing mines  need  to  be  found  and  put  in  production  to meet  the  ever increasing  industrial  demand.  In  recent  years,  great  progress  has  been  made  in  the understanding of the geological processes that produce  industrially viable concentrations of critical metals in igneous systems and supergene environments. At the same time, many aspects  of  metal  transport  and  enrichment  in  the  mantle  and  lower  crust  remain inadequately understood, as do the driving forces and mechanisms behind hydrothermal and metasomatic processes  leading  to  the  formation of  such  important deposits as Oyu Tolgoi  (Mongolia), Bayan Obo  (China) and Strange Lake  (Canada). CM2013 will provide a forum for further discussion of the origin and evolution of rare‐earth, tantalum, niobium, molybdenum  and  indium  deposits,  and  related  processes  in  igneous,  hydrothermal, metamorphic and supergene deposits. 

The  previous  Workshop,  held  in  Beijing  in  September  of  2012,  was  a  great  success, attracting many  distinguished  speakers  from  China, USA, UK, Germany,  Canada,  Japan, Czech  Republic,  Slovakia  and  Russia.  We  hope  that  you  will  enjoy  CM2013  and  look forward to helping you make it a fulfilling academic and cultural experience! 

 

CM2013 Organizing Committee    Organizing Committee O. Gerel, Chair of the Local Organizing Committee B. Munkhtsengel Y. Majigsuren B. Bold‐Erdene D. Bat‐Ulzii S. Oyungerel M. Badamtsetseg J. Kynicky, Chair of the CM Workshop series (Organization, Finance)  A.R. Chakhmouradian, Co‐chair of the CM Workshop series (Publications) Tsendjav (student volunteer) Javzmaa (student volunteer) 

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Venue 

The Technical Session on July 4 will take place on the campus of the Mongolian University of Science and Technology (MUST), located in the center of Ulaanbaatar, at 34 Baga Toiruu. The venue is just a short walk away from the city center (see maps on pp. 5 and 6). Ulaanbaatar, or UB City as it is colloquially known, is situated in the north‐central Mongolia at an elevation of 1300 m (4300 ft) above sea level and is one of the world’s sunniest capitals, with an average of less than 70 mm of precipitation and mostly clear skies throughout mid‐summer. 

In the 225 years since the present‐day site on the Tuul River at the foot of Bogd Khan Uul mountain was settled permanently, Mongolia’s capital has grown into a bustling modern city of over 1.2 million people. Its rich cultural heritage is embodied in the numerous historic sites and monuments, including the splendid Gandantegchinlen Monastery (est. 1727) and Winter Palace of the Bogd Khan, the impressive architectural ensemble of the Sukhbaatar Square, the Zaisan War Memorial, and the Museum of Fine Arts, which features silks, paintings and metalwork dating back to the 1600s. 

In the past twenty years, Mongolia has become a true Mecca for eco‐ and geotourists. Its innumerable  natural  attractions  include  pristine  lakes  and  forests  in  the mountainous western and northern parts of the country (such as Gorkhi‐Terelj National Park which we will visit on July 5), to mineral springs, wild horses and waterfalls in the Khangai Mountains of central Mongolia, to singing dunes, petrified forests, canyons and dinosaur sites  in the Gobi  region.  The  country  is  richly  endowed with mineral  resources  and  in  the  past  40 years, has experienced several exploration booms that led to the discovery of world‐class Au, Cu, Mo, fluorite and rare‐earth deposits. Some of these deposits will be visited during our fieldtrip on July 6‐9), which will also provide multiple opportunities for exploring the unique life, culture and natural beauty of Mongolia’s countryside and its people. 

 

Workshop Program July 3, 2013 (Wednesday)   Arrival and registration. On‐site registration will take place in Building 2 of    the MUST (34 Baga Toiruu), starting at 3:00 pm 4:00‐5:00 pm  Tour of the Museum of Geology and Natural Resources   Guide: Professor J. Lkhamsuren, DSc, Museum Director  6:00 pm  Icebreaker at Black Pearl (this restaurant is located just down the road 

from the Ulaanbaatar Hotel on Baga Toirog, see no. 5 on the city map)  July 4, 2013 (Thursday)  Conference Hall, Mongolian University of Science and Technology (MUST) 8:30‐8:40 am  Welcome message from Professor B. Ochirbat, Vice‐rector of MUST 8:40‐9:00 am  B. Baatartsogt and D. Altankhuyag   Mineral resource policy in Mongolia 9:00‐9:30 am  Keynote talk: Ochir Gerel* and B. Munkhtsengel   Alkaline magmatism and related mineralization in Mongolia (p.8) 

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9:30‐10:00  Anton Chakhmouradian*, E. Reguir, A. Zaitsev   Rare‐earth enrichment processes in igneous systems: An overview (p. 9‐10) 10:00‐10:20  Jindrich Kynicky*, O. Gerel, U. Kempe, M. Vašinová Galiová, V. Králová,    M. Smith, C. Xu   The diversity of rare‐earth element (REE) deposits… (p. 11) 

10:20‐10:40  Refreshment break 

10:40‐11:00  Baatar Munkhtsengel*, O. Gerel, S. Iizumi, D. Batbold   Petrology of the REE‐bearing Lugiin Gol nepheline syenite complex (p. 12) 11:00‐11:20  Ekaterina Reguir*, A. Chakhmouradian, V.S. Kamenetsky, B.M. Osovetskii,    I.V. Veksler, P. Yang, A. Camacho   Perovskite: An important petrogenetic and exploration indicator... (p. 13) 11:20‐11:40  Veronika Králová* and J. Kynicky   Application of integrated mineral analysis... (p.14) 11:40‐12:00  Shaun Spelliscy*   Claim staking: The first step to wealth and riches in Canada (p. 15‐16) 12:00‐12:20  Tamiraa Altangerel*   Rare earth deposits in Mongolia (p. 17) 

12:20‐2:00 pm  Lunch break 

2:00‐2:20  Martin Ondrejka*, I. Broska, P. Uher, M. Kohút, M. Putiš, P. Konečný   Accomodation of S and As by primary magmatic monazite... (p. 18) 2:20‐2:40  Wei Zhang*, C.R.M. McFarlane, D.R. Lentz, K.G. Thorne   Comparing and contrasting W‐Mo mineralization... (p. 19) 2:40‐3:00  Ochir Gerel*   Large Porphyry‐type Deposits in Mongolia... (p. 20) 3:00‐3:20  Jindrich Kynicky*, A. Chakhmouradian, C. Xu, M. Brtnicky, M. Vašinová    Galiová, V. Králová   Evolution of rare‐earth mineralized carbonatites at Lugiin Gol... (p. 21) 3:20‐3:40  Sven Hönig*, R. Škoda, R. Čopjaková, J. Leichmann, M. Novák   Garnet, a major yttrium and heavy rare earth carrier... (p. 22) 

3:40‐4:00  Refreshment break 

4:00‐4:20  Lin Ye*, T. Bao, L. Li, Y. Yang   The patterns of indium distribution in sphalerite... (p. 23) 4:20‐4:40  Dagva Batbold*   Mineralogy of carbonatite from the Lugiin Gol alkaline pluton... (p. 24) 4:40‐5:00  S. Amar‐Amgalan and Dash Bat‐Ulzii*   Alkaline granites geochemical studies of the Khanbogd pluton (p. 25) 5:00‐5:20  Mihoko Hoshino*, Y. Watanabe, M. Tsunematsu   A new type of REE‐bearing deposit... (p. 26) 5:20‐5:40  Simon Blancher* and I. Duhamel‐Achin   The Mabounié carbonatite (Gabon)... (p. 27) 5:40‐6:00  S. Oyunbat*   Geology and mineralization at the Ulaan Del  Zr‐Nb‐REE occurrence (p. 28) 

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July 5, 2013 (Friday) Geological excursion to Gorkhi Terelj Park 9:00 am  Departure (pick up at your hotel) 7:00 pm  Dinner at Mongol Shiltgeen (Hotel Mongolia); return to UB City 

 

Fieldtrip Program

July 6, 2013 (Saturday)   8:45 am  Arrival in the Oyu Tolgoi mine camp (550 km, see map on p. 5)  9:30  Safety briefing; introduction to the Oyu Tolgoi project 9:50  Oyu Tolgoi site tour, part I (main lookout hill, open pit lookout,     concentrator, core shack) 1:00 pm  Lunch in the Conference Center 2:00  Khanbogd complex (alkaline granites and pegmatites) 6:30  Arrival to the Big Ger Oyu Tolgoi visitor camp, check‐in and dinner  Overnight accommodation in the Big Ger camp 

July 7, 2013 (Sunday)  

 6:30 am  Breakfast 7:30  Trip to the Lugiin Gol by car (180 km)    Lugiin Gol REE deposit (syenites, carbonatites, REE ores) 

Overnight accommodation in the Lugiin Gol Reo camp 

July 8, 2013 (Monday)   

7:30 am  Trip to Ulgii Khiid by car (100 km)    Ulgii Khiid complex (syenites and carbonatites) 6:00 pm  Arrival to the Big Ger camp, dinner 

 Overnight accommodation in the Big Ger camp 

July 9, 2013 (Tuesday) 

6:45 am  Breakfast at the Big Ger 7:30  Check‐out and departure for the Oyu Tolgoi airport 9:15  Return flight to Ulaanbaatar          

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 Downtown  Ulaanbaatar  map  (from  http://maps.google.com/),  showing  the  location  of  the Mongolian University of Science and Technology (1), nearby hotels (2 Puma Imperial; 3 Zaluuchuud; 4 Ulaanbaatar), Black Pearl restaurant (5) and selected attractions (6 Sukhbaatar Square; 7 Museum of Natural History; 8 Zanabazar Museum of Fine Arts).     

 Schematic map of Mongolia  (from www.e‐mongol.com) showing  the  fieldtrip  route  (Ulaanbaatar – Oyu Tolgoi – Khan Bogd – Lugiin Gol – Ulugei Khiid – Oyu Tolgoi – Ulaanbaatar). 

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  ““SSpphhiinnxx  rroocckk””,,  KKhhaannnnbbooggdd  ggrraanniitt

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UlaanbaaJuly

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STRACTS13 Worksatar, Mony 4, 2013

                                 

GGoobbii,,  MMoonngg  

S shop ngolia 3 

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Alkaline magmatism and related mineralization in Mongolia

OCHIR GEREL1 and BAATAR MUNKHTSENGEL2 1 Department of Geology, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia; gerel@must.edu.mn 2 Department of Geology, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia;

munkhtsengel@must.edu.mn Large alkaline provinces are recognized in southern and north-western Mongolia. Volcanic-plutonic alkaline complexes in southern Mongolia are controlled by large sublatitudinal structures and occur within grabens. Rb-Sr ages of several complexes (Mushgai Khudag, Bayan Khoshuu, Khetsuu Teeg, Dorvon Dort, Tsogt Ovoo, Ulgii and Khavtgai Uul) range from 139.9±5.9 Ma to 135–111 Ma. The alkaline complexes consist of predominant alkaline trachytes, latites and syenites with subordinate melanephelinites, phonolites, nepheline syenites and quartz syenites that form flows, stocks and dikes, and silicic and carbonate pyroclastic rocks, magnetite-apatite, apatite rocks and mineralized breccias, hosted by Paleozoic sedimentary-volcanic sequences and Carboniferous granitoids. Carbonatites, carbonate-fluorite, fluorite, carbonate-fluorite-celestine-barite REE-ore-bearing rocks are associated with this complex [1]. The Lugiin gol carbonatite-bearing nepheline syenite yielded a Rb-Sr age of 244-222 Ma [2]. Granite-related (Nb-Zr-)REE deposits are associated with peralkaline rocks. Examples are the REE-Zr-Nb Khalzan Buregtei deposit in north-western Mongolia [3], REE pegmatite in the Khan Bogd pluton in southern Mongolia, and a number of occurrences in north-western Mongolia. These deposits occur in the apical part of granitic cupolas, and are generally associated with highly fractionated magmatic phases including peralkaline pegmatite. The host granites are composed of potassium feldspar, quartz, albite, arfvedsonite, aegirine, fluorite, and various minerals enriched in trace elements and REE, such as elpidite, gittinsite, zircon, pyrochlore, monazite, REE fluorocarbonates and polylithionite. Quartz-epidote metasomatic rocks contain zircon, fergusonite, allanite, chevkinite and titanite in vein-like zones. Fergusonite and zircon contain heavy REE and Y. Accessory minerals include amphibole, magnetite, zircon, epidote, ilmenite, fluorite, beryl, chevkinite, pyrite, and galena. Rare-earth pegmatites and quartz-fluorite veins may also occur. The genesis and tectonic environment of alkaline rocks and associated REE mineralization is controversial; some previously proposed ideas include a within-plate hot spot, rift-related, and post-collisional environment. Initial Sr and Nd isotope ratios show that these complexes originated from enriched sources similar to EMII and show geochemical characteristics typical of post-collisional arc tectonic settings. These characteristics can also result from low-degree melting of subduction-modified lithospheric mantle metasomatized by a volatile (CO2, H2O, etc.) rich fluid to produce rocks extremely enriched in large-ion-lithophile elements and light REE and depleted in high-field-strength elements [4].

References cited: [1] Samoilov, V.S. and Kovalenko, V.I. (1983) Complexes of Alkaline Rocks and Carbonatites in Mongolia. Moscow, Nauka,

200 p. (in Russian). [2] Munkhtsengel, B. and Iizumi, Sh. (1996) Petrology and geochemistry of the Lugiin gol Nepheline Syenite Complex in the

Gobi-Tien Shan fold belt, South Mongolia: a post collisional potassic magmatism. Mongolian Geoscientist. Special Issue, International Geological Symposium on East Asia, no. 14, 12-13.

[3] Kovalenko,V.I., Tsaryeva, G.M., Goreglyad, A.V., Yarmolyuk, V.V., Troitsky, V.A. (1995) The Peralkaline granite-related Khaldzan-Buregte rare metal (Zr, Nb, REE) deposit, Western Mongolia. Economic Geology, 90, 530-547.

[4] Gerel, O., Munkhtsengel, B., Enkhtuvshin, H., Iizumi, Sh. (2005) Mushgai Khudag and Bayan Khushuu volcanic-plutonic alkaline complexes with REE Ta, Nb, Fe carbonatite mineralzition. In: Geodynamics and Metallogeny of Mongolia with Special Emphasis on Copper and Gold Deposits (R. Seltmann, O. Gerel, D. Kirwin, eds.) London, 215-221.

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Rare-earth enrichment processes in igneous systems: An overview

ANTON R. CHAKHMOURADIAN1, EKATERINA P. REGUIR2, ANATOLY N. ZAITSEV3 1 Department of Gelogical Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; chakhmou@cc.umanitoba.ca 2 Department of Gelogical Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; umreguir@cc.umanitoba.ca 2 Department of Mineralogy, St. Petersburg State University, St. Petersburg, Russia; burbankite@gmail.com Currently, some 70% of advanced rare-earth exploration projects are focused on mineral deposits hosted by igneous rocks or found in their associated weathering products (such as lateritic profiles and proximal placers). Three groups of igneous rocks have been on the exploration radar the most in recent years: (1) carbonatites; (2) peralkaline silica-undersaturated rocks; and (3) peralkaline anorogenic granites. The first two groups accounted for ~50% of the global production of rare earth elements (REE) between the mid-1960s and mid-1990s. Deposits associated with carbonatites show the lowest levels of Y and heavy REE in their budget [Y/Nd ≤ 0.3; (La/Yb)CN = 40-2000] among the three groups, but are least depleted in Eu (Eu/Eu* ~1.0); these characteristics are in marked contrast to those of REE-enriched anorogenic granites [Y/Nd ≥ 0.8; (La/Yb)CN ≤ 13; Eu/Eu* ≤ 0.3]. Silica-undersaturated rocks show intermediate values [Y/Nd = 0.2-3.0; (La/Yb)CN = 2-200; Eu/Eu* = 0.2-0.4]. Although it is commonly assumed that that heavy REE (HREE), which include the critical metals Y, Tb and Dy, are far more valuable than light REE (LREE), note that the level of consumption of the light lanthanide Nd is double that of Y, Tb and Dy combined [1]. Postorogenic carbonatites associated with weakly undersaturated to saturated alkali syenites (e.g., Mountain Pass, California) sometimes show economic levels of enrichment in bastnäsite, LREECO3F (± other fluorocarbonates and monazite, LREEPO4) and have shown the greatest promise so far as an igneous REE source. A significant proportion of REE in carbonatites is found in minerals not amenable to processing, like amphiboles and clinopyroxenes (in some cases, up to 10% of the whole-rock HREE budget; [2]). Extreme enrichment of carbonatitic magmas in LREE cannot be explained satisfactorily with simple melting models. The presence of REE phases in their mantle source, low degree of contamination by crustal material, and fortuitous interplay between aF- and a(PO4)3- appear to be prerequisite to the formation of a commercially viable carbonatite-hosted deposit [3]. Appreciable REE mineralization commonly develops at postmagmatic stages through breakdown of apatite, calcite, burbankite and other primary low-grade REE hosts during their hydrothermal reworking (fluorocarbonates, ancylite, monazite; [4]) or lateritic weathering (secondary REE phosphates; [5]). A variety of minerals forming large-tonnage deposits in peralkaline undersaturated rocks have been proposed as the ultimate solution to looming critical-metal shortages. However, to date, only loparite [(Na,LREE,Ca,Sr)(Ti,Nb,Ta)O3] has been extracted at Lovozero (Kola, Russia) and processed industrially to yield REE, Nb, Ta and Ti products. It remains to be seen if any of the other REE-bearing minerals (e.g., eudialyte or apatite) are amenable to profitable metal recovery. These minerals crystallize from extremely evolved melts derived by protracted fractional crystallization of aegirine, feldspars and nepheline (± sodalite) from basanitic magma in anorogenic extensional settings. Their compositional variation within the deposit largely depends on the partitioning behavior of the elements of interest. For example, the REE content of the Lovozero loparite decreases, whereas its Nb content increases with progressive fractionation [6].

References cited: [1] Hatch, G. (2012) Dynamics in the global market for rare earths. Elements, 8, 341-346. [2] Reguir, E.P., Chakhmouradian, A.R., Pisiak, L.K., Halden, N.M., Yang, P., Xu, C., Kynicky, J., Couëslan C.G. (2012) Trace-

element composition and zoning in clinopyroxene- and amphibole-group minerals: implications for element partitioning and evolution of carbonatites. Lithos, 128-131, 27-45.

[3] Chakhmouradian, A.R., Zaitsev A.N. (2012) Rare-earth mineralization in igneous rocks: Sources and processes. Elements, 8, 347-353.

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[4] Ruberti, E., Castorina, F., Censi, P., Comin-Chiaramonti, P., Gomes, C.B., Antonini, P., Andrade, F.R.D. (2002) The geochemistry of the Barra do Itapirapuã carbonatite (Ponta Grossa Arch, Brazil): a multiple stockwork. Journal of South American Earth Sciences, 15, 215-228.

[5] Lapin, A.V. and Tolstov, A.V. (1995) Mineral Deposits in Carbonatite Weathering Crusts. Nauka, Moscow, 208 pp. (in Russian).

[6] Mitchell, R.H. and Chakhmouradian, A.R. (1996) Compositional variation of loparite from the Lovozero alkaline complex, Russia. Canadian Mineralogist, 34, 977-990.

