the supergene thorium and rare-earth element deposit at

Journal of Geochemical Exploration, 45 ( 1992 ) 113-157 113 Elsevier Science Publishers B.V., Amsterdam

The supergene thorium and rare-earth element deposit at Morro do Ferro, Pogos de Caldas,

Minas Gerais, Brazil

N. W a b e r

Mineralogisch-Petrographisches Institut, Universitiit Bern, Baltzerstrasse I, 3012 Bern, Switzerland

(Received 13 March 1991 ; accepted after revision 12 May 1992 )

ABSTRACT

Waber, N., 1992. The supergene thorium and rare-earth element deposit at Morro do Ferro, Poqos de Caldas, Minas Gerais, Brazil. In: N.A. Chapman, I.G. McKinley, M.E. Shea and J.A.T. Smellie (Editors), The Pogos de Caldas Project: Natural Analogues of Processes in a Radioactive Waste Repository. J. Geochem. Explor., 45:113-157.

The thorium and rare-earth element (Th-REE) deposit at Morro do Ferro formed under supergene lateritic weathering conditions. The ore body consists of shallow NW-SE elongated argillaceous lenses that extend from the top of the hill downwards along its south-eastern slope. The deposit is capped by a network of magnetite layers which protected the underlying highly weathered, argillaceous host rock from excessive erosion. The surrounding country rocks comprise a sequence of subvolcanic phonolite intrusions that have been strongly altered by hydrothermal and supergene processes.

From petrological, mineralogical and geochemical studies, and mass balance calculations, it is inferred that the highly weathered host rock was originally carbonatitic in composition, initially enriched in Th and REEs compared to the surrounding silicate rocks. The intrusion of the carbonatite caused fenitic alteration in the surrounding phonolites, consisting of early potassic alteration followed by a vein-type Th-REE mineralization with associated fluorite, carbonate, pyrite and zircon. Subse- quent lateritic weathering has completely decomposed the carbonatite forming a residual supergene enrichment of Th and REEs.

Initial weathering of the carbonatite has created a chemical environment that might have been conducive to carbonate and phosphate complexing of the REEs in groundwaters. This may have appreciably restricted the dissolution of primary REE phases. Strongly oxidic weathering has resulted in a fractionation between Ce and the other light rare earth elements (LREEs). Ce 3+ is oxidized to Ce 4+ and retained together with Th by secondary mineral formation (cerianite, thorianite), and by adsorption on poorly crystalline iron- and aluminium-hydroxides. In contrast, the trivalent LREEs are retained to a lesser degree and are thus more available for secondary mineral formation (Nd- lanthanite ) and adsorption at greater depths down the weathering column. Seasonally controlled fluctuations of recharge waters into the weathering column may help to explain the observed repetition of Th-Ce enriched zones underlain by trivalent LREE enriched zones.

Correspondence to: N. Waber, Mineralogisch-Petrographisches Institut, Universit~it Bern, Baltzerstrasse 1, 3012 Bern, Switzerland.

0375-6742/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

1. I N T R O D U C T I ( ) N

The thorium and rare-earth element deposit at Morro do Ferro ("iron hill'" ) is located some 10 km to the south of the city of Po9os de Caldas in the state of Minas Gerais, Brazil (Fig. l ). It is closely related to a circular, internal structure within the large ring structure of the Pogos de Caldas alkaline complex (Almeida Filho and Paradella, 1977). This Mesozoic complex comprises a suite of alkaline volcanic, subvolcanic and plutonic rocks, generally containing background concentrations of uranium, thorium and rare-earth elements (REEs). Regional postmagmatic (deuteric) hydrothermal alteration of the complex has resulted in widespread pervasive argillation and zeo- litization of the rocks (Ulbrich, 1984; Schorscher and Shea, 1992). Subse- quently, several different hydrothermal events of local extent have led to the formation of a number of radioactive anomalies. Such hydrothermal events are either related to the formation of volcanic breccia pipes, as in the case of the Osamu Utsumi uranium mine (Magno Jr., 1985; Waber et al., 1992 ), or to the intrusion of a carbonatite, as is proposed here for the Morro do Ferro area. Subsequent major stages in the geological evolution of the Polos de Cal- das complex included the emplacement of mafic-ultramafic dyke rocks, the onset of weathering resulting in a lateritic weathering zone and, in the case of Morro do Ferro, in the supergene enrichment of Th and REEs in a shallow zone just below the surface.

The fact that Morro do Ferro is still largely unperturbed by human activi- ties makes it extremely suitable for study in this present natural analogue project. Within the scope of the Polos de Caldas Natural Analogue Project (Chapman et al., 1992 ), current work at Morro do Ferro entailed the drilling of four boreholes ranging in depth from 40 to 75 m. Two of these boreholes (MF 10 and MF 11 ) penetrated the ore body into the underlying completely weathered host rock. Drilled at the bot tom of the hill near the south stream, the fourth borehole (MF12 ) encountered phonolitic country rock after pen- etrating 27.5 m of a lateritic weathering zone. In this present study an attempt has been made to describe the magmatic, hydrothermal and supergene evolution of the Morro do Ferro area based on material recovered from these boreholes, from surface sampling, and from a review of the available literature.

2. A N A L Y T I C A L M E T H O D S

Thin and polished sections were prepared of the core samples for micro- scopic investigations and microprobe analyses. The remaining core material was then crushed and milled using agate. After milling, the rock powder was split into equal aliquots used for different analytical methods. Special care was taken during the preparation to avoid any trace element contamination of the samples.

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POOOS DE CALDAS, BRAZIL I 15

Eudialite nepheline syenite b~ Osomu Utsurni Mine (OUI~)

Nephe[ine syenifes A Morro do Ferro(MF;,

Phonolite lava trows N

Phonolife breccia f

Phonotites; non subdiv*dec~

,1, Volcanic agglomerates, t u f t s , eft

Clostic sediment ( Upper Mesozoic )

Fenite Scc~le

F - ~ Crystalline bclsemenf 0 ~ 1Okra ( Pre cambrian)

Fig. 1. Simplified geological map (after Ellert, 1959) of the Polos de Caldas alkaline complex showing the location of the Morro do Ferro T h - R E E deposit and the Osamu Utsumi uranium mine. The circular structures within the caldera are after Almeida Filho and Paradella ( 1977 ).

[ i 6 WAB~ i~

A Philips diffractometer system with CuKce radiation was used fro' whole- rock mineral determination and clay mineral identification. Clay mineral abundance was estimated using the intensity ratios of the corresponding basal reflections. This method allows only a semi-quantitative determination of the clay mineral content due to variations in crystallinity and mineral chemistry. Illite crystallinity was determined in order to estimate the temperature of formation according to the method of Kubler (1968), and for a smaller set of samples, corrected according to the methods proposed by ~rodrn (1980, 1984). Trace mineral identification was performed using a Guinier camera with FeKa radiation.

Mineral chemical analyses were performed on an ARL-SEMQ microprobe equipped with 6 crystal spectrometers and an energy-dispersive system (EDS). Raw data were corrected for beam current drift, dead time and background (COMIC-ED, Sommerauer, 1981 ). Final data reduction was performed using ZAF-correction (EMMA-5/1-86 ).

Bulk chemical composit ion was determined by automated X-ray fluores- cence (XRF) spectrometry on a Philips PW1400 system using glass pellets of fused rock powder for major elements and pressed rock powder pills for trace elements. Sulfur and CO2 were determined by coulometric methods (Cou- lomat 702 ). REE analyses were performed with Inductively Coupled Plasma Source Optical Emission Spectroscopy (ICPOES) and neutron activation; radiochemical separation and alpha-spectrometry were used for Th and U analyses.

Scanning electron microscope (SEM) investigations were performed on a CamScan $4 microscope equipped with a Tracor Norton TN 5600 energy dispersive system (EDS); carbon and gold coatings were used for the samples.

Cathodoluminescence investigations were carried out on a hot cathode in- strument with a directly heated tungsten filament (Ramseyer et al., 1989). Working conditions were 70 keV, 30 Amps with exposure times varying from 30 to 240 seconds.

Bulk density was determined by immersion of the dried and weighed samples into mercury to measure their displaced volume. Average grain density was determined using He gas in a Multipycnometer. Total porosity was calculated from grain and bulk density.

The complete analytical data sets are given in Waber (1990a).

3. PREVIOUS W O R K

Morro do Ferro initially drew the attention of mining prospectors due to the abundant occurrence of magnetite layers and veins. In 1935 the Compan- hia Geral de Minas obtained the concession for the extraction of iron and associated minerals at Morro do Ferro. It was not until 1953, however, that the anomalous radioactivity of the deposit was detected during an aerial ra- diometric survey carried out for the Conselho Nacional de Pesquisas (CNPq)

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, PO(~OS DE CALDAS, BRAZIL 117

on behalf of the U.S. Atomic Energy Commission (Tolbert, 1966; IPT-Re- port, 1984). In the 1950s and early 1960s preliminary ground investigations of the Morro do Ferro area were subsequently carried out by the CNPq, the Comis~ao Nacional de Energia Nuclear (CNEN) and the Departemento Na- cional da P r o d u ~ o Mineral (DNPM) in collaboration with the U.S. Geolog- ical Survey (USGS). These investigations revealed that this unique surface radioactive anomaly is almost entirely due to thorium and its daughter products in association with rare-earth elements and minor uranium (Tolbert, 1955, 1966; Frondel and Marvin, 1959; Frayha, 1962, 1966a, b; Penna Franca et al., 1965; Wedow, 1967). These early investigations included ground sur- veys, drilling, trenching, gallery construction, geochemical analyses and lim- ited mineralogical studies, with a view to evaluating the potential of the ore body. Estimates of ore reserves give values of 50000 metric tons of ThO: and 300000 metric tons of total REEs (Frayha, 1962) and 12500 metric tons of ThO: and 35000 metric tons of total REEs (Wedow, 1967), respectively, de- pending on the cut-off grade and depth of the ore considered. By the mid 1960s it was verified that the near-surface ore body at Morro do Ferro did not conceal any additional uranium mineralization of interest, that no pros- pect existed for the use of thorium in the near future, and that the REE ores were too refractory for conventional extraction methods, thus making recovery costly. The idea, however, that some deeper seated uranium deposit might still exist at Morro do Ferro persisted until 1976 when the Empresas Nucle- ares Brasileiras SA (Nuclebr~is, today Ur~nio do Brasil) drilled an inclined 463 m deep borehole from near the summit of the hill through the ore body in a south-west direction. Unfortunately, uranium contents were generally low and radioactivity decreased drastically below 40 m; only background values were measured below 200 m, intercalated by a few minor highly active zones. As a result, all further exploration activity ceased.

New interest in the Morro do Ferro deposit arose in the 1970s with the suggestion that the deposit could be used as a natural biosphere analogue for a radioactive waste repository. For example, radioactivity in soils at Morro do Ferro is so high that natural foliage can easily be autoradiographed. This led to studies dealing with the exposure of plants and animals to Th and the uptake of ::SRa by flora and fauna (Drew and Eisenbud, 1966 ). These initial investigations on the mobility of thorium and its daughter products were mainly carried out by the Institute of Biophysics, Federal University of Rio de Janeiro and the Institute of Environmental Medicine, New York Univer- sity Medical Center, in collaboration with the Pontificial Catholic University of Rio de Janeiro and the CNEN. This work resulted in several reports and publications (Eisenbud, 1979, 1980; Eisenbud et al., 1982; Fujimori, 1982, 1983; IPT-Report, 1984; Lei, 1984; Miekeley et al., 1982, 1985; Lei et al., 1986; Barretto and Fujimori, 1986; Miekeley and Kuechler, 1987; and references cited therein).

4. GEOLOGICAL SETTING

Morro do Ferro is roughly elliptical in shape, trending north-east, and has a max imum diameter of about 1 km. With an altitude of 1541 m Morro do Ferro represents one of the highest elevations on the Pogos de Caldas plateau, rising some 140 m above the base. The hill is drained by two streams, one draining to the northern side (north-s t ream) and the other rising in the south (south-stream) and curving around the south-eastern periphery (Fig. 2 ). The hill is devoid of vegetation except for grass and small bushes.

The local geology of the Morro do Ferro area mainly consists of hydrothermally altered ("potass ic") rocks overlain by a deep lateritic weathering cover (Fig. 2 ). Hydrothermal ly unaltered rocks in the area include phonolites, tinguaites and foyaites of the agpaitic suite with subordinate miascitic counterparts (Barretto and Fujimori, 1986 ). Mineralogically and geochemically these rocks are comparable to the various alkaline rocks generally exposed within

" ~ ~ "potassic rocks" tinguaites (altered)

I I weathered carbonatile ~,~.:=:~==~,o. /'~2] magnetite layers

Fig. 2. Interpretative geological map of the Morro do Ferro area (modified after Barretto and Fujimori, 1986) with the approximate boundary of the weathered carbonatite, and the loca- tions of the Project's boreholes MF10 to MFI3 and the deep borehole SC 038 drilled by Nuclebras.

