Igarape Bahia Research Articles

40Ar-39Ar dating of Archean iron oxide Cu-Au and Paleoproterozoic granite-related Cu-Au deposits in the Carajás Mineral Province, Brazil: implications for genetic models

40Ar-39Ar dating of biotite from IOCG and granite-related Cu-Au deposits in the Carajas Mineral Province provides evidence for the timing of mineralization and constraints on genetic models of ore formation. Ages of biotite from greisen and quartz-rich vein and breccia deposits, Alvo 118—1885 ± 4 Ma, Breves—1886 ± 5 Ma, Estrela—1896 ± 7 Ma, and Gameleira—1908 ± 7 Ma, demonstrate the close temporal relationship between Cu-Au mineralization and subjacent A-type granites. Mineralization is hosted within granite cupolas (Breves) or in vein/breccia systems emanating from the cupolas (Estrela and Gameleira), consistent with a genetic relationship of mineralization to the B-Li-F-rich granites. Plateau and minimum ages of biotite from IOCG deposits, including Igarape Bahia, Cristalino, Corta Goela, and GT34, range from 2537 ± 6 Ma to 2193 ± 4 Ma. The 40Ar-39Ar age of biotite from Igarape Bahia (2537 ± 6 Ma) is similar to a previous SHRIMP 207Pb-206Pb age for monazite of 2575 ± 12 Ma when the uncertainties in the respective analyses and standards are taken into account. The age spectrum for biotite from Cristalino shows increasing ages for successive steps, consistent with post-crystallization Ar loss, and the age of 2388 ± 5 Ma for the last three steps is considered a minimum age for Cu-Au mineralization. The age of biotite from the GT34 prospect (2512 ± 7 Ma) coincides with a previously identified period of basement reactivation and may indicate the formation of Cu-Au mineralization at this time or resetting of biotite from an older mineralization event at this time. At Corta Goela, within the Canaa Shear Zone, the biotite age of 2193 ± 4 Ma lies between the ages of IOCG (2.57–2.76 Ga) and granite-related Cu-Au (~ 1.88 Ga) deposits elsewhere in the Carajas district but is similar to previously reported 40Ar-39Ar ages for amphibole from Sossego, possibly indicating that mineralization at both Sossego and Corta Goela was affected by a thermal event at this time. The Paleoproterozoic Cu-Au deposits are commonly hosted within Neoarchean IOCG alteration systems and the common occurrence of potassic alteration (especially biotite) in both types of deposits means that special care is required in interpreting the paragenesis of alteration in both types of deposits. The Paleoproterozoic Cu-Au deposits are reduced, and sulfur- and quartz-rich deposits lacking in major amounts of iron oxides and are therefore unlike IOCG deposits. Instead, they share many characteristics in common with widespread Paleoproterozoic Sn-W deposits in the Amazon Craton, including close spatial and temporal relationships with reduced A-type B-Li-F granites, and the occurrence of greisen and quartz-rich vein/breccia systems within and above granite cupolas. The occurrence of sericitic alteration in the Paleoproterozoic Cu-Au deposits is not evidence for an upward transition to sericitic alteration in IOCG deposits in the Carajas Mineral Province.

