Archean Rio Das Velhas Greenstone Research Articles

Paleoenvironmental reconstruction of gold-bearing BIF from the Archean Cuiabá deposit based on petrographic and geochemical studies

In the world-class Cuiabá gold deposit, located in the Archean Rio das Velhas greenstone belt in Brazil, metamorphosed carbonate-dominated banded iron formations represent the main lithology hosting gold mineralization. The deposit's stratigraphy consists of a basal metavolcanic unit with carbonaceous metasedimentary rocks, followed by a layer of gold-bearing BIF and thin layer of carbonaceous metapelite, a upper metavolcanic unit, and turbiditic metapelites associated with metavolcanoclastic rocks. The BIF is essentially composed of fine-grained quartz and siderite, with variable contents of carbonaceous matter. It is assumed that these chemical sedimentary rocks, classified as Algoma-type BIF, have been formed in the deep-sea environment, relatively close to volcanic spreading centers. This hypothesis is supported by the BIF's geochemical signature, which in general presents a positive anomaly of La (La/La*paas = 0.64–3.43), Eu (Eu/Eu*paas = 1.27–4.05) and Y, reflecting characteristics inherited from seawater and high-temperature hydrothermal fluids (>250 °C). In addition, the commonly observed superchondritic Y/Ho ratio values reinforce the influence of seawater on the rock's geochemistry. The absence of Ce anomalies concerns the oxygenation conditions of seawater, indicating low oxygen availability. The low content of Al2O3 (<0,27%) and other immobile elements, typical of detrital sediments, corroborate the interpretation that these rocks have been deposited in a deep marine environment with low siliciclastic influence. Subordinately, ferruginous metachert, BIF with magnetite and BIF with silicates are also present in the Cuiabá gold deposit. This diversity may be related to specific changes in the basin during the deposition of ferruginous chemical sedimentary rocks - changes that may indicate, for example. variations in the detritic support or availability of carbonaceous matter. Furthermore, geochemical studies from the lower and upper mafic metavolcanic units of the Cuiabá gold deposit helped to constrain and provide a broader understanding of the depositional basin's geotectonic context. Consequently, a genetic model that includes the paleoenvironment of this sequence is proposed. Both rocks of the lower and upper units have very similar mineralogical composition, but register significant geochemical differences. The lower unit is a calc-alkaline basalt/andesite from a continental arc, interpreted as having been developed during the Rio das Velhas II event (2800-2760 Ma), while the upper unit is defined as basalt with a transitional affinity to tholeiitic, possibly corresponding to an ocean floor volcanism.

The Archean Rio das Velhas greenstone belt revisited: new insights into the stratigraphy

The Neoarchean Rio das Velhas greenstone belt (RVGB), southeast Brazil, contains major tectonic and stratigraphic discontinuities that allow its division into three fault-bound tectonostratigraphic blocks named Santa Barbara, Nova Lima-Caete and Sao Bartolomeu. Detailed 1:25,000 scale mapping of the RVGB northern part revealed that each block contains important particularities and were subdivided into different lithostratigraphic formal units. The Santa Barbara block comprises the basal ultrabasic rocks of the Quebra Osso Group, followed by metavolcano-sedimentary successions of the Nova Lima Group (Vigario da Vara, Santa Quiteria, and Corrego do Sitio formations), and metasedimentary rocks of the Maquine Group (Palmital and Casa Forte formations). The Nova Lima-Caete block comprises the basal ultrabasic/basic volcanism (Ouro Fino and Morro Vermelho formations), metavolcano-sedimentary sequences (Mestre Caetano and Ribeirao do Brumado formations) of the Nova Lima Group, and the upper clastic sedimentary rocks of the Maquine Group (Palmital and Casa Forte formations). The Sao Bartolomeu block has a distinct rock assemblage given by metasedimentary rocks of the Nova Lima Group (Fazenda Velha, Corrego da Paina, and Pau D'Oleo formations), and clastic sedimentary rocks of the Maquine Group (Andaimes Formation). Despite some remarkable differences between these blocks, it is possible to recognize stratigraphic correlations and similar depositional evolution. The authors suggest volcanic architecture and sedimentary depositional models for the studied area, which includes: i) fissure eruption of komatiite lavas of the Quebra Osso Group in the SB block with subordinate clasto-chemical sedimentation; ii) ultrabasic/basic volcanism related to multiple volcanic centers in a subaqueous environment, characteristic from the basal portion of the Nova Lima Group in the SB and NLC blocks; iii) intermediate to acid volcanic and volcaniclastic rocks from the Nova Lima Group, present only in the NLC block, related to explosive eruptions; iv) clasto-chemical sedimentation, well recorded in the SB block; v) thick sequence of clastic sediments recorded in the three blocks with a predominance of greywacke-argillite cycles, deposited by turbidity currents in tectonically active submarine basins; vi) metasediments formed in alluvial-fluvial, coastal and shallow marine environments of the Maquine Group.

Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits

The Quadrilátero Ferrífero region is located in the extreme southeast of the Brasiliano São Francisco craton, Minas Gerais state, Brazil. It is composed of (i) Archean TTG granite-gneaissic terranes; (ii) the Archean Rio das Velhas greenstone belt; (iii) the Proterozoic metasedimentary and metavolcano-sedimentary covers. The Rio das Velhas rocks were deposited in the synformal NW–SE-directed Nova Lima basin. The Archean deformation converted the Nova Lima basin into an ample synclinorium with an eastern inverted flank. Archean orogenic gold mineralization within the Rio das Velhas greenstone belt rocks is controlled by NNW–SSE-directed, Archean regional shear zones subparallel to the strata of the Nova Lima synclinorium borders. Transamazonian and Brasiliano orogenies are superposed onto the Archean structures that control gold mineralization. In the eastern domain, Brasiliano fold-and-fault belts prevail, whereas in the western domain Archean and Transamazonian structures abound. The present study focus mainly is the western domain where the Cuiabá, Morro Velho, Raposos, Lamego and Faria deposits are located. Gold orebodies plunge to the E–NE and are tectonically controlled by the Archean D1–D2 deformation. The D3 Transamazonian compression—Which had a SE–NW vector sub-parallel to the regional mineralized Archean foliation/bedding—Buckled these structures, resulting in commonly open, synformal and antiformal regional folds. These are well documented near the gold deposits, with NE–SW axial traces and fold axes plunging to E–NE. Such folds are normal to inverted, NW-verging, with an axial planar foliation dipping moderately to the SE. The Transamazonian compression has only been responsible for the reorientation of the mineralized Archean gold ores, due to coaxial refolding characterized by an opposite tectonic transport. It has therefore not caused any other significant changes. Thrust shear zones, sub-parallel to the strong Transamazonian foliation, have given rise to localized metric segmentation and to the dislocation of gold orebodies. Throughout the region, along the towns of Nova Lima to Sabará, structures pertaining to the Brasiliano Araçuaí orogeny are represented only by gentle folding and by a discrete, non-pervasive crenulation cleavage. Thrust-shear zones and small-scale normal faults have caused, at most, metric dislocations along N–S-oriented planes.

Depositional model for banded iron formation host to gold in the Archean Rio das Velhas greenstone belt, Brazil, based on geochemistry and LA-ICP-MS magnetite analyses