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POffOS DE CALDAS, BRAZIL [ 19

the Pogos de Caldas plateau (Ulbrich, 1984; Schorscher and Shea, 1992). In the near vicinity of Morro do Ferro, these alkaline rocks suffered intense hydrothermal alteration similar to that observed for the Osamu Utsumi ura- n ium mine (Utsumi, 1971; Magno Jr, 1985; Waber et al., 1992). The hydrothermally altered country rocks were termed "potassic rocks" to stress their main geochemical characteristic (Utsumi, 1971 ). Intense weathering under semitropical conditions led to a 20-50 m thick lateritic weathering zone on top of the different country rocks. This zone is mineralogically and geochemically comparable to the one developed in the Osamu Utsumi uranium mine area (Waber et al., 1992 ). Assuming that the hydrothermally unaffected lam- prophyric dykes exposed in the Osamu Utsumi uranium mine, one of them yielding an age of 75.7+_0.3 Ma (Shea, 1992), represent the final state of magmatic and hydrothermal activity in the area, meteoric weathering would have lasted over a period of about 75 Ma.

At Morro do Ferro weathering of the host rock took place down to a depth of more than 130 m, and fresh host rock has never been recovered from any drillcore. The weathered material below the ore body differs markedly from the weathering zone on top of the surrounding hydrothermally altered phonolitic country rocks ("potassic rocks" ). Rock fragments described as strongly hydrothermally altered and brecciated "potassic rocks" were only recovered at depths below 130 m in two vertical boreholes drilled at the north-western periphery of the ore body (Frayha, 1962, 1966a, b). In the inclined 463 m deep borehole drilled by Urhnio do Brasil, core recovery only started at a depth of 132 m where the drill encountered an 8m thick magnetite layer. Down to 254 m, the recovered material is described macroscopically as oxidized, very fine-grained, homogeneous effusive rocks and tuffs variably hydrothermally altered and brecciated with intercalated minor reduced zones (Ca- margo dos Santos, 1984). In this borehole, the first occurrences of pyrite and fluorite are reported from a depth of 161 m in a reduced zone, and the first K-feldspar phenocrysts are described at a depth of 208 m. From 254 m down to the bot tom of the hole, the core consists of strongly altered and brecciated tinguaites. Fracture-related oxidized zones occur at several intervals. Micro- scopic investigations of samples from 380 m and 426 m revealed strongly brecciated and hydrothermally altered tinguaites ("potassic rocks" ), consisting of about 60% alkali-feldspar, 30% clay minerals with additional pyrite, sphalerite, fluorite, cryptocrystalline REE-phases and gypsum, all occurring as fracture and pore infillings. It was concluded that these rocks had under- gone a potassium-rich hydrothermal alteration and pyritization prior to their brecciation, the latter related to a second hydrothermal stage which also led to impregnation with cryptocrystalline REE- and Th-phases (IPT-Report, 1984).

The Th-REE ore body is a shallow zone of NW-SE elongated argillaceous lenses extending from the summit of the hill down along its south-eastern

12() x WABE.~

slope. Using a cut-offgrade of 0.5 wt.% ThO2, the ore body is 410 m long, 2 ! 5 m wide and 10-35 m thick (Barretto and Fujimori, 1986). Within the orebody the ThO2 concentration ranges between 0.1 and 5 wt.% ThO2~ and the total REE concentration varies between 1.5 and 21 wt.%. Vein-type mineralized zones may be observed in the weathered host rock down to more than 100 m depth, intercalated by thick barren zones (IPT-Report, 1984). The magnetite layers and veins are the dominant feature at the surface of Morro do Ferro, appearing to be concentrated in the uppermost 40 m of the hill. The thickness of such veins and layers ranges from a few m m up to several meters. Major magnetite layers at the surface strike about N50 °-60 ° W and dip 50 °- 85 ° NE. Minor veins strike N 10 °-35 °E with an inclination of 45 ° or more to the north-west or south-east (IPT-Report, 1984). The presence of this magnetite network is generally believed to have prevented Morro do Ferro from excessive erosion.

5. C O U N T R Y R O C K

Country rock was only encountered in borehole MF 12. Below the lateritic weathering zone oxidized and reduced subvolcanic phonolites were recovered in this borehole. The phonolites have suffered intense hydrothermal and metasomatic alteration that has resulted in a very potassium-rich composition ("potassic rocks" ). The type of alteration is comparable to the fenitic alteration found around certain carbonatite intrusions, as for instance at Songwe, Tanzania (Brown, 1964), at Kaiserstuhl, Germany (Sutherland, 1967 ) and in Zambia (Bailey, 1966 ).

The recovered phonolites are genetically closely related and represent different phases of emplacement during the magmatic activity in the Polos de Caldas alkaline complex. Despite their intense hydrothermal overprint, the country rocks may be subdivided into a Ieucocratic phonolite and a leucocratic tephri-phonolite according to differences in their modal and chemical composit ion and in textural features (Waber, 1990a). Contact phenomena are lacking between the two single phonolite stocks but may be observed around some of the small hololeucocratic dykes that intruded both of them in a later stage. The phonolitic rocks are quartz undersaturated with pseudo- morphed nepheline being the major foidic mineral, besides subordinate replaced leucite and pseudoleucite. They are porphyritic in texture with a fine- grained matrix containing phenocrysts of alkali-feldspar, nepheline and rare pseudoleucite. The matrix mainly consists of alkali-feldspar with subordinate nepheline and replaced clinopyroxene in different ratios. Xenoliths of phonolitic and nepheline-syenitic composition, together with amygdaloidal structures made of alkali-feldspar and clay minerals, frequently occur in the two phonolites. The xenoliths display the same alteration features as their host rocks. Both phonolite stocks are intruded by several hololeucocratic dykes

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POLOS DE CALDAS, BRAZIL 121

that mainly consist of alkali-feldspar with subordinate pseudomorphic nepheline. The dykes, too, display the same mineralization features as the surrounding phonolites, indicating emplacement before the main mineralization event. Completely brecciated zones are composed of a very fine-grained clay matrix with associated pyrite, fluorite, very fine-grained to cryptocrystalline U-Th-REE phases and alkali-feldspar. The xenoliths in these zones are hydrothermally altered in the same way as the phonolites and display pyrite impregnation to various degrees. Brecciation, therefore, took place at a late stage of the mineralization event.

Two subsequent, hydrothermal mineral assemblages can be discerned in the different phonolites: (1) the potassic alteration assemblage, and (2) the hydrothermally induced mineralization assemblage. A third mineral assemblage, superimposed on the hydrothermal ones, is produced by low-temperature weathering processes in the oxidized zone of the rocks. Such processes finally led to the complete decomposition of the phonolitic rocks and to the formation of the lateritic weathering cover on top of them. A compilation of the modes and the relative abundances of the clay minerals in the < 2 #m fraction is listed in Table 1. For a more detailed description the reader is referred to Waber (1990a).

T A B L E 1

Borehole MF12; average modal content of the hydrothermally altered phonolites and the lateritic weathering zone

Reduced Reduced Reduced Oxidized O x i d i z e d

leucocratic tephri- phonolitic leucocratic weathered phonolite phonolite dyke phonolite zone

Whole rock (wt. %) alkali-feldspar 85 50 7 4 65 0 - 2 8

albite - 10 4 - -

c l i n o p y r o x e n e (replaced) 2 10 -

o t h e r s * 3 5 10 7 5 - 2 2

gibbsite . . . . 1-12

Total clay fraction ( < 2 jzm ) 10 25 12 28 6 5 - 9 0

Relative abundance in the clay fraction in %: illite 7 0 55 6 0 4 0 1 0 - 3 8

kaolinite 27 12 15 56 6 0 - 9 0

smectite tr 21 15 - -

c h l o r i t e tr 10 7 - -

m i x e d - l a y e r - tr tr tr -

o t h e r s ** tr tr tr minor minor

* Others: pyrite, fluorite, zircon, REE-minerals and magmatic accessories in the reduced zone; F e -

a n d Mn-oxyhydroxides. alunite, j arosite and crandallite-group minerals in the oxidized zone. ** Others: florencite, goyazite, gorceixite, alunite, jarosite and hydrous ferric oxides.

5.1. Potassic alteration assemblage

This assemblage includes all the magmatic minerals and their hydrothermal alteration products. Alkali-feldspar occurs in four textural variations: ( 1 ) sanidine-shaped euhedral alkali-feldspar of the matrix; (2) lathy to tabular phenocrysts; (3) euhedral tabular alkali-feldspar in the amygdaloidal structure, and (4) xenomorphic to euhedral K-feldspar that occurs interstitially and as infillings of voids and fractures and belongs to the mineralization assemblage. In the leucocratic phonolite, alkali-feldspar suffered almost complete alkali-exchange and structural rearrangement and is today an intermediate microcline with a nearly pure orthoclase composition (Or > 98%). These intermediate microclines normally show an optical zonation from a core rich in minute hematite and fluid inclusions to a clear growth rim free of hematite. All the intermediate microclines show slight sericitization in their core but not of the growth rim. In the tephri-phonolite the alkali-exchange did not reach completion, as observed with the leucocratic phonolite, and the intermediate microclines still display microperthitic exsolution structures. Sericitization and zonation are the same as in the leucocratic phonolite. Under cathodoluminescence the bulk of alkali-feldspars in the leucocratic phonolite show an irregular patchy orange-red colored luminescence in the center, changing from light lilac color to a nearly non-luminescent rim. The same color zoning occurs in the feldspars of the amygdaloidal structures. In addition, the tephri- phonolite alkali-feldspars exhibit a dark blue colored core which yields an XRD pattern for high sanidine. This blue luminescence color is characteristic of such magmatic alkali-feldspar (Marshall, 1988). Red luminescence colors in alkali-feldspar are due to lattice impurities such as Fe 3+. Such colors are, among others, characteristic in rocks that underwent fenitization (Marshall, 1988 ). The weakly luminescent dull yellow-brown color of the growth rims is a characteristic feature of K-feldspar precipitated from low temperature so- lutions (Ramseyer et al., 1989). Nepheline has been completely replaced by illite and kaolinite in the intitial stages of the hydrothermal alteration. The pseudomorphs frequently display a zoned arrangement from a kaolinite-rich core to an illite-rich border zone. Pseudoleucite phenocrysts of the leucocratic phonolite are orientated intergrowths of alkali-feldspar and pseudomorphically replaced nepheline. Both minerals show the above described features. Plagioclase only occurs in tephri-phonolite and in the hololeucocratic phonolitic dykes. It is of pure albite composition and is exclusively found in the matrix, occurring interstitially between the alkali-feldspars, and as microperthitic exsolutions in alkali-feldspar. Clinopyroxene is pseudomorphically replaced by chlorite, smectite, and iron- and titanium oxides. Preserved green clinopyroxenes occur as inclusions in some alkali-feldspar phenocrysts and proved to be complexly zoned aegirine-augites. In all the recovered country. rocks magmatic accessories are rare. Sphene and apatite are the most abun-


dant besides zircon and a few opaque phases. Sphene is always completely pseudomorphically replaced by leucoxene and calcite. Magmatic apatite and zircon occur as hypidiomorphic to idiomorphic single needles and prisms.

The majority of the clay minerals found in the phonolitic country rocks have been formed during the potassic alteration event. However, clay formation has also taken place during the subsequent main mineralization event, as exemplified by intimate intergrowths of clay minerals and minerals of the mineralization assemblage. Further, clay minerals are also formed during weathering of the hydrothermally altered phonolites (sections 5.3 and 5.4). The clay mineralogy is very distinct between the leucocratic phonolite and the tephri-phonolite, in terms of total abundance as well as composition. In the leucocratic phonolite, illite and kaolinite dominate in the < 2/tm fraction with smectite, chlorite, halloysite and alunite occasionally occurring as traces. In the tephri-phonolite~ smectite and chlorite become major components of the < 2 #m fraction with additional traces of crandallite group minerals (Ta- ble 1 ). Illite occurs as an alteration product of nepheline and alkali-feldspar, and as a hydrothermal precipitate in veins. Coarser illite/sericite (2-60/zm) is of late-stage hydrothermal origin (i.e. during the main mineralizing event) replacing kaolinite in the border zone of nepheline pseudomorphs. There, fine- grained pyrite and fluorite sporadically occur as inclusions intimately intergrown with illite/sericite. Indications for mixed-layers are lacking in the leucocratic phonolite, but were found in a few samples of the tephri-phonolite. However, the smectite content of these illite-smectite mixed layers does not exceed 5%. Crystallinity measurements of illites from the center of nepheline pseudomorphs yield, for the leucocratic phonolite, values less than 0.25, and for the tephri-phonolite, in mixed layer free samples, values higher than 0.25. According to the Kubler-scale (Kubler, 1968) this would indicate a lower temperature for the illite formation in the tephri-phonolite (anchizonal) than for the leucocratic phonolite (epizonal). This seems to be rather unreasona- ble for the Morro do Ferro country rocks where the difference in the illite crystallinity reflects more the changes in the K + activity during the different stages of the hydrothermal alteration. Thus, illite formation in the leucocratic phonolite most probably has taken place under a higher K + activity compared to the illite formation in the tephri-phonolite. This coincides very well with the fact that the alkali-feldspar is completely exchanged in the leucocratic phonolite but not in the tephri-phonolite. Kaolinite almost exclusively occurs as a replacement product of magmatic nepheline and as vein infillings. In the reduced samples selected only a few millimeters from the redox front, and in the oxidized phonolite itself, secondary kaolinite is formed by the decomposition of K-feldspar. Dioctahedral smectite (montmorillonite) is found together with chlorite in pseudomorphs after primary mafic components (clinopyroxenes) in the tephri-phonolite.