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The diversity of rare-earth element (REE) deposits: An example of Mongolia JINDRICH KYNICKY1, OCHIR GEREL2, ULF KEMPE3, MICHAELA VAŠINOVÁ GALIOVÁ4,5, VERONIKA

KRÁLOVÁ6, MARTIN SMITH7, CHENG XU8 1 Department of Geology and Pedology, Mendel University in Brno, Czech Republic; kynicky@mendelu.cz 2 Department of Geology, Mongolian University of Science and Technology, Mongolia; gerel@must.edu.mn 3 Institut für Mineralogie, TU Bergakademie Freiberg, Freiberg, Germany; kempe@mineral.tu-freiberg.de 4 Department of Chemistry, Faculty of Science, Masaryk University, Czech Republic; galiovam@seznam.cz 5 Central European Institute of Technology, Masaryk University, Czech Republic 6 TESCAN, a.s., Brno, Czech Republic; veronika.kralova@tescan.cz 7 School of the Environment and Technology, University of Brighton, United Kingdom; martin.smith@brighton.ac.uk 8 School of Earth and Space Sciences, Peking University, Beijing, China; xucheng1999@hotmail.com

Rare-earth deposits of Mongolia show a wide spatial distribution and are associated with rocks of carbonatitic affinity, primary and metasomatically modified peralkaline granitic rocks and associated placer and residual deposits. The distribution of these deposits is controlled by the large-scale tectonic structures of the Northern Gobi Rift Zone, Gobi-Tien Shan Fold Belt and Lake Tectonic Zone, as well as by the local geomorphology. The deposits of the Northern Gobi Rift Zone are associated with Early Cretaceous rifting (120-139 Ma). Associated REE fluorocabonate mineralization in carbonatites and britholite in nelsonites at Mushgai Khudag, Bayan Khushuu, Khotgor and Tsogt Ovoo deposits hosts up to 12 wt. % REE2O3 (predominantly LREE). The carbonatites of the Gobi-Tien Shan Fold Belt are associated with Triassic rifting (240-244 Ma) along the southeastern side of the Gobi-Tien Shan fold belt, near the boundary with the Sulinheer (Solonker) suture zone. The REE contents in the majority of these carbonatites (e.g., Lugiin Gol and Omnot Olgii) are ten times higher than global averages for carbonatites (up to 15 wt. % REE2O3); these rocks are significantly enriched in LREE. The Khalzan Buregte peralkaline granitic massif hosts one large Zr-Nb-REE deposit in the Lake tectonic zone. This and three other, temporally and spatially associated, massifs (Ordovician Ulaan Khuren, Gurvan Uneet and Ulaan Uneet) are situated in north-western Mongolia and represent the westernmost occurrences in the Mongolian-Transbaikalian alkaline granitoid province. The majority of ore-bearing samples are significantly enriched in HREE and Y (up to 1.2 wt % REE2O3), Nb (up to 1.8 wt %), and Zr (up to 11.5 wt %; on average, 2.5 wt %). REE-bearing placers are associated with strongly eroded peralkaline granites of the Khalzan Buregte massif. The ore mineralogy consists of bastnasite and synchysite (up to 96 g/m3), monazite (up to 39 g/m3), xenotime (up to 38 g/m3) and zircon (up to 2445 g/m3). The deposits of REE in Mongolia represent a possible source of critical metals to meet the growing demand for these commodities outside of China. Our data from a range of different deposits in Mongolia show that not only primary, but also (and, in some cases, especially) secondary processes create economic concentrations of REE with characteristic distribution patterns. Advanced exploration for REE is currently underway at Lugiin Gol [1]. Possible low-cost extraction of the economically valuable heavy REE as a by product of Zr and Nb mining from the placers related to Khalzan Buregte may become viable in the near future. There are also other, smaller placer deposits that are presently under consideration. Although Mongolia has no history of REE mining, the aforementioned deposits are of strategic significance and potentially high economic value owing to their high grade and the current trends in the global REE market. This work was supported by the European Regional Development Fund, project “CEITEC” (CZ.1.05/1.1.00/02.0068).

Reference cited: [1] Altangerel, T. (2013) Rare earth deposits in Mongolia. CM2013 Program with Abstracts (this volume).

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Petrology of the REE-bearing Lugiin Gol nepheline syenite complex

BAATAR MUNKHTSENGEL1, OCHIR GEREL1, SHIGERU IIZUMI2, DAGVA BATBOLD3 1 Department of Geology, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia; munkhtsengel@yahoo.com 2 Department of Geosciences, Shimane University, Japan 3 Orkhon Exploration Company, Ulaanbaatar, Mongolia 

The Lugiin Gol nepheline syenite complex occurs in the South Gobi (Gobi-Tien Shan) Fold Belt, intruding Permian accretionary sedimentary rocks. The complex is mainly composed of nepheline syenite stock and phonolite, tinguaite and REE-bearing carbonatite dikes. The nepheline syenite stock is circular in plan view, and crops out over an area of approximately 12 km2. The stock consists mainly of nepheline-bearing syenite, nepheline-bearing alkali feldspar syenite and nepheline syenite. These syenites show an Rb-Sr whole rock isochron age of 244.9±22.4 Ma and a whole rock-mineral isochron age of 222.2±3.2 Ma, which are consistent with previously reported K-Ar whole rock and mineral ages. The Lugiin Gol complex is intruded by an alkali granite porphyry dike, which shows an Rb-Sr whole rock isochron age of 210.3±10.9 Ma. This alkali granite porphyry dike shows lower initial Sr and higher initial Nd isotope ratios, a younger Rb-Sr isochron age and different petrographical and petrochemical characteristics with respect to the Lugiin Gol nepheline syenite complex. These features suggest that the alkali granite porphyry dike was derived from a different magma source involving a significant lower crust component. Main constituent minerals of the Lugiin Gol syenites are clinopyroxene (diopsite-hedenbergite, Wo49-

51En35-17Fs16-32), amphibole (ferroan-pargasite to ferropargasite), biotite (An11-45), plagioclase (An40-0), potassium feldspar (Or98-70) and nepheline (Ne80-68). Apatite, zircon, titanite, allanite, pyrochlore, fluorite, cancrinite, sodalite, white mica, calcite and magnetite are accessory phases. The nepheline syenites, which range from 52 to 59 wt.% SiO2, are characterized by high contents of alkalis, Al2O3, large ion lithophile elements, and by low MgO contents and mg-numbers, indicating that these rocks crystallized from a fractionated magma. The rocks are depleted in high field strength elements, such as Ta, Nb, P and Ti, showing geochemical characteristics similar to those of island arc volcanics. In primitive mantle normalized REE diagram, they show enrichment in light REE and moderately flat patterns across the heavy REE. Some rocks show a weak Eu negative anomaly. Although the syenites are low in MgO contents, they have similar petrochemical characteristics to Group III Roman-type (ultra)potassic rocks. Most dike rocks at Lugiin Gol, including REE-bearing carbonatite, plot on the 244.9 Ma Rb-Sr whole rock isochron, indicating that the Lugiin Gol complex was derived from a single magma batch. These rocks have initial Sr isotope ratios around 0.7080 and initial Nd isotope ratios around 0.5122, which are distinctly more radiogenic than those of the country sedimentary rocks. Taking these geochemical data and previously reported experimental results into account, it seems to be difficult to derive such magma by the melting of crustal materials. Crustal metasomatism cannot explain the origin of the complex, since it shows clear petrographical and petrochemical evidence supporting an igneous origin. The most probable magma source for the complex would be an enriched mantle source similar to EMII. The magma derived from such enriched mantle may have evolved by fractional crystallization of minerals such as olivine, clinopyroxene and calcic plagioclase during its ascent to the upper crust. The Lugiin Gol complex may have crystallized from such an evolved magma. Crustal contamination would not have played an important role in the evolution of that magma. In South Mongolia, calc-alkaline and alkaline magmatism continued from the Carboniferous to the Permian. South Mongolia was an active continental margin during the late Paleozoic. It has been suggested that the continent-continent collision may have occurred at the end of the Permian and Sulinkheer Sea closed. The Lugiin Gol complex intruded around the time of collision. Primary magma of this complex may have been derived from enriched mantle, which was metasomatized by long-term subduction.

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Perovskite: An important petrogenetic and exploration indicator mineral EKATERINA P. REGUIR1, ANTON R. CHAKHMOURADIAN2, VADIM S. KAMENETSKY3,

BORIS M. OSOVETSKII4, ILYA V. VEKSLER5, PANSEOK YANG6, ALFREDO CAMACHO7,

1 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; umreguir@cc.umanitoba.ca 2 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; chakhmou@cc.umanitoba.ca 3 CODES, University of Tasmania, Australia; dima.kamenetsky@utas.edu.au 4 Perm State University, Perm, Russia; opal@psu.ru 5 GFZ, Helmholz Centre, Potsdam, Germany; veksler@gfz-potsdam.de 6 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; panseok.yang@ad.umanitoba.ca 7 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; camacho@cc.umanitoba.ca Perovskite-group minerals [general formula, (Ca,Na,REE,Sr,Th)(Ti,Nb,Fe3+)O3; REE = rare-earth elements] are common constituents of such economically important rocks as kimberlites, nepheline syenites and carbonatites. These minerals are resistant to weathering in a temperate climate and potentially can be used to track their source rocks using regional-scale heavy-mineral surveys. A large spectrum of trace elements present in perovskites and detectable by standard mass-spectrometry techniques (K, Mn, Zn, Ba, Pb, Sc, V, Cr, Y, Zr, Hf, U, Ta) makes these minerals a valuable source of information about the provenance and evolution of their parental magmas, including the sources and emplacement ages of these magmas. Perovskites can be also used to discriminate among superficially similar rock types (e.g., kimberlites and lamproites) and ascertain the relations between associated igneous rocks [1]. In the present work, we demonstrate the utility of perovskite for this type of petrogenetic studies and explore potential applications of this mineral in mineral exploration. A detailed comparative analysis of trace-element enrichment in perovskite from carbonatite and associated clinopyroxenite from the Afrikanda complex (Kola, Russia), showed that samples from the carbonatite contain significantly higher levels of Na, Pb, REE, Zr, Hf, Th and U relative to those from the clinopyroxenite. Model evolutionary trends, based on our calculated partition coefficients for several key elements (La, Y, Zr, Hf, Th, U), show that these two rocks cannot be related to each other by crystal fractionation. The mantle-like 87Sr/86Sr ratios of perovskite from both rock types and their virtually identical U-Pb ages (~370 Ma) can be explained by their derivation from the same magma by liquid immiscibility, or from the same isotopically equilibrated, but mineralogically complex mantle source [1]. A detailed study of perovskite from heavy mineral concentrates extracted from clastic sediments in the eastern part of the East European platform (Perm Region) showed that its major-element composition is similar to that of perovskite from carbonatites, kimberlite and other alkali-ultramafic rocks worldwide. However, we identified several trace-element criteria that can be reliably used to discriminate between kimerlitic perovskite and its counterparts from other rock types, and used these criteria to demonstrate that the composition of Perm perovskite is most consistent with a carbonatitic source and certainly not a kimberlitic one. The U-Pb geochronological study of the Perm perovskite yielded a U-Pb age of 364 Ma. Our data thus imply the existence in this part of the East European platform of an undetected carbonatite intrusion coeval to the Devonian Afrikanda and other Kola carbonatites. References cited: [1] Reguir E.P., Camacho, A., Yang, P., Chakhmouradian, A.R., Kamenetsky, V.S. and Halden, N. (2010) Trace-element study

and uranium-lead dating of perovskite from the Afrikanda plutonic complex, Kola Peninsula (Russia) using LA-ICP-MS. Mineralogy and Petrology, 100, 95-103.

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Application of integrated mineral analysis to the study of disseminated REE mineralization in silicate rocks

VERONIKA KRÁLOVÁ1 and JINDRICH KYNICKY2

1 TESCAN, a.s., Brno, Czech Republic; veronika.kralova@tescan.cz 2 Mendel University in Brno, Czech Republic; kynicky@mendelu.cz Whereas carbonatites of the Lugiin Gol alkaline complex and rare-earth deposit [1] have been investigated in a fair amount of detail [2-4], their associated silicate rocks are not as well understood. The predominant type of silicate rock at Lugiin Gol is leucocratic nepheline(-bearing) syenite, which lacks any economic potential and hence, has been largely neglected in detailed mineralogical studies of Lugiin Gol. The present paper is the first detailed report of unusual rare-earth mineralization associated with primary silicates, carbonates and fluorite and hosted by the silicate rocks. As a first step, it was critical to choose a method which would enable us to perform a detailed, yet rapid, inspection of a large sample set. This method of choice should also be able to deliver quantitative results and allow their comparison among individual samples. For these reasons, we chose an automated mineral analyzer, a specialized type of scanning electon microscope offering automated and rapid data acquisiton and processing, providing the required level of detail, and allowing direct cross-comparison of analytical results. In this work, the capabilities of TIMA (TESCAN Integrated Mineral Analyzer, [5]) for analysis of REE-mineralized samples were tested. The analysis was done using a TIMA instrument based on a thermionic emission scanning electron microscope equipped with ultra-fast YAG scintillator back-scattered electron detector and two silicon drift energy-dispersive X-ray detectors. The sample was analyzed with a conductive carbon layer 10 nm in thickness in high-vacuum mode at standard operating conditions (accelerating voltage of 25 kV and working distance 15 mm). The nepheline(-bearing) syenites are composed predominantly of potassium feldspar, nepheline, sodalite, plagioclase, amphibole, biotite, cancrinite and minor quantities of calcite, titanite, magnetite, apatite and zircon. Primary carbonate phases usually occur close to drop-shaped fluorite clusters and contain potassium feldspar in melt inclusions. A large set of silicate samples from the Lugiin Gol deposit showing high levels of rare-earth elements (REE) was selected to examine their mineralogy in detail. The examined silicate rocks are paragenetically and texturally diverse and serve as an ideal model for testing the capabilities of TIMA. In addition to the minerals that were expected to be present in these samples based on petrographic data, all examined rocks contain primary carbonates (burbankite and/or Sr-rich calcite I), fluorite and later-deposited Sr-poor Mn-rich calcite II associated with strontianite, barite and celestine. The primary carbonates were identified as melt inclusions and glomerocrysts of burbankite-calcite-fluorite, sometimes present as cores overgrown by a mantle of pure Sr-rich calcite. The primary textures and parageneses were partially modified and overprinted by late-stage processes involving recrystallization, break-down and replacement of burbankite and other early-crystallized carbonates, followed by precipitation of the secondary carbonate-sulfate paragenesis. References cited: [1] Altangerel, T. (2013) Rare earth deposits in Mongolia. CM2013 Program with Abstracts (this volume). [2] Batbold, D. (2013) Mineralogy of carbonatite from the Lugiin Gol alkaline pluton, South Mongolia. CM2013 Program with

Abstracts (this volume). [3] Kynicky, J., Chakhmouradian, A.R., Xu, C., Brtnický, M., Vašinová Galiová, M., Králová, V. (2013) Evolution of rare-earth

mineralized carbonatites at Lugiin Gol and Omnot Olgii, southern Mongolia. CM2013 Program with Abstracts (this volume). [4] Munkhtsengel, B., Gerel, O., Iizumi, S., Batbold, D. (2013) Petrology of the REE-bearing Lugiin Gol Nepheline Syenite

Complex. CM2013 Program with Abstracts (this volume). [5] http://www.tescan.com/en/products/tima/tima-mineral-analyzer

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Claim staking: The first step to wealth and riches in Canada SHAUN SPELLISCY1

1 GemOil, Regina, Saskatchewan, Canada; gemoil@sasktel.net

All is fair in love, war and claim staking. F.H. Heidman

I recently reminded Juan Garcia in Mascota, Mexico, that the most important step in the mining process is the claim staker. Without a validly located and registered claim the rest of the process is meaningless. If someone comes along later and disputes the claim, all the hard work that followed to develop a mine is lost, and disputes do happen. There has never been a discovery in Canada that resulted in a mine where someone did not try and dispute the underlying claim. There are many different words for claim; tenement, disposition, lease, licence, land or ground. To simplify things I will refer to all of them as claims. In the California gold rush of 1849, miners made up their own rules and essentially adopted Mexican mining law. Mexican law gave the right to mine to the first one to discover the deposit and begin mining it. The area that could be claimed by one person was limited to that which could be mined by a single person or small group. The idea of actually filing a claim with the government goes against the secretive nature of prospecting and even today some prospectors operate without registering a claim. Many prospectors believe they have no chance to fight against a big company and their entourage of lawyers, and open the door to inquiry by filing a claim with the government agent. Perhaps they made a mistake in their area calculations and it will be disputed? Maybe, the inscription on the application is missing a letter to what is written on the monument? There are also rules and regulations that need to be obeyed to the letter, including filing deadlines. Most prospectors do not want to be bothered with administrative nonsense, and who can blame them. There is no school to teach you how to stake a claim but there is a movie called “The Treasure of the Sierra Madre”, a feature film adaptation of B. Traven's 1927 novel of the same name that will prepare you for most of the things associated with exploration. I recommend the film and book to anyone seriously interested in a mining career. In Canada, once you receive a certificate saying your claim is validly located, you may with the permission of the government and local indigenous people begin to explore it. Certain groups in Canada are against this free entry method of claim staking and want prospectors to consult prior to staking a claim. This of course would spill the beans as to where something might be found. Needless to say there is a lot of objection to this manner of advance consultation. Canada has 13 different mining jurisdictions, each with different rules and regulations for the staking of claims: British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland and Labrador, Yukon, Northwest Territories, and Nunavut. The odds are always quite low that a discovery will happen. If you examined the odds of winning you would quit claim staking and head to the nearest casino. However, the excitement and thrill of making a discovery keep people actively exploring. I have staked over 10,000 claims and while I have found several interesting showings, not one has been a mine. No point quitting now! The Province of Saskatchewan has been governed since December 12, 2012 by the MARS system and is 100% computer based. Anyone over the age of 18 can stake and own a claim in the province, there is no residency requirement, all you have to do is register, own a computer and have a credit card. Claims can be anywhere from 16 hectares to 6000 hectares in size and have a lifespan of two years. Outside of the application fee the first year is free and the second year you apply work or cash credits to keep your claim in good standing. In year ten of your claim, the cash and work requirements double [1]. In the transition between the old system in Saskatchewan and the new system, I recall a government director would send letters to claim stakers announcing changes which always began as follows: After extensive

16

consultation with industry the Department has decided to... And that meant after being taken out for lunch with some executive from a major mining company she decided to implement something. The people on the ground, the people that make the discoveries are passed by in our technocratic world.

Figure caption: (left) A claim staker hard at work (Juan Garcia, Mexico); (right) an example of claim map, showing 72,179 hectares (!) staked around a small mineral discovery that covers only about 0.5 hectares. Reference cited: [1] https://mars.isc.ca/MARSWeb/default.aspx

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Rare earth deposits in Mongolia TAMIRAA ALTANGEREL1

1 Mineral Resources Authority of Mongolia, Barilgachdyn talbai 3, Ulaanbaatar; atamiramn@yahoo.com Since the 1970-1980s, several prospective areas for rare-earth mineralization have been recognized in Mongolia, stimulating further prospecting activities. The following types of REE ore formations are recognized in Mongolia: REE-bearing carbonatite, Nb(Ta)-Zr-REE-bearing alkaline metasomatic rocks, alkaline intrusive rocks hosting REE-bearing pegmatites, sediments hosting monazite and uranium minerals. There are four major REE deposits (see below) and a number of occurrences that need further exploration activities. Mushgai hudag (Mushugai Khuduk). During the most recent exploration campaign, which took place in 2007-2012, six ore zones were recognized (Tumurtei, Khuren Khad, Main, High Grade, Monazite and Jonshit) comprising tens of discrete ore bodies, which range from steeply dipping tabular or lenticular bodies to breccias, stockworks, veins and subhorizontal volcanic sheets. The reserves and resources of the deposit as of June 2012 are: 232,000 tonnes of REO (combined rare-earth oxide), plus 82,490 t REO in the newly discovered Monazite and Jonshit zones. The average REO grade in the Mushgai hudag ore is 1.36%, but reaches 6.15% on average in the newly discovered High Grade zone. Lugiin Gol is an intrusive complex [1-3] comprising hundreds of mineralized veins and dikes, of which 21 have been found to be of potential economic use. However, their average thickness is only 35 cm. According to the findings of the most recent exploration campaign conducted between 2005 and 2009, the total resources amount to 505,822 tonnes of ore grading 2.67% REO on average (to a total of 13,505 tonnes REO). Khotgor was at the focus of mineral exploration between 2005 and 2009, which established an average REO grade of 1.22%. The estimated total REO content is 486,720. While this tonnage value makes Khotgor the largest REE deposit in Mongolia, it is still only a small to medium-sized deposit on the global scale. Khalzan Burged (Khalzan Buregte) is an intrusion of alkali granite [4], for which a technical report and recommendation for further exploration was issued by Micromine in the first half of 2012. An extended drilling program by Mongolian National Rare Earth Corporation LLC (MNREC), under the supervision of Micromine, was conducted and the results are not yet available. Russian geologists, who had worked on this deposit previously, reported an inferred resource of 160 million tonnes of ore to a depth of 250 m. The economic potential of REE resources in Mongolia is still being evaluated, and their links to alkaline and carbonatite magmatism is being investigated by national and international groups [1-4]. References cited: [1] Batbold, D. (2013) Mineralogy of carbonatite from the Lugiin Gol alkaline pluton, South Mongolia. CM2013 Program with

Abstracts (this volume). [2] Kynicky, J., Chakhmouradian, A.R., Xu, C., Brtnický, M., Vašinová Galiová, M., Králová, V. (2013) Evolution of rare-earth

mineralized carbonatites at Lugiin Gol and Omnot Olgii, southern Mongolia. CM2013 Program with Abstracts (this volume). [3] Munkhtsengel, B., Gerel, O., Iizumi, S., Batbold, D. (2013) Petrology of the REE-bearing Lugiin Gol Nepheline Syenite

Complex. CM2013 Program with Abstracts (this volume). [4] Kynicky, J., Gerel, O., Kempe, U., Vašinová Galiová, M., Králová, V., Smith, M., Xu, C. (2013) The diversity of rare-earth

element (REE) deposits: An example of Mongolia. CM2013 Program with Abstracts (this volume).