Petrographic and geochemical data were obtained for selected carbonaceous and siderite-bearing chert and banded iron formation (BIF) from the Archean Rio das Velhas greenstone belt (RVGB), Quadrilátero Ferrífero, including seven orogenic gold deposits (Cuiabá, Lamego, Raposos, Roça Grande, São Bento, Urubu and Ápis) and two barren BIFs (Sumidouro and Campo Grande). The RVGB has Algoma-type BIFs that differ in terms of lithological association, mineralogical assemblage and geochemistry. Trace element geochemistry of the BIFs is used to investigate their depositional environment, and to evaluate if primary geochemical and petrographic features have implications for gold mineralization. The chert layers of the Lamego and Urubu deposits exhibit significant, positive Eu anomalies (Eu/Eu*PAAS = 1.57–4.01) and low contents of immobile elements (Al2O3 = <0.34 wt%, TiO2 = <0.02 wt%), suggesting a high contribution of hydrothermal fluid in their formation, with relatively low detrital input. The carbonate- and the magnetite-rich BIF of the Cuiabá, Roça Grande, Ápis and Raposos deposits display Eu/Eu*PAAS = 1.34–3.25; Al2O3 = 0.04–1.47 wt%, TiO2 = <0.03 wt%, interpreted to represent an intermediate contribution of hydrothermal fluid, as well as limited detrital input. The São Bento, Campo Grande and Sumidouro BIF show a significant mineralogical variability including magnetite, carbonate and iron-rich silicates. Their Eu/Eu*PAAS = 1.1–2.6; Al2O3 = 0.11–6.82 wt%, TiO2 = 0.03–0.33 wt% suggest high detrital input and low hydrothermal fluid influence. Textural relations and mineral assemblages of magnetite-rich samples indicate three main generations of this mineral: (i) diagenetic (very low grade metamorphic), (ii) early and (iii) main-stage hydrothermal. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses show that Ti, V, Ni and Co contents of magnetite systematically fluctuate. Their concentrations of Ti (50–677 ppm), V (11–207 ppm), Ni (8–166 ppm) and Co (3–14 ppm) in magnetite (i) are the highest comparatively to types (ii) and (iii) (Ti = 7–352 ppm; V = 0.2–59 ppm; Ni = 1–22 ppm; Co = 0.1–7 ppm). Even higher trace element content of the Sumidouro (i) is related to important clastic input to seawater during BIF deposition. The composition of magnetite types (ii) and (iii) is dependent on coexisting minerals (sulfides, carbonates and silicates), and on the type of mechanism of their formation (dissolution-reprecipitation, or re-equilibration). The mineralogical, geochemical and geological diversity of the RVGB BIFs, as well as magnetite composition in BIF, point to a depositional model for the RVGB BIF within the Rio das Velhas basin related to both proximity to the hydrothermal vent and of terrigenous sedimentary input.Deposition of chert layers (e.g., Lamego and Urubu deposits) was favored near the hydrothermal vents on the abyssal plain due to high concentration of silica and organic carbon. Away from the hydrothermal source, chert transitioned to carbonate-rich BIF, typical of Cuiabá, Roça Grande and Ápis, which still formed under the influence of organic matter. Magnetite-rich BIF, i.e. Raposos deposit, was deposited further away from the volcanic center, with limited supply of organic carbon and silica. In a submarine environment with greater clastic input, such as continental shelfs and island arcs, BIF composition was influenced by detrital contamination, and iron-rich silicates (i.e., stilpnomelane and chlorite) are common, besides magnetite and carbonate; this is the case for the São Bento, Campo Grande and Sumidouro BIFs. The BIF types with abundant iron-rich carbonate and-or magnetite host gold mineralization more efficiently since they are especially favorable to sulfides replacement accompanied by gold precipitation.

Mineralogical characterization and origin of uranium mineralization in Witwatersrand-like metaconglomerate of the Moeda Formation, Quadrilátero Ferrífero, Brazil