124 ~ WABER

5.2. Hydrothermal mineralization assemblage

This mineral assemblage includes all minerals formed during the hydrothermal overprint, i.e. pyrite, sphalerite, fluorite, carbonate, zircon and Zr minerals, U-Th phases, REE phases, and newly precipitated K-feldspar and clay minerals (mainly illite/sericite and kaolinite) in voids and fractures.

Pyrite is by far the main opaque phase present. The major amount of pyrite is very fine-grained (0.005-0.5 ram) and finely disseminated throughout the phonolites. Coarser-grained pyrite occurs in void infillings and on fracture planes (0.1-2 ram), where it is intimately intergrown with all the other minerals of the mineralization assemblage. Pyrite is slightly more abundant in the tephri-phonolite (3-4 vol%) than in leucocratic phonolite ( 1-3 vol% ) or the phonolitic dykes. Fluorite occurs in interstices, as void and vein infillings, and on fracture planes. The majority of fluorites are violet colored, often display radiation damage and contain tiny opaque inclusions. Inclusions and/or intergrowths with carbonate are frequently observed. Sideritic carbonate occurs in close association with pyrite and fluorite. Carbonates penetrate from veins into the rock matrix, partly replacing primary minerals. In conductive fractures carbonate minerals display corrosion, indicating dissolution. The modal abundance of carbonate reaches about 2 vol%. Monazite is the most abundant REE mineral present in the phonolitic rocks besides the potential REE-bearing minerals ofmagmatic origin (e.g. clinopyroxene, sphene). It occurs disseminated as very fine-grained euhedral crystals ( 10 to 150/~m) and coarser-grained in void and vein fillings associated with pyrite, fluorite, Ti- oxides and sphalerite. In such infillings, additional cheralite and bastnaesite occur. Zircon is mainly present as vein infillings together with the REE phases, fluorite and pyrite. In several cases it forms intimate intergrowths with an- other Zr mineral, most probably baddeleyite. Xenomorphic to euhedral, fully ordered, triclinic low microcline occurs interstitially in void and fracture infillings. The clear low microcline macroscopically displays a light bluish color that is commonly attributed to triclinic potassium feldspar of low temperature origin (< 270°C, Oftedal, 1957). Such K-feldspars have a dull yellow- brownish luminescence, similar to the thin growth rims around the intermediate microclines of the potassic alteration assemblage.

5.3. Oxidized zone

Orange-brown colored oxidized leucocratic phonolite was encountered in borehole MF12 from 27.5 to 37 m depth. The color change is mainly attributed to the ferrous-to-ferric oxidation due to the downward diffusion of oxidizing groundwaters during weathering. The encountered redox front is much more disperse (about 1 m in thickness ) than the redox fronts observed in the

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POffOS DE CALDAS, BRAZIL 125

Osamu Utsumi mine (Waber et al., 1992 ); this might be due to lateral input of oxygenated groundwaters (compare Holmes et al., 1992 ).

Mineralogically, the oxidized phonolite is characterized by the absence of pyrite, fluorite, sphalerite and reduced U-Th-REE- and Mn phases. K-feldspar decomposes to kaolinite and subordinate illite. This transformation becomes more complete further up the vertical profile in the lateritic weathering cover. Minerals formed in the oxidized zone include Fe-hydroxides (mainly goethite and ferrihydrite), kaolinite, illite, Mn-oxyhydroxides (pyrolusite, nsutite, manganite?), crandallite group minerals (gorceixite, goyazite, florencite) and alunite-jarosite group minerals. The oxidic iron phases, alunite and jarosite are mainly a product of pyrite oxidation. Amorphous to low crystalline Fe-hydroxides and goethite are the most abundant among the newly formed iron phases. Hematite occurs in trace amounts. The oxidation of pyrite is already complete 2 to 3 cm from the redox front within the oxidized zone. Strongly corroded relict pyrite may still be present in the center of Fe- hydroxide-clay aggregates within that narrow zone. Mn-oxides/hydroxides are specially abundant on the oxidized side of the disperse redox front. There they occur as fracture fillings migrating dendrite-like in the rock matrix. Part of the Mn-phases in such fractures clearly display replacement by other Mn- oxides/hydroxides. The Mn-oxides/hydroxides are always in close association with Fe-hydroxides. Gorceixite and goyazite occur in fractures forming thin films and in pore spaces. Florencite is observed in close association with bastnaesite and other cryptocrystalline REE phases. All the crandallite-group minerals are most probably of secondary weathering origin.

In the oxidized zone, the < 2/~m fraction constitutes an average of about 28% of the bulk rock (Table 1 ). Kaolinite is the most abundant clay mineral (average 56%), followed by illite (average 40%). Chlorite is absent and smectite sporadically occurs in trace amounts together with alunite-jarosite group minerals and crandallite group minerals.

5.4. Lateritic weathering cover

The transition between the lateritic weathering cover and the massive oxidized country rock encountered in borehole MF12 at a depth of 27.5 m is sharp, to within a few centimeters. The first 0.68 m of the core consists of friable brown colored soil material rich in organics. Further down, the core consists of yellowish-red colored clay/silt material with intercalated red-brown ferricretes in the first 5 m. Down to the lateritic weathering zone/country rock junction, the clay-silt material is predominantly yellowish-white in color. The few intercalated dark red-brown colored zones with strongly oxidized magnetite represent former magnetite veins that had originally intruded the unweathered country rock.

The mineralogy of the lateritic weathering zones is dominated by very fine-

120 N WABEi<

grained minerals and the < 2 #m fraction comprises 65-90% of the total rock volume (Table 1 ). Coarser components ( > 100/lm) include iron and manganese concretions, newly formed gibbsite and relict preserved minerals of the hydrothermal!y altered phonolite such as sericite and, in lower levels, K- feldspar. Secondary REE-Th phases formed by decomposition of the hydrothermal REE-Th minerals are always present in trace amounts, reaching mi- nor contents in certain zones.

Kaolinite is the most abundant silicate mineral phase formed during weathering. The kaolinite-illite ratio is about 9:1 in the first 6 m of the core, decreases to 4:1 down to 24 m and is close to 1:1 at the junction of the lateritic weathering zone/oxidized country rock. Secondary weathering kaolinite tends to form more sparry aggregates with somewhat less well-defined crystals than in the bedrock. Densely packed kaolinite "booklets", typical of the hydrothermally altered country rock, disappear completely in the uppermost 10 m of the profile, where the transformation of kaolinite to gibbsite occurs. Relicts of illite/sericite are mainly preserved in nepheline pseudomorphs intimately intergrown with kaolinite. Only in samples most adjacent to the oxidized country rock does illite occur intimately intergrown with K-feldspar, suggest- ing growth at the expense of the latter. Further up the vertical profile illite/ sericite becomes decomposed to kaolinite and its modal abundance decreases drastically. Ill i te/smectite mixed layers are no longer present in the weathering zone, and neither is chlorite. Gibbsite is present in samples down to 16 m below the surface. Its major abundance, however, is in the first 10 m of the core, composing slightly more than 10% of the lateritic weathering zone. Tex- tural relationships point to a formation of gibbsite mainly at the expense of kaolinite, and more subordinately at the expense of illite; its formation from amorphous Al-gel is also indicated. K-feldspar is not stable under weathering conditions. However, relicts of coarser-grained K-feldspar are found up to 6 m below the surface. Crystallographically orientated etch pits on the surface clearly give evidence of the dissolution of K-feldspar. A marked change in the K-feldspar abundance occurs between 24 and 28 m depth, where the K-feldspar content rises from about 30% to more than 60% of the rock.

In contrast to the oxidized country rocks, the Fe-hydroxides are no longer dispersed over the whole rock but tend to be much more concentrated in spe- cific zones and layers (ferricretes). A high abundance of Fe-minerals ( 7-14%) is observed in the first 6 m of the core, decreasing down to about 4%, with the exception of the thin ferricrete layers and magnetite veins where the Fe-minerals can easily make up 10 to 15% of the lateritic weathering zone, Amor- phous to poorly crystalline Fe-hydroxides are the most abundant iron phases in the uppermost 6 m of the core; ferryhydrite, goethite and subordinate hematite are predominant further down in the weathering profile. Ageing of poorly defined Fe-hydroxides to goethite occurs in zones that are no longer accessible to water due to m m thick coatings of kaolinite. A few centimeters


further into more porous, conducting zones the decomposition of goethite and hematite to less well-defined Fe-hydroxides may be observed. In one par- ticular sample from a former magnetite vein (MF 12-16-1A ) relict magnetite is preserved in the innermost center of originally cm-sized magnetite nodules. Mn-oxyhydroxides like pyrolusite and nsutite are often associated with Fe- hydroxides.

Alunite and jarosite are the most important trace components in the lateritic weathering zone, both occurring in close association with Fe-hydroxides and in pyrite pseudomorphs. No uranium minerals could be identified in the weathering zone; uranium probably occurs incorporated in Th-minerals and/ or adsorbed on Fe-hydroxides. Thorianite and thorogummite were detected in several samples and are the most important Th-minerals of secondary origin. Relict zircon and bastnaesite as potential Th- (and U-) bearing minerals were sporadically observed. Florencite occurs in nearly all the investigated samples and is one of the most important Ce-minerals. Furthermore, cerianite and goyazite, together with relict (?) monazite and bastnaesite, are important REE-bearing minerals.

5.5. Fracture mineralogy

The entire country rock intercepted by borehole MF12 is intensely frac- tured. Two major fracture systems occur, one dipping 75-85 ° to the horizontal plane, the other with an inclination between 50-60 ° to the horizontal plane. Additionally, a great number of fractures and fissures occur which were produced through local brecciation. No correlation could be established between inclination, relative age, and infilling of the single fractures. All the investigated fractures are potentially water-bearing and their infillings are controlled by the present groundwater system. In general, the observed fracture infillings are in very good agreement with the mineral stabilities calculated from the groundwater composition (see Nordstrom et al., 1992).

In the tephri-phonolite down to 68 m, illite and kaolinite are the predominant infillings, invariably accompanied by smectite; chlorite was detected in two fractures at a depth of about 50 m (Table 2). Both illite and kaolinite found in water-bearing fractures display a less well-defined X-ray pattern compared to their matrix counterparts and are, therefore, believed to be of secondary origin. This is further sustained by the textural relationships evidenced by SEM investigations. Between 58 and 70 m there is a zone where abundant illite-smectite mixed-layers occur besides kaolinite. The illite to smectite ratio of these mixed-layers is about 9 to 1. In this zone calcite relicts also occur. Hydrothermal K-feldspar is sporadically present on fracture planes. Etch pits on the surface of such K-feldspars clearly indicate dissolution. The first fluorite occurs 2 m below the redox front; the idiomorphic crystals have corroded surfaces, indicating the initiation of dissolution. Pyrite, though ox-

i 2 8 ~ WABr.t~

T A B L E 2

B o r e h o l e M F 1 2 ; q u a l i t a t i v e m i n e r a l o g i c a l composit ion of fracture infillings in the h y d r o t h e r m a l i y

a l t e r e d p h o n o l i t e s ; s a m p l e n u m b e r s r e f e r t o d e p t h in m e t e r s

S a m p l e M i n e r a l k f f lu cc py r u t ch l ill k a o m l s m e g ib h f o

origin 1 1 1 i 1 J 1,2 1,2 1,2? 1,2? 2 2

3 1 - 1 A x x x x x

3 4 - 1 A x xxx x

3 7 - 1 A xx xx xx x x

4 2 - 1 B xx x xx x xx x

4 8 - 1 B x x x x xxx x x

5 3 - 1 A - 1 xx x xxx x x

5 3 - 1 A - 3 x x x x xxx x

5 9 - 1 A x xxx xx x x

6 2 - 1 B - 1 xxx x x x x

62 - l B-2 xx xx x x x x

6 5 - 1 B x x x x x x x

6 8 - 1 A - 1 x x x xxx x

7 1 - 1 A - 1 xxx

71-1A-2 xxx

Abbreviations: k f = K-feldspar, f lu = fluorite, cc = calcite, py = p y r i t e , r u t = futile, chl = chlorite, ill = iUite, k a o = k a o l i n i t e , m l = i l l / s m e m i x e d layers, sme = smectite, gib = gibbsite, hfo = hydrous ferric oxides; x = trace, xx = minor, xxx = major infilling; 1 = hydrothermal origin, 2 = secondary, weathering origin.

idized at its surface, was first detected in a fracture at 47 m (about 12 m below the redox front). Pyrite totally devoid of surface oxidation, does not occur until a depth of 65 m. Fe-hydroxide coatings on fracture planes are observed in the core down to 62 m,

In the fractures of the leucocratic phonolite (below 68 m), kaolinite is again the predominant infilting phase (Table 2). Pyrite, here without surface oxidation, and fluorite and calcite, are often present in fractures at these depths.