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Accomodation of S and As by primary magmatic monazite in Western Carpathian granitic rocks: The role of minor element substitutions and redox

conditions of parental melt M. ONDREJKA1, I. BROSKA 2, P. UHER1, M. KOHÚT3, M. PUTIŠ1, P. KONEČNÝ3

1 Comenius University, Faculty of Natural Sciences, Mlynská dolina G, Bratislava, Slovak Republic; mondrejka@fns.uniba.sk 2 Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05, Bratislava, Slovak Republic 3 Dionýz Štúr State Geological Institute, Mlynská dolina 1, 817 04, Bratislava, Slovak Republic A detailed microprobe study of REE-bearing accessory minerals in the Western Carpathian granitic rocks revealed the characteristic presence of sulfur and arsenic in orthomagmatic monazite. Over 1300 spot analyses were compiled from the different granitoid suites involving I-, S-, specialized S- and A-type granitic rocks and their pegmatitic derivates. The highest average concentration of S and As in monazite was determined in I-type granitic rocks (0.35 wt.% SO3, 0.18 wt.% As2O5 with a max. value of 1.6 wt.% SO3) and in related pegmatites (0.23 wt.% SO3, 0.16 wt.% As2O5), while the specialized S-type granitic rocks registered the lowest S and As concentrations (≥0.02 wt.% SO3, 0.10 wt.% As2O5). Similar very low average values of S were recorded in S-type granitic rocks (≥0.04 wt.% SO3), while A-type granites exhibited only slightly increased S content (≥0.1 wt.% SO3). The accomodation of As in the remaining granitic suites is more homogeneous and varies within the limited range of 0.14-0.16 wt.% As2O5. In general, the highest contents of S and As were documented from leucocratic I-type granitic members rich in titanite and magnetite, while the more fractionated and specialized S-type granites recorded the lowest average values. It is obvious that monazite from pegmatites and aplites is depleted in S and As in comparison with monazite from their parental rocks. The higher S and As concentrations in primary magmatic monazite may be connected with a specific oxidized environment during its crystallization from a melt, where higher oxygen fugacity (fO2) and S activity are more likely. Similar features are apparent in primary magmatic apatite from I-type granites, where S content can reach 0.5 wt.% SO3 in more basic granitic rocks such as tonalites [1]. Sulfur is accommodated via the coupled substitution SiSP-2 [2] involving the (SO4)2- sulphate anion group. Increased sulphur content in apatite from I-type granites correlates with a relatively oxidized magma [3]. On the other hand, the sulfur content can be controlled by the formation of late-magmatic sulfides (mainly pyrite accompanied by minor arsenopyrite). These sulfides occur especially in some I- and S-type granitic rocks, and their presence indicates more reduced conditions during crystallization [4]. This work was supported by the Slovak Research and Development Agency under contract APVV-0081-10 and VEGA Agency No. 1/0255/11 and 1/0257/13. References cited: [1] Broska, I., Williams, C.T., Uher, P., Konečný, P., Leichmann, J. (2004) The geochemistry of phosphorus in different granite

suites of the Western Carpathians, Slovakia: the role of apatite and P-bearing feldspar. Chem. Geol. 205, 1-15. [2] Piccoli, P.M. and Candela, P.A. (2002) Apatite in igneous systems. In: Kohn, M.J., Rakovan, J., Hughes, J.M. (Eds.),

Phosphates –geochemical, geobiological, and materials importance. Rev. Mineral. Geochem., 48, 255-292. [3] Tepper, J.H. and Kuehner, S.M. (1999) Complex zoning in apatite from the Idaho batholith: a record of magma mixing and

intracrystalline trace element diffusion. Am. Mineral., 54, 581-595. [4] Broska, I., Petrík, I. and Uher, P. (2012) Accessory minerals of the Western Carpathian granitic rocks, VEDA Press,

Bratislava, 236 pp.

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Comparing and contrasting W-Mo mineralization from the Sisson Brook, Mount Pleasant, Lake George and Burnthill deposits, New Brunswick,

Canada WEI ZHANG1, CHRISTOPHER R.M. MCFARLANE1, DAVID R. LENTZ1, KATHLEEN G. THORNE2

1 Department of Earth Sciences, University of New Brunswick, Frederiction, New Brunswick, Canada; Wei.Z@unb.ca 2 New Brunswick Department of Energy and Mines, Fredericton, New Brunswick, Canada The Sisson Brook (W-Mo-Cu, ca. 378 Ma, Re-Os molybdenite, unpublished), Mount Pleasant (W-Mo-Bi and Sn-Zn-In, ca. 363 Ma, U-Pb zircon), Burnthill (W-Mo-Sn) and Lake George (Sb-Au and W-Mo, ca. 412 Ma, U-Pb zircon) deposits are related to metaluminous to peraluminous granitoids, which exhibit transitional I-type to A-type granite signatures. They are generally evolved (> 65% SiO2) and enriched in incompatible elements. The anomalous behavior of these elements was caused by a combination of crystal fractionation followed by aqueous phase saturation and separation, a process whose activity is corroborated by field and petrographic evidence (e.g., miarolitic cavities, myrmekite) [1-3]. The faults and fractures, in both endo- and exo-granitic settings, offer conduits for hydrothermal ore fluid flow and deposition of mineralization. The contact metasomatic rocks vary considerably and control the proportion of wolframite and scheelite in stockwork veins and replacement zones. The skarn-like alteration at Lake George promoted scheelite deposition, whereas the decrease of the aFe/aCa in the Sisson Brook deposit produced scheelite rims on wolframite in some vein systems. At the Lake George deposit, the scheelite and molybdenite deposition was controlled by decreasing temperature (a function of distance from cupola) and increasing pH (H metasomatism and CO2 effervescence). For the Sisson Brook deposit, the consumption of K in the fluids during alteration destabilized the hydrothermal HKWO3 and liberated Ca2+ ions to form scheelite. The formation of the porphyry W-Mo orebodies at Mount Pleasant was related to high fluid pressures caused by crystallization of the underlying granitic magma that led to fracturing and breccia formation [4-6]. References cited: [1] Yang, X.M., Lentz, D.R., Chi, G.X. and Thorne, K.G. (2008) Geochemical characteristics of gold-related granitoids in

southwestern New Brunswick, Canada. Lithos, 104, 355-377. [2] McLeod, M.J., Johnson, S.C. and Krogh, T.E. (2003) Archived U-Pb (zircon) dated from southern New Brunswick. Atlantic

Geology, 39, 209-225. [3] Tucker, R.D., Bradley, D.C., Ver Straeten C.A., Harris, A.G., Ebert, J.R. and McCutcheon, S.R. (1998) New U-Pb zircon

ages and the duration and division of Devonian time. Earth and Planetary Science Letters, 158, 175-186. [4] Seal II, R.R., Clark, A.H., and Morrissy, C.J. (1987) Stockwork tungsten (scheelite) – molybdenum mineralization, Lake

George, Southwestern New Brunswick. Economic Geology, 82, 1259-1282. [5] Nast, H.J., and Williams-Jones, A.E. (1991) The role of water-rock interaction and fluid evolution in forming the porphyry-

related Sisson Brook W-Cu-Mo deposit, New Brunswick. Economic Geology, 86, 302-317. [6] Kooiman, G.J.A., McLeod, M.J., and Sinclair, W.D. (1986) Porphyry tungsten-molybdenum orebodies, polymetallic veins

and replacement bodies, and tin-bearing greisen zones in the Fire Tower Zones, Mount Pleasant. New Brunswick. Economic Geology, 81, 1356-1373.

 

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Large porphyry-type deposits in Mongolia

OCHIR GEREL1 1 Department of Geology, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia; gerel@must.edu.mn Porphyry systems supply most of the copper and significant gold to Mongolia’s economy. Porphyry Cu-Au-Mo deposits and occurrences are associated with typical calc-alkaline metaluminous, oxidized, I-type, magnetite series granitoids. This type is the dominant granitoid type in Mongolia formed during subduction processes. The largest porphyry-type deposits are the Devonian porphyry-Cu-Au deposits of Oyu Tolgoi, Tsagaan Suvarga in South Mongolia, and late Paleozoic and early Mesozoic Cu-Mo deposits of Bayan Uul and Erdenet in central and north Mongolia. Ore mineralization in the Oyu Tolgoi is associated with monzodiorite porphyry and, in part, with a granodiorite porphyry intrusion. The intrusive complex is intensely overprinted by advanced argillic alteration. Copper mineralization is bornite-dominant, with subordinate chalcopyrite, minor chalcocite, pyrite, enargite and tennantite. The Erdenet Cu-Mo porphyry deposit is situated within the Orkhon-Selenge trough, filled by Permian and early Mesozoic volcanic and sedimentary rocks, intruded by late Permian-Triassic Selenge Complex granitoids of calc-alkaline series. Mineralization of the Erdenet Cu-Mo deposit is represented by chalcopyrite and molybdenite associated with granodiorite porphyry, the latest intrusion cuts the host Triassic Erdenet pluton of Selenge Complex [1]. Three mineralization stages are distinguished: (1) quartz-chalcopyrite-pyrite and quartz-pyrite-molybdenite-chalcopyrite; (2) quartz-chalcopyrite-tennantite; (3) quartz-chalcopyrite-galena-sphalerite and overprinting bornite-chalcocite-covellite. Porphyry deposits formed in an island or a continental arc environment and are associated with calc-alkaline porphyry stocks, intruded volcanic and granitoid rocks in island arc or orogenic series in continental arcs. Porphyry stocks are latest to be emplaced, and close to their host granitoids [1]. Host granitoid series and mineralized stock granitoids are enriched in large-ion lithophile elements and depleted in high-field-strength elements. All of these rocks plot in the field of volcanic-arc granites, suggesting that they formed during subduction. A fluid source is in the mantle, but very important also are melting-assimilation-storage-homogenization processes in the crust. Intrusive rocks have 87Sr/86Sr = 0.703-0.705 [1,2]. Multistage porphyries are typical. Cu-Mo mineralization is likely associated with subalkaline granitoids of Na-series, and Cu-Au mineralization with K-series, which is exemplified by Oyu Tolgoi and Erdenet. References cited: [1] Gerel, O. and Munkhtsengel, B. (2005) Erdenetiin Ovoo porphyry copper-molybdenum deposit in Northern Mongolia. In: Geodynamics and Metallogeny of Mongolia with special emphasis on copper and gold deposits (R. Seltmann, O. Gerel and D. Kirwin, eds.). London, 85-105. [2] Gerel, O. and Oyunchimeg, R. 2008. Granitoids and associated mineralization in Mongolia: with emphasize to porphyry systems. In: Granites and Earth’s Evolution: Geodynamic Setting, Petrogenesis and Ore Content of Granitoid Batholiths, Proceedings of the 1st International Geologic Conference, Ulan-Ude, 283-286.

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Evolution of rare-earth mineralized carbonatites at Lugiin Gol and Omnot Olgii, southern Mongolia

JINDRICH KYNICKY1, ANTON R. CHAKHMOURADIAN2, CHENG XU3, MARTIN BRTNICKY1, MICHAELA VAŠINOVÁ GALIOVÁ4, VERONIKA KRÁLOVÁ5

1 Department of Geology and Pedology, Mendel University in Brno, Czech Republic; kynicky@mendelu.cz 2 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada; chakhmou@cc.umanitoba.ca 3 Laboratory of Materials of the Earth's Interior and Geofluid Processes, Institute of Geochemistry, Chinese Academy of

Sciences, Guiyang, China 4 Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic 5 TESCAN, a.s., Brno, Czech Republic; veronika.kralova@tescan.cz A large suite of fresh and metasomatically modified carbonatites from the Lugiin Gol and Omnot Olgii complexes in South Gobi (Mongolia) was examined. These carbonatites have a clear economic potential owing to their overall high content of rare-earth elements (REE). Exploration is currently underway to determine the commercial viability of these deposits. The samples studied in this work were collected from outcrop and drill-core. Representative chondrite-normalized REE profiles of the Lugiin Gol and Omnot Olgii carbonatites have a steep negative slope and lack any anomalies. In addition, all analyzed samples are characterized by extremely high abundances of Ba and Sr, coupled with low levels of Nb, Ta, Ti, K, Rb and Cs. Their age of emplacement was determined by U-Pb dating of zircon [1] as Early Triassic (ca. 240 Ma), i.e. post-orogenic. The carbonatites show also unusually low, mantle-derived carbon isotopic characteristics (on average, –8.6‰ δ13CV-PDB) and normal oxygen isotopic characteristics (on average, 10.4‰ δ18OV-SMOW), indicating an igneous origin with a possible contribution from reworked oceanic crust. The carbonatites are represented predominantly by coarse-grained sövite composed of magmatic calcite, minor to accessory Na-Sr-REE-bearing apatite, and a plethora of rare-earth carbonates whose modal content locally reaches 30%. The rocks are paragenetically diverse and contain both primary carbonates (burbankite–calcioburbankite series and REE fluorocarbonates) and hydrothermally or metasomatically derived phases associated with strontianite, fluorite, barite, celestine and quartz. The primary fluorocarbonates are represented by zoned synchysite-(Ce) crystals (synchysite I) with domains of bastnäsite-(Ce) and röntgenite-(Ce). Some samples contain phenocrysts of euhedral parisite-(Ce) sometimes mantled by synchysite I. All primary carbonates and primary fluorite are also important components of feldspathoid silicate rocks associated with the carbonatites [2] and indicate protracted magma differentiation involving immiscibility of silicate, carbonate and fluoride melts at lower temperatures, and late magmatic processes. It appears that this complex evolution was key to the formation of strongly evolved REE-mineralized carbonatites. References cited: [1] Kynicky, J., Chakhmouradian, A.R., Davis W., Xu, C., Vašinová Galiová, M., Králová, V., Brtnický, M. (2013) Origin and

evolution of the Lugiin Gol carbonatites (southern Mongolia) and associated rare-earth mineralization. Ore Geology Reviews (in preparation).

[2] Králová, V., Kynicky, J., Chakhmouradian, A.R. (2013). Application of integrated mineral analysis to the study of disseminated REE mineralization in silicate rocks. CM2013 Program with Abstracts (this volume).

22

Garnet, a major yttrium and heavy rare earth carrier in A-type granites: mineral mass-balance calculation study

SVEN HÖNIG1, RADEK ŠKODA1, RENATA ČOPJAKOVÁ1, JAROMÍR LEICHMANN1, MILAN NOVÁK1

1 Department of Geological Sciences, Faculty of Science, Masaryk University Brno, Kotlářská 2, Brno, Czech Republic; honig@mail.muni.cz

Magmatic garnet (spessartine-almandine) often occurs as an accessory mineral in S-type granites, less frequently in I- and A- type granitic rocks. Although garnet considered a very sensitive recorder of metamorphic history of the rock, very little studies on the behavior of yttrium and rare earth elements (REE) in garnet-bearing magmatic rocks were published up to date [1-2]. The residence of Y+REE within the post-collisional anorogenic granite bodies is studied here, highlighting individual Y+REE fractions of the individual mineral phases such as rock-forming minerals (quartz, plagioclase, potassium feldspar) and accessory minerals (garnet, muscovite, magnetite, epidote, monazite to secondary phosphates) and their contribution to the Y+REE rock budget. Minor to accessory garnet (< 2 vol.%; almandine-spessartine with Sps41-46Alm28-44And0-13Grs6-12Prp0-1), enriched in Y+REE (up 1.54 wt.% Y, 1 wt.% ΣREE) was distinguished as a very significant Y+REE reservoir in the rock. The studied garnet hosts 84% of the total Y and 61% of the REE in the rock. Zircon is another important carrier of REE in the granite and accounts for 13% of the whole-rock Y budget and 11% of the REE budet. Based on these results, at least 63% of light REE (LREE) is probably hosted by monazite, which is altered to the mixture of secondary REE-bearing phosphates and clay minerals. The Y+REE fractions of major rock-forming minerals (quartz and feldspars), despite their high modal amount (94 vol.%), is low (1% Y, 10% LREE, 1% heavy REE, HREE), excluding Eu which resides predominantly in the feldspars (90%). Minor to accessory muscovite and magnetite contribute 1% Y and 2% REE each to the whole-rock budget. The amount of Y+REE residing in accessory epidote is negligible. These data are consistent with the previous mass-balance calculation studies, where mineral fractions were calculated [3-4]. However, the garnet has never been considered a major Y+HREE cerrier in granites. The studied granite is part of the Hlína suite within the Cadomian Brno batholith, Czech Republic. The Hlína rocks are metaluminous to slightly peraluminous (alumina saturation index = 0.90-1.08), garnet-bearing leucogranites of anorogenic affinity and occur as small irregular plutons intruding older I-type granites. Garnet is arranged in train-like textures concordant to the intrusive contacts [5]. References cited: [1] Wang, R.C., Fontan, F., Chen X. M., Hu, H., Liu, C.S., Xu, S.J., de Parseval, P. (2003) Accessory minerals in the Xihuashan

Y-enriched granitic complex, southern China: a record of magmatic and hydrothermal stages of evolution. Canadian Mineralogist, 41, 727-748.

[2] Müller, A., Kaersley, A., Spratt, J., Seltmann, R. (2012) Petrogenetic implications of magmatic garnet in granitic pegmatites from southern Norway. Canadian Mineralogist, 50, 4, 1095-1115.

[3] Bea, F. (1996) Residence of REE, Y, Th and U in granites and crustal protholiths; implications for the chemistry of crustal melts. Journal of Petrology, 37, 521-552.

[4] Dahlquist, J.A. (2001) REE fractionation by accessory minerals in epidote-bearing mataluminous granitoids from the Sierras Pameanas, Argentina. Mineralogical Magazine, 65, 463-475.

[5] Hönig, S., Leichmann, J., Novák, M. (2010) Unidirectional solidification textures and garnet layering in Y-enriched garnet-bearing aplite-pegmatites in the Cadomian Brno Batholith, Czech Republic. Journal Geosciences, 55, 113-129.

23

The concentration regularity of indium in sphalerite in some different type Pb-Zn deposits, China

LIN YE1, TAN BAO, LIZHEN LI, YULONG YANG

1 State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China; yelin@vip.gyig.ac.cn

The crustal abundance of indium is estimated to be about 0.056 ppm [1]. Normally, In is enriched in minerals containing tetrahedrally coordinated cations in their structure, among which sphalerite is the most economically significant In-bearing mineral. Sphalerite scavenges a wide variety of elements, such as Fe, Mn, Cd, Ge, Ga and In [2,3]. We studied the trace element composition of sphalerite from different Pb-Zn deposits in China, including Mississippi-Valley-type (MVT), skarn, sand-hosted, syngenetic massive sulfide and magmatic hydrothermal-type deposits, and made the following conclusions regarding the distribution of In in this mineral: 1. Indium is enriched in high-temperature sphalerite. Normally, the ore-forming temperature is above 250 °C, and the Fe content of sphalerite is higher than 10 wt% in this type of sphalerite. For example, in syngenetic massive sulfide deposits, the concentration of In is 111-415 ppm (mean 227 ppm, n = 26), 58-566 ppm (mean 196 ppm, n = 38) and 3-262 ppm (mean 71 ppm, n = 18) at Dabaoshan, Laochang and Bainiuchang, respectively. In magmatic hydrothermal Pb-Zn deposits, the concentration of In is 10-566 ppm (mean 98 ppm, n = 41) and 118 ppm [4] at Baoshan and Meng'entaolegai. 2. The concentration of indium is very low in low-temperature sphalerite. The ore-forming temperature is usually below 200 °C in MVT Pb-Zn deposits, and the content of In in sphalerite is less than 10 ppm; some examples include Huize (mean 0.7 ppm, n = 24), Niujiaotang (mean 0.1 ppm, n = 26), Mengxing (mean 0.7 ppm, n = 24). Jinding is a sand-hosted Pb-Zn deposit; its ore-forming temperature is between 110 to 150 °C, and the content of In in sphalerite is lower than 0.5 ppm. Although the ore-forming temperature of skarn-type Pb-Zn deposits is higher relative to MVT deposits (200-250 °C), the content of indium in sphalerite is lower; a few examples are Hetaoping (mean 0.05 ppm, n = 24) and Luziyuan (mean 0.1 ppm, n = 57). 3. A number of tin minerals occur in indium-rich Pb-Zn deposit, showing the possible relationship between In and Sn. The correlation coefficients between these two elements are 0.64 and 0.41 at Baoshan and Dabaoshan, respectively. 4. Yanshanian-Himalayan intermediate-acidic intrusive rocks may have played an important role in the enrichment of In in the Chinese Pb-Zn deposits.