The Moeda Formation is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Brazil, and has quartz-pebble metaconglomerate that hosts Au–U mineralization, commonly assumed to be detrital in origin. The Au–U mineralization occurs immediately above an unconformity with the underlying Archean Rio das Velhas greenstone sequence. In this work, we focus on the Moeda metaconglomerate in its U-bearing portions in the Gandarela syncline: the southern sector, near the syncline axis; and the western limb, near the Palmital gold deposit. The metaconglomerate has mica seams that host zircon, xenotime, rutile, nodules of organic matter and sulfide minerals. The sulfide assemblage includes four generations of pyrite along with Fe–Ni–Cu–Co–Zn–Pb–Mo sulfides. A Sb–As sulfosalt assemblage of gersdorffite, stibnite and ullmannite is late in relation to pyrite and other sulfides. Gold is argentiferous with minor Hg. Uranium minerals include uraninite, coffinite, brannerite and uranyl arsenates. An early generation of uraninite and brannerite occurs as inclusions in organic-matter nodules, mainly in the southern sector. A late generation of uraninite formed due to a low-temperature hydrothermal overprint. Coffinite and late-stage brannerite are products of hydrothermal alteration of zircon and rutile, respectively. Uranyl arsenates have Ni as the prevalent cation; they formed as replacement of arsenic minerals, such as gersdorffite, by oxidized fluids containing U. Microstructural data and mineral chemistry favor a hydrothermal hypothesis analogous to an unconformity-related setting, rather than a detrital, placer-deposit model for the occurrence of U in the Moeda metaconglomerate.

Fluid inclusion and sulfur and oxygen isotope studies on quartz-carbonate-sulfide veins of the Carvoaria Velha deposit, Córrego do Sítio gold lineament, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Córrego do Sítio lineament is defined as a 16-km long, NE-SW-trending ductile shear zones, which controls fourteen gold deposits, including the Carvoaria Velha deposit. The dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the Archean Rio das Velhas greenstone belt, which host gold mineralization. Metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence. All these rocks exhibit alteration to quartz, carbonate and sericite, besides sulfides and sulfosalts. The main gold mineralization styles at the Carvoaria Velha deposit, and at Córrego do Sítio as a whole, are quartz-carbonate-sulfide±sulfosalt veins of varied distribution. The veins are classified as: V1 veins – quartz-ankerite-pyrite-berthierite-gold – parallel to the main regional foliation Sn,; V2 veins – quartz-ankerite-pyrite – developed at extensional crenulation cleavage Sn+1, and rarely gold mineralized; V3 veins – quartz-ankerite – filling Sn+3 fractures, usually free of sulfides and sulfosalts; V4 veins – quartz-calcite – of restricted occurrence in metamafic dikes and sills. The latter crosscut the metasedimentary sequence, are extensional and display no preferential orientation. The most common ore minerals in all vein types are arsenopyrite, pyrite, berthierite, and pyrrhotite. Microprobe analyses reveal the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena, cobaltite, which commonly overgrow the sulfides. Fluid inclusion studies trapped in quartz from V1, V2 and V4 veins have identified a H2O-CO2±CH4-NaCl mineralizing fluid that may contain KCl, MgCl2 e FeCl2. The presence of CH4 in fluid inclusions of the V1 and V2 veins reflect interaction of the fluid with the Córrego do Sítio and Santa Quitéria carbon-bearing metapelitic host units.Based on the microthermometric data, the hydrothermal fluid is interpreted to have evolved in at least two stages: i) an early fluid stage, trapped in smoky quartz, of moderate salinity (~8.5eq. wt% NaCl), and minimal trapping temperature of 330±17°C; and ii) a late-stage fluid trapped in recrystallized quartz with lower salinity (~4.6eq. wt% NaCl), and a minimal trapping temperature of 365±33°C. Isotopic values of δ18Ofluid (+7.9 and +13.0‰), Δ33S (+0.3 to +3.5‰) and δ34S (−2.9 to +6.1‰) suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the Carvoaria Velha deposit must have had a metamorphic origin, and interacted with metasedimentary sequences during its ascending path. The addition of CH4 during fluid-rock interaction may have caused some decrease in ƒO2 of the fluid which, as a consequence, destabilized gold-bearing sulfur complexes, liberating S−2 for the formation of Fe sulfides and sulfosalts (arsenopyrite, pyrite, pyrrothite and berthierite, etc.), and outcome gold deposition.