5.6. Rock physical properties

The variation of porosity, bulk and grain density in borehole MFI2 is shown in Fig. 3. In the reduced rocks porosity is an average of about 5%, displaying only minor variation over the whole profile with the exception of the strongly brecciated zones (12-20%). A strong increase of porosity occurs in the oxidized phonolite towards the lateritic weathering zone. The deepest investigated oxidized phonolite sample (MF12-35-1A) has a porosity of 10%; the sample adjacent to the lateritic weathering zone junction has an increased porosity of 33%. The porosity of the lateritic weathering zone varies between 45-50%. Bulk density decreases down to values below 1.5 g/cc compared to an average of 2.4 g/cc for the hydrothermally altered, reduced phonolites. The observed porosity changes are a product of hydrothermal alteration


B~ehole MF12 Borehole MFIO

0

-10 I -20

30

~ . 4O

-50

-60

70

-80

k.

bulk densily ~

grain density 10 2O 3O 4O 2

0

-10

-20

30

60

70

80 30 40 50

g -4o

50

l bulk density grain dens~

2 3 4

porosity % density (g/cc) porosity % density (g/cc)

Fig. 3. Variation of porosity, bulk and grain density in the hydrothermally altered country rock and its weathering zone (borehole MF12), and in the weathered carbonatite and orebody (borehole MF10).

(brecciation, argillic alteration) and superimposed weathering processes (dissolution of alkali-feldspar, sulfides, fluorite, and carbonates ).

5.7. Geochemistry

The recovered country rocks from borehole MF12 mineralogically resem- ble the hydrothermally altered phonolites of the Osamu Utsumi uranium mine in many of their features. Considering the general chemical composition, there also exists a strong relationship, which is more pronounced between the leucocratic phonolite and the Osamu Utsumi mine phonolites. In order to explain the geochemical changes that took place during the hydrothermal alteration, the Morro do Ferro country rocks are compared to hydrothermally unaltered regional phonolite samples as discussed by Schorscher and Shea ( 1992 ) and to the phonolitic rocks of the Osamu Utsumi uranium mine (Wa- ber et al., 1992). These comparisons seem to be reasonable: (1) due to the mineralogical similarity of the regional phonolites with the unaltered phonolite samples found at the Osamu Utsumi mine, inspire of the slightly greater peralkaline affinity of the regional rocks, and (2) due to the similarity of the hydrothermally altered Morro do Ferro country rocks with the altered phonolitic rocks at the Osamu Utsumi mine. Mean values of the different phonolitic country rocks recovered from borehole MF 12 are given in Table 3.

TA

BL

E 3

Geo

chem

ical

mea

n va

lues

of

hydr

othe

rmal

ly a

lter

ed p

hono

lite

s, l

ater

itic

wea

ther

ing

zone

, w

eath

ered

ca

rbon

atit

e, a

nd o

rebo

dy.

Mea

n va

lues

of

the

oreb

ody,

wea

ther

ed c

arbo

nati

te a

nd la

teri

tic

wea

ther

ing

zone

are

sta

tistic

ally

irr

elev

ant d

ue to

the

larg

e sc

atte

r of

the

anal

yses

with

res

pect

~o

the

num

ber

of a

naly

zed

sam

ples

Bor

ehol

e M

FIO

B

oreh

ole

MF

I 2

Wea

ther

ed h

ostr

ock

Lat

erit

ic w

eath

erin

g L

euco

crat

ic

Tep

hri-

I,

euco

cra~

++

O

rebo

dy

(car

bona

tite

) zo

ne

phon

olit

e ph

onol

ite

phon

olit

c ox

idiz

ed

redu

ced

redu

ced

aver

age

rang

e av

erag

e ra

nge

aver

age

rang

e n

= 3

n

-- 5

n

= 3

n-

--12

n

=8

n

=5

SiO

: T

iO2

A12

03

Feto

t M

nO

MgO

C

aO

~a~O

K

20

P20~

L

OI

CO

2

wt.%

20

.59

(6.0

1-39

.41)

33

.52

(26,

73-4

0.64

) 38

.49

(28.

1-48

.16)

54

.17

51.3

1 57

.87

2.0

5

(0.9

7-3.

12)

1.9

3

(1.0

9-5.

42)

1.0

2

(0.7

7-1.

41)

0.71

0.

57

0.62

25

.95

(14.

11-3

8.06

) 27

.36

(22.

07-3

2.84

) 32

.60

(28.

4-38

.84)

23

.00

20.5

5 21

.61

18

.95

(6

.49-

36.2

1)

15

.71

(4

.5-2

9.44

) 6.

85

(2.1

1-12

.94)

3.

67

6.27

1.

62

2.69

(0

.09-

10.6

3)

2.05

(0

-6.9

1)

1.0

3

(0-4

.11

) 0.

00

1.30

0.

00

0.23

(0

.1-0

.51)

0.

13

(0.0

7-0.

22)

0.08

(0

,04-

0.12

) 0.

07

0.27

0.

10

0.0

3

(0-0

,04)

0

+0

2

(0,0

1-0,

04)

0.03

(0

.01-

0.05

) 0.

03

1.43

0.

08

0.30

(0

.2-0

:55)

0.

16

(0.0

8-0.

27)

0.26

(0

,18-

0.37

) 0.

37

1.25

0,

51

2.93

(1

,3-4

.77)

2

,91

(2

.7-3

.34)

3

.92

(2

.1-7

.93)

11

.35

8.69

13

+33

0.77

(0

.18-

1.19

) 0,

15

(0.0

6-0.

32)

0.33

(0

.14-

0.79

) 0.

12

0,09

0.

11

13

.25

(9

.15-

20.4

3)

11

.30

(9

.66-

12.7

) 1

1.9

0

(7.5

1-15

.19)

2.

57

3,95

2,

28

n.a.

n.

a.

n,a,

n.

a.

0.62

(0

.4-1

.0)

1.65

2.

46

0~27

F pp

m

1708

B

a 15

5 R

b 11

3 Sr

69

8 Pb

13

6 T

h 87

67

U

27

Nb

1084

L

a 41

54

Ce

7628

N

d 20

11

Y

404

Zr

2318

V

59

7 C

r 54

N

i 22

C

o 17

C

u 25

Z

n 72

4 H

f 62

Sc

3

S 44

8

612-

2585

) 69

3 (2

42-1

144)

10

-529

) 95

(1

3-18

7)

40-1

69)

101

(74-

136)

14

6-13

59)

85

(42-

175)

65

-253

) 55

(9

-95)

14

04-1

4843

) 40

8 (1

65-8

49)

< 5

-64)

7

( < 5

-45)

73

1-22

06)

11

17

(5

04-3

144)

12

86-1

1100

) 43

14

(123

9-85

73)

1516

-139

62)

16

20

(8

42-2

632)

70

1-51

42)

2058

(6

38-3

758)

20

0-41

6)

297

(163

-581

) 14

69-3

619)

24

45

(147

3-52

53)

95-1

156)

63

0 (3

06-1

259)

17

-99)

34

(2

7-44

) 7

(< 5

-18)

<

5-5

8)

17

(<5

-34

) <

5-4

4)

b.d.

b.

d.

135-

1039

) 67

5 (1

25-1

836)

28

-78)

41

(2

1-91

) <

2-7

) b.

d.

b.d.

84

-164

9)

379

(119

-104

4)

1134

19

2 12

6 89

1 24

9 21

9 92

543

1882

17

95

1410

22

8 21

33

220 19

8 15 7

196 39 3

134

(599

-177

7)

(36-

360)

(9

3-22

0)

(51-

3189

) (4

3-82

2)

(103

-554

) (4

5-17

3)

(368

-751

) (1

405-

8649

(1

470-

2406

(3

87-4

723)

(1

36-3

78)

(130

4-26

69

(110

-347

) (6

-25

) (<

5-1

9)

( <

5-4

0)

(<5-

18)

(60-

363)

(2

0-51

) (<

34

) (7

2-24

5)

1734

14

7 33

9 21

1 29

77

33

355

2148

12

54

489

118

1402

13

6 5 b.

d. 9

b.d.

78

20

5 13

9

5936

15

0 20

8 68

1 31

52

25

274

928

1106

24

4 97

933 97

8 b.

d.

14

b.d.

56

2 11 3

3459

1706

15

8 32

8 27

8 28

4O

19

291

615

667

168 85

1148

18

7 b.

d.

b.d.

5 b.

d.

163 12

6 31

40

m

m

M z ,=

m

m

m

> K

o o o m P ©

Ana

lyse

s by

XR

F;

n.a.

= n

ot a

naly

sed,

b.d

. =be

low

det

ecti

on li

mit

. m

> t"-' t~

N F

1132 u \~,A B E_~,

The leucocratic phonolite is characterized by its extremely elevated K20 content (average of 13.33 wt. % ), reflecting the potassium-rich hydrothermal alteration. In contrast, Fe,ot, Na20, MgO, CaO and MnO are strongly to completely depleted in this rock, compared to the unaltered regional phonolites (Fig. 4a). The tephri-phonolite is characterized by having less SiO2 (average of 51.3 wt.%) and K20 (average of 8.69 wt.%) but has higher concentrations of Feto,, MnO, CaO, Na20, crystalline water and CO2 compared to the leucocratic phonolite. Mineralogically, these differences are reflected in a lower abundance of alkali-feldspar which, in addition, did not suffer complete al-

100.00

10.oo ; (a)

0.01 ~ - I I I - - -4.- . . . . . . I - ~ I I - - - - - 4

SiO2 T i O 2 AI203 Fe tot MnO MgO CaO Na20 K20 P205 H20

(b)

• MF12-42 IA

[] MF12-46-1A

• MF12-47 1A

O MF12-51-1A

• MF12-58-1A

A MF12-68-1B

X MF12,70-18

{ >K UF12-71-IC I

• MF12-42-1A

1 0 . 0 0 / ~ ~ [] MF12-46-1A

• MF12-47-1A

i,oo oMF12-511A

A MF12-68-18 :

0.10 X MF12-70qB '

f )K MF12-711C

0.01 t I I ~ I I I

SiO2 TiO2 AI203 Fe tot MnO I~jO CaO Na20 K20 P205 H20

Fig. 4. Borehole MF 12; major element contents of the reduced hydrothermally altered phonolites normalized to (a) the average of regional phonolites (data from Schorscher and Shea, 1992 ) and (b) the average of hydrothermally altered phonolites of the Osamu Utsumi uranium mine (data from Waber et al., 1992). Tephri-phonolite: samples 42-1A, 47-1A, 51-1A, and 58-1A. Leucocratic phonolite: samples 68-1B, 70- i B, and 71- t C. Phonolitic dyke: sample 46-1A.


kali-exchange in the tephri-phonolite, a higher abundance of relict primary mafic minerals, and a stronger impregnation with pyrite and carbonates compared to the leucocratic phonolite. In comparison with the hydrothermally unaltered regional phonolites, the tephri-phonolite has increased contents of Feto~, MnO, C O 2 and crystalline water (Fig. 4a). SiO2, Na20 and, in the majority of the samples, CaO, are lower than in the regional phonolites. K20 , although slightly increased in some samples, displays similar concentrations to those in the regional phonolites. The leucocratic dyke (sample MF12-46- 1A) displays in its major element composition a strong affinity to the leucocratic phonolite. Differences include a slightly lower K20 content and higher values for MnO, CaO and Na20, intermediate between tephri-phonolite and leucocratic phonolite (Fig. 4 ).

In Fig. 4b, the major elements of the Morro do Ferro country rocks are normalized to the mean value of the leucocratic phonolite of the Osamu Ut- sumi mine. As can be seen from this figure, there occur only minor differences between the two leucocratic phonolites. The tephri-phonolite, in contrast, has higher contents of Fetot, MnO, MgO, CaO, Na20, P205, crystalline water and CO2, but much lower K20 and SiO2 concentrations.