References cited: [1] Rudnick, R.L. and Gao, S. (2005) Composition of the continental crust. In: R.L. Rudnick, Editor, The Crust. Treatise on

Geochemistry, Volume 3, Elsevier, Amsterdam, 1-64. [2] Cook, N.J., Ciobanu, C.L., Pring, A., Skinner, W., Shimizu, M., Danyushevsky, L., Saini-Eidukat, B. (2009) Trace and minor

elements in sphalerite: A LA-ICPMS study. Geochimica et Cosmochimica Acta, 73, 4761-4791. [3] Ye, L., Cook, N.J., Ciobanu, C.L., Liu, Y.P., Zhang, Q., Liu, T.G., Gao, W., Yang, Y.L., Danyushevsky, L. (2011) Trace and

minor elements in sphalerite from base metal deposits in South China: a LA-ICPMS study. Ore Geology Reviews, 39, 188-217.

[4] Zhang, Q., Liu, Z.H., Zhan, X.Z., Shao, S.X. (2003) Specialization of ore deposit types and minerals for enrichment of indium. Mineral deposits, 22(1), 309-316 (in Chinese with English abstract).

24  

Mineralogy of carbonatite from the Lugiin Gol alkaline pluton, South Mongolia

DAGVA BATBOLD1 1 Orkhon Exploration CoLtd, Ulaanbaatar, Mongolia; email: dbbold62@yahoo.com

Minerals of calcitic carbonatite rich in lanthanides (rare earth elements, REE) from the Lugiin gol alkaline pluton, South Mongolia, were investigated in this study. This carbonatite deposit is located in the eastern part of the Gobi desert, within a Paleozoic folded belt. The carbonatite from the Lugiin gol pluton is calcitic in composition and forms exclusively veins or dikes. Spatially and genetically, it is related to autometasomatically altered nepheline syenites. Mineralogically, this carbonatite consists mainly of calcite, synchysite, fluorite, quartz and pyrite. Uniquely high concentrations of synchysite were observed in this carbonatite. Representative samples were collected from almost all of lateral and vertical parts of the carbonatite. Electron microprobe analysis was applied as a major method for the study of minerals. Microscopic observation and X-ray powder diffractometric analysis were used as well. More than 27 mineral species were investigated in detail using these methods. Among them, several species, such as Nd-synchysite and Sr–roentgenite, which were not previously reported from carbonatites, were discovered in this study. Many of the minerals, such as boulangerite, mica, siderite and some other identified and analyzed in this study, are also described for the first time from the Lugiin gol deposit. Unique compositions and combinations of REE fluorocarbonates, such as synchysite-bastnäsite syntaxial intergrowths, were observed and documented. Most of the REE, which are represented almost entirely by the cerium group (light rare earths from La to Gd), are concentrated in the fluorcarbonates, mainly synchysite and parisite. Mineralogical and geological observations provide evidence for moderate-temperature hydrothermal origin of the studied carbonatite. The Lugiin gol carbonatite is interpreted as representative of the chlorite-sericite-ankerite T-facies of Samoilov [1]. In general terms, it is REE-rich sövite. It is notable that the present study is the first case of complex mineralogical investigation of Mongolian carbonatites and also is the first example where carbonatite mineralogy was studied by a Mongolian geologist.

References cited: [1] Samoilov, V.S., and Kovalenko, V.I. (1984) Complexes of alkaline rocks and carbonatites in Mongolia. The joint Soviet-Mongolian Scientific-Research Geological Expedition, Transactions, vol. 35, 196 pp.

25

Alkaline granites geochemical studies of the Khanbogd pluton, South Mongolia

SERJLKHUMBE AMAR-AMGALAN1 and DASH BAT-ULZII2

1 Oyu Tolgoi LLC., Monnis Tower, Chinggis Avenue 15, Sukhbaatar district, Ulaanbaatar, Mongolia; amaramgalans@ot.mn 2 School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, Baga Toiruu-34,

Sukhbaatar district, Ulaanbaatar, Mongolia; ulziid@yahoo.com

Geochronological and geochemical data for alkaline granites reveal geochemical features of Permian alkaline granites of the Khanbogd pluton in South Mongolia. The Khanbogd pluton occurs in an island arc terrane [1] that comprises Silurian-Devonian and Carboniferous sedimentary and volcanic rocks. The main alkaline granites are aegirine, arfvedsonite-bearing granites. Ekerite, pantellerite and syenite compositions were described from small bodies and dykes [3, 5]. They show an Rb-Sr whole rock isochron age of 295± 7 Ma [2], which is consistent with previously reported data, including a zircon U-Pb age of 290 ±1 Ma by evaporation method [3] and K-Ar hornblende age [4].

Main constituent minerals of the alkali granites are aegirine, arfvedsonite, potassium feldspar, albite and quartz. Accessory minerals are fluorite, apatite, zircon, titanite, allanite and elpidite. Two types of ekerite are identified, coherent and foliated. The coherent ekerite comprises fine-grained quartz, albite and arfvedsonite. The foliated one is characterized by albite and arfvedsonite zones. Pegmatite varieties are: arfvedsonite-rich, elpidite-rich and albite-arfvedsonitic. Elpidite is an as accessory mineral in all types of granite. The elpidite content ranges from 0.1 to 5%. In pegmatites, elpidite is an REE ore mineral. The albite, arfvedsonite and elpidite associations were also noticed in fissures within the granites. The zirconium silicate armstrongite and niobium silicate mongolite were first identified at this pluton [4].

The alumina saturation index ranges from peralkaline through metaluminous to weakly peraluminous in the granites. The Khanbogd granites are characterized by high K, Zr, Nb (27.8-221 ppm), Ga, Y and F contents, but low MgO, CaO and Sr contents and high FeO*/MgO and Ga/Al ratios. These rocks show flat chondrite-normalized REE patterns with a strong negative Eu anomaly [2]. The εNd value ranges from +5.08 to +6.93 and εHf from +12.8 to +15.9 [5].

The geochronological and geochemical data show that the pluton formed in the post-accretion period of the South Mongolian arc evolution. References cited: [1] Badarch, G., Cunningham, W.D., Windley, B.F. (2002) Journal of Asian Earth Sciences, 21, 87-110. [2] Amar-Amgalan S. (2004) Petrological and geochemical studies on the Khanbogd alkaline complex, South Mongolia.Master

thesis, Shimane University, Japan. 96 pp. [3] Kovalenko, V.I., Yarmoluyk, V.V., Salnikova, E.B., Kozlovsky, A.M., Kotov, A.B., Kovach, V.P., Savatenkov, V.M.,

Vladykin, N.V., and Ponomarchuk, V.A. (2007) Geology, geochronology, and geodynamics of the Khan Bogd alkali granite pluton in Southern Mongolia. Geotectonics, 450-446.

[4] Vladykin, N.V., Kovalenko, V.I. and Dorfman, M.D. (1981) Mineralogical and geochemical features of Khanbogd pluton of alkaline granites. Nauka Press. 134 pp.

[5] Amar-Amgalan S. (2008) U-Pb geochronology and multi-isotopic systematic of granitoids from Mongolia, Central Asian Orogenic Belt: Implications for granitoid origin and crustal growth during the Phanerozoic. Doctoral thesis. Institute for Study of the Earth’s Interior, Okayama University, Japan. 138 pp.

26

A new type of REE-bearing deposit associated with the Blockspruit fluorite mine, Republic of South Africa

MIHOKO HOSHINO1, YASUSHI WATANABE1, MAIKO TSUNEMATSU1 1 National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan; hoshino-m@aist.go.jp

Volatile components, such as fluorine, enhance rare earth element (REE) enrichment in melts, explaining why some fluorite deposits contain high REE contents. Fluorite deposits may be viable exploration targets as REE resource. Many fluorite deposits occur in the Bushveld granitic complex, consisting of the Proterozoic Nebo granite, in the Republic of South Africa. Our research group has carried out a field survey of the fluorite deposits since 2007. The REE-bearing fluorite deposits are classified into two types based on whether the host rock is alkaline ultramafic (Vergenoeg, Blockspruit, Ruigterpoort) or granite (Buffalo, Slipfontein). The former type is enriched in heavy rare earth elements (HREE), and the latter in light rare earth elements (LREE). The Blockspruit fluorite deposit is located in the western part of the Bushveld complex. HREE-rich amphibolites intrude into the Nebo granites at the Blockspruit deposit. In this study, the surface survey of the Blockspruit deposit was conducted in order to clarify the wide-area distribution of the HREE-rich amphibolites. Mineralogical and geochemical analyses (portable X-ray fluorescence, inductively-coupled-plasma mass-spectrometry, mineral liberation analysis and X-ray diffraction) were performed for the collected samples. The results of the surface survey show that the fresh HREE-rich amphibolite intruding into the Nebo granite extends over at least 600 m2. In the Blockspruit deposit, there are some old trenches remaining from fluorite mining operations. Thick weathered amphibolites from the trenches show high REE contents (3395 ppm or 0.40 wt.% REE2O3). Analysis by energy-dispersive spectrometry reveals that fresh amphibolites contain a large proportion of REE-free apatite [Ca5(PO4)3(OH,F,Cl)] replaced by xenotime (HREEPO4) and monazite (LREEPO4). Generally, monazite-(Ce), which is a relatively commonly mineral, has Ce as the predominant element among the REE. However, the composition of monazite from the Blockspruit amphibolite is dominated by Nd and the mineral should be classified as monazite-(Nd). Interestingly enough, the weathered amphibolites contain phosphate minerals such as xenotime-(Y) and monazite-(Nd), but lack apatite. Monazite in the weathered amphibolite is also monazite-(Nd). These data suggest that REE-free apatite grains replaced by xenotime-(Y) and monazite-(Nd) in the fresh amphibolites were completely decomposed by F-rich hydrothermal fluid and only the xenotime-(Y) and monazite-(Nd) grains remain in the weathered amphibolites near the surface. At Blockspruit, the residual type deposit formed by weathering of HREE-rich amphibolite affected by F-rich hydrothermal fluid and representing a new type of REE deposit. The HREE ratio to the total REE content in the weathered amphibolite is 41% with 4% (209 ppm) Dy. This ratio is comparable with those calculated for other HREE exploration projects in the world, although the average grade is lower at Blockspruit than in the projects involving alkaline rocks. Most of the HREE in the weathered amphibolites are included in phosphate mineral phases, such as xenotime and monazite. The REE ores in this deposit are HREE-rich amphibolites and their weathered residues. Because the weathered amphibolites contain high xenotime content (3%), these minerals are amenable to ore mineral concentration.

27

The Mabounié carbonatite (Gabon), the source of a polymetallic lateritic ore deposit

SIMON B. BLANCHER1 and ISABELLE DUHAMEL-ACHIN1 1 ERAMET Research, 1 av. Albert Einstein, Trappes, France; simon.blancher@erametgroup.com

The carbonatite of Mabounié (Gabon), first described in 1988 [1], is a complex igneous intrusion approximately 2.3 km in diameter hosted in Paleoproterozoic fenitized gneisses and migmatites. Numerous mineralogical and geochemical studies were conducted in the twenty years following its discovery ([2-4], BRGM, pers. comm.) that led to the discovery of a polymetallic lateritic ore deposit above the intrusion. In the present work, new petrographic studies are reported on fresh drill core samples obtained within the framework of the Maboumine project. The carbonatite is heterogeneous sövite, crosscut by a few dikes of rauhaugite (dolomite-rich carbonatite). The mineralogy varies strongly along magmatic bedding, schlieren patches and hydrothermally recrystallized areas [4]. Apatite-sövite and phlogopite-sövite are common and intermixed with each other, and contain a variable amount of Ti-rich magnetite (in some cases, up to 40 vol.% of the rock). Other accessory minerals include rare-earth carbonates, ilmenite, pyrochlore, zircon, baddeleyite, calzirtite and zirconolite. In the lateritic profile developed above the carbonatite suite, the lowermost horizon consists of massive phosphates and is characterized by the dissoluion of carbonates; it is followed by “ribbon” laterites characterized by the breakdown of apatite, and finally by an iron-oxide-rich horizon at the surface of the deposit. Even though pyrochlore is affected by different processes and its composition changes slightly across the profile [3], this mineral remains the main carrier of rare metals (Nb, Ta) and rare earth elements, which are concentrated upward mainly in the “ribbon” ore.

References cited: [1] Laval, M., Johan, V., Tourliere, B. (1988) The Mabounié carbonatite: example of the formation of a residual deposit with

pyrochlore, Chron. Rech. Min., no. 491, 125-136. [2] Makanga, J.F. and Edou-Minko, A. (2003) Etude petrographique et geochimique du complexe annulaire de Mabounié

(Gabon). Afr. J. Sci. Technol., 4, 67-77. [3] Piantone, P., Itard, Y., Pillard, F., Boulingui, B. (1995) Compositional variation in pyrochlores from the weathered Mabounié

carbonatite (Gabon). In J. Pasava, B. Kribek, and K. Zak, Eds., Mineral deposits; from their origin to their environmental impacts: Rotterdam, A.A. Balkema Publishers, Third Biennial SGA Meeting, 629-632.

[4] Boulingui, B. (1997) Minéralogie et géochimie du gisement résiduel de phophore et niobium de Mabounié (Moyen-Ogooue, Gabon). Unpubl. PhD. Thesis, Institut National Polytechnique de Lorraine, 150 pp.

28  

Geology and mineralization at the Ulaan Del Zr-Nb-REE occurrence, Western Mongolia

S. OYUNBAT1

1 Geo-Info LLC, Mongolia; oyunbat@geo-info.mn

The host geology of the Ulaan Del Zr-Nb-REE occurrence comprises granodiorite and diorite of the Middle-Late Cambrian Togtohynshil complex, dikes of the Middle-Late Devonian Halzan complex and Quaternary to Recent unconsolidated sediments. The Devonian dikes vary in composition from acidic to subalkaline rhyolite, rhyolite-porphyry, microdiorite, syenite, dacite, trachyrhyolite and pyroclastic breccias related to tectonic and hydrothermal(-metasomatic) events associated with the proximal Tsagaan Shiveet deep fault zone. Metasomatically altered dikes consisting of feldspar and quartz are 0.5-2 m in width and up to 500 m in length. Mineralization is represented by isolated grains and thin veinlets of Zr-bearing minerals, some of which are enriched in Ti and Mn and are commonly associated with lilac fluorite. The predominant type of alteration is potassic feldispar-albite with minor fluorite, sericite, quartz and clay. Weak and restricted brecciation developed along the junctions of dikes and faults, or zones of weakness. The potassic alteration is mostly developed as red to pinkish-red spots at the exocontact of the dikes. Greisenization overprints this reddish alteration, but is less prominent. Fluorite was noted in all areas affected by potassic alteration, where it occurs as small phenocrysts and pore-filling material in the host rocks. Rarely, fluorite also occurs as very short and thin veinlets. Fluorite is dark lilac to purple and cryptocrystalline. Isolated grains, spots and thin veinlets of dark brown to brownish(-yellow), tabular to isometric minerals are observed where their host rocks is turned reddish or reddish-brown. Sometimes, these minerals are developed as veins or irregular lense-like plates in the rock. The Zr grade varies from 0.2 wt.% to 1%; noteworthy associated elements are Nb (0.03-0.15%), Hf (0.002-0.02%), Mn (0.05-0.2%), Ti (0.05-0.3%), Mo (0.015-0.07%), Cu (0.015-0.02%), Pb (0.015%) and Zn (0.02-0.1%; all values from the Central Geological Laboratory, Mongolia) in samples taken from various dikes. The altered rocks consist of feldspars, quartz, muscovite, zircon, fluorite, titanite, apatite, albite and sericite. In thin section, red and reddish ore minerals completely replaced by hematite and magnetite were identified. Hematite occurs as space-filling grains, or replaces biotite. Synchysite and xenotime were identified by electron-microprobe analysis.

29

The nature of ore-forming fluids at the Huanglongpu carbonatite-hosted Mo deposit, Shanxi, China: Evidence from fluid inclusions and stable isotopes

CHENG XU1 and WENLEI SONG2

1 Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China; xucheng1999@pku.edu.cn

2 Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China; wlsong99@163.com

The global Mo supply is derived almost completely from porphyry-type ore deposits. Economic Mo mineralization associated with carbonatites has so far been reported only from the Lesser Qinling orogen in central China [1]. Several types of fluid inclusions are found in calcite and quartz from carbonatite dikes at the large Huanglongpu Mo deposit in Shanxi. Based on a petrographic study of primary and pseudosecondary fluid inclusions, they can be divided into six types, including those consisting of (1) a pure vapor phase (H2O-enriched and CO2-dominanted gases), (2) an aqueous phase, (3) aqueous-carbonic phase, (4) CO2 vapor, (5) a solid-bearing aqueous phase, and (6) solid-bearing aqueous-carbonic phase. Microthermometric results gave homogenization temperatures of 208-425 °C and 227-338 °C for types (2) and (3) and salinities of 12-22 wt.% NaCl equivalent and 2.2-7.9 wt.% NaCl equivalent, respectively. Crystal habits and results of micro-Raman spectroscopic analysis demonstrate that the transparent daughter minerals in fluid inclusions in quartz are mirabilite (Na2SO4⋅10H2O), anhydrite (CaSO4), glauberite [Na2Ca(SO4)2], glaserite [K3Na(SO4)2], halite, sylvite, celestite and calcite. The opaque solid minerals characterized by triangle- or hexagon-shaped habits are identified as molybdenite and Pb-bearing minerals. The Raman studies reveal that there is abundant (SO4)2- in the liquid phase of aqueous fluid inclusions, and (HCO3)- in the liquid phase in CO2-bearing aqueous fluid inclusions. The δ18OV-SMOW values of quartz from the carbonatite dikes range from 8.8 to 10.2‰ and are consistent with the composition of associated calcite (7.22 to 9.19‰). The δ34SCDT ratio of molybdenite, galena and pyrite ranges from -6.69 to -7.68‰, -8.87 to -10.54‰ and -6.55 to -7.15‰, respectively. Barite gave a δ34SCDT range of 4.61 to 5.12‰. Based on S isotopic fractionation between sulfide and sulfate minerals, the total δ34S of the ore-forming fluids is about 1-2‰, suggesting a mantle-derived source. The results of the present study suggest that the ore-forming fluids at Huanglongpu were characterized by moderate to high salinity, represented a CO2-dominanted system, were derived from mantle-derived carbonatitic liquids, and had the capacity to transport Mo.

Reference cited: [1] Xu, C., Kynicky, J., Chakhmouradian, A.R., Qi, L., Song, W.L. (2010) A unique Mo deposit associated with carbonatites in

the Qinling orogenic belt, central China. Lithos, 118, 50-60.