O DEPÓSITO BAHIA: UM POSSÍVEL EXEMPLO DE DEPÓSITO DE SULFETO VULCANOGÊNICO TIPO BESSHI ARQUEANO EM CARAJÁS

The Bahia ore deposit is hosted by the Archean Igarape Bahia Group which is one of the volcano-sedimentary sequences that have filled the Carajas basin (SE Para state). This group consists of basic metavolcanic, metapyroclastic and clastic metasedimentary rocks, besides banded iron formations and breccias. These rocks are cut by basic dikes and have been altered by hydrothermal fluids to mineral assemblages compatible with the greenschist facies metamorphic conditions. The Bahia deposit is formed by the Acampamento Norte, Acampamento Sul, Furo Trinta and Alemao ore bodies. The first three ones show disseminated mineralization that is hosted mainly by breccias. The last one was recently discovered and is partially made up of massive sulfide lenses. The breccias occur in the verticalized contact between the basic metavolcanic and the clastic metasedimentary rocks and usually display gradational boundaries with the host rocks. The lithoclasts are either from basic metavolcanic rocks or banded iron formations. Their shapes vary form angular to subangular. The matrix is composed of chlorite, siderite, chalcopyrite, quartz, magnetite, tourmaline and calcite. The matrix/clasts ratio is highly variable producing clast-supported to matrix-supported breccias, some of which showing slight foliation and orientation of the lithoclasts. To these breccias it is suggested a phreatic origin whose fragments were reworked and transported into deeper zones as debris flow. Compared to the host rocks, Cu and Au are more enriched in the breccias, in which the sulfides also occur in veins and pods together with quartz and/or siderite. Chalcopyrite and the pyrite are the main sulfides, but in the massive magnetite and sulfide-rich beds bornite is also present. Magnetite is an abundant phase found not only in banded iron formation clasts, but also in the breccia matrix as disseminations or in massive beds. Chloritization, carbonation, magnetitization and sulfidation are the most important types of hydrothermal alteration, although silicification and tourmalinization may also occur. Pervasive Chloritization has affected ali rocks in greater or lesser degree. Microthermometric studies in quartz crystals present in the breccias revealed aqueous fluids chemically represented by the H 2 O-NaCl-CaCl 2 system with variable salinities (5.5-41.5 weight % eq. NaCl). It was also observed pure CO 2 inclusions. The more frequent homogenization temperature ranges were 110-140°C and 150-225°C respectively for the two-phase and three-phase aqueous inclusions. The aqueous fluids have been assumed to be convectively driven sea water whereas the carbonic fluids were probably mantle-derived. The most saline aqueous fluids (>23% wt equiv. NaCl) are believed to have had a magmatic contribution. The geological characteristics of the Bahia deposit allow it to be interpreted as a Besshi-type volcanogenic deposit in which uranium and rare-earth elements, that occur in anomalous concentrations, have been introduced by later events.

Selected ore deposits of Brazil

This series of papers on a variety of ore deposits in Brazil is the outgrowth of the “First Brazilian Symposium on Metallogeny” held in Gramado, Rio Grande do Sul, in May 2005. The selection of papers mirrors quite well the range of commodities and fascinating diversity of ore deposit types Brazil has to offer. The thematic issue starts with a paper by Monteiro et al. who use detailed petrography to establish a consistent paragenetic sequence of alteration and mineralization throughout the Sossego iron–oxide–copper–gold (IOCG) deposits in the giant Carajas Mineral Province. Combined with stable isotope data, it allows the authors to constrain the spatial and temporal zoning of hydrothermal alteration and mineralization. They conclude that the Pista-SequerinhoBaiano and Sossego-Curral ore bodies formed at a deep and high structural level of an IOCG system, respectively. Significantly, the copper–gold mineralization was late in the alteration history and broadly synchronous in both ore bodies. Another contribution to the ore deposit geology of the Carajas Mineral Province is by Dreher et al. who use new geologic, fluid inclusion and stable isotope data to establish that the Igarape Bahia Cu–Au deposit displays both characteristic syngenetic VHMS and epigenetic IOCG style geological features. This controversial paper provides evidence via recent B isotope data on tourmaline that there is the distinct possibility of an involvement of a marine evaporitic source in the hydrothermal system of Igarape Bahia. The authors carefully evaluate evidence for both genetic models and correctly agree that more detailed work needs to be conducted to resolve this fascinating genetic conundrum. Souza Neto et al. provide an overview of W–Au skarns in the Serido mobile belt in the Borborema Province in northeast Brazil. Special emphasis is put on the reduced Bonfim Au–Bi–Te skarn deposit. The majority of skarns in the Serido mobile belt are oxidized tungsten skarns even though the Itajubatiba and Bonfim gold-bearing skarns display reduced features, such as pyrrhotite as predominant sulfide, garnet with high almandine plus spessartite component, and elevated gold content. PIXE data revealed that significant amounts of F (up to 9,490 ppm) are present in prograde titanite, and retrograde clinozoisite–zoisite and gold-related epidote, suggesting significant F activity in the skarn forming hydrothermal fluids. Beurlen et al. in a second paper on mineralization in the Borborema Province use Nb–Ta–(Ti–Sn)–oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the pegmatite-rich Borborema Province. The Borborema Pegmatite Province, with over 750 registered mineralized rare-element granitic pegmatites, is a historically important tantalum province and is currently well known for top-quality gems, such as the Paraiba Elbaite. Detailed electron microprobe analyses of pegmatite Miner Deposita (2008) 43:127–128 DOI 10.1007/s00126-007-0174-y

