geology and characteristics of pb–zn–cu–ag...

J. SE Asian Appl. Geol., Jan–Jun 2011, Vol. 3(1), pp. 54-63

GEOLOGY AND CHARACTERISTICS OFPb–Zn–Cu–Ag SKARN DEPOSIT AT RUWAI,LAMANDAU REGENCY, CENTRALKALIMANTAN

Arifudin Idrus∗1, Lucas Donny Setijadji1, Fenny Tamba2, and Ferrian Anggara1

1Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia2PT. Kapuas Prima Coal - Jakarta

Abstract

This study is dealing with geology and charac-teristics of mineralogy, geochemistry and physico-chemical conditions of hydrothermal fluid respon-sible for the formation of skarn Pb-Zn-Cu-Ag de-posit at Ruwai, Lamandau Regency, Central Kali-mantan. The formation of Ruwai skarn is geneti-cally associated with calcareous rocks consisting oflimestone and siltstone (derived from marl?) andcontrolled by NNE-SSW-trending strike slip faultsand localized along N 70° E-trending thrust fault,which also acts as contact zone between sedimen-tary and volcanic rocks in the area. Ruwai skarnis mineralogically characterized by prograde alter-ation (garnet and clino-pyroxene) and retrograde al-teration (epidote, chlorite, calcite and sericite). Oremineralization is characterized by sphalerite, galena,chalcopyrite and Ag-sulphides (particularly acan-thite and argentite), which formed at early retro-grade stage. Geochemically, SiO2 is enriched andCaO is depleted in limestone, consistent with silicicalteration (quartz and calc-silicate) and decarbona-tization of the wallrock. The measured reserves ofthe deposit are 2,297,185 tonnes at average grades of14.98 % Zn, 6.44 % Pb, 2.49 % Cu and 370.87 g/tAg. Ruwai skarn orebody originated at moderatetemperature of 250-266 °C and low salinity of 0.3-0.5 wt.% NaCl eq. The late retrograde stage formed

∗Corresponding author: A. IDRUS, Department ofGeological Engineering, Faculty of Engineering, GadjahMada University, Jl. Grafika 2 Yogyakarta, 55281, Indone-sia. E-mail: [email protected]

at low temperature of 190-220 °C and low salinity of˜0.35 wt.% NaCl eq., which was influenced by mete-oric water incursion at the late stage of the RuwaiPb-Zn-Cu-Ag skarn formation.Keywords: Geology, skarn, mineralogy, geochem-istry, Ruwai, Central Kalimantan.

1 Introduction

1.1 Background

Geological framework and characterization interm of mineralogy, rock geochemistry andphysicochemical conditions of responsible hy-drothermal fluid of the Ruwai Pb-Zn-Cu-Agskarn deposit has been investigated. This studyis needed for a better understanding of theore deposit, particularly on the genetic aspectsincluding mineral assemblages, textures, geo-chemistry and natures of hydrothermal fluidsinvolved. The genetic aspects combined withunderstanding of geological framework of thedeposit could be guidance for the further ex-ploration and mining development of the de-posit. Some previous works in the area are re-ported, particularly emphasizing on the geol-ogy of the deposit for exploration, for instance,Ayson (1997), Baratang (1997) as well as Cookeand Kitto (1997). No studies in details on themine geology and characterization of the de-posit were previously conducted.

54

GEOLOGY AND CHARACTERISTICS OF Pb-Zn-Cu-Ag SKARN DEPOSIT AT RUWAI

Figure 1: Location map of the study area sit-uated in Ruwai, Lamandau regency, CentralKalimantan