30  

Preliminary results on major, trace and REE chemistry of the Dancherla syenite body, Ananthapur district, Andhra Pradesh, India

P.V. SUNDER RAJU1 and D.V.SUBBA RAO1 1 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad, India; perumala.raju@gmail.com Preliminary assessment of REE (rare earth elements) and rare-metal potential and geochemical characterization, with implications for metallogeny, is currently underway at alkaline syenite complexes in Andhra Pradesh (coordinates for the Dancherla alkaline complex: 77°25′–30′ E; 15°0–10′ N). The Dancherla alkaline complex is located to the west and south-west of the Paleoproterozoic Cuddapah basin in the Ananthapur district. The Dancherla complex hosts several syenite bodies, but our efforts focused on Dancherla, Peddavaguru and Danduvaripalle localities. The main Dancherla syenite body is oval-shaped and has a total area at the current level of exposure of around 18 km2. It consists of medium- to fine-grained mesocratic syenite, leucocratic, porphyritic and quartz syenite in cross-cut relations with diabase-hornblendite dyke swarms. Major element distributions show simple trends of decreasing MgO, FeO, P2O5 and TiO2 contents with increasing SiO2. Bulk-rock chemical analyses gave > 8 wt.% Na2O, > 5 wt.% K2O, low CaO and MgO contents (< 5 wt.%) and ~16 wt.% Al2O3. In binary Na2O+K2O vs. SiO2 diagrams, the samples plot in the nepheline syenite field. Our preliminary geochemical data suggest enrichment in light REE and flat heavy-REE distribution in chondrite-normalized patterns. In some samples, positive Eu anomalies were detected, suggesting plagioclase enrichment. The maximum La and Ce concentrations recorded thus far are 200 and 350 ppm, respectively. Binary diagrams showing variations in Hf-Zr and Nb-Ta ratios show a positive correlation and indicate an overprint from externally derived fluids. The microprobe and electron-microscopy work is currently in progress.

 

Figure caption: (left) Dancherla syenite outcrop; (right) Yerrakonda syenite body.

 

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GEOLOGICAL HISTORY OF SOUTH MONGOLIA: AN OUTLINE Geotectonic setting of South Mongolia It has been widely accepted that South Mongolia underwent the accretion of a number of arcs and continental blocks during the Paleozoic, including island– and Andean-type magmatic arcs, rifted basins, accretionary wedges, and continental margins (Ruzhentsev et al., 1985; Zonenshain et al., 1990; Ruzhentsev and Pospelov, 1992; Sengör et al., 1993, 1994; Sengör and Natal’in, 1996). Paleozoic rocks of South Mongolia record a significant part of this process. Most authors agree that the Paleozoic and early Mesozoic time interval represents the growth of South Mongolia through the development and accretion of volcanic-arc material and fragments of continental crust (Ruzhentsev and Pospelov, 1992; Zorin et al, 1993; Sengör et al., 1993b). Several authors have proposed various tectonic models for the formation of Asian continent. For example, Sengör et al (1993a, 1993b, 1996) developed a palinspastic reconstruction for the Altaids, the region bounded by the Siberian platform, North China block, Tarim basin, and Ural Mountains. According to this tectonic model, South Mongolia is located in the eastern part of the Altaid collage (Fig. 1), which expanded outward from a single, long-lived arc system. Many workers interpreted Ordovician to Permian rocks in southern Mongolia as elements of volcanic arcs and microcontinents that developed and accreted as the southern margin of Mongolia evolved from a passive to an active setting (Ruzhentsev and Pospelov, 1992; Zorin et al., 1993). Each author, however, offered a different interpretation for the detailed tectonic evolution of South Mongolia. Lamb and Badarch (1997) suggested that the Ordovician – Silurian sequences of South Mongolia were formed in ocean margin environments, and the Devonian - Carboniferous units record the existence of a mature island arc or continental arc. Sengör and Natal’in (1996) stated that arc construction commenced in the Late Silurian with subduction of oceanic crust beneath the Tuva-Mongol arc, and that the large Kipchak and Tuva-Mongol magmatic arcs were active for much of the Paleozoic. They suggested that complicated collage tectonic units had formed throughout Central Asia by the Permian, by collision, shortening and strike-slip displacement, and that the Manchuride-Altaid suture at the Solonker zone was finally completed in the Late Permian. In the late Paleozoic, southern Mongolia was a period of rifting, which is similar in style to the Basin-and-range structural province of North America, and the rifting was accompanied by bimodal basalt, peralkaline granite and comendite magmatism (Kovalenko and Yarmolyuk, 1995) in a mature continental setting (Lamb and Badarch, 1997). Amalgamation of continental blocks and magmatic arcs took place by the end of the Paleozoic (Sengör et al., 1993) or beginning of the Mesozoic (Ruzhentsev et al., 1985; Zonenshain et al., 1990) with attendant unroofing of perisutural areas. Zorin et al. (1993) suggested that an Andean-style arc was formed along the southern edge of Mongolia during the middle Devonian, with a northward-dipping subduction zone and a possible back-arc basin to the north. They speculated that this back-arc basin might have opened far enough for the arc to become a detached island arc. By the end of the Early Carboniferous, the South Gobi Microcontinent accreted from the south, and the subduction zone stepped southward to the new southern edge of Mongolia, creating another Andean- style active margin. By the late Permian, the North China block collided with Mongolia, and since then thick terrestrial sedimentary rocks were deposited in South Mongolia.

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Fig. 1. Generalized tectonic map of the Altaids. Sengör and Natal’in, (1996) suggested that arc construction commenced in the Late Silurian with the subduction zone stepping southward to the new southern edge of Mongolia, creating another Andean-style active margin, which existed until the late Permian, when the North China block collided with Mongolia. Ruzhentsev and Pospelov (1992) proposed a more complex model for tectonic evolution of South Mongolia. They suggested that rifting of proto-Asia occurred in the early Paleozoic, creating several small continental blocks separated by ocean basins, and also suggested that these ocean basins began to collapse in the Middle Paleozoic, creating active volcanic arcs. Final collision with the North China block and accretion of all tectonic elements was completed in the Triassic.

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Fig. 2. Summary diagram of the tectono-stratigraphic, magmatic and mineralization history of the Gurvansayhan terrane. Data from Kovalenko and Yarmolyuk (1995), Hendrix et al. (1996), Lamb and Badarch (1997), Lamb and Cox (1998) and Watanabe and Stein (2000); simplified by Perello et al. (2001). Regional geology of South Mongolia The vast territory of Mongolia lies in the heart of the Central Asian Orogenic Belt (CAOB), one of largest provinces of Phanerozoic continental growth on Earth. The CAOB is fringed by the Precambrian Siberian Craton in the north and by the Tarim and North China-Korean cratons in the south. The CAOB is called the Altaid tectonic collage by Sengör et al. (1993). Mongolia is tectonically divided into northern and southern domains by the main Mongolian lineament, and it was previously thought that the former was formed mainly by the Caledonian orogeny and latter by the Hercynian (Variscan) orogeny. This idea, however, has been revised recently, because the terms Caledonian and Hercynian are inappropriate as time indicators for the CAOB. However, the basic two-fold subdivision for Mongolia still holds (Badarch et al., 2002). The southern domain, or South Mongolia, is important for clarification of the tectonic growth and geological history of Central Asia. Badarch et al. (2002) stated that South Mongolia is dominated by Lower to Middle Paleozoic arc-related volcanic and volcaniclastic rocks with fragments of ophiolites and serpentinite mélanges, and that Silurian and Devonian fossil-rich reef limestones, associated with terrigenous and volcaniclastic rocks, are distributed along its northern margin. They also noted that Pennsylvanian to Permian volcanic rocks are widespread in South Mongolia. Permian limestones, turbidites and minor volcanic rocks in the eastern part of the South Mongolia are interpreted as fragments within a suture zone between the Altaid collage and Manchuride

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by numerous Mesozoic rare-metal granite plutons and is unconformably overlain by Upper Jurassic to Cretaceous non-marine volcanic and sedimentary rocks. Badarch et al. (2002) presented alternative views on the tectonic evolution of Mongolia. They distinguished 44 tectonostratigraphic terranes and classified them into cratonic, metamorphic, passive margin, island arc, forearc/backarc, accretionary complex and ophiolite types (Fig. 2).

Fig. 3. Tectonostratigraphic terrane map for Mongolia after Badarch et al. (2002). Gurvansaykhan terrane: Host rocks to the Oyu Tolgoi deposit and Khan Bogd complex Lamb and Badarch (1997) studied three Silurian sequences in the Gurvansaykhan terrane, and clarified that the lower horizons of the Silurian sequences are dominantly composed of fine-grained cross-bedded sandstone and siltstone, indicative of low-energy marine conditions and that the upper horizons are largely made up of imbricated, matrix-supported pebble conglomerate and pebbly sandstone deposited in higher-energy, fan-delta environments. They also showed that sandstones in the uppermost horizons are mostly mature and have quartzose compositions with significant amounts of volcanic grains, whereas the conglomerate horizons are dominated by plutonic, volcanic and quartz fragments (Fig. 2). According to Lamb and Badarch (1997), middle to upper Devonian strata consists of basalts, massive limestone, siltstone and volcaniclastic pebble to cobble conglomerates with interbedded andesitic basalt flows and pillow lavas.

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Geochemical characteristic of the Devonian basalts indicate that this volcanism took place in an arc environment (Badarch et al., 2002). The terrane contains porphyry copper deposits, such as Tsagaan Suvarga and Oyu Tolgoi. The Tsagaan Suvarga porphyry copper ore has an Ar–Ar sericite age of 364.9±3.5 Ma (Lamb and Cox, 1998), and a Re–Os molybdenite age of 370.4±0.8 Ma (Watanabe and Stein, 2000). The age of the Oyu Tolgoi hypogene copper-gold deposit, located in the southeastern margin of the terrane, was initially determined as 411±3 Ma by K-Ar biotite geochronology (Perello et al., 2001), but subsequently revised as 388-382 Ma (U-Pb) to 372±1.2 Ma (Re-Os on molybdenite). The structure of the terrane is complex and dominated by imbricated thrust sheets, dismembered blocks, mélanges and high-strain zones. There are several mélange zones containing blocks of pillow lavas, fossiliferous limestone, sandstone, gabbro, diabase dikes and amphibolite. Carboniferous rocks consist of laterally changing facies dominated by volcaniclastic sandstone and conglomerate, as well as welded lithic-vitric rhyolite tuff, rhyolite- dacite flows, basalt and trachyrhyolite (Perello et al., 2001, Kirwin et al., 2005). These sequences are intruded by Late Devonian monzodiorite, and Carboniferous diorite, granite, granodiorite and syenite bodies ranging in size from dikes to batholiths. The Permian peralkaline Khanbogd granite complex (Kovalenko and Yarmolyuk, 1995; Garamjav, 1996, Kovalenko et al., 2006), which is characterized by high REE, Zr and Nb concentrations, occurs at the southern margin of the terrane, intruding the Siluro-Devonian and Carboniferous volcanic and sedimentary rocks in the southeastern part of the area. The terrane is overlain by Carboniferous, Permian, Jurassic and Cretaceous volcanic and sedimentary rocks. Petrographic characteristics and timing of granitic magmatism in the southern part of the CAOB The Central Asian Orogenic Belt (CAOB) is well-known for massive generation of juvenile crust in the Phanerozoic. It is voluminous granitic intrusions, mostly of juvenile character, that distinguish the CAOB from other classical Phanerozoic orogenic belts (Kovalenko et al., 1996; Jahn et al., 2000a,b, 2002; Jahn, 2003). The alkali-rich intrusive rocks in the CAOB are exposed in two separated belts (northern and southern). The northern belt extends from Transbaikalia in Russia to northern Mongolia and the southern belt from NE China through southern Mongolia to Xinjiang in western China. Based on geological and geochronological data, Zanvilevich et al. (1995) and Wickham et al. (1995, 1996) divided the alkaline (mainly potassic) magmatism in the northern zone into five stages: Ordovician to Silurian (stage 1), Devonian (stage 2), Early Permian (stage 3), Late Permian (stage 4) and Triassic (stage 5). The northern belt is characterized by the occurrence of peralkaline intrusive rocks and abundant A-type granites (Jahn et al., 1996). Granitic rocks in northern Xinjiang district of the southern belt are mostly syn-orogenic I-type granites, with minor post-orogenic A-type granites, and the granitic magmatism is divided into two stages, late Caledonian (377-408 Ma) and Hercynian (290-344 Ma) (Liu, 1990). Granitic rocks in NE China (southern zone) are calc-alkaline I- and A-type granites, divided into two intrusive stages, Late Paleozoic (290-260 Ma) and Mesozoic (200-120 Ma) (Wu et al., 2002). Calc-alkaline and volumetrically minor alkaline intrusive rocks forming the southern belt are widely distributed in South Mongolia. This magmatism is considered to have initiated in the early Paleozoic and continued to the middle Mesozoic (Zorin, 1993; Ruzhentsev and Pospelov, 1992). Most of the granitic rocks in this area are of calc-alkaline I-type, but alkali-rich intrusive rocks, such as A-type granites, alkaline granites, syenites and nepheline-bearing syenites are also found.

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LG

LG - Lugiin Gol Complex

KhB ULG

KhB – KhanbogdULG- Ulgii

Fig. 4. Distribution of alkaline complexes in South Mongolia (Samoilov and Kovalenko, 1983).

SITE 1. KHANBOGD INTRUSIVE COMPLEX

History of exploration and geology Khanbogd (Lat. 43°13′ N; Long. 106°53′ E) is located in the South Gobi aimag (province) of Mongolia, approximately 600 km south of Ulaanbaatar. The area has an average elevation of 1160 m above sea level and a relatively flat to gently undulating topography typical of the Gobi Desert and the Southern Mongolia-Inner Mongolia plateau. The highest hypsometric elevation in this complex is Khanbogd Mountain (>1450 m above sea level) (Vladykin et al., 1981). First studies of the geology of the Khanbogd complex were made by Russian scientists, who conducted a geological survey of the area and prospected for mineral resources at Khanbogd. Subsequently, several workers (Kovalenko, 1977; Vladykin et al., 1981; Kovalenko and Yarmolyuk, 1995, Kovalenko et al., 2006) studied the complex, focusing mainly on the geology, geochemistry and related mineralization. Vladykin et al. (1981) studied in detail the mineralogy and geochemistry of the complex, and reported their data in a monograph concerning the genesis of rare-metal granites. This group (Vladykin et al., 1985) also discovered here two new minerals, armstrongite and mongolite. Vladykin et al. (1981) also reported the first Rb-Sr whole-rock isochron age for the Khanbogd rocks. New geochronological data were obtained within the framework of the Uudam Tal project by the Japan International Cooperation Agency and Metal Mining Agency of Japan (JICA and MMAJ, 1992). Garamjav (2007) proposed a spiral-vertical structure for the Khanbogd pluton.

 

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The Khanbogd pluton is located in the Gurvansaykhan terrane, which is a part of the South Mongolian domain (Figs. 3, 4) (Badarch et al., 2002). The pluton is one of the largest alkaline granitic intrusions in the world. The shape and internal structure of the complex can be readily visualized from satellite imagery (Fig. 1.1); the complex has a concentric structure (Kovalenko, 1977, Kovalenko, et al., 1995, Kovalenko et al., 2006) and consists of two circular bodies, western and eastern, and several types of dike rocks. The Khanbogd granites intrude Siluro-Devonian volcanic and sedimentary rocks along the western and southwestern margins of the complex, and Carboniferous volcanic rocks (mostly basaltic andesite and andesite) along its southern and eastern margins. Within the complex, granites host xenoliths of Carboniferous volcanic rocks. The Khanbogd complex is uncomformably overlain by thick Cretaceous sedimentary rocks with dinosaur fossils.

Fig. 1.1. Satellite image of the Khanbogd complex. Landsat TM6.

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Khanbogd complex of alkali granites and associated rocks The intrusive rocks of the Khanbogd pluton were emplaced in the following order: (1) phase-I granites (western body); (2) phase-II granites (eastern body); (3) ekerite and ekerite-porphyry dikes; (4) pantellerite dikes; (5) fine-grained syenite and monzonite dikes; (6) an alkali granite-porphyry dike (Fig. 1.2). According to the current classification, based on the modal composition of these rocks, rocks emplaced at phases I and II should be classified as alkali-feldspar granite. Radiometric U-Pb ages of accessory zircon and titanite (Kovalenko et al., 2006; Vaglio, 2007) show that the main-phase granites were emplaced at 290-292 Ma, nearly contemporaneously with the granite-hosted pegmatites and aplites (290.0±0.4, 290.9±0.3, 290.7±0.9 Ma). The granite porphyry crosscutting the main-phase granites is distinctly younger (272.5±1.2 Ma), indicates protracted magmatic activity in the area.

The alkali granites of phase I are coarse-grained and show hypidiomorphic equigranular texture. These rocks consist mainly of potassium feldspar (37-71 vol.%) with a perthitic texture, quartz (22-57%) and plagioclase An1-4 (<2%) with subordinate aegirine (Ae86-97; ≤12%) and alkali amphibole (≤10%). Fluorite, apatite, zircon, titanite, allanite and opaque minerals are accessory constituents. Alkali feldspar occurs as subhedral to anhedral crystals ranging in size from 2 to 4 mm. Microperthite contains narrow sinuous albite lamellae or braided patterns. The albite lamellae commonly show albite twinning. Rarely, plagioclase occurs in the core of large feldspar grains, and is usually heavily sericitized. Most of the Khanbogd alkali granites consist only of perthite, indicating crystallization at high temperatures. Plagioclase rarely occurs along the rim of potassium feldspar crystals. Quartz shows weak undulatory extinction and occurs as subhedral to anhedral grains ranging in size from 1 to 3 mm, and is rarely enclosed in the perthite. Some samples show micrographic texture between quartz and potassium feldspar and granophyric textures composed of radiating quartz grains along the margin of feldspar crystals. Subhedral to anhedral alkali amphibole, ranging in size from 0.3 to 2.0 mm, usually fills interstices among the felsic minerals but, in some cases, occurs as euhedral prismatic crystals. The amphibole exhibits strong pleochroism from bluish green to dark blue. Aegirine (≤2 mm in length) occurs as euhedral to subhedral grains pleochroic in shades of green. Colorless to purple fluorite occurs as euhedral inclusions in alkali amphibole, but anhedral crystals filling interstitial spaces among the feldsic mineral are also common. Zircon occurs as subhedral to euhedral crystals commonly included in the rock-forming silicates and surrounded by a pleochroic halo. Aggregates of alkali amphibole, fluorite and opaque minerals developed interstitially crystallized in the waning stages of magma crystallization, probably near the solidus.

The phase-II alkali granites show a medium-grained equigranular texture with graphic intergrowth between quartz and potassium feldspar. It consists mainly of alkali feldspar (51-66% by volume) with a perthitic texture, quartz (30-47%), alkali amphibole (0.5-23%), plagioclase (1-2%) and aegirine (0.1-0.9%). In some cases, mafic minerals are altered to chlorite and opaque minerals. Accessory minerals are fluorite (<0.5% of volume), zircon (<0.1%), titanite (<0.1), opaque minerals (<0.2%) and apatite. In these granites, mafic minerals are less abundant than in the phase-I granites. Colored minerals (alkali amphibole and aegirine) occur as interstitial segregations in potassium feldspar and quartz.

Ekerite dikes are common in the Khanbogd complex; these are fine-grained rocks of massive or trachytic texture. They are composed of alkali feldspar, quartz, albite, alkali amphibole and aegirine. Quartz occurs as equate grains up to 1.3 mm across. Amphibole and aegirine occur as prismatic crystals interstitial with respect to quartz in massive ekerite. In trachytic ekerite, the

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mafic minerals, alkali feldspar and albite are aligned subparallel to one another (Vladykin et al., 1981).

Pantellerite dikes are composed of potassium feldspar, quartz, aegirine (rarely alkali amphibole), and exhibit granophyric and spherulitic textures. Phenocrysts are represented by quartz and potassium feldspar. The latter also forms anhedral grains commonly intergrown with quartz to form a granophyric texture. Spherulites consist of skeletal needles (dendrites) of riebeckite. The groundmass is crypto- to microcrystalline and consists of alkali feldspars, acicular riebeckite and aegirine grains.

Alkali granite porphyry dike is petrographically distinct from the rest of the intrusive rocks, including the presence of calcic amphibole, biotite and abundant plagioclase. This rock is composed predominantly of quartz, potassium feldspar, plagioclase, amphibole and biotite, and shows a porphyric texture. Quartz occurs as equant grains (0.2-2.5 mm in size) with undulatory extinction. Potassium feldspar occurs as tabular and anhedral grains up to 3 mm across commonly intergrown with quartz. Feldspars also form well-developed phenocrysts and give the rock its porphyritic appearance. Amphibole (up to 0.8 mm in size; pleochroic from dark green to tan) is subhedral in habit and altered to chlorite and opaque minerals. Biotite (up to 0.4 mm up to in size) occurs as individual grains and occasionally as clusters. The groundmass is composed of quartz, potassium feldspar, amphibole and biotite (up to 0.05-0.1 mm across). Titanite, zircon, apatite and opaque minerals are accessory constituents.