Agalmatolito do Quadrilátero Ferrífero, MG

Agalmatolite is a peraluminous rock type of industrial use composed essentially of pyrophyllite. Northwest of the Quadrilatero Ferrifero mineral province in Minas Gerais, agalmatolite occurrences and mines are distributed over an area of about 350 km 2 . There are several rock types commercially named agalmatolite and which are made up of minerals other than pyrophyllite, such as muscovite, quartz, kyanite and andalusite showing variable degrees of replacement by pyrophyllite and diaspore. Analyses by SEM/EDS, optical microscopy and X ray diffraction showed that it ispossible to distinguish pyrophyllite from muscovite by their distinct textures, pyrophyllite occurring as randomly oriented and extremely fine-grained masses, while muscovite constitutes two textural types: large flakes or fine-grained but always preferentially oriented aggregates. The country rocks, which at least in part correspond to the protolith of the agalmatolite, are metavolcanic and metasedimentary rocks interpreted as belonging to the archean Rio das Velhas greenstone belt. The metavolcanic rocks are composed of relict phenocrysts of alkali-feldspar in a metamorphic matrix of chlorite, muscovite, epidote and quartz. The metasedimentary rocks are banded iron formations, quartzites and schists with variable amounts of quartz, muscovite, kyanite, andalusite, corundum, garnet, staurolite, chloritoid, chlorite, tourmalines and rutile/leucoxene. The stable association of staurolite and chloritoid enabled to establish the temperature conditions of the regional Archean metamorphic event, which formed the country rocks and the protolith of the agalmatolite, at T~500-550 o C. The observed replacement of kyanite by andalusite indicates an episode of decreasing pressure to less the 4kbar, possibly during the regional metamorphic event. The agalmatolite was formed at T ~350-400 o C at the expenses of peraluminous metassedimentary rocks with variable amounts of kyanite, andalusite, muscovite, and quartz. Pyrophyllite was generated by hydrothermal alteration of these rocks along shear zones of possible Transamazonian age, which acted as sinks for H 2 O-rich fluids.

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures. Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.

The geology of the Morro Velho gold deposit in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil

The Morro Velho gold deposit, Quadrilátero Ferrífero region, Minas Gerais, Brazil, is hosted by rocks at the base of the Archean Rio das Velhas greenstone belt. The deposit occurs within a thick carbonaceous phyllite package, containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites. Considering the temporal and spatial association of the deposit with the Rio das Velhas orogeny, and location in close proximity to a major NNW-trending fault zone, it can be classified as an orogenic gold deposit. Hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates, sulfides, chlorite, white mica±biotite, albite and quartz, as described in other Archean lode-type gold ores. Two types of ore occur in the deposit: dark gray quartz veins and sulfide-rich gold orebodies. The sulfide-rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies. The sulfide assemblage comprises (by volume), on average, 74% pyrrhotite, 17% arsenopyrite, 8% pyrite and 1% chalcopyrite. The orebodies have a long axis parallel to the local stretching lineation, with continuity down the plunge of fold axis for at least 4.8 km. The group of rocks hosting the Morro Velho gold mineralization is locally referred to as lapa seca. These were isoclinally folded and metamorphosed prior to gold mineralization. The lapa seca and the orebodies it hosts are distributed in five main tight folds related to F1 (the best examples are the X, Main and South orebodies, in level 25), which are disrupted by NE- to E-striking shear zones. Textural features indicate that the sulfide mineralization postdated regional peak metamorphism, and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed. Lead isotope ratios indicate a model age of 2.82 ± 0.05 Ga for both sulfide and gold mineralization. The lapa seca are interpreted as the results of a pre-gold alteration process and may be divided into carbonatic, micaceous and quartzose types. The carbonatic lapa seca is subdivided into gray and brown subtypes. Non-mineralized, gray carbonatic lapa seca forms the hanging wall to the orebodies, and is interpreted as the product of extreme CO 2 metasomatism during hydrothermal alteration. This dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite. The brown carbonatic and micaceous lapa secas are the host rocks to gold. These units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and/or volcanic rocks of varying composition from dacitic to andesitic, forming various types of schists and phyllites. The high-grade, massive sulfide orebodies occur at the base of the gray carbonatic lapa seca. Both disseminated mineralization and quartz veins are hosted by micaceous lapa seca. The data are consistent with a model of epigenetic mineralization for the lapa seca, from a hydrothermal fluid derived in part from the Archean basement or older crust material.