2 Regional Geology

The Ruwai Pb-Zn-Cu-Ag skarn deposit is aproduct of hydrothermal process resulted fromLate Cretaceous dyke/stock, which intrudedthe Triassic-Middle Cretaceous volcanic andsedimentary rocks (Figure 2; Ayson, 1997;Baratang, 1997; Cooke and Kitto, 1997). Sed-imentary rocks consist of siltstone, sandstoneand limestone, which are included into LateTriassic-Middle Cretaceous Ketapang com-plex. Siltstone has been locally altered toskarn/hornfels, whereas limestone has beensilicified. Two volcanic rocks are recognizedin the field including felsic volcanic and acidintrusive rocks (dykes/stock). These volcanicrocks are a member of Late Triassic-MiddleCretaceous Matan complex. The youngest rockoutcropped in the field is granodiorite, a mem-ber of Late Cretaceous-Early Tertiary Sukadanagranitoid complex. Figure 2 also shows that theore deposit prospects including Southwest Gos-san, Ruwai, Central Gossan, Karim and Gojoare obviously localized between the lithologi-cal contact between volcanic and sedimentaryrocks along N 70º E-trending fault. It is inter-preted that the regional fault is of thrust type

resulted from regional east-west compressionduring the late of Tertiary. In addition, otherprominent structures are the NNE-SSW trend-ing strike-slip faults.

3 Methods of Study

Two ‘normative’ methods were used in thisstudy including geological fieldwork and labo-ratory analysis of selected samples taken. A to-tal of 21 rock and quartz vein samples were se-lected for analyses of petrography (6 samples),ore microscopy (6 samples), rock geochemistry(4 samples) and microthermometry of fluid in-clusion (5 samples), respectively. Petrographicanalysis on thin section and ore microscopicanalysis on polished section were conductedat Department of Geological Engineering, Gad-jah Mada Unversity. Bulk rock geochemistrywas analysed using XRF (X-Ray Fluorescence)at Kyushu University, Japan. Microthermomet-ric analysis of fluid inclusion was performedusing LINKAM THMS 600 freezing and heatingstage at Centre of Research and Developmentof Geotechnology, National Institute of Sciences(LIPI), Bandung.

4 Results and Analysis

4.1 Geology of Ruwai Skarn Deposit

As outlined before, Ruwai Pb-Zn-Cu-Ag skarndeposit is localized along the contact betweenvolcanic rocks in the south and sedimentaryrocks in the north. The lithological contactis also interpreted to be a N 70º E-trendingthrust fault, which caused the volcanic rocksemplaced overlying the sedimentary rocks (Fig-ure 3).

Stratigraphically, volcanic rocks are the old-est rocks, but in the field the rocks are em-placed on top of the sedimentary rocks due tofault movement. The type of volcanic rocks isdifficult to be identified due to strongly weath-ering. However, according to previous workers(e.g. Ayson, 1997), there are two groups ofvolcanic rocks have been recognized, i.e. LateTriassic-Middle Cretaceous Matan volcaniccomplex and Late Cretaceous-Early Tertiary

© 2010 Department of Geological Engineering, Gadjah Mada University 55

IDRUS et al.

Figure 2: Map of regional geology of Ruwai area and its vicinity (after Ayson, 1997; Baratang, 1997;Cooke and Kitto, 1997)

Kerabai volcanic complex. The Matan com-plex is characterized by felsic volcanic rocks,whereas Kerabai complex is typified by basicvolcanic rocks.

Sedimentary rocks recognized in the Ruwaiprospect consist of siltstone, sandstone andlimestone, which are correlated to Late Triassic-Middle Cretaceous Ketapang complex. Oremineralization is closely associated with silt-stone and limestone. Siltstone is probablyderived from marl that has been undergonedecalcification and silicification. Limestonealso has been partially changed to marble andoutcropped obviously in the centre of Ruwaiprospect. Locally, the rock has been silicified toform calc silicate alteration.

Granodiorite is outcropped in Rada River sit-uated between Karim and Gojo hills. The in-trusive rock is correlated with Late Cretaceous-Early Tertiary Sukadana granite. Genetically,this intrusion is probably related to the forma-tion of the Gojo and Karim skarn Fe deposits.In Ruwai mine (prospect), strong altered mon-zonite is recognized, which may be related tothe formation of the Pb-Zn-Cu-Ag skarn de-

posit. In the Ruwai mine, some young intru-sions in form of dyke and sill including micro-diorite, andesite, basalt and rhyolite cross cutthe ore body (Figure 4).