Orbicular granite was identified only in the south-western part of the intrusion. It is coarse-grained and contains orbicules composed quartz and large potassium feldspars crystals in the core and showing concentration of mafic minerals, such as arfvedsonite and aegirine, in the rim. Pegmatites and associated rare-metal mineralization There are 12 mineralized prospects in the Khanbogd pluton associated with granitic pegmatites (Vladykin et al., 1981). Two of these prospects, “Armstrongite” and “Arfvedsonite”, are shown in Figs. 1.3. and 1.4. The pegmatites occur as zoned lenses 5 to 100 m in length, or as layers alternating with aplite and ekerite (Vaglio, 2007). Pegmatites of both types are localized in the cupola of the western granite body, commonly occurring near its contact with the wall-rocks at the roof of the pluton. Two types of pegmatites, mineralized and non-mineralized, are recognized in the western body. They are generally associated with aplite rocks and commonly with ekerite dikes. Pegmatites are readily recognizable from a distance on flat areas of the Khanbogd plateau due to protruding remnants of weathering-resistant cores composed largely of massive white quartz (Fig. 1.5).

The mineralized pegmatites contain a quartz core 1-5 m across. They contain large crystals of arfvedsonite, along with microcline, plagioclase and quartz, and are commonly enriched in armstrongite (CaZrSi6O15⋅3H2O) in the matrix, whose presence imparts a pinkish color to the rock. Large crystals of alkali zirconosilicates are absent and aegirine is rare in these rocks. The content of Zr is ~100 ppm, and varies among the individual zones of a pegmatite body. In pegmatites and aplites, large crystals of arfvedsonite are oriented with their growth vector pointing toward the pegmatite core.

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The mineralized pegmatites occur as dikes of variable thickness and lack a quartz core. Armstrongite occupies the space among larger crystals of microcline and arfvedsonite. The texture of the mineralized pegmatites is poikilitic, and their modal composition consists of quartz, potassium feldspar, aegirine, arfedsonite and a variety of alkali zirconosilicates, such as armstrongite, elpidite (Na2ZrSi6O15⋅3H2O) and gittinsite (CaZrSi2O7). Arfvedsonite crystals are sometimes transformed to aegirine along their margin. The “Dorozhny” (“Roadside”) pegmatite was reported to contain composite pseudomorphs of quartz and lilac kovalenkoite [(Ca,Sr,K,Ba)7 Nb12Si8O52(O,OH)⋅15H2O] after an unknown mineral, and foliated aggregates of mongolite [Ca4Nb6Si5O24(OH)10⋅nH2O] replacing the kovalenkoite (Vladykin et al., 1981; Minerals of Mongolia, 2006). Other accessories include titanite, zircon, fluorite, calcite and fluorocarbonates of rare-earth elements, REE [bastnäsite-synchysite series: REExCay(CO3)x+y(F,OH)x]. Aplites are similar to the alkali granites in terms of their major-element geochemistry, including their Zr/Hf, Th/U, Nb/Ta, Y/Ho and (La/Yb)CN values in the aplite samples are very close to those in the main-phase granites. In the aplite–pegmatite series, the index of peralkalinity increases from 1.4 in the aplite to 2.5 in the blocky zone of pegmatites, which reflects a drop in Al2O3 content toward the interior of pegmatite veins. This is accompanied by a drastic increase in Mn, Sr, Ba, Y, REE, Zr, Nb, Ta, Th and U contents and a decrease in Na content, whereas the abundances of other elements do not follow a consistent pattern of variation. In the blocky zone, all key trace-element ratios greatly deviate from chondritic values, i.e. Zr/Hf = 1083, Th/U = 1.7, Nb/Ta = 70, Y/Ho = 93, Yb/Y = 0.04 and (La/Yb)CN = 11 (Kynicky et al., 2011). Lead isotope data indicate a common origin of the Khanbogd pegmatites and granites (Vaglio, 2007). Mass-balance modeling shows that the pegmatites probably differentiated from a peralkaline granitic melt such as that which produced the main-phase granites through fractionation of ~70 wt.% of alkali feldspar plus quartz with subordinate contributions from alkali amphibole and aegirine. Geochemical variations within the aplite-pegmatite series cannot be accounted for exclusively by fractionation. The two most notable characteristics of the pegmatites are their enrichment in light rare-earth elements, LREE [(La/Yb)CN = 11] coupled with enrichment in Y relative to the similar-sized heavy rare-earth elements, HREE (Y/Ho = 93). Although the enrichment of granitic rocks in Zr and other HFSE can be attributed to magmatic differentiation, there is no doubt that these elements were remobilized during the postmagmatic evolution of the complex. This postmagmatic alteration affected both main-phase and aplite–pegmatite units, but is most extensive in apical parts of the intrusion. The extent of alteration, reflecting variations in fluid-to-rock ratio, ranges from isolated thin veinlets of secondary zirconosilicates cross-cutting the precursor elpidite to pseudomorphs lacking any relict material. The alteration appears to have been a two-step process, involving replacement of the primary elpidite by Ca-rich elpidite or armstrongite, and development of zircon at the expense of the elpidite and armstrongite. The late-stage zircon is associated with calcite or gittinsite plus quartz. Enrichment of secondary zirconosilicates in Ca and their association with calcite, fluorocarbonates and fluorite indicates that the fluid responsible for remobilization of Zr and other high-field-strength elements was enriched in Ca, CO2 and F. Preferential partitioning of the LREE+Y and Zr into that fluid relative to HREE and Hf would explain the observed trend of increasing whole-rock (La/Yb)CN, Y/Ho and Zr/Hf values from granites to pegmatites, as well as comparatively lower (La/Yb)CN, Y/Ho and Zr/Hf values in the primary elpidite (Kynicky et al., 2011). This interpretation is consistent with the published trace-element compositions for hydrothermal zircon and fluorite, which indicate that CO2-bearing fluids facilitate Y–Ho and Zr–Hf decoupling in granites.

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Fig. 1.3. Schematic map of the Armstrongite prospect and cross-section I-II (Vladykin et al., 1981). 1 Elements of attitude; 2 mafic volcanic rocks, 3 rhyolites and breccias of mafic volcanic rocks at the foot of rhyolite sequence; 4 phase-1 alkali granites; 5 fine-grained pegmatoid alkali granite; 6 “layered” ekerite and pegmatite; 7 silicified alkali granite; 8 quartzose rock; 9 fine-grained ekerite; 10 ekerite with radial aegirine aggregates; 11 alkali granitic pegmatite; 12 anchimonomineralic armstrongite zone.  

Fig. 1.4. Geologic structure of the Arfvedsonite pegmatite prospect (Vladykin et al., 1981). 1 Phase-1 alkali granite; 2 medium-grained granite; 3 fine-grained quartz-feldspar-arfvedsonite zone; 4 coarse-grained quartz-feldspar-arfvedsonite zone; 5 giant arfvedsonite crystals; 6 elpidite-enriched rock; 7 aegirine pseudomorphs after arfvedsonite; 8 coarse (up to 5 cm) crystals of REE zirconosilicates; 9 crystals of REE silicates; 10 quartzose zone; 11 (shown only in the inset) quartz-feldspar zone with elpidite and arfvedsonite.  

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Fig. 1.5. Exposed remnant of the quartz core of a large pegmatite vein. Western body, Khanbogd plateau.

Mineral chemistry Potassium feldspar is the principal rock-forming mineral of the Khanbogd granites. Its compositional range is very restricted, Or95-97Ab2-4. Plagioclase composes less than 2 modal percent of the granites, and occurs as anhedral grains at the margin of alkali feldspar crystals or as lamellae in the alkali feldspar.

Amphiboles are one of the major constituents in the Khanbogd rocks. On the basis of their chemical composition, these amphiboles can be classified into two distinct groups: sodic amphibole in the alkali granites and (ekerite) dikes and calcic amphibole in the granite porphyry dike. The sodic amphiboles from the phase-I granites and ekerites are arfvedsonite, whereas the calcic amphibole from the granite porphyryis ferro-edenite. The arfvedsonite is quite uniform in composition irrespective of the host rock type, and ranges from 29.3 to 34.1 wt.% FeOΣ and from 7.8 to 8.7 wt.% Na2O. It has a high Fe3+ content (>1 atoms per formula unit), representing a solid solution from arfvedsonite to riebeckite. Aegirine occurs in the phase-I and -II alkali granites and dike rocks, with the exception of alkali granite porphyry. It has a Na2O content ranging from 11.5 to 13.47 wt %, but low Ca and Al contents. The overall compositional range is Ae87-96Jd0.2-

3.3Q0.5-13.0.

Elpidite is the major zirconosilicate phase in all of the granitic rocks with the exception of granite porphyry. Primary elpidite is characterized by uniform composition with small variations in Ca content (<1.5 wt.% CaO), and contains several thousand ppm REE+Y, but samples from pegmatites and, especially, aplites, are significantly enriched in HREE relative to those from the main-phase granites. Late-stage elpidite replacing the primary variety contains high Ca levels (up to 5.7 wt.% CaO). Chondrite-normalized REE profiles of elpidite are essentially flat with a prominent negative Eu anomaly.

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Whole-rock major-element chemistry All rocks of the Khanbogd complex are characterized by high silica and alkali contents (>73 wt.% SiO2; >7.7 wt% Na2O+K2O). The phase-I granites are high in SiO2 (72.9-76.7 wt.%) and total alkali contents (8.1-9.6 wt.% Na2O+K2O), but low in Al2O3 (10.3-12.3 wt.%), MgO (0.1-0.4 wt.%) and CaO (0.1-0.7 wt.%). The FeOΣ content is relatively high, ranging from 3.7 to 5.2 wt.%, and so is the FeOΣ/MgO ratio (12.3-42.3). These geochemical data indicate that the Khanbogd alkali granites are highly evolved. The magnetite-ilmenite series classification, based on the presence or absence of magnetite (and thus on the oxygen fugacity of crystallizing granitic magma; Ishihara, 2003) can be used to classify the phase-I alkali granite as “ilmenite-series granites” because of their low magnetic susceptibility (0.2-1.3×10-3

SI units, compared to 2-2.5×10-3

SI units for the “magnetite series”; Amar-Amgalan, 2004). In the Q’-ANOR classification diagram of Streckeisen and Le Maitre (1979), the Khanbogd granites plot in the alkali granite field (Fig. 1.6). Fig. 1.6. Classification of the Khanbogd plutonic rocks using their normative compositions (after Streckeisen and Le Maitre, 1979). The numbered fields correspond to those in the modal QAPF diagram of Streckeisen (1976). The named rock types are plotted at the position of their concentration in the data file used by Streckeisen and Le Maitre. Q’ = Q/(Q+Or+Ab+An)×100; ANOR = 100×An/(Or + An). Whole-rock trace-element chemistry The Khanbogd alkali granites contain high levels of Ga, Zr, Nb, Y and REE (except Eu), but low Ba and Sr contents. Samples of the alkali granite porphyry dikes have much higher Sr and Ba, but lower Ga, Nb and Zr concentrations than the Khanbogd alkali granites. High Ga concentrations in these rocks are in accord with Ga enrichment in peralkaline rocks worldwide (19-31 ppm). Fig. 1.7 shows REE patterns of the Tsokhiot andesite lavas normalized to the chondrite values of Sun and McDonough (1989). The REE patterns of the lavas exhibit

46  

characteristics typical of island-arc calk-alkaline magmas with enrichment in LREE relative to HREE. These rocks lack Eu anomalies (Eu/Eu* = 1.0). The Khanbogd alkaline granites have extremely high REE contents, which is reflected in the presence of REE mineralization in the complex. The REE patterns of the phase-I alkali granites are generally strongly fractionated [(La/Yb)CN = 4.1-13.2], whereas the phase-II rocks show relatively flat patterns with (La/Yb)N = 2.3-4.4. Both granite phases exhibit a strong negative Eu anomaly (Eu/Eu* = 0.07- 0.18).

Fig. 1.8 shows a trace-element abundances in the Khanbogd rocks normalized to the normal mid-ocean-ridge basalt of Sun and McDonough (1989). This diagram indicates that the Khanbogd magmas were enriched in Rb and K and depleted in Sr, Ba and Eu. Some high-field-strength elements (HFSE = Nb, Ta, P and Ti) are depleted with respect to trace elements of similar compatibility, whereas other HFSE (U, Th, Zr and Hf) are enriched.

Fig. 1.7. Chondrite-normalized REE pattern of the Khanbogd alkali granites and granite porphyry dike.

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Fig. 1.8. N-MORB-normalized spider diagram for the Khanbogd granites.

Fig. 1.9. Variation in initial 143Nd/144Nd and 87Sr/86Sr isotope ratios of granites from the Khanbogd complex and volcanic wall-rocks of the Tsokhiot formation. The Bulk Silicate Earth (BSE) composition at 295Ma is shown for comparison. Main oceanic-basalt isotopic reservoirs of Zindler and Hart (1986) are also shown. The dashed line field shows the range of initial 87Sr/86Sr ratios in the Khanbogd alkali granites.

Tsokhiot Formation

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Isotopic compositions Strontium and Nd isotopic compositions were measured for selected samples of Khanbogd alkali granites. The granites give a Rb-Sr whole-rock isochron with an initial Sr isotopic ratio of 0.7028±0.0029. The initial 143Nd/144Nd ratio of the Khanbogd granites ranges from 0.51259 to 0.51267 (for 295 Ma, εNd = + 6.9-8.0). The initial 87Sr/86Sr and 143Nd/144Nd isotope ratios and εSr and εNd values of the alkali granite porphyry were calculated using a Rb-Sr whole-rock isochron age of 236 Ma. The porphyries show initial 87Sr/86Sr isotope ratios ranging from 0.70526 to 0.70724 and 143Nd/144Nd values ranging from 0.51249 to 0.51260. The calculated εSr and εNd values range from +14.7 to +43.4 and from + 3.9 to + 5.9, respectively, showing much higher initial Sr and lower Nd isotope ratios than the main-phase alkali granites. The Upper Carboniferous Tsokhiot basaltic andesites and andesites in the Khanbogs area have comparable initial Sr and Nd isotope ratios to those of the Khanbogd granites (Fig. 1.9). Khanbogd rocks in granite classification The term A-type granite was proposed by Loiselle and Wones (1979) to distinguish high-alkali granitic rocks in anorogenic settings from typical calc-alkaline granitic rocks found in orogenic settings, such as active continental margins. These authors showed that A-type granites are geochemically distinct from both I- and S-type granites and occur in rifted zones and stable continental environments. The letter “A” indicates the anhydrous, alkalic and anorogenic nature of their parental magmas.

The Khanbogd granites are composed predominantly of potassium feldspar, quartz, aegirine and interstitial alkali amphiboles (riebeckite-arfvedsonite) and, for the most part, lack plagioclase, indicating the hypersolvus nature of their parental magmas. These petrographic characteristics are consistent with those of other A-type granites around the world, although not all A-type granites are hypersolvus rocks. The Khanbogd granites are characterized by moderately high alkali abundances (especially Na2O) and low CaO and Al2O3 contents at high FeOS/MgO ratios (see above). They are also characterized by high Rb, Nb, Y, REE, Ga, Zr and F abundances and low levels of Ba and Sr. The granites show relatively flat chondrite-normalized REE patterns with strong Eu negative anomalies. These geochemical characteristics are also seen in other A-type granites. Pearce et al. (1984) proposed discrimination diagrams for granitic rocks of different tectonic provenance based on trace-element abundances, such as Rb, Y and Nb (Fig. 1.10). On these discrimination diagrams, the Khanbogd rocks plot in the field of A-type granite (Amar-Amgalan et al., 2005). Other geochemical characteristics allow conclusive identification of the Khanbogd rocks as A2-type (Fig. 1.11). A2-type granitoids are believed to derive from (underplated) continental crust that underwent arc magmatism (Eby, 1992). This interpretation is consistent with the reconstructed tectonic history of the Gurvansayhan terrane (see above).

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Fig. 1.10. (a) Zr+Nb+Ce+Y (ppm) versus FeOΣ/MgO discrimination diagram (after Whalen et al., 1987) showing compositional variation of the Khanbogd granites; FG = fractionated I-type granitoids, OGT = I-, S- and M-type granitoids; A = average A-type granite, Australia. (b) Y vs. Nb discrimination diagram (Pearce et al., 1984). Fields for syn-collisional (COLG), volcanic-arc (VAG), within-plate (WPG) and ocean-ridge (ORG) granites are indicated.

Fig. 1.11. Khanbogd granites plotted on the A-type granitoid discrimination diagram of Eby (1992).

(a)

(b)

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SITE 2. LUGIIN GOL COMPLEX History of exploration Russian geologists discovered the Lugiin Gol nepheline syenite complex in 1971 and reported petrological and geochemical data on this locality in Kovalenko et al. (1974). A joint Mongolian-Polish expedition examined the complex for its potential as a REE deposit (Uberna et. al., 1988). The Uudam Tal and Altan Tal projects, carried out by the Japan International Cooperation Agency and Metal Mining Agency of Japan, provided new geochronological and petrographic data for the Lugiin Gol complex (JICA and MMAJ, 1992). D. Batbold (1997) studied the mineralogy of carbonatites, and B. Munkhtsengel (2001) the petrology and geochemistry of the complex at the Department of Geology, Shimane University, Japan. J. Kynicky’s group at Mendel University (Czech Republic) has been actively studying the REE mineralization in carbonatites and associated silicate rocks since the early 2000s, most recently in collaboration with A.R. Chakhmouradian (University of Manitoba, Canada), C. Xu (Peking University, China) and M. Smith (Brighton University, UK). Presently, Reo Co. Ltd. holds a mineral exploration license for the Lugiin Gol area. Geology

The Lugiin Gol Complex is situated in the Lugiin Gol district (Lat. 42°58′ N, Long. 108°35′ E), which occupies an area of ~70 × 50 km in the southeastern part of the Gobi-Tien Shan fold belt, South Mongolia. This area is a highland with an elevation of 1040-1140 m above sea level, extending across the southern part of the Gobi desert. In terms of main geomorphological features, the district is underlain by clastic sedimentary rocks along valleys and monad rocks with relative height of about 100 m, forming a gentle hilly landscape (JICA and MMAJ, 1992).

The Lugiin Gol district is located near the boundary between two geotectonic units, the Gobi-Tien Shan Caledonian belt and Sulinkheer (Solonker) suture zone. The Gobi-Tien Shan belt is composed mainly of Late Precambrian limestones, amphibolites and gneisses crosscut by Late Paleozoic granitoids and intermediate volcanics. The Sulinkheer zone is comprised of Paleozoic ophiolitic mélange in the southern part and of Late Paleozoic sediments in the northern part. The northern part is underlain chiefly by black shale of the called Lugiin Gol Formation. The eastern Lugiin Gol district represents a large body of deep-marine deposits derived from a magmatic-arc source to the north (in present-day coordinates), indicating that an ocean basin (Sulinkheer or Solon Sea) once existed between the South Gobi (South Mongolian) microcontinent and North China block, which collided in the Late Permian (Amory et al., 1994).

The Lugiin Gol complex is emplaced into clastic sedimentary rocks of the Lugiin Gol Formation composed predominantly of black shales, siltstones, sandstones and conglomerates with rare limestone beds, and deformed into E-W- and NW-SE-trending folds. The age of the Formation is debated; Suetenko (1973) found Late Permian fauna (Brachiopods in olistostroms) in these rocks, whereas Petrenko proposed a Middle Devonian age on the basis of spore-pollen analysis. The intrusive contacts are sharp, and the country sedimentary rocks are thermally metamorphosed to hornfels ranging 300-600 m in width in most areas, but reaching 1300 m along the western contact.

 

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It is noteworthy that Uberna et al. (1988) grouped into the “hornfels rim” all contact metamorphic and metasomatized rocks, including such diverse lithologies as hornfelses, fenites and skarns. True hornfelses are represented by biotite, cordierite-biotite and andalusite-corundum assemblages. Xenoliths and fragments of contact hornfels are found in the central and northern parts of the complex. Other noteworthy contact-metamorphic assemblages are corundum-biotite and spinel (hercinite)-garnet rocks. Skarns are limited in distribution and typically contain scapolite and calcic amphiboles.