The most striking differences in the trace element composition include the higher mean values for the tephri-phonolite of F (factor 3.5 ), Zn (factor 3 ), Co (factor 2.8), Sr (factor 2.4), total LREEs (factor 1.6, XRF data), U and Th (both by a factor 1.3 ) and Cr which is below detection in the leucocratic phonolite. All these elements, with the partial exception of Sr, occur in mineral phases of the hydrothermal mineralization assemblage and corroborate the mineralogical observations. By analogy to the unaltered regional rocks, where Sr is also incorporated into alkali-feldspar (M.N.C. Ulbrich, 1983 ), Sr might still be partly present in the incompletely exchanged alkali-feldspar of the tephri-phonolite. In addition, smectite, which has a high capacity to in- corporate Sr, occurs in major amounts in the clay fraction in the tephri-phonolite. Rb (factor 1.6), V (factor 1.9) and Sc (factor 2) are enriched in the leucocratic phonolite. Rb, V and Sc are mainly incorporated in alkali-feldspar and the enrichment factors reflect the difference in modal abundance of alkali-feldspar in the two phonolites. The phonolitic dyke displays an intermediate trace element composition between the leucocratic phonolite and the tephri-phonolite. The dyke is, however, lower in U, Th, Pb and S than both phonolites.

In comparison with the unaltered regional phonolites, the strong enrichment in both Morro do Ferro phonolites of U, Th, LREEs, Y and Pb and the depletion of Ba and Sr are the most obvious features (Fig. 5a). F, Cr, Co and Zn are enriched in the tephri-phonolite whereas they are slightly to strongly depleted in the leucocratic phonolite. Rb, V and Sc exhibit higher values in the leucocratic phonolite but more or less equal values in the tephri-phonolite compared to the regional samples. Nb, Zr and Hf display similar values to the

134 r,~ WABEF.

1000 00

F L

•0000 i

lOOO !

100

0.10 L

0 01 - 4 - - + ~ I I - - + - - - ÷ - - + - 4 I ~---~. -~--.4. 4 ~ F - 4 ~ - . ' F Ba Rb Sr Pb Th U Nb La Ce Nd Y Zr V Cr Ni Co Cu Zn Hf S

• MF12-42 L a,

[] MF12-46 !A

• MF12-47 1A

O MF12-51-1A

• MF12 58 1A

A MF12 68-1~

X MF12 70-1B ! I i MF12-71-!C i

1000.00

1o0oo I (b)

10.00

0.10 ~ " ' ~ ~

001 I t t ~ 4 + - ~ I - - I - I I I Jr ~ I I I I - -4 F Ba Rb Sr Pb Th U Nb La Ce Nd Y Zr V Cr Ni Co Cu Zn Fir S

• MF12-42-1A

[] MF12-46 1A t

• MF12-47 1A

O MF12-51-1A

• MF12-58-1A

A MF12-68-1B

X MF12-70 1B

)K MF12-71-1C

10000.00

1000.00

100.00

10.00

1.00

O10

0.01

(c)

I I + I I I ~ I I I I I 1 I I I t I t

Ba Rb Sr Pb Th U Nb La Ce Nd Y Zr V Cr Ni Co Cu Zn Hf S

• MF12-42-1A

O MF12-46-1A

• MF12-4TIA

o. MF12-51-1A

• MF12-58-1A

A MF12-68-1B

X MF12,70-1B

:~ MF12-71-IC

Fig. 5. Borehole MF 12; trace element contents of the reduced hydrothermally altered phonolites normalized to (a) the average of regional phonolites (data from Schorscher and Shea, 1992), (b) the average of hydrothermally altered phonolites of the Osamu Utsumi uranium mine (data from Waber et al., 1992 ), and (c) in comparison with the highly mineralized volcanic breccias of the Osamu Utsumi uranium mine (shaded area). Tephri-phonolite: samples 42-1 A, 47-1 A, 51-1A, and 58-1A. Leucocratic phonolite: samples 68-1B, 70-1B, and 71-1C. Phonolitic dyke: sample 46-1A.

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POCOS DE CALDAS, BRAZIL 13 5

regional phonolites. Sulfur is enriched in both phonolites with the exception of two strongly argillically altered samples and the phonolitic dyke.

In Fig. 5b the trace elements of the Morro do Ferro country rocks are normalized to the mean value of the reduced, hydrothermally altered rocks of the Osamu Utsumi mine. Here too, the country rocks are enriched in LREEs, Y, Th and Pb and depleted in Ba. Compared to the Osamu Utsumi mine phonolites, however, all the country phonolites of Morro do Ferro are depleted in U (with one exception), V and Zr. On the other hand, F, Co and Zn are increased in the tephri-phonolite but decreased in the leucocratic phonolite; S occurs in much lower concentrations in both if compared to the Osamu Utsumi mine phonolites.

In comparison with the highly mineralized volcanic breccia samples of the Osamu Utsumi mine, the Morro do Ferro country rocks are much poorer in U, Th, Pb, Nb, Zn, Ba and particularly in Y, Zr, Hf and S. When normalized to the unaltered regional rocks, however, the general distribution pattern is very similar, with the exception of higher LREEs but lower Y, Zr, Hfand S in the Morro do Ferro rocks (Fig. 5c).

Interelement correlations are not obvious among major and trace elements in the Morro do Ferro country rocks. The better correlation in both phonolitic rocks between CaO and F (r=0.99, n = 8 ) than between CaO and CO2 (r=0.64, n = 8) sustains the mineralogical observations that fluorite is the major fluorine and calcium bearer, whereas CO2 is preferentially present in carbonates other than calcite, for example, siderite (rFe_CO2 ----0.75, n = 8 ) and REE carbonates (e.g. bastnaesite; rce-co2 = 0.82, n = 8 ).

In the oxidized leucocratic phonolite the increase in the A1203 content and the decrease in the SiO2 and K20 contents, compared to its reduced counterpart, reflect the decomposition of K-feldspar to clay minerals. MgO, CaO and Na20 are also lower, whereas Fetot, crystalline water and CO2 display increased values. Fluoride displays equal values as in the reduced leucocratic phonolite, although no fluorite is present any longer in the oxidized zone. This indicates the presence of hydrothermal fluoro-carbonates (e.g. bastnaesite) that are less sensitive to oxidation than fluorite. Furthermore, F might be retained by secondary minerals formed under oxidizing conditions. Zn and S are the only elements that show the same drastic depletion due to overall dissolution of the sulfides, as observed in the Osamu Utsumi mine. Sr, although depleted, is still fairly high in the oxidized phonolite and is partly retained in secondary mineral phases (goyazite). U and Th are more abundant than in both reduced phonolites. This can partly be explained by higher primary contents and, for U, additionally by the retention by adsorption on iron-hydroxides. That multiple processes govern the distribution of U and Th in the oxidized phonolite is also supported by the natural decay series (see Linsalata and Morse, 1992 ).

In the vertical profile of the lateritic weathering zone the differem evolu-

] 3 6 J WABEP,

tion of SiO: and A1203, the latter increasing towards the surface, is most characteristic (Fig. 6). K~O and SiO~ are strongly depleted towards the surface. This reflects the overall kaolinitization and, in the uppermost part, where the A1203 content becomes higher than the SiOe content, the formation of gibbsite. Fetot shows an irregular distribution, as does MnO, in response to the concentration of Fe- and Mn-oxyhydroxides in certain zones. MgO, CaO and Na~O are further depleted in the lateritic weathering zone, whereas TiO, and

M O R R O DO FERRO BOREHOLE MF-12:

A E

00

o o

I0 ~. e

ZO

i" -50

~ e

60 •

0 5 IP 1 5

,e . m

m

[]

u

I~ o..

m ~

! e

[] T 02

~nO

• P205

,)

ii o •

IIi ~-e

m o

m

weight • %

2

l

o

3 10 2[~ 30 40 rg; 0(

J

.m • o

• i ) '

• o • c

II mo:o

SaC ~ o ~r ~'.

Na2C Fe203 K20 A,203 5 0 ? [ ~ ]

ppm

0 50C 1000 1500 0 100 200 300 400 500 2000 4000 6000 0 1000 2000

!0

20 '-:.. J : / " I

40 .q ,~ 0 "ub J Q J ~ "J ",o

- - . ' , / r -5O I ~

• Nd I e o Sr i ~, U o Y • Zr , . . . .

Fig. 6. Borehole MF12; geochemical variation with depth for selected major and trace elements (analyses by XRF ).


P205 display an increase towards the surface reflecting a residual enrichment of refractory minerals. Rb becomes depleted towards the surface and its good correlation with K20 ( r= 0.98, n = 5) shows that Rb is mainly incorporated in K-feldspar. Sr is depleted in the lower part of the weathering zone but shows a strong increase in the uppermost 6 m of the profile. Similar trends can be observed for Pb and Th. Both these elements show a close correlation with Sr ( rSr_Pb -~ 1 ; rsr_Th = 0.99, n = 5 ) and P 2 0 5 ( rpEOs-Pb = 0.99; rp2os_Th ---- 0.98, n = 5 ) reflecting the formation of the observed secondary Sr-Th-Pb-phosphates goyazite, p lumbogummite , and thorogummite. V displays a similar behavior, but is even better correlated with Nb and TiO2 which are both residually enriched towards the surface. V, Nb and TiO2 are mainly present in association with the hydrotherrnal alteration products (leucoxene) which are more resistant to weathering. Zr shows an irregular distribution with highest contents in the weathered magnetite vein sample (MFI2-16-1A). Towards the surface, Zr is residually enriched. It correlates closely with Hf and, surprisingly, with Ce. A possible explanation for this might be the residual enrichment of zircon and cerianite (see below) in the same horizons. With the exception of the very first sample below the surface U displays a similar distribution as Th in the lateritic weathering zone. The average content of U is higher in the lateritic weathering zone (average 92 ppm, n - 5 ) than in the oxidized (average 33 ppm, n = 3 ) and reduced country rock (average 15 ppm, n = 8 ). There are no well established correlations between U and other elements in the lateritic weathering zone. The best affinity for U is shown by Fetot ( r=0.85, n = 5 ) and crystalline water ( r=0.86, n = 5) indicating the possible absorption of U on Fe-hydroxides. F, Ba and S are all depleted towards the surface, with Ba displaying the most heterogeneous distribution. High Ba concentrations occur in samples with elevated MnO contents where Ba is incorporated in psilomelane.

5.7.1. Rare-earth elements In the reduced phonolites La, Ce and Nd exhibit a very good interelement

correlation, best pronounced between La and Ce (rL~_Ce = 0.99, rL~-Nd = 0.96 and rce_Nd= 0.96, n = 8; XRF data). This indicates mineralogical control of the LREEs by the same mineral phases in these rocks. Normalized to ordinary chondrite, the reduced phonolite samples show a very similar distribution pattern for the complete light rare-earth elements (LREEs) series from La to Sm (Fig. 7, ICPOES data from Linsalata and Morse, 1992). Compared to both the unaltered regional phonolites and the hydrothermally altered leucocratic phonolites of the Osamu Utsumi mine, the Morro do Ferro country rocks are strongly enriched in LREEs and have a less steep decline from La to Sm. The chondrite normalized La-Sm ratio varies between 3 and 8 for the Morro do Ferro phonolites compared to values between 15 and 17 in the phonolites of the Osamu Utsumi mine.

138 ,, W~,~E~

m

100000

j / - - / • • . . . . . . . . . . . • 10000

1000

[

100

10 + 4 + 4

La Ce Pr Nd Sm

• MF12-16q A

[3 MF12-28-1A

• MF12-31-lB

o MF12-35-1A

• MF12-37-1B

/', MF12-58-1A

Fig. 7. Borehole MF12; LREEs normalized to ordinary chondrite (analyses by ICPOES, data from Linsalata and Morse, 1992). Lateritic weathering cover: sample 16-1A. Oxidized leucocratic phonolite: samples 28-1A, 31-1B, and 35-1A, Reduced tephri-phonolite: sample 58-1A. Reduced leucocratic phonolite: sample 37- I B.

In the oxidized phonolite the average LREE concentrations are enhanced compared to its reduced counterpart. This can be explained by a stronger hydrothermal impregnation with highly birefringent cryptocrystalline REE- mineral phases in these levels of the original rock profile. The chondrite normalized distribution patterns are still comparable to the ones of the reduced rocks (Fig. 7 ), however, the correlation between La, Ce and Nd is less well pronounced in the oxidized phonolite.