4.2 Mineralogical Characteristics

Ore minerals

Field observation and ore microscopy of six se-lected samples indicates that the Ruwai skarnore body is characterized by the presence ofpyrite (FeS2), galena (PbS), sphalerite (ZnS),lesser chalcopyrite (CuFeS2) as well as Ag-bearing sulphides particularly acanthite (AgS)and argentite (AgS). Iron oxides minerals suchas magnetite and hematite are also present.Sphalerite is predominantly observed and mi-croscopically often exhibits reddish brown in-ternal reflection (Figure 5). Galena is the sec-ond abundant ore mineral within the ore bodyand showing a typical texture of triangular pits(Figure 5B).

Pyrite is mostly present in form of subhedralgrain and locally it replaces the margin of spha-lerite. However, occasionally pyrite is replaced

56 © 2010 Department of Geological Engineering, Gadjah Mada University


Figure 3: Mine of Pb-Zn-Cu-Ag skarn at Ruwaishowing ore mineralization localized along thecontact between volcanic rocks and sedimen-tary rock. Dashed line indicates suspected faultzone

by sphalerite suggesting that pyrite occurs inthe broad stability conditions. Chalcopyrite israrely present and it seems that the mineralformed in the early stage in term of paragen-esis sequences. Silver-bearing sulfides (proba-bly acanthite and argentite) mostly occur as in-clusion and micro fractures filling in sphalerite(Figure 5C). The measured reserves of the de-posit are 2,297,185 tonnes at average grades of14.98 % Zn, 6.44 % Pb, 2.49 % Cu and 370.87 g/tAg.

Hydrothermal alteration minerals

Two groups of hydrothermal alteration miner-als are identified on the basis of field inves-tigation, handspecimen description and petro-graphic analysis, including (1) prograde alter-ation minerals, and (2) retrograde alterationminerals. Prograde alteration minerals are rep-resented by typical calc-silicate minerals partic-ularly garnet and clino-pyroxene.

Prograde alteration minerals are commonlyrecognized in the country rocks of meta-limestone and meta-siltstone (marl?). Progrademinerals were also found in many other skarndeposit types worldwide, for instance, KingIsland, Sheelite (Kwak, 1986) and Batu Hijau,Sumbawa (Idrus et al., 2009). Garnets in theRuwai skarn deposit are mostly identified in

Figure 4: Micro-diorite cross cutting sedimen-tary rock bedding and Pb-Zn-Cu-Ag skarn orebody

massive form with coarse crystal grains fillingin the fractures of meta-limestone (Figure 6A)and meta-siltstone. However, in some casesgarnets are locally disseminated as fine-grainedcrystals in the sedimentary rocks. Optically,garnets frequently reveal a keliphytic struc-ture, i.e. a zoned structure developing in therims of garnet (Figure 6B). Garnets are mostlylight brown in color and they are interpreted asandradite (Ca-Fe-rich garnet type). Megascop-ically, clino-pyroxenes are present as greenishfine-grained crystals together calcite, layer-ing in siltstone and limestone (Figure 6C).Microscopically, clino-pyroxenes are dissem-inated (Figure 6D) and occasionally occurredas vein/veinlet in the sedimentary rocks andlocally in monzonite. We interpret that theclino-pyroxene is of wollastonite type.

Retrograde alteration minerals are character-ized by the presence of epidote, chlorite, cal-cite and sericite. Epidote exhibits a yellow-ish green in color, whereas chlorite is darkgreen, both minerals are identified in sedimen-tary rocks overlapping with prograde mineralphases. The retrograde minerals also widelyoccurred in young intrusions such as micro-diorite and andesite. Calcite commonly oc-curred as ‘white layer’ in the sedimentary rocksand partly formed as vein/veinlet. Sericite re-placing plagioclase in volcanic rocks and intru-sion is the latest stage of the hydrothermal min-


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eral formation in the deposit. It is formed as aproduct of hydration reaction between the min-eral phase and meteoric water.