The Lugiin Gol complex consists of a stock comprising feldspathoid syenites and geochemically equivalent dike rocks. The nepheline syenite stock crops out in an area of approximately 12 km2, showing a circular outline in plan view a diameter of ~3.5 km (Figs. 2.1, 2.2). The stock is composed of nepheline-bearing syenites, nephenline syenites and nepheline-bearing melasyenites (Fig. 2.3). Dike rocks are attributed to two phases of emplacement, “pre-plutonic” and “post-plutonic”. The “pre-plutonic dikes” intruded the sedimentary country rocks before the intrusion of the nepheline syenite stock, whereas their “post-plutonic” counterparts intruded both sedimentary rocks and the nepheline syenite stock. A dike of alkali granite porphyry averaging ~20 m in width crosscuts the entire stock (Fig. 2.2).

Petrography and mineralogy The modal composition of different igneous rocks exposed at Lugiin Gol is summarized in Figs. 2.3 and 2.4, where sample numbers refer to points in Fig. 2.2.

Medium- to coarse-grained, equigranular to porphyritic nepheline-bearing syenite is an abundant rock type in the stock. It is composed predominantly of potassium feldspar, plagioclase, nepheline, amphibole, biotite, clinopyroxene, with minor amounts of apatite, titanite, cancrinite, zircon, allanite, fluorite, muscovite and opaque minerals. The color index of these rocks ranges from 12 to 20.

Potassium feldspar crystals (Or86-97, 45-70 vol.%, up to 9 mm across) are anhedral, perthitic and occur as an interstitial phase between plagioclase and mafic minerals. Plagioclase (An2-29, 6-14%, 0.5-3 mm in size) is weakly zoned, and usually occurs as prismatic crystals, some of which are poikilitically included in potassium feldspar. Sericitization of feldspars was observed. Nepheline (Ne60-77, 2-12%, up to 2 mm across) forms anhedral and interstitial grains. In some cases, nepheline is partly replaced by cancrinite, sodalite and zeolites. Clinopyroxene (0.4-2.8%, up to 1 mm in size) is pale green and usually occurs as anhedral inclusions in amphibole. Amphibole (8-17%, 0.2-2 mm in size) is pleochroic in shades of green; it occurs as prismatic or anhedral grains and is commonly intergrown with biotite. Biotite (0.2-4%, 0.2-0.8 mm) is pleochroic from dark brown to tan, and occurs usually as discrete platy grains, although clusters were also observed. The biotite is poikilitically included in larger feldspar crystals. Apatite (locally reaching concentrations of ~1.5%), titanite, zircon, allanite, fluorite and opaque minerals are accessory phases, whereas cancrinite, calcite, muscovite and muscovite are secondary minerals. These accessory phases are typically present as inclusions (0.01-0.5 mm in size) in the rock-forming silicates. Fluorite occurs as inclusions in the feldspars and its content increases with a decrease in color index.

Coarse- to medium-grained, hypidiomorphic leucocratic nepheline syenite is the predominant rock type in the stock. It is mainly composed of potassium feldspar, nepheline, plagioclase,

53  

amphibole, biotite, cancrinite and minor proportions of titanite, apatite, zircon, fluorite and other minerals. Sericite, muscovite, calcite and zeolite are secondary alteration products. The color index of the nepheline syenites ranges from 2 to 9. Mafic minerals represent a small proportion of these rocks and include amphibole and biotite. Amphibole (1-6%, 0.2-4 mm across) forms subhedral crystals pleochroic from brownish green to light green. Biotite (0.4-3%, 0.3-0.9 mm in size) is fresh, pleochroic from brown to tan, and occurs as anhedral crystals occasionally intergrown with amphibole. Plagioclase (An0-1, 2-3%, 0.05-2 mm) occurs as prismatic crystals, which are typically included in potassium feldspar. Potassium feldspar (Or81-97, 40-60%, 0.9-6.2 mm in length) occurs as a subhedral elongate crystals that poikilitically enclose plagioclase, nepheline and mafic minerals. Nepheline (Ne70-78, 20-38%, 0.5-2 mm) occurs as anhedral interstitial grains, in some cases partly replaced by cancrinite and zeolites (natrolite and yugawaralite). Cancrinite (3-8%) is interstitial with respect to nepheline and potassium feldspar. Apatite, titanite and opaque minerals are common accessory phases occurring as inclusions in mafic silicates. Pyrochlore (NaCaNb2O6F) was observed in a single sample as euhedral crystals up to 1.2 mm across associated with fluorite.

According to the observed variations in color index, some of the nepheline-bearing syenites can be classified as nepheline-bearing melasyenites, which range from medium-grained porphyritic to fine-grained equigranular varieties. These rocks usually occur in the central part of the complex as xenoliths. The medium-grained porphyritic variety consists predominantly of potassium feldspar, amphibole, plagioclase, clinopyroxene, biotite, nepheline and minor proportions of titanite, apatite, allanite, zircon, cancrinite, fluorite, zeolite and opaque minerals. Phenocrysts are represented by potassium feldspar and amphibole; potassium feldspar, amphibole, clinopyroxene, biotite and nepheline make up the groundmass. Potassium feldspar (Or50-80, 45-50%, up to 6 mm in phenocrysts) occurs as euhedral tabular crystals which poikilitically enclose other minerals. Potassium feldspar grains in groundmass are 0.05-0.9 mm in size. Plagioclase (An22-39, 10-20%, 1.5-2.5 mm) occurs as tabular, prismatic crystals typically included in potassium feldspar. Nepheline (Ne75-78, 0-2.5%, 0.5-3 mm) occurs as subhedral grains. Clinopyroxene (2.5-3.8%, 0.05-0.6 mm) is pale green, and usually occurs as anhedral inclusions in amphibole, showing appearing to be in disequilibrium with their host. Amphibole (17-27%, 0.2-5 mm) occurs as euhedral prismatic crystals showing dark greenish brown to pale brown pleochroism. Biotite (1.5-8%, 0.3-3 mm) occurs as platy crystals pleochroic from dark brown to pale brown or tan.

Fine-grained, equigranular, nepheline-bearing melasyenites occur as xenoliths showing evidence of recrystallization. The rock is composed predominantly of potassium feldspar, nepheline, amphibole, biotite, clinopyroxene with minor proportions of titanite, apatite, zircon, allanite, fluorite, zeolites, cancrinite and opaque minerals. Potassium feldspar (~41%, 0.3-4 mm) occurs as subhedral crystals poikilitically enclosing mafic minerals, usually at the rim. Nepheline (~10%, 0.02-0.5 mm) occurs as subhedral crystals. The mafic minerals amphibole, biotite and clinopyroxene occur together as granular aggregates of subhedral crystals. Clinopyroxene (~5%) is enclosed in amphibole and shows reaction relations. Amphibole (~27%, 0.04-0.9 mm) is pleochroic from pale green to brownish. Biotite (~3%, 0.02-0.8 mm) occurs as euhedral inclusions in clinopyroxene, but sometimes also occurs as short tabular crystals. Titanite (~2.5%) is a very common accessory phase, which in some cases, occurs as relatively large (≤ 1.2 mm in length) crystals; other accessory phases occur as inclusions in other minerals, and do not exceed 0.06 mm in size.

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The most common “pre-plutonic” rock is phonolite, which was emplaced as dikes in the sedimentary rocks and hornfelses, showing an E-W strike. The dikes range in width from 0.5 to 1.0 m, and can be traced for tens of meters and, in some cases, for up to 1 km. The rock is pinkish gray and porphyritic owing to the presence of abundant (up to 15%) phenocrysts of potassium feldspar, plagioclase and biotite some 1-4 mm across.

The “post-plutonic” dike rocks can be divided into three distinct petrographic types: nepheline monzogabbro, tinguaite and carbonatite. These dikes occur in a radial (and rarely, concentric) pattern. They range in width from several cm to 3 m, do not exceed 1 km in length and crosscut both the nepheline syenite plutonic rocks and hornfelses. The contacts between the “post-plutonic” dikes and nepheline syenites are sharp and commonly feature a recognizable chilled margin.

A dike of fine-grained, porphyritic nepheline monzogabbro ranges from 1 to 2 m in width and is only a few m long. The dike has a chilled margin ~1 cm in width. The rock consists of plagioclase, potassium feldspar, amphibole, nepheline, biotite, clinopyroxene, pyrochlore and minor amounts of titanite, apatite, fluorite, allanite and an opaque mineral. The phenocrysts (15-20%) comprise potassium feldspar, plagioclase, amphibole, clinopyroxene and pyrochlore. The groundmass is composed predominantly of plagioclase, amphibole and minor proportions of potassium feldspar, nepheline and biotite. Plagioclase is the major constituent in the groundmass (45%, 1.1-4.5 mm in size) and also occurs as rare prismatic crystals with subtle zoning. Potassium feldspar (20-35%, 0.5-2 mm) occurs as euhedral tabular crystals, which locally poikilitically enclose plagioclase in their core. Nepheline (4-5%, 0.1-0.4 mm) occurs as an interstitial phase in the groundmass. Amphibole (15-20%, 1.1-2.2 mm) is pleochroic from dark to pale green and prismatic in habit. It poikilitically enclosed small platy grains of biotite and relict grains of clinopyroxene. Biotite (4-5%, 0.02-0.8 mm) is pleochroic from tan to light brown, and clinopyroxene (1.5-2%, 0.04-0.5 mm) is present exclusively as relict grains in amphibole. Pyrochlore (locally, up to 6%, 0.6-3 mm) is isotropic, pale brown, and occurs as anhedral grains usually associated with fluorite.

Tinguaite dikes are common “post-plutonic” rocks that occur as radial and rarely concentric bodies up to 2 km in length and 1.0-1.5 m in width crosscutting the nepheline syenite stock and hornfelses. They are composed predominantly of potassium feldspar, nepheline, plagioclase, amphibole, biotite and minor proportions of cancrinite, muscovite, titanite, fluorite, apatite and opaque minerals. Potassium feldspar (25-60%, 0.02-1 mm) occurs as tabular euhedral crystals containing inclusions of mafic minerals. Nepheline (8-30%, 0.01-1 mm) occurs as interstitial grains among other silicates. Plagioclase (4-20%, 0.01-0.8 mm) forms tabular elongate crystals with distinct polysynthetic twinning. Amphibole and biotite occur as subhedral to euhedral crystals ranging from 0.01 to 0.8 mm in size. Fluorite is a common accessory phase found as inclusions in potassium feldspar. Some tinguaite dikes show a porphyritic texture due to the presence of pseudoleucite phenocrysts. These dikes range from 0.3 to 1.6 m in length and are composed of potassium feldspar, nepheline, amphibole, biotite, clinopyroxene and minor amounts of fluorite, sericite, muscovite, calcite and opaque minerals. Pseudoleucite phenocrysts (15%, up to 3 cm in diameter) comprise orthoclase, microcline and analcime. Potassium feldspar (46%, 0.02-0.9 mm) is present in the groundmass as subhedral crystals, commonly containing inclusions of accessory minerals. Nepheline occurs as small (0.01-0.2 mm) subhedral grains partly altered to sericite. Clinopyroxene (~5%, 0.2-0.9 mm) occurs as anhedral crystals showing weak pleochroism in shades of green. It encloses grains of potassium feldspar and nepheline.

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Amphibole (~12%, 0.04-0.6 mm) occurs as subhedral grains enclosing minute crystals of accessory minerals. Biotite forms small (0.01-0.2 mm) tabular crystals pleochroic from reddish brown to yellow brown.

Calcite carbonatites occur as dikes and veins in the contact zone with the host rocks, mainly in the northern, eastern and western parts of the Lugiin Gol complex. Carbonatite bodies range from a few cm to 1 m in width, and can be traced over a distance of 1 km in some cases. The carbonatites are, for the most part, coarse- to medium-grained massive rocks composed of primary calcite, Na-Sr-REE-bearing apatite and a plethora of rare-earth carbonates whose modal content locally reaches 30%. Of note are primary carbonates of the burbankite–calcioburbankite series [(Na,Ca)3(Sr,Ca,REE,Ba)3(CO3)5] and intergrowths of REE fluorocarbonates (synchysite, bastnäsite and röntgenite), oveprinted by a hydrothermal-metasomatic assemblage which includes strontianite, fluorite, barite, celestine and quartz (Kynicky et al., this volume, p. 21). In total, over 25 minerals, including such unusual species as synchysite-(Nd), have been identified in the Lugiin Gol carbonatites using a variety of techniques (Batbold, this volume, p. 21). The rare-earth mineralization in the carbonatite has been the subject of much exploration interest. To date, 21 dikes and veins have been recognized to define an indicated resource of ~506,000 t of ore grading 2.67% REE2O3 (Altangerel, this volume, p. 17). Batbold (1997) subdivided the carbonatites into three groups depending on the degree of oxidation.

The alkali granite porphyry dike is crosscutting the central part of the Lugiin Gol complex in a roughly N-S direction. The dike ranges in width from 10 to 20 m and is about 3.5 km long. This rock is different from any of the spatially associated igneous rocks in petrography and geochemistry, and is appreciably younger. The syenites were dated at ~245 Ma using a variety of techniques, whereas the dike gave an Rb-Sr whole-rock isochron age of 210±11 Ma (Munkhtsengel et al., this volume, p. 12; Kynicky et al., this volume, p. 21). The granite porphyry dike is composed of quartz, potassium feldspar, plagioclase, biotite and accessory titanite, apatite, zircon and opaque minerals; sericite, calcite and muscovite are secondary phases. Phenocrysts consist of quartz, plagioclase and biotite. Quartz (20-22%, 0.2-2.5 mm) occurs as euhedral and rounded crystals, which sometimes poikilitically enclose altered potassium feldspar. Potassium feldspar (14-20%, 0.4-2.5 mm) is subhedral and tabular, and locally encloses quartz grains. Plagioclase (50-55%, 0.4-1.8 mm) occurs as prismatic and tabular crystals locally altered to sericite and carbonates. Biotite (5-8%, 0.2-1.5 mm) forms discrete platy crystals and clusters showing pleochroism from dark brown to tan. Quartz, feldspars and biotite also make up the groundmass. Mineral chemistry In the nepheline-bearing syenites (Fig. 2.4), clinopyroxene is hedenbergite showing a limited compositional variation: Wo47-49Fs25-30En19-24; the Mn content is low (≤ 0.03 atoms per formula unit, apfu). The composition of amphiboles is largely ferro-pargasitic, with Mg/(Mg+Fe) values between 0.2 and 0.3. The biotite is annite characterized by a limited range of FeΣ/(FeΣ+Mg) values (0.7-0.75) and low Mn contents (0.1 to 0.18 apfu), at 0.24-0.36 apfu Ti. Plagioclase ranges from albite to oligoclase (Ab70-96An2-29Or1-9) and shows a normal zoning pattern involving a decrease in Ca and Al contents (An18 to An2) from the crystal core outward. The compositional range of potassium feldspar can be described as Or86-97Ab3-14An0-1. Nepheline is close to Ne72Ks23Q5 in composition and lacks zoning.

56  

In the nepheline syenites (Fig. 2.4), plagioclase is unzoned albite with a compositional range of Ab97-99An0.5-1.5Or0.4-1.6; potassium feldspar is orthoclase showing a much greater compositional variation, Or81-97Ab2-18An0-0.1. Nepheline compositions span the range Ne73-77Ks22-20Q5-2. Amphibole is ferropargasite with a Mg/(Mg+Fe) ratio of 0.13-0.14. Biotite is annitic in composition, with FeΣ/(FeΣ+Mg) values ranging from 0.70 to 0.89 and appreciable Mn content.

The porphyritic nepheline-bearing syenites contain microcline showing significant compositional variation from Na-rich anorthoclase-like compositions (Or50Ab49An1) to microcline moderately enriched in Na (Or80Ab19An1). Plagioclase compositions span the range Ab60-77An39-22Or0.7-1.5 showing progressive depletion in Ca toward the rim. Nepheline approached Ne75-78Ks19-20Q5-2 in composition. Pyroxene ranges from diopside to hedenbergite (Wo50-51Fs16-30En19-33). Amphibole is ferro-pargasite, with a Mg/(Mg+Fe) ratio ranging from 0.38 to 0.42. Mica is annitic, and FeΣ/(FeΣ+Mg) ranges from 0.55 to 0.64; the Mn content is low.

The nepheline monzogabbro dike is composed of zoned potassium feldspar (core, Or50-51Ab47-

45An3-4; rim, Or74-79Ab26-21An0.2-0.4), diopsidic clinopyroxene (Wo51.6-52.5Fs16-19En28-30), ferropargasitic amphibole with Mg/(Mg+Fe) = 0.28-0.41 and borderline phlogopite-annite micas with FeΣ/(FeΣ+Mg) = 0.4-0.5. Major- and trace-element geochemistry The rocks of the Lugiin Gol complex are rich in alkalis (Na2O+K2O = 10.1 to 16.7 wt.%). The SiO2 contents range from 52.0 to 58.7 wt.%, FeOΣ/MgO ratios from 2.3 to 26.4, and mg# = 100×Mg/(Mg+Fe) = 3-22, indicating that these rocks are highly evolved. Most of the rocks are nepheline-normative, with the exception of the “pre-plutonic” phonolite dikes which are slightly quartz normative. Nepheline-bearing melasyenites are the least fractionated rock type (FeOΣ/MgO = 2.3-4.2; mg# = 16-22; Na2O+K2O = 10.1-12.8 wt.%) and contain 3-15 wt.% of normative nepheline. The most evolved rocks are represented by coarse- to medium-grained, leucocratic nepheline syenites (FeOΣ/MgO = 10.4-15.4; mg# = 5-7; Na2O+K2O = 14.9-16.7 wt.%). The medium- to coarse-grained equigranular and porphyritic nepheline-bearing syenites are compositionally intermediate (FeOΣ/MgO = 5.0-6.9; mg# = 10-13; Na2O+K2O = 11.8 to 14.3 wt.%).

The “pre-plutonic” phonolite dikes (FeOΣ/MgO = 13.8-26.2; mg# = 3-5) are quartz-normative, and have high levels of Al2O3 (~24 wt.%), K2O (10.2-10.6 wt.%), at low Na2O contents (2.5-2.7 wt.%) and relatively high SiO2 (57.5-58.7 wt.%) contents. Two samples of “post-plutonic” dikes, represented by fine-grained porphyritic nepheline monzogabbro, have similar SiO2 content (52.6 and 52.8 wt.%) and high normative nepheline content (22-23 wt.%). The mg# of these two samples is 15, whereas the FeOΣ/MgO ratio is 4.5.

The tinguaite dikes contain 53.9-56.2 wt.% SiO2 and 15.6-17.0 Na2O+K2O at FeOS/MgO = 14-26 and mg# = 3-5. The pseudoleucite-phyric dike has high K2O and Al2O3 contents (10.9 and 20.9 wt.%, respectively) at a low Na2O content (2.4 wt %) and a normative nepheline content of 11 wt.%.

57  

Fig. 2.3. Simplified intrusive relations and AFP classification diagram for the Lugiin Gol igneous rocks.

Fig. 2.4. Mineralogy of rocks of the Lugiin Gol Pluton

F

A

foid-bearing monzonite

foid-monzosyenite foid-monzodioritemonzogabbro

foidolite

foid

-dio

rite,

foid

gabb

ro

foid s yeni te

foid-bearing syenitefoid-bearing monzodioritefoid-bearing monzogabbro

foid-bearing alkalifeldspar syenite

foid-bearing diorite,gabbro, anorthozite

P

A-alkali feldsparP-plagioclaseF-feldspathoid

LUGIIN GOL COMPLEX

Tinguaite dike

Phonolite dike

Carbonatite dike

Nepheline syenite stock

Alkali granite porphyry dike

Sample No (LG-) 70507 82316 82308 82304 70512 82313 82306 82301 82312 82302 82303 82315 82311 82307 82314Mg# 22 16 13 13 12 12 11 11 10 7 6 5 5 5 5

Clinopyroxene Wo50-51Fs16-29mg#14-13 mg#14-11

Amphibole mg#42-38 mg#31-20 mg#28-21

Biotite Ann 38-45 Ann24-27 Ann26-30 Ann11-16 core rim core rim core rim core rim core rim

Plagioclase An39 - 22 An38 - 29 An33 - 28 An3 - 0 An2 - 1core rim core rim core rim core rim core rim

K-feldspar Or79 - 95 Or76 - 98 Or58 - 92 Or77 - 89 Or83 - 90

Nepheline Ne 77 Ne72-78 Ne68-76

Apatite

Zircon

Fluorite

Titanite

Allanite

Pyrochlore

Opaque mineral

Nepheline-bearing syenites Nepheline syenites

Wo49-50Fs32-30

Ann6-10

Ne75-78 Ne71-80

58  

The alkali granite porphyry dike is geochemically distinct from the rocks described above in that it has a high SiO2 content (~72.5 wt.%), but a relatively low total-alkali content (7.3-8.3 wt.%) and FeOΣ/MgO value (2.4-3.4) at high mg# (19-24). The content of normative quartz is 31-32 wt.%.