In the lateritic weathering zone La and Nd display a very good correlation between each other ( r= 1, n=5 , XRF data) and with Y (r=0.95, n = 5 ) . Ce now behaves completely differently (rL~_Ce = 0.44, rNd-Ce = 0.43, n = 5 ) due to the oxidation of trivalent Ce to the less soluble Ce 4+ and its incorporation into minerals being less susceptible to weathering (cerianite). Thus, Ce correlates much better with the residually enriched Zr ( r = 0.96, n - 5 ). The complete LREE-series is only available for one lateritic sample. This sample has the highest absolute REE concentrations of all the analysed samples and displays in its chondrite normalized pattern a strong negative Ce-anomaly (Fig. 7, MF 12-16-1A), similar to the ones observed for the weathered host rock samples (compare Fig. 11 ). The distinctly higher concentrations of REEs, and also Fe, Mn, Th, U and Zr in this sample are explained by the fact that this sample is a former magnetite vein derived from the carbonatitic host rock.

In three samples from oxidized phonolite and lateritic weathering zone Tb and Yb have been analysed by neutron activation (Fig. 8 ). Compared to oxidized phonolites of the Osamu Utsumi mine the absolute abundance of these HREEs is higher, too, in the Morro do Ferro country rocks. However, the Morro do Ferro country rocks display a much stronger enrichment in LREEs

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POLOS DE CALDAS, BRAZIL

100000

139

10000

1000

100 '~iii!ii!~iii'~'~iii~i~ii~i~=:~:~% ~ •

I I I I I I I I L I 4 I I

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

• MF12-16-1 1 [] MF12-28 1A

* MF12-31-1A

Fig. 8. Borehole MFI2; REEs normalized to ordinary chondrite compared to oxidized hydrothermally altered phonolites of the Osamu Utsumi mine (shaded area, data from Waber et al., 1992; analyses by neutron activation and ICPOES, respectively). Lateritic weathering cover: sample 16-1A. Oxidized leucocratic phonolite: samples 28-1A and 31-1A.

compared to HREEs as observed in the chondrite normalized La-Yb ratio which varies between values of 27 and 119 for the oxidized Morro do Ferro country rock compared to values around 18 for the oxidized phonolites of the Osamu Utsumi mine.

6. SUPERGENE Th-REE-MINERALIZATION

6. I. Host rock

The term "host rock" here applies to the completely weathered material below the Th-REE ore body of Morro do Ferro. Such host rock was encountered in all the boreholes drilled at Morro do Ferro and its lower boundary is still not clearly defined. In some features the host rock resembles the lateritic weathering zone produced on top of the hydrothermally altered phonolitic country rocks. There are, however, important differences in the mineralogical and geochemical composition that cannot be explained by weathering of the same parent rock. It is proposed that the original host rock at Morro do Ferro was a carbonatite, the weathering of which led to the present supergene Th- REE deposit. The existence of a primary carbonatitic host rock at Morro do Ferro is supported by:

( 1 ) The favorable geological environment of the Poqos de Caldas plateau that consists of a suite of more or less differentiated alkaline rocks (M.N.C. Ulbrich, 1983; H.H.G.J. Ulbrich, 1984; Schorscher and Shea, 1992) which may often have associated carbonatites, for example as in other alkaline com- plexes in Brazil (Heinrich, 1966; Ulbrich and Gomes, 1981; Woolley, 1989, and literature cited therein).

1 4 0 - ~ A ~ E ~ ~

(2) The primary and secondary mineral parageneses that are typical for unweathered and weathered carbonatites (e.g. Jaffe and Selchow, 1960; Eby, 1975; Reedman, 1984; Lottermoser, 1988; Mariano, 1989a; Le Bas, 1989; Hogarth, 1989; Mariano, 1989b ).

( 3 ) Chemical mass balance calculations according to the method of Brim- hall and Dietrich (1987), that include chemical, physical and mechanical changes during weathering in consideration of the estimated original overburden (Ulbrich, 1984, 1989) and the erosion rate (Holmes et al.~ 1992)~ reveal a geologically meaningful thickness only for rocks with average Th concentrations of more than 250 ppm (Waber, 1990b). Besides the Morro do Ferro host rock, such high Th contents are only reported from two chibinite samples (Rocha et al., 1984; IPT-Report, 1984) and four volcanic breccia samples with vein mineralizations from the Osamu Utsumi mine (Waber et al., 1992 ) in the Pogos de Caldas alkaline complex. Average values for these latter rocks are, however, much lower. In contrast, many carbonatites, especially ferro-carbonatites, are known worldwide to have such high Th concentrations (average 276 ppm, n = 13; Woolley and Kempe, 1989).

(4) The occurrence of magnetite veins; "Highly characteristic of nearly all carbonatites are accessory to locally essential amounts of magnetite. The magnetite tends to occur as disseminated euhedra, not uncommonly in flow bands" (Heinrich, 1966, p. 182). Magnetite ores are well known from many carbonatite occurrences e.g. Ipanema and Jacupiranga, Sao Paulo, Brazil (Derby, 1891 ), Bukusu, Uganda (Taylor, 1955), Kaiserstuhl, Germany (van Wambeke et al., 1964).

(5) The much deeper weathering of the Morro do Ferro host rock compared to the surrounding hydrothermally altered country rocks. The suscep- tibility of a carbonatite to weathering is much greater than for the silicate country rocks due to the high carbonate content of the former. This subsequently led to its complete alteration and allowed the formation of the present supergene Th-REE mineralization.

(6) The occurrence of secondary gypsum as fracture fillings in the hydrothermally altered rocks below the completely weathered host rock (Camargo dos Santos, 1984 ); gypsum was observed nowhere else in Morro do Ferro and is thought to have been precipitated from waters dissolving calcite and sulfides from the original overburden.

(7) Ultramafic dykes at the nearby Osamu Utsumi mine which have carbonatitic affiliations (Waber et al., 1992).

6.1.1. Petrography and mineralogy The non-mineralized carbonatitic host rock was encountered in borehole

MF10 from 24 to 75 m, in MF11 from 20 to 40 m, and in MF13 from 5 to 60 m (compare Fig. 5 in Waber, 1990a). The completely weathered host rock is characterized by its very fine grain size with 35-55% of the whole rock being


less than 20 #m. It is of yellowish-brown to reddish-brown color and lighter than the material of the ore body but darker than the weathering zone capping the country rock. A well developed horizontal layering is present throughout the host rock which contrasts with the more sparry texture of the MF12 weathering zone. Accumulations of partly to completely oxidized magnetite nodules often associated with Mn-oxyhydroxides occur randomly distributed in the host rock. Such dark brown colored zones represent former magnetite layers and range from less than a centimeter in width, with nodules of several millimeters in diameter, to layers of several centi- to decimeters in width with cm-sized nodules. The mineralogical composition of the host rock is rather simple in terms of its major components but becomes extremely variable in its minor and trace components, including an abundance of normally very rare minerals (Table 4).

Kaolinite and illite/sericite are by far the most abundant components of the host rock. They both occur in nearly equal amounts over the whole MF 10 profile which strongly contrasts with the lateritic weathering zone capping the country rocks. Gibbsite occurs throughout the host rock mainly in the < 2/~m fraction. Coarser grained gibbsite, often as pore infillings, is sporadically present and more abundant in the upper part of the host rock. Low crystalline Al-hydroxides and amorphous Al-phases are frequently distributed throughout the host rock. In several places, the formation of gibbsite from Al-gels has been observed. Alunite and jarosite both occur throughout the host rock. Both these minerals are indicators for a highly acid environment and may have formed through oxidation of primary sulfides, e.g. pyrite.

Among the secondary iron-phases, goethite, ferryhydrite and amorphous iron-gel are the most abundant. They are very heterogeneously distributed and may be absent in certain zones, whilst being the predominant minerals only a few centimeters below. Goethite and ferryhydrite are always in close association with amorphous to low crystalline iron-hydroxides. Hematite is far less abundant and it only seems to be stable in zones where it is protected from further water access. In such zones, it may also be formed through ageing of iron-hydroxides. The crystalline to amorphous iron-hydroxides display a very complex behaviour of formation, dissolution, and recrystallization, all changing over very short distances. Such behaviour is well known from lateritic weathering zones and the processes taking place are most probably related to fluctuations in the water input (dry-wet season changes; Boulangr, 1984; Tardy and Nahon, 1985; Schwertmann, 1985; Muller, 1987; Boudeulle and Muller, 1988). Primary magnetite is restricted to certain layers and is only preserved in nodules larger than 1 cm in diameter. From core to rim, such nodules display alteration to hematite, goethite and low crystalline Fe-hydroxides, with rhythmically intercalated Mn-hydroxides. Nodules of cryptomelane are always associated with magnetite nodules but are much less abundant; sporadically, psilomelane, lithiophorite and lithiophyllite occur.

TABLE 4

Borehole MF10; qualitative mineralogical bulk composition of the orebody and the weathered carbonatitc

S a m p l e d e s c r i p t i o n S a m p l e i l l / s e r kao l i l l / k a o l g i b b a l u n jar ( ) f lo r bas t

o r i g i n 2 2 r a t i o 2 2 2 i '/, 2

Orebody homog, red-brown c lay l- 1A x x x xxx 6 6 / 3 4 xx x xx

h o m o g , r e d - b r o w n c lay l - l B xx × 8 0 / 2 0 xxx x xxx

red-brown c lay ~- l C xx x 8 8 / 1 2 xxx xx ~ ×x

red and white c lay 3 -1A xx xx 5 8 / 4 2 xxx x xx

red clay 4 - 1 A x xx_x 2 5 / 7 5 xx x x ~ xx

white clay with red l aye r s 6 - 1 A xx xx 5 5 / 4 5 x x ~ ×

dark red clay 10-1A xx x 8 4 / 1 6 x x x x ××

h o m o g , w h i t e c lay 12-1A xx xx 4 3 / 5 7 x x x xx

white clay with red l aye r s 18-1A x x x ×xx 4 8 / 5 2 x ~

white and red c lay 19-1A xx x 6 2 / 3 8 xx x ×

Weathered carbonatitic host rock b r i g h t b r o w n c lay 2 5 - 1 A xx x 8 6 / 1 4 x xx x xx

white clay with red l a y e r s 3 6 - 1 A xx xxx 3 4 / 6 6 x x x

white c lay 4 9 - 1 A - A xxx x x x 5 0 / 5 0 x ×

alteration zone around fracture 4 9 - 1 A - B xxx x x x 5 5 / 4 5 x x x

white clay, magnetite layer 5 4 - 1 A x xxx 3 0 / 7 0 x x

white and brown c lay 5 6 - 1 A xx xx 4 2 / 5 8 x x x ×

white c lay 6 4 - 1 A x x x x x x 5 5 / 4 5 x x x

white c lay 7 3 - 1 A x x x xxx 4 4 / 5 6 x

Abbreviations: ill = d l i t e , ser= sericite, k a o l = k a o l i n i t e , g i b b = g i b b s i t e , a l u n = a l u n i t e - g r o u p , j a r o =jarosite- group, flor = florencite, bast = bastnaesite, mon = monazite, pych = pyrochlore, cher = cheralite, thor = thorite, cer = cerianite, thoria = thorianite, lanth = Nd-lanthanite, mag = magnetite, hem = hematite, goe = goethite, hfo = hydrous ferric oxides; x x x = d o m i n a n t , xx = m a j o r , x = minor component; M n O x = M n -

o x y h y d r o x i d e s ; 1 = primary origin, 2 = s e c o n d a r y o r i g in .

Cryptomelane is often rimmed by Mn-hydroxides such as nsutite, birnessite and pyrolusite. Inclusions in cryptomelane include idiomorphic florencite, thorianite and cerianite. The manganese-oxides/hydroxides display a similar complex behaviour as the iron phases.