Ore mineralization is interpreted to beformed immediately post of prograde stageduring the slightly decrease of temperature.This interpretation is proven by the occurrencesof galena and sphalerite inclusions in massivegarnet filling the fractures as well as in the formof base metal veins/veinlets crosscutting thefragments/crystals of massive garnet.

Rock geochemical characteristics

Geochemical characterization of meta-limestone and young intrusions is performedby XRF (X-Ray Fluorescence) as shown by theanalysis result in Table 1. Limestone reveals ahigh SiO2 content of 12.41 wt.% and low CaOcontent of 50.73 wt.% in average. The high con-centration of SiO2 and the low content of CaOcompared to ‘ideal’ limestone compositionare due to silicification and decarbonatizationprocesses of the wall rock. Silicification andcarbonatization may cause the volume lost of

the rock. The chemical composition of youngintrusion including micro-diorite and rhyolitewas also analysed (Table 1). The young in-trusions are slightly altered. This is provenby their chemical composition showing rela-tively high SiO2 contents of 54.8 wt.% in dioriteand 74.92 wt.% in rhyolite. The similar behav-ior is also shown by high total alkali (K2O +NaO) contents of diorite (5.57 wt.%) and rhy-olite (7.73 wt.%). Few trace elements are alsoincluded in Table 1.

Fluid inclusion microthermometry

Three quartz+ore samples, one barren quartzsample and one calcite sample were microther-mometrically analysed using freezing andheating stages. In term of fluid inclusionphases present, there are petrographicallyno differences among the samples. The sam-ples are dominantly composed of liquid-richmonophase and liquid-vapour-biphase fluidinclusions. The fluid inclusions are geneticallycategorized into primary and secondary types.The primary fluid inclusions are commonly



Figure 6: Prograde alteration minerals: (A). Handspecimen of coarse-grained garnet (Grt) hostedby meta-limestone. It is also shown that sphalerite and galena (Sph+Gn) stringers crosscut garnetcrystals, (B). Photomicrograph of garnet (Grt) with a typical keliphytic structure, (C). Outcrop ofsilicified siltstone containing wollastonite (Wo), and (D). Photomicrograph of wollastonite (Wo)-enriched siltstone


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Table 1: Geochemical data using XRF of meta-limestone as a host of ore mineralization and ofyoung intrusion (micro-diorite and rhyolite)

represented by a negative crystal, tabular orprismatic forms, isolated, and mostly take placenear the crystal growth zone. The secondaryfluid inclusions are mostly placed on alongmicro fractures during their trapping.

Microthermometric analysis indicates thattemperatures of homogenization (Th) of fluidinclusions in quartz+ore samples vary from 250to 266 °C (moderate), temperature of melting(Tm) of -0.2 to -0.3 °C corresponding to salinityof 0.3 to 0.5 wt.% NaCl eq. Fluid inclusions incalcite sample reveals Th of 190–220 °C, Tm of-0.2 °C and averaging salinity of 0.35 wt.% NaCleq. Fluid inclusions in barren quartz show Thof 180 °C, Tm of -0.8 °C and salinity of 1.42 wt.%NaCl eq. The temperature of homogenizationis interpreted to be temperature of trappingand it doesn’t need to be corrected. In general,the temperature and salinity of hydrothermalfluid are relatively low, and this may representthe physicochemical condition of hydrothermalfluid during the retrograde alteration of theRuwai skarn deposit.

5 Discussion

5.1 Geological controls on the deposit forma-tion

Two important geological aspects which controlthe formation of the Ruwai skarn deposit in-clude lithology and geological structures. TheRuwai skarn deposit is originated by metaso-matism process of calcareous wallrocks (lime-stone and siltstone/marl). Monzonite is inter-preted to be mineralization-bearing intrusion inthe area. Monzonite also acted as host of en-doskarn mineralization, whereas limestone andsiltstone/marl were the host of exoskarn miner-alization. A NNE–SSW-trending strike-slip andN 70º E-trending thrust faults are interpreted tobe pathway for the localization of the Pb-Zn-Cu-Ag skarn deposit. The Karim and Gojo Feskarn deposits were also developed along thestructures. Some minor N–E trending strike-slip faults formed during post-mineralizationcrosscutting the ore body and taking part toshape the current geometry of the deposit.