In the total alkali versus silica classification diaram (e.g., Cox et al., 1979; Wilson, 1989), the Lugiin Gol rocks plot mostly in the nepheline syenite field, whereas the alkali granite porphyry plot at the boundary between the alkali and subalkalic granite fields (Fig.2.5). In terms of their K2O versus Na2O variation, the rocks from the nepheline syenite stock plot mostly in the potassic field, the phonolite dikes fall in the highly potassic field, and tinguaite compositions straddle across the potassic, highly potassic and transitional fields (Fig. 2.6).

The rocks are enriched in large-ion lithophile elements and depleted in high-field-strength elements (such as Nb, Ta, P and Ti). In normalized diagrams, they exhibit a strongly enriched Pb signature (Fig. 2.7), LREE-enriched rare-earth patterns and a weak negative Eu anomaly (Fig. 2.8).

Fig. 2.5. Total alkalis (Na2O+K2O) versus silica Fig. 2.6. Na2O vs K2O diagram (SiO2) diagram (after Cox et al., 1979) for the for the Lugiin Gol Complex Lugiin Gol igneous rocks.

0

2

4

6

8

10

12

14

16

18

20

35 40 45 50 55 60 65 70 75

Na2O+K2O wt%

SiO2 wt%

Nepheline-bearing syeniteNepheline-bearing K-feldspar syeniteNepheline syeniteTinguaite dikePhonolite dikeAlkali granite porphyry dike

syenodiorite

Ultrabasic Basic Intermediate Acid

Alkalic rocks

Subalkalicrocks

Nephelinesyenite

Syenite

Alkali granite

GraniteQ-dioritegranodiorite

Diorite

Diorite

Syenodiorite

monzonites

Gabbro

Gabbro

Ijolite

Nepheline

syenite

Nepheline

syenite

Ijolite

Essexite

Theralite

Monzodiorite

59  

Fig. 2.7. Primitive-mantle-normalized spider diagrams for the Lugiin Gol rocks.

Fig. 2.8. Primitive-mantle-normalized REE patterns of the Lugiin Gol rocks.

0,1

1

10

100

1000

10000

Ba Rb Th U * K Nb* Ta* La Ce Sr Nd P Hf * Zr Sm Ti Tb Y Pb

Nepheline-bearing syenite

Nepheline-bearing K-feldspar syenite

Nepheline syenite

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Nepheline-bearing syenite

Nepheline-bearing K-feldspar syenite

Nepheline syenite

60  

Fig. 2.9. Isochron diagrams for intrusive rocks from the Lugiin Gol complex.

Fig. 2.10. εSr vs. εNd diagram for the Lugiin Gol igneous rocks.

- 22

- 18

- 14

- 10

- 6

- 2

2

6

10

- 25 - 15 - 5 5 15 25 35 45 55 65 75 85 95

Nep h e lin e Sy en ite Sto ckT in g u a ite d ikeCarb o n a tite d ikePh o n o lite d ikeA lka li g ran ite p o rp h y ry d ikeBu lk Ea rth (LG-245 M a)

DM

Bu lk Ea rth

N -M O RB

EM I

EM II

Lu g iin Go l Co mp lex

HIM U

man tlea rray

0.700

0.710

0.720

0.730

0.740

0 2 4 6 8 1087Rb/86Sr

87Sr

/86Sr

Whole rockBiotite+amphibolePlagioclase rich fractionK-feldspar rich fractionAge: 222.2±3.2 MaSrI: 0.70811±0.00009MSWD: 0.33

0.700

0.704

0.708

0.712

0.716

0.720

0 0.2 0.4 0.6 0.8 1 1.287Rb/86Sr

87Sr

/86Sr

Nepheline syenite stockAge: 244.9±22.4 MaSrI: 0.70800±0.00017MSWD: 1.53

0.700

0.704

0.708

0.712

0.716

0.720

0 0.5 1 1.5 2 2.587Rb/86Sr

87Sr

/86Sr

Nepheline syenite s tockTinguaite dikeCarbonatite dikePhonolite dike

0.700

0.705

0.710

0.715

0.720

0.0 0.5 1.0 1.5 2.0 2.5

87Rb/86Sr

87Sr

/86Sr

A lkali granite porphyry

Age: 210.3±10.9 Ma

SrI: 0.70763±0.00027

MSW D: 0.12

(a) (b)

(c) (d)

61  

Geochronology Kovalenko et al. (1974) and JICA and MMAJ (1992) reported K-Ar whole-rock and mineral ages for the Lugiin Gol rocks as Jurassic and Triassic, respectively. A whole-rock isochron gave an age of 245±22 Ma and an initial Sr isotope ratio of 0.70800±17; the whole rock-mineral isochron calculations gave 222±3 Ma with an initial Sr isotope ratio of 0.70811±9 (Figs. 2.9a-c). All rock samples plot on the reference isochron (Fig. 2.9c). The alkali granite porphyry dike gave a Rb-Sr isochron age of 210±11 Ma and an initial Sr isotope ratio of 0.70763±27 (2σ) (Fig. 2.d).

Isotopic compositions

Initial 87Sr/86Sr and 143Nd/144Nd isotope ratios and εSr and εNd values were calculated for the major rock types on the basis of the whole-rock isochron age (245 Ma). The initial 87Sr/86Sr and 143Nd/144Nd ratios range from 0.707723 to 0.708234, and from 0.512156 to 0.512256, respectively. The calculated εSr values range from +45 to +60, whereas the εNd values range from –3 to –1.

The carbonatites show unusually low, mantle-derived carbon isotopic characteristics (on average, –8.6‰ δ13CV-PDB) and normal oxygen isotopic characteristics (on average, 10.4‰ δ18OV-SMOW), indicating an igneous origin with a possible contribution from reworked oceanic crust (Kynicky et al., this volume, p. 21).

The alkali granite porphyry has an initial 87Sr/86Sr isotope ratio of 0.70763±27 and initial 143Nd/144Nd isotope ratio of 0.512430, translating into an εSr values of 47-49, and εNd value of ~1 (calculated assuming an emplacement age of 210 Ma).

62  

SITE 3. ULGII COMPLEX The Ulgii (Ulgii Khiid, Ulugei Khid) alkaline complex is situated in Manlai soum (Lat. 43°65′ N, Long. 108°21′ E), in the eastern part of South Gobi aimag, near the Ulgii khid (monastery) some 265 km from Sainshand. Geology The Ulgii complex is hosted by Carboniferous to Permian volcanic-sedimentary series. In the central part of the area, there are Lower Carboniferous mafic and intermediate volcanic rocks, tuffs, tuffaceous sandstones, siltstones and conglomerates. To the north, Upper Carboniferous to Permian volcano-sedimentary rocks are represented by tuffs of acidic composition, sandstones, siltstones, conglomerates and intermediate volcanic rocks. Faults intersect these rocks, separating them into tectonic blocks. In the south-western part of the area, basalts and basaltic andesites are associated with comendites, and in the northern part, basalts are amygdaloidal and contain chalcedony (agate) nodules. Two gabbro bodies crosscut the Paleozoic rocks. Both Paleozoic rocks and late Mesozoic alkaline volcanic rocks are bordered by depressions filled by Lower to Upper Cretaceous sediments. The Ulgii igneous series are covered by Cretaceous sediments.

The Ulgii complex is composed of volcanic, subvolcanic and intrusive rocks, including trachytes, latites, syenites, quartz syenites, syenite porphyries, etc.), silicate and carbonate-silicate pyroclastic rocks, silicate-carbonate breccias, magnetite-apatite rocks and carbonatites. All these rocks are controlled by a ring structure measuring up to 20 km2 in area. Its southern part is disrupted by a NE-trending fault and covered by Upper Cretaceous sediments. The ring structure is confined to large graben of E-NE orientation. Crater-facies rocks, represented by trachytes, carbonatites and mineralized zones of brecciation are confined to small ring structures distinguished within the complex. The largest one, some 2 km in diameter, hosts the Ulgii syenite pluton, situated 4.5 km NE from the monastery of Ulgii Khiid.

The major rocks of the Ulgii complex are alkaline volcanic rocks and plagioclase-bearing trachyte. There are two areas of volcanic rocks, Northern (60 km2) and Southern (8.5 km2). Besides trachytes, subvolcanic and extrusive pyroclastic rocks (tuff-breccias, agglomerates), and rarely nepheline-bearing trachytes with plagioclase are observed. The latter comprise layers up to 20-25 m in thickness and subvolcanic veins up to 2-2.5 m in thickness.

The most complete cross-section of alkaline volcanic rocks is exposed in the central and eastern parts of the Southern area, where a 90-95-m thick sequence is exposed. It unconformably overlies Permian basaltic andesites and comendites, and gently dips to the south. At the base of the sequence, there is a cobble layer with gravel-stone interbeds up to 20 m in thickness. In addition to the bedrock volcanic (basalti andesites and comendites), cobbles and other clasts contain alkali granites analogous to those that make up the Khanbogd pluton (see pp. 37-49). Gravel-stone in the upper part of the sequence has a tuffaceous matrix of trachytic composition. The bed contains petrified wood probably entrapped in the volcano-sedimentary material during the eruption. The upper part of the sequence also contains flows of aphyiric nepheline-bearing trachytes. Also present are nepheline-bearing lenses composed of trachytic pyroclastic material (agglomerate) with a silicate and carbonate-silicate matrix. Pyroclastic material also occurs on the surface of cobble and gravel beds, where volcanic bombs up to 1.5 m in diameter were found. The aphyric nepheline-bearing trachyte flow is overlain by a second cobble bed 8-10 m in

63  

thickness, where clasts are composed of both host rocks and alkaline rocks. This bed is overlain by trachytic pyroclastic deposits up to 11 m in thickness, composed of grey agglomerates. The third bed of cobbles overlies the pyroclastic unit and pinches out to the west and east. This bed also contains clasts of alkaline volcanic rocks. Trachytic pyroclastic deposits form lenses up to 6-7 m in thickness pinching out to the west, covering the third cobble bed. Above, there is a 7-8-m thick flow of biotite-clinopyroxene trachytes, some containing nepheline, blanketed by a 10-12-m thick grey unit composed of porphyritic clinopyroxene-biotite trachyte and latite. These rocks are overlain by a trachytic pyroclastic (agglomerate) unit up to 8 m in thickness.

The upper part of the alkaline volcanic sequence is made up of a 20-m thick flow of grey porphyritic clinopyroxene-biotite trachyte with subordinate latite, overlain by a bed of conglomerates containing pebbles of both alkaline volcanic and their host rocks. Tuffs were deposited in a forested area, preserving numerous petrified remnants of Jurassic vegetation.

The volcano-sedimentary sequence is interrupted by subvolcanic intrusions. In the central part of the complex, there is a body of porphyritic biotite trachyte crosscutting aphyric nepheline-bearing trachytes, pyroclastic rocks and clastic sedimentary units. There are also dikes of clinopyroxene phonolites and trachytes reaching a thickness of 2.5 m and a length of 1 km. They crosscut trachyte flows and pyroclastic trachytes. Within the volcanic package, discrete eroded volcanic structures can be recognized representing crater facies (agglomerates, porphyritic sanidine trachytes, lava breccias). Ulgii syenite pluton Intrusive rocks form a number of plutons up to 6 km2 in area, one of which is the Ulgii syenite pluton exposed in the central part of the area (Fig. 3.1). The pluton is confined to an anticline, and crosscuts the Lower Carboniferous volcano-sedimentary sequence composed of andesites and basaltic andesites. The pluton is 3 km2 in area, and has a circular shape in plan view modified by apophyses at its eastern and north-eastern contacts. The pluton is composed of plagioclase-bearing syenite, nepheline syenite, leucocratic monzonite, quartz syenite porphyry, quartz syenite, various dikes (syenite, syenite-porphyry, microsyenite), apatite-magnenite rocks, carbonatites and breccias with a carbonate matrix. The core part is composed of syenites and bordered by a syenite-porphyry ring. The pluton is cut by large EW- and NW-trending faults associated with zones of cataclasis and brecciation in the syenites and quartz syenite porphyries. Apatite-magnetite rocks, carbonatites and carbonatite breccias are also confined to the faults.

In the core part, intrusive rocks are represented by greenish-grey medium-grained plagioclase-bearing biotite-clinopyroxene, biotite-clinopyroxene-amphibole and biotite-amphibole varieties of syenite. Biotite-clinopyroxene syenite is associated with nepheline-bearing and nepheline syenite. Porphyritic mesocratic syenites with a fine-grained groundmass occur in two areas, the of which in the western part of pluton measures 160 × 250 m. It is interpreted as a chilled margin of the syenite core.

 

Fig. 3.1. (Samoilovbreccias; syenites oporphyritiCarbonife

Schematic gv and Koval3 zone of c

of the central ic syenites anerous volcano

eological malenko, 1983)

cataclasis; 4 dpart of the pl

nd monzoniteo-sedimentary

ap of the Ulg): 1 carbonadykes and vluton; 6 quart

es; 8 syenites y sequence; 10

64 

gii syenite pluatites and maeins of syentz syenite porand alkaline

0 faults.

uton (a), andagnetite-apatiite porphyrierphyries of th

syenites of t

d (b) central ite rocks; 2 es and microhe peripheral zthe central pa

part of the pcarbonate-si

osyenites; 5 qzone; 7 mesoart of the plut

pluton ilicate quartz ocratic ton; 9

65  

Quartz syenite porphyry making up the marginal part of the Ulgii pluton forms a gently dipping ring 25-800 m in width, crosscutting the host rocks and the syenite core. In the northeastern part of the pluton, there is a series of apophyses showing a very gentle dip. The largest apophyses of quartz syenite porphyry occur in the host rocks, some 2.5 km from the contact. A series of apophyses form a large intrusive body of complex irregular shape to the northeast of the pluton. Quartz syenite porphyries also occur within the host rocks at the southwestern contact of the pluton. Gently dipping (5-15°) contacts of the quartz syenite porphyry with the core syenites are intrusive. Steeply dipping dikes of syenite porphyry have also been documented. An irregularly shaped of biotite quartz syenite crosscuts the syenite core, forming sharp intrusive contacts with the latter. Within the Ulgii pluton, gently and steeply dipping dikes of leucosyenite, syenite porphyry and microsyenite crosscut the core syenites, syenite porphyries and quartz syenite. Their thickness does not exceed 2 m and length 200 m.

Carbonate-silicate breccias, magnetite-apatite rocks, carbonatites and other mineralized rocks occur predominantly in the core part within the syenites and are rare in the peripheral zones composed of syenite porphyry and quartz syenites. Carbonate-silicate breccias occur at the intersection of WNW and NNW faults, forming a body of oval shape 150 × 180 m in size, and three linear zones 15-20 m in thickness and 160 m in length. This body and linear zones are elongated in N-NW direction along the fault strike. Smaller lenticular bodies of carbonate-silicate breccias (15 × 35 m) occur in latitudinal orientation along other faults. The breccias contain slightly carbonatized fragments of syenites (rarely syenite porphyry) set in a calcitic matrix (10-60 vol.%). Within the oval breccia body, there is a steeply deeping dike of massive medium-grained leucocratic syenite, indicating that brecciation occurred prior to the cessation of magmatic activity. Magnetite-apatite rocks and carbonatites occur within a large zone of cataclasis reaching 60 m in width and 350 m in length.

Magnetite-apatite rocks are relatively rare and form irregular and veining bodies up to 2 × 3 m in size within cataclastically deformed syenites, Their contact with the host syenites is sharp. Very thin magnetite veinlets with apatite crosscut the quartz syenite and are, in their turn, crosscut by leucocratic syenite. In the south-eastern part of the syenite core, chalcedony-apatite rocks have been recognized. In the Ulgii complex, calcite carbonatites are the most common carbonate rock type occurring as a stockwork of veins up to 1.5 m in width and 90 m in length; both gently (15-25°) and steeply dipping (70-80°) veins have been observed. The carbonatites crosscut all alkaline intrusive rocks and magnetite-apatite rocks. Contacts between the carbonatites and earlier-emplaced rocks are sharp, and the magnetite-apatite rocks are carbonatized near the contact. The carbonatites are commonly silicified to produce carbonate-quartz (chalcedony) rocks. The latest rocks to crystallize are veins composed of brecciated hematite-quartz (chalcedony) massive aggregates.

The inferred order of emplacement of the above rocks is: (1) syenites forming the core part; (2a) quartz syenite of the peripheral zone; (2b) quartz syenite; (3) magnetite-apatite rocks and carbonate-silicate breccias; (4) carbonatites; (5) silicified rocks and quartz (chalcedony) veins. The syenite dikes formed, for the most part, before the magnetite-apatite rocks and carbonate-silicate breccias, but younger syenite dikes have also been documented. Mineralized rocks containing fluorite, barite, celestine and enriched in REE are relatively rare. The age of the Ulgii rocks was determined using K-Ar techniques for trachyte (151-171 Ma), syenite (149-153 Ma); quartz syenite (147-158 Ma); quartz syenite porphyry (151 Ma), latite (152 Ma) and carbonatized

66  

breccia (159 Ma). 87Sr/86Sr ratios of 0.7054±0.004 and 0.7065±0.003 were measured for phlogopite-bearing and apatite-bearing carbonatite, respectively (Samoilov and Kovalenko, 1983).

In the north-eastern part of the Ulgii area, late Mesozoic basalts form thin flows covering the conglomerates that contain trachyte pebbles and are associated with younger rhyolite flows. These series are similar to other areas of late Mesozoic magmatism north of Dalanzadgad (Khetsuu Teeg, Mushgai Khudag, Durven Dert).

Geochemistry and mineralization The Ulgii alkaline rocks can be grouped on the basis of their geochemical characteristics into three groups. Subalkaline trachytes (group 1) contain elevated concentrations of Rb, Nb, Zr and Hf, but low levels of Sr, Ba, Li, Cu, and exhibit low K/Rb and high Rb/Sr ratios. Group 2, comprising plagioclase trachytes and latites, was probably derived by differentiation of trachytic magmas with accompanying enrichment of Zr, Hf and Pb, and depletion of Sr, Ba, Cu, Zn and K (Samoilov and Kovalenko, 1983). Group 3 consists of plutonic rocks (including the Ulgii syenite stock) originated differentiation of syenitic magmas.

As mentioned earlier, mineralization is confined to magnetite-apatite rocks, breccias, carbonatites and silicified carbonatites within the syenite pluton, mainly in the southern part of the core area, and rarely within the syenite porphyry in the peripheral zone (Fig. 3.1). The location of mineralization is controlled by the NNW and WNW faults, cataclasis and fragmentation. The largest zone of cataclasis in the syenites is 60 m thick and 350 m long. Major occurrences of magnetite-apatite ores and carbonatites are associated with this zone. Ores veins have irregular shapes and are composed predominantly of apatite and magnetite; they contain 20.6-24.2 wt.% P2O5, 14.8-36.2 wt.% FeOΣ and 0.34-0.68% TiO2. In the south-eastern part of the Ulgii pluton, carbonate-chalcedony-apatite rocks, probably representing a product of carbonatization and silicification of magnetite-apatite rocks, contain 18% P2O5 and 8% FeOΣ.

Mineralized breccias have relatively low contents of Sr, Ba and REE+Y (0.26, 0.37 and 0.053 wt.%, respectively) in comparison with other alkaline complexes in South Mongolia (e.g., Mushgai Khudag). Carbonatites contain up to 1.94 wt.% and, on average, 1.56 wt.% SrO, but are low in Ba (0.27 wt.%). Their REE+Y budget (0.34 wt.%) is dominated by LREE (Samoilov and Kovalenko, 1983). The mineralized rocks at Ulgii have low levels of Nb (≤10 ppm) and Ta; the Zr content reaches 32 ppm in the carbonatites, 230 ppm in the breccias, and 350 ppm in the magnetite-apatite rocks. The mineralization is believed to be of no economic interest, which is in contrast to similar magnetite- and apatite-rich rocks at Mughgai Khudag, where several prospective REE area have been delineated (Altangerel, this volume, p. 17).

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