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D 1, D 2 and D 3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D 3. The orebodies constitute sulfide-bearing D 1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage. Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H 2O-rich containing CO 2 + Na + and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H 2O is indicated. The CO 2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H 2O to CO 2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.

Rochas metaultramáficas de Lamim, sul do Quadrilátero Ferrífero, MG: contribuição ao conhecimento do protólito da pedra-sabão

A região de Lamim, sudeste do Quadrilátero Ferrífero (QF), é constituída de gnaisses e rochas metamáficas e metaultramáficas, estas do Grupo Nova Lima, base do greenstone belt Rio das Velhas. As principais rochas metaultramáficas são pedra-sabão, amplamente explotada no QF, e serpentinito. A região distingue-se por rochas com olivina preservada, interpretadas como o protólito magmático das metaultramáficas. São olivina-anfibólio fels com textura semelhante a cumulus, com inclusão de vários grãos de olivina em anfibólio de porte maior. O teor relativamente alto de ferro da olivina (Fo75Fa25) indica a sua origem magmática. Há dois tipos de anfibólio, cristais centimétricos de tremolita, envolvendo vários grãos de olivina, interpretados como pseudomorfoses, substituindo o piroxênio intercumulus original, e antofilita acicular. Os teores de serpentina, clorita, dolomita e talco variam. A composição química e as texturas sugerem que o provável protólito das rochas metaultramáficas são komatiitos peridotíticos plutônicos e não komatiitos vulcânicos encontrados em outras regiões do QF. O peridotito foi variavelmente metamorfizado em condições de fácies xisto verde alto a anfibolito baixo durante a infiltração de fluidos aquosos que acompanharam o processo metamórfico-metassomático.

Brazil's premier gold province. Part II: geology and genesis of gold deposits in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero

Orogenic, gold deposits are hosted by rocks of the Archean Rio das Velhas greenstone belt in the Quadrilatero Ferrifero region, Minas Gerais state, Brazil, one of the major gold provinces in the world. The gold deposits occur at the base of the mafic–ultramafic succession, with the most important orebodies controlled by E–W-striking, strike-slip faults. The main mineralization styles are (1) structurally controlled, sulfide replacement zones in banded iron formation (BIF); (2) disseminated sulfide minerals and gold in hydrothermally altered rocks along shear zones; and (3) auriferous quartz–carbonate–sulfide veins and veinlets in mafic, ultramafic, and felsic volcanic rocks, and also in clastic sedimentary rocks. The most common host rocks for ore are metamorphosed oxide- and carbonate-facies banded iron (± iron-rich metachert) formations (e.g., the Cuiaba, Sao Bento and Raposos deposits) and the lapa seca unit, which is a local term for intensely carbonatized rock (e.g., the giant Morro Velho mine with >450 t of contained gold). Metabasalts host most of the remaining gold deposits. Mineralogical characteristics and fluid inclusion studies suggest variations in the H2O/CO2 ratio of a low-salinity, near-neutral, reducing, sulfur-bearing, ore fluid. The presence of abundant CH4-rich inclusions is related to reduction of the original H2O–CO2 fluid via interaction with carbonaceous matter in the wallrocks. Oxygen fugacity was close to that of graphite saturation, with variations likely to have been influenced by reaction with the carbonaceous matter. Carbon-rich phyllites and schists, which commonly bound ore-bearing horizons, seem to have played both a physical and chemical role in localizing hydrothermal mineral deposition. Microtextural studies indicate that gold deposition was mainly related to desulfidation reactions, and was paragenetically coeval with precipitation of arsenic-rich iron sulfide minerals. Carbon isotope data are compatible with dissolution of CO2 from pre-existing mantle-derived carbonation zones, and indicate fluids of metamorphic origin. A major episode of hydrothermal fluid introduction into different rock types caused epigenetic gold formation and wallrock alteration at about 300 to 400 °C during the late stages of regional deformation and metamorphism of the greenstone belt. The age of gold mineralization is constrained to be younger than 2,698 and perhaps closer to 2,670 Ma.