New geologic, fluid inclusion and stable isotope studies on the controversial Igarapé Bahia Cu–Au deposit, Carajás Province, Brazil

The Igarape Bahia Cu–Au deposit in the Carajas Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks of the Igarape Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiunas Supergroup, in which other important Cu–Au and iron ore deposits of the Carajas region are also hosted. The orebody at Igarape Bahia is a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals, and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite, amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarape Bahia is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl2 equiv), together with carbonic (CO2 ± CH4) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from the fragmental layer have δ13C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ34S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ18O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction with 18O-rich, probably sedimentary rocks. The late veins of the Igarape Bahia deposit area were formed from saline aqueous fluids (2 to 60 wt% NaCl + CaCl2 equiv) with δ18Ofluid compositions around 0 per mil that indicate contribution from meteoric fluids. With respect to geological features, Igarape Bahia bears similarity with syngenetic, volcanic-hosted massive sulfide (VHMS)-type deposits, as indicated by the volcano-sedimentary geological context, stratabound character, and association with submarine volcanic flows, hyaloclastite, and exhalative beds such as BIF and chert. On the other hand, the highly saline ore fluids and the mineral assemblage, dominated by magnetite and chalcopyrite, with associated gold, U- and LREE-minerals and scarce pyrite, indicate that Igarape Bahia belongs to the Fe oxide Cu–Au (IOCG) group of deposits. The available geochronologic data used to attest syngenetic or epigenetic origins for the mineralization are either imprecise or may not represent the main mineralization episode but a later, superimposed event. The C, S, and O isotopic results obtained in this study do not clearly discriminate between fluid sources. However, recent B isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit (Xavier, Wiedenbeck, Dreher, Rhede, Monteiro, Araujo, Chemical and boron isotopic composition of tourmaline from Archean and Paleoproterozoic Cu–Au deposits in the Carajas Mineral Province, 1° Simposio Brasileiro de Metalogenia, Gramado, Brazil, extended abstracts, CD-ROM, 2005) provide strong evidence of the involvement of a marine evaporitic source in the hydrothermal system of Igarape Bahia. Evaporite-derived fluids may explain the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit. Evaporite-derived fluids also exclude a significant participation of magmatic or mantle-derived fluids, reinforcing the role of nonmagmatic brines in the genesis of Igarape Bahia. Considering this aspect and the geological features, the possibility that the deposit was generated by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated.

Fragmental rocks of the Igarapé Bahia Cu-Au deposit, Carajás Mineral Province, Brazil

The Igarape Bahia primary Cu-Au deposit in the Carajas Mineral Province, northern Brazil, is hosted by hydrothermally altered fragmental rocks that occur at the contact between the volcanic and the sedimentary units of the Igarape Bahia Group. This group is considered as a low greenschist facies unit of the major Archean (~2.75 Ga) metavolcano-sedimentary Itacaiunas Supergroup, which hosts the most important Cu-Au and iron ore deposits of the Carajas Province. The fragmental rocks constitute an extensive, approximately concordant layer 2 km long and 30-250 m thick. They are heterolithic, usually matrix-supported rocks, made up mainly of coarse, angular to rounded, basalt, BIF and chert clasts set in a matrix of fine magnetite, siderite, Fe-chlorite and disseminated to massive chalcopyrite. Amphibole, quartz, gold and a series of accessory minerals containing elements such as Co, Mo, W, F, P, U, Mn, Pb, Sn, Ag, B, Cl, and LREE are also present in the matrix. The footwall metavolcanic rocks to the fragmental unit are flow basalts, autobreccia and in situ hyaloclastite, lacking evidence of explosive volcanism. The hanging wall metasedimentary sequence contains mainly coarse and fine-grained metaturbidites showing abundant slump and disruption features indicative of a deep, tectonically active marine setting. The fragmental rocks were probably deposited after a period of stability and volcanic quiescence – as evidenced by the presence of BIF and chert clasts - and define a sudden extensional event related to syndepositional faulting that resulted in deepening of the basin. According to this model, the fragmental layer represents a debris flow deposit made up of collapsed material that accumulated in a submarine depression adjacent to the fault zone.