A large variety of primary and secondary Th-REE minerals have been detected in the host rock and the list given in Table 5 is probably far from being complete. Textural relationships are virtually the only indications to distinguish between Th-REE minerals of primary (magmatic or hydrothermal) and secondary (weathering) origin. Thus, inclusions of such minerals found in well preserved magnetite cores are considered to be of primary origin. All the Th-REE phases reported by previous workers (Frondel and Marvin, 1959; Wedow, 1967; Fujimori, 1982; Barretto and Fujlmori, 1986) from the ore body could also be confirmed in the weathered host rock, however, with certain modifications concerning the origin of these minerals. Bastnaesite, cheralite, monazite, thorite, zircon and pyrochlore occur as more or less idiomorphic inclusions in magnetite, with bastnaesite, monazite and zircon being


rnon pych cher thor cer thor ia lanth m a g h e m goe hfo MnOx 1,2 1 1 1 2 2 2 1 1?,2 2 2 1,2

XX XX X X XX

X X? X X XX XX X X X XX XX X X XX X XX X XX X

x x? xx x xx x X X X XX X XX X XX XX X

X X X X X X X X X XX XX X XX

X XX X X X X X

X X? XX X X X X X X XX X

X X X XX X XX X X

x? x x x x xx x x

x x x x x

x x

x x x x? x xx x x x x

x xx xx x x x x x x xx x

x x? x x x

T A B L E 5

T h - R E E m i n e r a l s o f the o r e b o d y a n d w e a t h e r e d c a r b o n a t i t e at M o r r o do F e r r o

T h - R E E m i n e r a l s o f p r i m a r y o r ig in T h - R E E m i n e r a l s o f s e c o n d a r y or ig in

b a s t n a e s i t e ( C e , L a ) ( C O 3 ) F c e r i a n i t e

t h o r b a s t n a e s i t e ( T h , C e , L a ) 2 ( C O 3 ) 2 F 2 , n H 2 0 f lo renc i t e

che ra l i t e ( C a , C e , T h ) ( P , S i ) 0 4 goyaz i t e

r n o n a z i t e R E E ( P O 4 ) gorce ix i t e

t ho r i t e T h (S iO4) N d - l a n t h a n i t e

z i r c o n ( Z r , T h , U ) (S iO4) m o n a z i t e

p y r o c h l o r e ( N a , C a , T h , U , R E E ) 2- t h o r i a n i t e

(Nb,Ti,Ta) 206 (OH,F) thorogummite

( C e 4 + , T h ) O 2

C e A 1 3 ( P O 4 ) 2 ( O H ) 6

( S r , R E E ) AI3 ( P O 4 ) 2 ( O H ) 5 " H 2 0

(Ba ,REE)A1 3 ( P O 4 ) 2 ( O H ) 5" H 2 0

(La,Nd) 2 ( ( C O 3 ) 3" 8 H 2 0

R E E ( P O 4 )

( T h , U ) O 2

T h ( S i O 4 ) ~ _ x ( O H ) 4 x

the most abundant. All these minerals are therefore of magmato-hydrothermal origin. Additional Th-REE-phases, reported the first time from the Morro do Ferro host rock and ore body, include the crandallite-group minerals rio- rencite, gorceixite and goyazite, secondary monazite, and the REE-carbonate Nd-lanthanite. All these Th-REE minerals mainly occur as fine grained crys-

144 ~ WABt3~

Fig. 9. Scanning electron micrograph ol secondary Nd-lanthanite idiomorphically grown in a pore space of the orebody ( sample M F 10-12-1 A ).

tals disseminated in the rock matrix and are considered to be of secondary origin. For the interpretation of the contrasting behaviour of LREEs in host rock and ore body, the abundant occurrence of secondary Nd-lanthanite as a pore infilling (Fig. 9) in the host rock is of great importance. The Nd-lanthanite is crystallographically comparable to the one found in the weathering zone of the Curitiba Basin, Paran~t, Brazil (Fortin, 1989). All the investigated Nd-lanthanites are free of Ce and the formation of this mineral might therefore help to explain the negative Ce-anomalies observed in most of the host rock samples (see below). Secondary Th-REE minerals are generally more abundant in the host rock than in the ore body, especially the hydroxyphosphates of the crandallite group and Nd-lanthanite. Oxidic secondary minerals like cerianite and thorianite only sporadically occur in the uppermost part of the host rock and in samples with abundant magnetite.

6.1.2. Geochemistry The geochemical variation with depth along borehole MF 10, which pene-

trates through the ore body and down into the weathered host rock, lis shown in Fig. 10 for selected elements. The very high concentrations of Th, total REE and total iron result in a high mass absorption and therefore the stan- dard correction procedures applied in routine XRF-analysis are no longer ap-

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POGOS DE CALDAS, BRAZIL

MORRO DO FERRO BOREHOLE MF-10:

145

0 1

0 + +:i+!]o + --~

I 0

+ 0 + , , . . . . . . . . . . . . . . : , .

,.c -40

" 0

50 m" • . . . . . . a

- 6 0

-70 = ' = [] TiO2 . o • - - • • Felol

-8o I • P 2 0 5 1 o MnO

w e i g h t - %

2 3 4 5 0 10 20 30 0 10 20 30 4.0

• . . e n • , .~

I .......... 'P: D / / /

e . . O

o -~

0

+,

K20

e. ~ e D . . . . "C

/ / ; , ]"

,, .- # Q"

e . t~ .

• " " 'E l

AI203 SiO2

IE .l=

] l lpm

0 500 1000 0 6000 12000 0 50 0 5000 I00(~

0

- 2 0 ~ ' 1 ° .. ~i~:::::::: ......... == +

40 : / :: o

-50

- ,ot~ .. ;, I~ ~ - ? ~ r F~ " ? ~ C o . , oc0 + • U ] • Nd

n ~ 0 I i

Fig. 10. Borehole MFI0; geochemical variation with depth for selected major and trace elements (analyses by XRF).

plicable for such extreme rock compositions. This in turn produces less accurate values for the light major elements evidenced by the low totals of the major element sums. Nevertheless, the analyses may be used for determining the general geochemical trends. Much more accurate results with respect to

146 ~WASE~

the absolute concentrations of Th, U and the REEs have been obtained by alpha-spectrometry and ICPOES (see Linsalata and Morse, 1992).

The most striking feature in the major element composition of the host rock is the extremely good correlation between SiO2 and A1203 over the whole profile, whereas K20 remains nearly stable over the same distance (Fig. 10). This is in strong contrast to all the investigated weathering profiles overlying hydrothermally altered phonolitic rocks (cf. Fig. 6 and Waber, 1990b). Fetot, MnO and TiO: display a rather heterogeneous distribution, reflecting the al- ternating occurrence of ferricretes and magnetite veins. Total iron is normally three times higher, and MnO and TiO2 more than two times higher, in the host rock than in the weathered zone overlying the MF 12 phonolitic country rocks (Table 3 ). MgO, CaO and Na20 are all present in very low concentrations similar to those found in the MF12 weathered zone.

One of the major characteristics of the trace element composit ion in the host rock are the higher concentrations of Th, REEs, Nb, V, Cr, Ce and Zn, when compared to the weathered zone of the country rocks. On the other hand, Sr, Pb and U are slightly decreased (Fig. 10). Th displays a rather heterogeneous distribution in the host rock with an average of 408 ppm (XRF data, n = 8 ) and 487 ppm (ICP data, n = 7 ), respectively, over a thickness of 50 m in the MF10 borehole. The concentration generally increases towards the ore body. Th is fairly well correlated with Ce ( r= 0.92, n--8 ), supporting the presence of Th-Ce minerals. Moreover, Th is correlated with Sr (r = 0.94, n = 8 ) and U (r=0.90, n = 8 ) . Nb is an important component in many minerals typical for carbonatites (e.g. columbite-group minerals) and remains rather immobile during weathering. Nb is increased by a factor 2 to 6 in the host rock compared to the weathered zone of the country rock and correlates closely with TiO2 (r = 0.95, n = 8 ), Zr (r = 0.98, n = 8 ) and Hf (r = 0.93, n = 8 ). Highest Nb contents are recorded in samples with abundant magnetite. The same applies for V that can be enriched by a factor 6 in such samples compared to the country rock weathering zone. In magnetite-free samples the contents are enriched by a factor about 1.5 to 2. Cr and Co are both considerably higher in the host rock than in the country rock weathering zone. Whereas Co is mainly bound in magnetite rich samples, Cr contents are increased in all samples. U displays a similar distribution as Th. The concentrations are com- parably low, around the detection limit with the exception of a magnetite-rich sample, where the U content is 45 ppm (Fig. 10). Sr and Pb occur in concentrations similar to those in the lower part of the country rock weathering zone. They both show only minor variation with depth, Sr being enhanced in samples with abundant magnetite. Sr is best related with P205 and Ba indicating the presence of goyazite.

6.1.2. I. Rare-earth elements Similarly to Th, the LREEs contents are at their lowest within the last 20 m

of the core, occurring in concentrations comparable to those in the weathered zone of the country phonolites. However, further up in the remaining 30 m of


the host rock, La and Nd are strongly increased (by a factor 3 to 4 and 2 to 3, respectively), whereas Ce displays a decrease in some samples and a moderate increase in others. In Fig. 11, the LREEs from La to Sm analysed by IC- POES (data from Linsalata and Morse, 1992) are normalized to ordinary chondrite. Four samples show a pronounced negative Ce-anomaly, one sample has a moderate negative Ce-anomaly (MF10-56-1A) and two samples (MFI0-64-1A, MF10-70-1A) do not display any anomaly. Samples with a negative Ce-anomaly also have higher total abundances in all the LREEs other than Ce, but lower Ce contents if compared to the samples without any Ce- anomaly. The different behaviour of Ce compared to the other LREEs is also expressed in the good correlation of Ce with Ba (r=0.96, n= 8, XRF data), F (r=0.93, n = 8 ) , and U and Th (for both r=0.92, n = 8 ) . In contrast, La and Nd correlate perfectly with each other ( r= 0.99, n = 8 ), reflecting mineralogical control by the same phases (Nd-lanthanite), and display a rather good relationship with Y (rLa_ ¥ = 0.85 and rNd-v = 0.90, n = 8 ).

6.1.3. Rock physical properties Total porosity varies between 37 to 48% in the host rock (Fig. 3). In the

ore body the variation of porosity is greater, between 20 to 52% of the rock volume. The average porosity is the same for host rock and ore body (41% ). Bulk density of the host rock is 15% higher compared to the weathering zone overlying the country rock, but only about 8% lower than that in the ore body. Grain density is about 15% higher in the ore body than in the host rock samples devoid of magnetite nodules. Compared to the weathering zone overlying the country rock the grain density is about 5% higher. These values reflect the higher abundance of residually enriched heavy minerals, both in host rock and ore body, compared to the MF 12 weathering zone.

1000000

100000 I

t

ooo I ~000

100

La Ce Pr Nd Sm

• MF10-36-1A

[] MF10-49-1A.A

• MF10-49-1A-B

O MF10-56-1A

• MF10-64-1A

A MF10-70-1A

× MFlO-73-1A

Fig. 11. Borehole MF10; LREEs normalized to ordinary chondrite of the weathered carbonatite (analyses by ICPOES, data from Linsalata and Morse, 1992).

6.2. Th-REE orebody

6.2.1. Petrography and mineralogy The Th-REE ore body consists of a fine-grained clay-silt matrix similar to

the underlying host rock; coarse-grained components are, however, much more abundant. Color plates from core sections of these unusual rock materials are illustrated in Waber (1990a). The most obvious feature of the ore body is the abundant magnetite veins of variable thicknesses which have stabilized the hill from excessive erosion, allowing a residual enrichment of Th and REE minerals to accumulate through dissolution of the more soluble mineral phases. The ore material is mainly dark reddish-brown in color. Magnetite nodules occur loosely arranged in layers and vein-like structures and also more randomly distributed throughout the matrix. The mineralogical composition of the ore body is given in Table 4, and the variation of the rock physical properties is shown in Fig. 3.

In the ore body the illite/sericite: kaolinite ratio increases from nearly 1:1 at the lower boundary to 4:1 towards the surface. This is in strong contradic- tion to the weathering profiles overlying the phonolitic country rocks in borehole MF 12, and overlying the hydrothermally altered rocks of the Osamu Ut- sumi mine (Waber et al., 1992). Furthermore, this also contrasts markedly with the expected stability of illite compared to kaolinite in a normal lateritic weathering profile. The reason for such behaviour is not yet clearly under- stood but two observations may play an important role: firstly, illite/sericite in the ore body is on average coarser-grained than in the host rock, and sec- ondly, illite/sericite in the ore body is always coated with an iron-hydroxide film and is very often completely included in iron-hydroxides and /o r aluminium hydroxides. This may protect the illite/sericite from water interaction and, therefore, from complete dissolution. Gibbsite and amorphous Al-hydroxide are most abundant in the first 10 m below the surface. Well crystal- lized gibbsite is always associated with poorly crystalline varieties and completely amorphous Al-gel. The formation of gibbsite from such Al-phases is of obvious importance. The very abundant magnetite nodules vary in size from some millimeters to several decimeters. In the small grain-size ranges the nodules are completely oxidized, in coarser-grained nodules ( > 1 cm ) the magnetite tends to be well preserved in the central parts. Otherwise, the iron phases occur in the same manner as described for the host rock, with the exception of Fe-hydroxides and amorphous Fe-gel, which are finely distributed throughout the ore material.