Fluid evolution and gold deposition at the Cuiabá mine, Se Brazil: fluid inclusions and stable isotope geochemistry of carbonates

The gold-bearing quartz-carbonate vein deposits of the Cuiaba Mine, northern sector of the Quadrilatero Ferrifero, State of Minas Gerais, SE Brazil, are hosted by a sequence of banded carbonaceous metasedimentary rocks within tholeiitic basalt flows of the Archean Rio das Velhas Greenstone Belt. A regional greenschist metamorphic grade characterizes rocks in this section of the Belt. Episodes of hydraulic fracturing enhanced the focussing of fluids percolating along the host primary layering, which promoted pervasive carbonatization and sulfidation associated with crosscutting fractures and quartz-carbonate veins/breccias. The gold mineralization is closely related to the sulfiderich zones. Fluid inclusion investigations show that the mineralizing fluids were dominantly aqueous, of low salinity (3-6 wt.% NaCleq.), with variable concentrations of CO (0 - 11.4 mol%), CH 4 (1.5 - 10.2 mol%), subordinate N 2 (0 - 0.7 mol%), and traces of H 2 S/HS'. Carbonates in the carbonaceous host rock display heavier 8 13 C compositions (-4.3 to -0.2 0 / oo ) than carbonates in the gold-related, carbonate-sulfide altered zones and associated veins and breccias (-8.1 to -S.5 o / oo ). The range of carbonate a 18 O values in the carbonaceous host rock (12.8 to 16 o / oo ) is broadly similar to the carbonates in the mineralized zones (10.6 to 15.2 o / oo ), with a slightly larger variation in the latter. Collectively, fluid inclusion and carbon isotope data indicate that: (1) the mineralizing fluid was originally composed of a H 2 O-CO 2 , solution, with significant concentrations of CH 4 added via hydrolysis of the carbonaceous matter during vein-fluid/wallrock interaction; (2) sedimentary carbonates, probably of marine origin, are readily distinguished from those carbonates formed by reaction between CO 2 -bearing vein fluids and wallrocks; (3) gold-related vein-fluid CO 2 was relatively homogeneous in isotopic composition, suggesting equilibration with carbonate and reduced carbon reservoir (e.g. mantle-derived magma); (4) gold deposition may have been triggered by a sharp decrease in f O 2 caused by the CH 4 enrichment of the fluid.

FLUID EVOLUTION AND GOLD DEPOSITION AT THE CUIABÁ MINE, SE BRAZIL: FLUID INCLUSIONS AND STABLE ISOTOPE GEOCHEMISTRY OF CARBONATES