Geology and SHRIMP U-Pb Geochronology of the Igarape Bahia Deposit, Carajas Copper-Gold Belt, Brazil: An Archean (2.57 Ga) Example of Iron-Oxide Cu-Au-(U-REE) Mineralization

A striking feature of the Carajas region, Brazil, is the clustering of a variety of different types of Cu-Au deposits. The most abundant in the belt are the >200 million metric tons (Mt) of Fe oxide Cu-Au-(U-REE) deposits, which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features, including (1) intense Fe metasomatism leading to the formation of grunerite, fayalite, and/or Fe oxides (magnetite and/or hematite); (2) intense carbonate alteration (mainly siderite); (3) sulfur-poor ore mineralogy (chalcopyrite and bornite); (4) quartz-deficient gangue; (5) extreme low REE enrichment, and (6) enrichment in U and Co. The Igarape Bahia deposit is perhaps the best documented Fe oxide Cu-Au-(U-REE) deposit of the belt, containing about 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The Cu-Au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano-sedimentary rocks. SHRIMP II zircon dating of the host metavolcanic rocks gives a 207Pb/206Pb age of 2748 ± 34 Ma. This suggests a correlation between the Igarape Bahia volcano-sedimentary sequence and the Grao Para volcanic rocks, which have published ages of ca. 2.75 Ga. SHRIMP dating of monazite from the matrix of ore-bearing magnetite breccias gives a 207Pb/206Pb age of 2575 ± 12 Ma, confirming the epigenetic nature of the mineralization and placing it ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The 2575 ± 12 Ma SHRIMP age of hydrothermal monazite from the Igarape Bahia mineralization is indistinguishable from published conventional 207Pb/206Pb ages for zircons from the Archean A-type granites of the Carajas belt, indicating that mineralization processes at Igarape Bahia were temporally related to these A-type Archean granites. The wide range of highly radiogenic 87Sr/86Sr ratios (0.714–0.755) of carbonates from the Igarape Bahia deposit suggests multiple crustal sources, consistent with a magmatic-hydrothermal origin. SHRIMP dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207Pb/206Pb age of ~2670 Ma, consistent with field evidence and the age of host rocks, but does not unequivocably constrain the age of the ores. The styles of hydrothermal alteration, mineralogy, and geochemistry of the Igarape Bahia ore, as well as published fluid inclusion and stable isotope data, support its classification as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) group of deposits, as previously argued by several authors. The SHRIMP age of 2575 ± 12 Ma for hydrothermal monazite indicates that Igarape Bahia is an Archean example of this deposit group.

A statistical approach to the experimental design of the sulfuric acid leaching of gold-copper ore

The high grade of copper in the Igarape Bahia (Brazil) gold-copper ore prevents the direct application of the classic cyanidation process. Copper oxides and sulfides react with cyanides in solution, causing a high consumption of leach reagent and thereby raising processing costs and decreasing recovery of gold. Studies have showm that a feasible route for this ore would be a pretreatment for copper minerals removal prior to the cyanidation stage. The goal of this experimental work was to study the experimental conditions required for copper removal from Igarape Bahia gold-copper ore by sulfuric acid leaching by applying a statistical approach to the experimental design. By using the Plackett Burman method, it was possible to select the variables that had the largest influence on the percentage of copper extracted at the sulfuric acid leaching stage. These were temperature of leach solution, stirring speed, concentration of sulfuric acid in the leach solution and particle size of the ore. The influence of the individual effects of these variables and their interactions on the experimental response were analyzed by applying the replicated full factorial design method. Finally, the selected variables were optimized by the ascending path statistical method, which determined the best experimental conditions for leaching to achieve the highest percentage of copper extracted. Using the optimized conditions, the best leaching results showed a copper extraction of 75.5%.