The same criteria as for the host rock have been applied to distinguish between primary and secondary Th-REE phases (Table 5 ). Secondary Th-REE minerals are commonly much more fine grained (0 .01-10/am) and occur disseminated throughout the ore body. Bastnaesite and thorbastnaesite were found as inclusions in magnetite and are, therefore, considered to be of pri-


mary origin. In the rock matrix these minerals always have strongly corroded crystal surfaces and decompose to cerianite and crandallite-group minerals. Monazite seems to be of primary as well as of secondary origin. Primary monazite inclusions found in magnetite are much more coarse-grained than secondary monazite which occurs as minute euhedral crystals ( 1-10/tm) forming earthy aggregates in the matrix. These features are similar to the secondary monazite known from the weathering zone overlying the Mt. Weld carbonatite, Australia (Lottermoser, 1988 ), and at Arax~l and Catalao I, Brazil (Mar- iano, 1989a). Cheralite is mainly decomposed to cerianite, and thorite alters to thorianite and thorogummite. Pyrochlore occurs as strongly metamict and corroded grains. Florencite pseudomorphically replaces pyrochlore, similar to the processes described from the weathering zone of the Mr. Weld carbonatite (Lottermoser and England, 1988). Zircon occurs as euhedral, slightly corroded crystals displaying variable concentrations of Th and U. In the ore body Nd-lanthanite was only observed in samples MF 10-1-1 B, MF 10-6-1A and MF 10-12-1 B, all of them displaying either a negative Ce-anomaly or none at all. As in the host rock, Nd-lanthanite does not occur as a clearly defined alteration product of a primary phase, but seems to have been precipitated from solution. Secondary oxidic Th-REE minerals are more abundant in the ore body than the secondary Th-REE-hydroxyphosphates; this is of special importance because cerianite exclusively incorporates Ce 4+ but not the trivalent REEs. The abundant formation of cerianite in the ore body, therefore, may be responsible for the observed positive Ce-anomalies (see below).

6.2.2. Geochemistry In the lower part of the ore body, SiO2 and A1203, in common with the host

rock, are positively correlated (Fig. 10 ); towards the surface SiO: drastically decreases whereas A1203 increases, reflecting the formation of gibbsite. K20, with values similar to those from the weathered carbonatitic host rock, only shows minor fluctuations up to the uppermost meter in the ore body, where it then becomes depleted. Fe203 is high throughout the ore body with concentrations ranging between about 6 and 36 wt.%. MnO displays a very heterogeneous distribution from zero in some samples to values of up to about 10 wt.% in other samples only a few decimeters apart. No relation between iron and manganese is observed. P205 is residually enriched towards the surface, reaching concentrations of more than 1 wt.%. Throughout the ore body, P205 is increased compared to the host rock reflecting the higher abundance of REE phosphates.

The Morro do Ferro ore body is characterized by its extremely high Th and REE concentrations, which vary from centimeter to centimeter. Apart from Th and REEs, strong enrichments of Sr, F and Pb occur in the ore body compared to the weathered carbonatitic host rock; U is also higher in overall abundance. All the other trace elements are present in similar concentrations

150 - ~ W A B E ~ ~

and display the same variations as observed in the host rock, with the exception of Ba which is depleted in the ore body. Th is strongly enriched in the first 10 m, then decreases slowly with depth to values representative for the host rock. Th shows a certain affinity to Ce but not to the other LREEs, which becomes even more obvious in the data obtained by alpha-spectrometry and ICPOES (Linsalata and Morse, 1992). Moreover, Th shows an affinity to samples rich in iron oxyhydroxides indicating possible adsorption of Th onto such phases. U displays a higher total abundance in the ore body than in the host rock, but is also extremely variable (Fig. 10). As for Th, samples nearly free of iron oxyhydroxides are also free of U. Autoradiographs of thin and polished sections show a finely disseminated distribution of radiation intensity in fine-grained, iron stained zones of the ore material. Strong intensity is recorded where highly birefringent minerals are observed in the matrix, along the oxidized rims of magnetite nodules, and around their primary inclusions. White colored ore samples display only very weak or negligible radiation in- tensities. In comparison to the host rock, Sr is strongly enriched in the ore body by a factor up to 10; the enrichment is highest from a depth of I. 5 to 6 m. At the surface, the Sr concentrations decrease once again. Similar behavior can be observed for F, except that F displays a more heterogeneous distribution. Compared to the host rock, F is enhanced by a factor 2 to 3. Good interelement correlations between trace elements in the ore body are rare. Affini- ties exist between U-Sr, Rb-V-K20 , Th-Ce and F-Ce.

6.2.2. 1. Rare-earth elements The behavior of the LREEs differs markedly between the ore body and the

host rock. La and Nd occur in equal or lower amounts in the ore body, whereas Ce is increased (Fig. 10). Ce shows no correlation witla the other LREEs. La and Nd show a good correlation in the ore body (r=0.99, n = 7, ICPOES data) whereas Ce shows a weak trend with Th ( r=0.9 , n = 7 ) . The chondrite normalized LREE patterns from La to Sm (ICPOES data from Linsalata and Morse, 1992 ) show a modest to strong positive Ce-anomaly for all samples except three (Fig. 12 ). Two of these exceptions are samples MF 10-6-1A and MF 10-12-1 A, which display a modest to strong negative Ce-anomaly as observed in most samples of the host rock. Similarly, these two samples also display higher concentrations in the other LREEs if compared to most overlying samples from the drillcore. Both these samples consist of white colored clay material and carry Nd-lanthanite. The third exception is sample MF10- 1-1B which comes from just below the surface and displays a considerable enrichment in all LREEs. The positive Ce-anomaly reported from the other samples in the ore body is in agreement with the occurrence of cerianite in these samples.

Summarizing, the behavior of Ce as compared to the other LREEs is observed to be different over the entire drillcore profile throughout the ore body and weathered carbonatitic host rock. The positive Ce-anomalies in the ore-

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POLOS DE CALDAS, BRAZIL | 51

1000000

100000

10000

-b

.~ 1000

100

I I I -t

La Ce Pr Nd Srn

• MFIO-I-IB

o MFIO-3-1A

• MF10-4-1A

C, MFIO-6-1A

• MF10-12-1A

A MFlO-18-1A

X MFlO-25-1A

Fig. 12. Borehole MF10; LREEs normalized to ordinary chondrite of the Th-REE orebody (analyses by ICPOES, data from Linsalata and Morse, 1992).

body can be explained mineralogically by the slow dissolution of the primary REE minerals, whereby C e 3+ is oxidized to C e 4+ and incorporated into secondary cerianite. Ce is therefore retained in the ore body, whereas the trivalent LREEs have migrated downwards in the profile, resulting in an apparent positive Ce-anomaly in the ore body. The faster-moving trivalent LREEs subsequently become more readily available for secondary mineral formation (e.g. Nd-lanthanite) and/or adsorptive processes, as exemplified by the negative Ce-anomalies and the increased total abundance of LREEs (except for Ce), in the samples from the host rock. These processes are further controlled by the abundance and type of occurrence (e.g. inclusion) of the primary REE phases and their differing solubilities during weathering.

7. CONCLUSIONS

Based on the regional geology study by Schorscher and Shea ( 1992 ), it ap- pears that the Morro do Ferro area underwent a late phase of subvolcanic activity and associated hydrothermal alteration. This event led to the formation of a carbonatite with associated magnetite layers and veins enriched in both Th and REEs, compared to the unaltered regional phonolites. Related to this event were the brecciation and hydrothermal alteration of the surrounding phonolitic country rocks. Xenolithic evidence of breccia veins in the phonolites indicate that brecciation and alteration took place subsequent to the phonolites being deuterically altered. This deuteric alteration is similar to that generally observed in the rocks of the Poqos de Caldas alkaline complex. Clay mineral parageneses, including kaolinite, indicate that the phonolites at Morro do Ferro had cooled down substantially before the hydrothermal alteration occurred. The recurrence of magmatic activity was almost

1 52 ~ WABER

contemporaneous with the emplacement of the carbonatitic host rock and the intrusion of yet a second stage of minor phonolitic plugs and dykes. The former existence of a primary carbonatite at Morro do Ferro is inferred from comparison of the weathered material with the weathering zone overlying hydrothermally altered and unaltered silicate rocks, from primary and secondary mineral parageneses (bastnaesite, cheralite, monazite, thorite, pyrochlore, and florencite, goyazite, monazite, Nd-lanthanite, cerianite, thorianite, respectively ), from geochemical mass balance calculations, and from the type of country rock alteration (fenitization). Furthermore, the identification of a carbonatite is supported by the occurrence of ultramafic dykes with carbonatitic affiliations at the nearby Osamu Utsumi uranium mine (Waber et al., 1992).

There is clear evidence that the hydrothermal alteration induced by the emplacement of the carbonatite was polyphase. It consisted of an early potassium metasomatism with associated disseminated pyritization, followed by a vein-type thor ium-REE mineralization with associated fluorite, pyrite, carbonate and zircon. During this second phase, small-sized phonolitic plugs and dykes were emplaced. These late-stage phonolitic rocks are compositionally almost identical with the older phonolitic country rocks at Morro do Ferro, however, they were not subjected to the same intensive, hydrothermal alkali- exchange alteration. Compared to the unaltered regional phonolites, the hydrothermal alteration of the phonolites at Morro do Ferro resulted in a strong enrichment of K, S, Th, U, Pb, Y and Rb and a depletion in Sr, similar to the hydrothermal alteration observed in the rocks of the Osamu Utsumi uranium mine. In addition, the Morro do Ferro phonolites are strongly enriched in REEs, especially in the LREEs, but depleted in Ba. Compared to the hydrothermally altered rocks of the Osamu Utsumi uranium mine, the phonolites show an enrichment in Th, REEs, Y, Pb, F, P and CO2 and a depletion in U, S and Ba. The intrusion of ultramafic dykes with carbonatitic affiliations at the Osamu Utsumi uranium mine marks the end of magmatic activity in the area so that weathering of the rocks under low-temperature conditions began about 75 Ma ago (Shea, 1992).

The carbonatite is much more susceptible to weathering than the silicate country rocks. Thus, subsequent weathering of the carbonatite has been so complete that no relicts now survive; the only primary feature preserved is a resistant stockwork of partially oxidized magnetite layers and veins which cap the deposit. This stockwork has probably stabilized the underlying 130 m of highly altered carbonatitic rock from greater erosion and allowed the supergene enrichment of Th and REEs to accumulate. In contrast, weathering of the phonolite country rocks resulted in a lateritic cover of only about 30 m and an oxidation zone of about 10 m in thickness over the same time period. Weathering of the hydrothermally altered country rocks at Morro do Ferro

THE SUPERGENE Th-REE DEPOSIT AT MORRO DO FERRO, POLOS DE CALDAS, BRAZIL 15 3

resulted in the dissolution of sulfides, fluorite, carbonates and K-feldspar, the formation of secondary kaolinite, gibbsite and secondary Th- and REE-phases, and the residual enrichment of P, Ti, Th, Ce, Sr, and Zr towards the surface.

The conclusion that the original rock of the present supergene Th-REE mineralization was carbonatitic in composition is also of importance to the long- term study of the mobility of Th and REEs. Initial weathering of the carbonatite (i.e. carbonate removal) created a chemical environment that may have appreciably buffered the dissolution and mobility of the primary Th-REE phases. With time, when oxidizing conditions had become established throughout the bedrock to considerable depth, a redistribution of Th and REE gradually occurred, resulting in the present-day profiles. The strong enrichment of Th and Ce in the ore body, and the enrichment of the other LREEs in the host rock below, can be explained by mineralogical controls. Thus, the formation of thorite has fixed Th, and the oxidation of Ce 3 + to the less soluble Ce 4+ has given rise to cerianite. In contrast, the trivalent LREEs were not retained and have migrated further down the weathering column. Hence, their higher concentrations at depth have resulted in the formation of secondary minerals such as Nd-lanthanite and trivalent REE-hydroxyphosphates, and their adsorption on poorly crystalline iron- and aluminium-hydroxides. This results in the observed enrichment oftrivalent LREEs in the lower samples of the ore body and the weathered carbonatitic host rock. Seasonally controlled fluctuations of water input into the weathering column may explain the observed repetition of Ce-enriched zones underlain by trivalent LREE enriched zones.

A C K N O W L E D G E M E N T S

This study would not have been possible without the hospitality and helpful discussions of many Brazilian colleagues, especially L. Barroso Magno Jr. (Nuclebr~is), N. Miekeley (PUC, Rio de Janeiro ), H.D. Schorscher (Univer- sity of S~o Paulo) and R. Frayha (Pogos de Caldas), who provided logistic support throughout the project, and made work in Polos de Caldas a pleas- ure. Considerable thanks for analytical support are due to Ms. D. Riesen and Ms. H. Haas, U. Kr/ihenbiihl, J. Megert, K. Ramseyer and A. Zweili (Univer- sity of Bern). The author is grateful to B.A. Hofmann (Natural History Mu- seum of Bern), P. Linsalata (New York University Medical Center), D.K. Nordstrom (USGS) and Tj. Peters (University of Bern) for close collaboration and stimulating discussions. Constructive reviews by J.-C. Petit (CEA, Paris), T. Milodowski (BGS) and J.A.T. Smellie (Conterra AB) are grate- fully acknowledged. The work was financially supported by the Polos de Cal- das Project and managed by Swedish Nuclear Fuel and Waste Management (SKB), Stockholm.

154 N WABE~

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