5.2 Mineral paragenesis

Generally, mineral paragenesis in the Ruwaiskarn deposit is grouped into two stages i.e.prograde and retrograde as shown by Figure7. Prograde stage formed at the tempera-ture of more than 300 ºC represented by gar-net (andradite), clino-pyroxene (wollastonite),quartz, pyrite, chalcopyrite and possibly mag-netite which occurred in both monzonite andwallrocks (limestone and siltstone). Garnet istypically characterized by keliphytic (coronas)structure, which is produced by a rim reac-tion of garnet crystals during postmagmaticstage/hydrothermal exsolution (cf. Williamset al., 1982). Retrograde stage is typified byepidote, chlorite, quartz, calcite and sericite aswell as pyrite, chalcopyrite, galena, sphalerite,hematite, acanthite and argentite. Pyrite andchalcopyrite possibly originated at the earlyretrograde stage, followed by galena, spha-lerite and Ag-bearing sulfides, respectively. Oremineralization occurred during the retrogradestage is common in the skarn deposit, for in-stance, Ertsberg (Meinert et al., 1997) and Batu



Hijau (Idrus et al., 2009). Quartz and pyrite arestable in a broad P-T condition, therefore, theyare identified in both prograde and retrogradestages.

Figure 7: Mineral paragenesis in the Ruwaiskarn deposit

5.3 Physicochemical conditions of ore for-

mation

Physicochemical conditions consisting of tem-perature, pressure, salinity and depth of theore formation is interpreted on the basis offluid inclusion analysis in quartz and calcitevein samples. Ore mineralization is associ-ated with quartz vein, hence, the fluid inclu-sion data represents ore formation at the earlyretrograde stage, whereas calcite vein is inter-preted to form at the late retrograde stage. As aresult, the Ruwai skarn ore deposit formed at amoderate temperature range of 250–266 °C witha relatively low salinity of 0.3–0.5 wt.% NaCleq. The skarn mineralization and alteration isculminated at a low temperature and salinityof 190–220 °C and 0.35 wt.% NaCl eq., respec-tively during the late retrograde stage. The for-mation temperature and salinity are relativelylower in comparison to those of the Batu Hi-jau porphyry-related skarn, which formed attemperature of 340–360 °C and salinity of 35–45 wt.% NaCl eq. (early retrograde stage) aswell as temperature of 280–300 °C and salin-

ity of 1–10 wt.% NaCl eq (late retrograde stage)(Idrus et al., 2009). On the basis of the temper-ature and salinity, it is interpreted that Ruwaiskarn deposit originated at ‘hydrostatic’ pres-sure (P) of 0.05 kbar, corresponding to pale-odepth of 0.5 km (cf. Hedenquist et al., 1998).

5.4 Recommendation for exploration

On the basis of geological field data, the devel-opment exploration of the Ruwai skarn mineis directed to southwest (N 250 °E) and north-east (N 70 °E), parallel to lithological contact be-tween sedimentary and volcanic rocks. More-over, the extension and geometry of ore bodyto south and north is still open. Therefore, ex-ploration program including detailed geologi-cal mapping, geophysics and drilling are pro-posed. Geophysical exploration e.g. IP (In-duced Polarization) and geomagnetic surveycould be applied. In addition, lithological dis-tribution and mineralogical characteristics in-cluding calc-silicate alteration and ore mineral-ogy could be a controlling factor in directing ex-ploration activities particularly geological map-ping and drilling.

Ruwai skarn tends to be categorized into ex-oskarn type rather than endoskarn, althoughfew endoskarn indications were recognized inthe field. Ore mineralization and calc-silicate al-teration are associated with meta-limestone andsiltstone (marl?). The understanding and recog-nizing of diagnostic minerals of calc-silicate al-teration particularly garnet (light brown, com-monly crystalline), clino-pyroxene (light green,fine-grained crystals) and epidote (yellowishgreen, fine-medium grained crystals) are cru-cial during exploration in the field. Ore-bearingsulfides (sphalerite, galena, chalcopyrite andAg-sulfides) are intimately related to the calc-silicate occurrences.