Sulfuric acid leaching of Igarapé Bahia gold-copper ore for copper extraction — An ore pretreatment for gold recovery by cyanidation

The high copper grade in the Igarape Bahia (Brazil) gold-copper ore restricts the direct application of the classical cyanidation process. Copper oxide and sulfide minerals react with cyanides in solution, causing high leach-reagent consumption, raising processing costs and decreasing gold recovery. According to studies, a feasible solution would be to pretreat the ore to remove copper minerals prior to cyanidation. The goal of this experimental work was to study the conditions for the removal of copper from the Igarape Bahia gold-copper ore by sulfuric acid leaching, applying a statistical approach to the experimental design. By using the Plackett Burman method, it was possible to select the variables that exhibited the largest influence on copper extraction at the sulfuric acid leaching stage. These variables were leach solution temperature, stirring speed, sulfuric acid concentration in the leach solution and ore particle size. The individual effect of these variables and their interaction influences on the experimental response were analyzed by applying the replicated full-factorial-design method. Finally, the selected variables were optimized by the ascending-path statistical method, which determined the best leach experimental conditions for maximum copper extraction. The optimized leaching conditions resulted in a copper extraction of 75.5%.

OCCURRENCE OF ILVAITE IN THE IGARAPÉ BAHIA Cu-Au DEPOSIT, CARAJÁS PROVINCE, BRAZIL

This paper describes the first known occurrence of the mineral ilvaite in Brazil. The mineral is sited in the matrix of magnetite-rich breccias and stockwork veins of the Igarape Bahia Cu-Au deposit. Electron microprobe analysis (EMPA) of the Igarape Bahia ilvaite reveals the following composition: Ca 0.96 Fe 2+ 1.96 Fe 3+ 1.00 Si 2.01 O 8 (OH) and X-ray diffraction (XRD) data indicate a structure compatible with the monoclinic polymorph (a 0 =13.032 A , b 0 =8.810 A , c 0 =5.850 A and a =90.254°). It is suggested that ilvaite crystallized from hydrothermal solutions, at a temperature range of 300-350°C, under conditions of high high f O 2 and low sulfur activity. The formation of ilvaite is tentatively related to the reaction magnetite + calcite + quartz + H 2 O = ilvaite + CO 2 + O 2 .

The Serra Pelada Au-PGE deposit, Serra dos Carajás (Pará State, Brazil): geological and geochemical indications for a composite mineralising process

The Serra Pelada Au-PGE-rich deposit is located in the Serra dos Carajas, a leading mining area in Brazil. This region is characterised by a complex geological and structural framework and is affected by deep lateritisation which has lasted for more than 70 Ma. The Serra Pelada deposit is emplaced in a late-Archean low-grade metasedimentary sequence (Rio Fresco/Aguas Claras Formation) which is host to other gold deposits in the region (Igarape Bahia, Aguas Claras). The Rio Fresco/Aguas Claras sequence was deposited in tectonic basins developed on Archean basement and Au-bearing greenstone terranes which were intruded by PGE-rich layered mafic complexes (e.g. Luanga). The Serra Pelada mineralisation is located along a regional, complex system of strike-slip faults (Cinzento-Carajas systems) which were active during the late Archean to early Proterozoic. The mineralisation appears to be concentrated along a faulted hinge zone of a fold. Ore zone rock facies are dominated by low-grade ferruginous to carbonaceous metasiltstones and minor sandstones, locally brecciated and cemented by quartz (±sulphide) stockwork. Supergene alteration led to partial to total transformation into friable aggregates of kaolinite, Fe oxide-hydroxides, silica and secondary phosphate-sulphates even at depths exceeding 200 m. Precious metals are exceptionally enriched, with up to more than 1,000 ppm Au+PGE in some peculiar ferruginous-graphitic zones locally called "hidrotermalito". Geochemistry shows complex patterns of major and trace elements, particularly rare-earth elements (REE), in mineralised vs. nonmineralised samples. These patterns are interpreted in terms of variable degree of superposition of hydrothermal and supergene alteration. Precious metals show progressive increase from samples with hydrothermal imprint to samples with supergene imprint. The geological evolution of the Carajas region and the characteristics of mineralisation at Serra Pelada may suggest a composite mineralising process: hydrothermal activity (by fluids likely originated from granitoids) was followed by supergene alteration during long-lasting lateritisation to develop extreme precious metal enrichments in a geological context probably already anomalous for Au and PGE.