Rock-geochemical data including ore chem-istry is useful to interpret the trend of Pb, Zn,Cu and Ag grades upon alteration and miner-alization zone in the field. The ore chemicaldata could also be used for isograde mappingand ore body modeling. Fluid inclusion data(T, P, depth, salinity) are mostly applied for ‘re-construction’ of genetic model of ore deposit interm of physical and chemical properties of hy-


IDRUS et al.

drothermal fluids responsible for the formationof the Ruwai skarn deposit.

6 Conclusions

1. Geological aspects which predominantlycontrolled the formation of the Ruwai Pb-Zn-Cu-Ag skarn deposit consist of litho-logical type (limestone and siltstone/marl)and the presence of structural elements i.e.NNE–SSW-trending strike-slip fault and N70 E-trending thrust fault, which also actsas lithological contact between sedimen-tary rock and volcanic rock. The economicore body is mostly localized along thethrust fault zone and associated with calc-silicate-altered wallrocks consisting of silt-stone (marl?) and limestone, thus, the oredeposit is categorized into calcic-exoskarntype. However, some evidences for thepresence of minor endoskarn hosted by thecausative monzonite intrusion have alsobeen recognized in the field.

2. On the basis of mineral paragenesis, theRuwai skarn deposit is genetically groupedinto 2 mineral assemblages, which consistof prograde-related mineral assemblages(high temperature), and retrograde-relatedmineral assemblages (low temperature).Prograde-related mineral assemblages aretypically characterized by the presence ofandraditic garnet (Ca-Fe-rich type) andclino-pyroxene (wollastonite), whereasretrograde-related mineral assemblagesare represented by epidote, chlorite, cal-cite and sericite which formed during thedecrease of temperature. Ore mineralstypified by sphalerite, galena, chalcopy-rite and Ag-bearing sulfides (acanthite andargentite) may be formed during earlyretrograde stage. Chalcopyrite was pre-cipitated in the first occasion, followed bygalena, sphalerite and Ag-bearing sulfides,consecutively. Pyrite is interpreted to beformed from early to late retrograde stageof the skarn formation.

3. Silicification and decarbonazation of wall-rocks particularly limestone has caused anincrease of SiO2 content (˜12.41 wt.%) and

a decrease of CaO content (˜50.73 wt.%) ofthe rock, respectively. The elemental con-centration change in limestone reveals thepresence of calc-silicate minerals particu-larly garnet and clino-pyroxene replacingcalcite. In addition, the alteration processesmay also decrease the volume (volume-loss) of the rock.

4. Microthermometric fluid inclusion data in-dicate that the Ruwai skarn ore body orig-inated at a moderate temperature of 250-266 °C and a relatively low salinity of0.3-0.5 wt.% NaCl eq., which correspondswith a hyrostatic presssure of 0.05 kbarand depth of 0.5 km below of paleosur-face. The moderate temperature of forma-tion coincides with petrographic/ore mi-croscopic data suggesting the ore body for-mation during the early retrograde stage.The origin of Ruwai skarn deposit is cul-minated at low temperature and salinityof 190–220 °C and 0.35 wt.% NaCl eq., re-spectively during the late retrograde stage.The relatively low temperature and salin-ity of hydrothermal fluid as shown by fluidinclusion data and the presence of sericitein altered wall rocks may imply a signifi-cant contribution of meteoric water in theRuwai ore body formation during the ret-rograde stage.

Acknowledgements

Field works and this publication is madepossible due to financial support and per-mission from management of PT. KapusPrima Coal, Jakarta; those are very acknowl-edged. Laboratory analysis is partly fundedby Faculty of Engineering, Gadjah Mada Uni-versity through Public Research Grant No.:UGM/TK/1820/M/05/01 given to AI and FA.Many thanks also go to Dr. Akira Imai and Dr.Wahyu Wilopo for their help in analyzing someselected samples by XRF at Kyushu University,Japan.

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