Gold deposits of the Carajás mineral province: deposit types and metallogenesis

The Carajas mineral province is located in the south-eastern part of the Amazon craton and is divided into the southern and northern tectonic blocks, which are referred to as the Rio Maria granitoid–greenstone terrane and the Itacaiunas shear belt, respectively. The geological evolution of this province occurred almost entirely in the Archean, with no tectonism from ~2.4 to 1.9 Ga and then an episode of very extensive intraplate granitic magmatism. Distinct types of gold deposits, most associated with Archean metavolcano-sedimentary sequences, including greenstone belts as old as 2.9 Ga, comprise (1) Fe-oxide-poor Cu–Au, (2) Fe-oxide–Cu–Au–U–REE, (3) shear-zone related lode-gold, and (4) sedimentary rock-hosted Au–PGE. The shear-zone related deposits are more typical of the Rio Maria granitoid–greenstone terrane, whereas the other types are more abundant in the Itacaiunas shear belt. The first type, represented by the Serra Verde deposit, shows most characteristics of volcanogenic massive sulfide (VMS) deposits, except for the dominance of chalcopyrite over pyrite and/or pyrrhotite. Some deposits of the second type (Igarape Salobo and Igarape Bahia) are also syngenetic, but their ore mineralogy and paucity of pyrite and/or pyrrhotite are inconsistent with, or at least unusual for, classic VMS deposits. Other Fe-oxide–Cu–Au–U–REE deposits, such as the Cristalino and Sossego, occur as epigenetic stockwork mineralization related to Archean granitoid stocks. Most shear-zone related lode-gold deposits are classified as orogenic gold deposits (e.g. Sapucaia, Babacu and Tucuma) that cut greenstone belt rocks. The Cumaru deposit is also hosted by a shear zone, but may have a genetic association with a granitoid intrusion. The Aguas Claras deposit, although also hosted in a shear zone and perhaps intrusion-related, differs from the orogenic gold deposits by its extensional tectonic setting, lack of CO2 in the ore fluids, and voluminous chalcopyrite. The sedimentary rock-hosted Serra Pelada Au–PGE deposit is structurally controlled and occurs in an Archean clastic-chemical sedimentary rock sequence.

Chloritization of the hydrothermally altered bedrock at the Igarap� Bahia gold deposit, Caraj�s, Brazil

The Igarape Bahia gold deposit has developed from weathering of a near-vertical hydrothermal Cu (Au) mineralization zone. The unweathered bedrock composed of chlorite schists is mainly metamorphosed basalts, pyroclastic and clastic sedimentary rocks and iron formation. Contents and Fe/(Fe + Mg) ratios of chlorites increase from distal country rock towards the mineralization zone, which can be attributed to different water/rock ratios and locations in a hydrothermal system. In the hydrothermal system high salinity fluids convected through basin-floor rocks, stripping metals from the recharge zones with precipitation in discharge zones. The chlorite with lower Fe/(Fe + Mg) ratios indicates alteration by relatively unreacted Mg-rich fluids, occurring within recharge zones. By contrast, the chlorite with higher Fe/(Fe + Mg) ratios in the mineralization zone formed from solutions rich in Fe, Mn, Au, Cu, H2S and SiO2 within a discharge zone. The iron formation could also be formed within the discharge zone or on the basin floor from the Fe-rich fluids. The distal country rock with less chlorite content is a hydrothermal product at low water/rock ratios whereas the proximal country rock and the host rock with more chlorite content formed at high water/rock ratio conditions. The Al(IV) contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 °C, with temperatures slightly increasing from distal country rock towards the mineralization zone.

A three-stage genetic model for the Igarape Bahia lateritic gold deposit, Carajas, Brazil

A significant gold deposit in laterite is under development in the Carajas region of Brazil. The lateritic profile of the Igarape Bahia deposit, Carajas, can be divided into six horizons: basal unweathered rocks followed by saprolite, a pallid zone, a mottled zone, a ferruginous zone, and topsoil. Native gold and electrum are found within iron oxide-hydroxide nodules in the ferruginous zone where gold orebodies have been delineated. Most of the gold grains are less than 10 mu in size. There are three morphologicial types: (1) rounded grains in voids, associated with colloform goethite and needlelike goethite; (2) irregularly shaped grains filling shrinkage cracks in amorphous goethite; and (3) subidiomorphic crystals associated with amorphous goethite or needlelike goethite. The morphological features and occurrences of the gold suggest that it has been mobilized and reprecipitated during lateritization.In the ferruginous zone, gold grains from the upper part contain more silver than those from the lower part, i.e., the fineness of gold increases downward in the profile. Quartz veinlets and sulfides in bedrock suggest that the primary rocks were enriched in gold. In the first weathering stage thiosulfate ions from weathering of sulfides are hypothesized to transport both gold and silver into the saprolite and mottled zones where secondary electrum and silver-rich gold were formed. In the second stage, gold and silver were remobilized and incorporated into the ferruginous zone as the weathering proceeded and the erosion surface was lowered. In the third stage, after uplift and incision of the landscape, chloride leaching separated silver from gold grains to purify the electrum. The survival of electrum in the upper part of the ferruginous zone can be attributed to lower chloride concentrations in near-surface ground water and to enrichment of organic matter which can lower the redox potential and increase the stability of native silver. Some silver was then adsorbed by manganese oxide minerals in the lower part of the ferruginous zone.

The significance of biogenic element cycling in ancient tropical soils

Many tropical soils on stable cratonic areas have developed over millions to 10's of millions of years. Some elements (e.g., alkalies, alkaline earths, Si) are extremely depleted by leaching, and others (A1, Fe, Ti) are greatly enriched. Commonly this enrichment is regarded as merely the accumulation of insoluble constituents, but inspection of the soil shows development of pisolites, nodules and other features indicating extensive dissolution/precipitation of Feand Al-oxides and -hydroxides. This paper suggests that cycling of elements through vegetation is an important part of soil development in these ancient soils. This project was initiated to investigate the processes forming lateritic gold deposits, in which a relatively small primary vein or similar Au deposit in bedrock mushrooms in the soil to become a significant Au enrichment in the laterite zone (Davy and E1-Ansary, 1986; Lecomte and Colin, 1989). The site of our investigation is the gold deposit of Igarape Bahia in the Serra Carajas mining region, Para Province, Brazil. In this region, a very ancient plateau surface furnishes the environment for major Fe, Mn and A1 deposits of the Carajas district (Beisiegel, 1982; Machamer et al., 1991 ) as well as a gold deposit at Igarape Bahia. The plateau surface is interpreted to have been developed in Cretaceous or early Tertiary (Beisiegel, 1982; Kotschoubey and Lemos, 1985 ). The entire region is covered by dense tropical rain forest. At the Igarape Bahia Au deposit, a narrow primary zone of Au-Cu mineralization at depth gives rise to a relatively rich Au deposit in soil. The deposit appears analogous to other Au accumulations in which the initial primary Agbearing gold grains are etched and become smaller, and new grains of Ag-poor gold grow in the laterite (Lecomte and Colin, 1989). A preliminary study indicated that considerable Au was present in vegetation over the deposit, so in a later sampling expedition an extensive group of vegetation samples was collected, along with soils and waters (Andrade et al., 1991; Bliss, 1991; Machesky et al., 1993). Vegetation sampling was concentrated along a traverse across the unmined F30 deposit, a satellite Au deposit --~ 1 km from the main Igarape Bahia deposit. Additional samples were collected at three background sites ~ 1, ~ 10 and ~ 30 km from the deposit. At the F30 deposit and at the 1 km background site, all species of tree and shrub were collected, with leaves, twigs and bark being collected from most species. Five specific species (a large tree, two bamboo species, and two bushes) were sampled along the traverse and at the background sites. Samples were washed, dried, chopped, homogenized and analyzed for ~ 40 elements by neutron activation (Activation laboratories Ltd., Ancaster, Ontario) and Xray fluorescence (Materials Characterization Lab, Penn State University).