The gold-bearing quartz-carbonate vein deposits of the Cuiabá Mine, northern sector of the Quadrilátero Ferrífero, State of Minas Gerais, SE Brazil, are hosted by a sequence of banded carbonaceous metasedimentary rocks within tholeiitic basalt flows of the Archean Rio das Velhas Greenstone Belt. A regional greenschist metamorphic grade characterizes rocks in this section of the Belt. Episodes of hydraulic fracturing enhanced the focussing of fluids percolating along the host primary layering, which promoted pervasive carbonatization and sulfidation associated with crosscutting fractures and quartz-carbonate veins/breccias. The gold mineralization is closely related to the sulfiderich zones. Fluid inclusion investigations show that the mineralizing fluids were dominantly aqueous, of low salinity (3-6 wt.% NaCleq.), with variable concentrations of CO 2 (0 – 11.4 mol%), CH 4 (1.5 – 10.2 mol%), subordinate N 2 (0 – 0.7 mol%), and traces of H 2 S/HS- . Carbonates in the carbonaceous host rock display heavier δ 13 C compositions (-4.3 to –0.2o/ oo ) than carbonates in the gold-related, carbonate-sulfide altered zones and associated veins and breccias (-8.1 to –5.5o / oo ). The range of carbonate δ 18 O values in the carbonaceous host rock (12.8 to 16o / oo ) is broadly similar to the carbonates in the mineralized zones (10.6 to 15.2o / oo ), with a slightly larger variation in the latter. Collectively, fluid inclusion and carbon isotope data indicate that: (1) the mineralizing fluid was originally composed of a H 2 O-CO 2 solution, with significant concentrations of CH 4 added via hydrolysis of the carbonaceous matter during vein-fluid/wallrock interaction; (2) sedimentary carbonates, probably of marine origin, are readily distinguished from those carbonates formed by reaction between CO 2 -bearing vein fluids and wallrocks; (3) gold-related vein-fluid CO 2 was relatively homogeneous in isotopic composition, suggesting equilibration with carbonate and reduced carbon reservoir (e.g. mantle-derived magma); (4) gold deposition may have been triggered by a sharp decrease in fO 2 caused by the CH 4 enrichment of the fluid.

99/03117 Kinetic behavior of the Ru/TiO 2 catalyst in the reaction of partial oxidation of methane

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.

U–Pb evidence of late Archean tectono-thermal activity in the southern São Francisco shield, Brazil

The São Francisco craton in eastern Brazil is one of the major shield areas in South America. In the Quadrilátero Ferrífero area (southern sector of the craton) the Archean Rio das Velhas greenstone belt is surrounded by granite–gneiss terrane and is overlain by Proterozoic sedimentary successions. The Bonfim Metamorphic Complex is the only area of the granite–gneiss terrane adjacent to the greenstone belt that has been mapped. It comprises two gneissic units, tonalites, and late granitic dykes, which crosscut the regional north–south shear fabric. Samples of gneiss, two tonalites and a granitic dyke were dated by U–Pb. A zircon core from the Alberto Flores gneiss yielded a minimum age of 2920 Ma, whereas the overgrowth is concordant at 2772 ± 6 Ma. Two tonalites from the vicinity of Serra da Moeda are [Formula: see text] old, and a late dyke yielded an age of [Formula: see text]. These data, together with previously published U–Pb ages, show that (i) greenstone belt volcanism was coeval with granitoid intrusion and with metamorphism of older units in the granite–gneiss terrane at ca. 2780 Ma and (ii) the main crust-forming event in the Quadrilátero Ferrífero area is about 2780 Ma old. Clear evidence is now available indicating that older magmatism is 2.8–3.2 Ga old. In addition, the last Archean deformation must have occurred in the interval 2780–2703 Ma. Finally, the presence of 2774 ± 6 Ma titanite in the tonalites indicates that the metamorphism associated with the Trans-Amazonian orogeny (ca. 2.0 Ga) did not reach amphibolite facies in the study area.

Use of carbon tetrachloride plus air for the decomposition of sulphides and related compounds

The Córrego do Sítio lineament is defined as a 16-km long, NE-SW-trending ductile shear zones, which controls fourteen gold deposits, including the Carvoaria Velha deposit. The dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the Archean Rio das Velhas greenstone belt, which host gold mineralization. Metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence. All these rocks exhibit alteration to quartz, carbonate and sericite, besides sulfides and sulfosalts. The main gold mineralization styles at the Carvoaria Velha deposit, and at Córrego do Sítio as a whole, are quartz-carbonate-sulfide ± sulfosalt veins of varied distribution. The veins are classified as: V1 veins – quartz-ankerite-pyrite-berthierite-gold – parallel to the main regional foliation Sn,; V2 veins – quartz-ankerite-pyrite – developed at extensional crenulation cleavage Sn + 1, and rarely gold mineralized; V3 veins – quartz-ankerite – filling Sn + 3 fractures, usually free of sulfides and sulfosalts; V4 veins – quartz-calcite – of restricted occurrence in metamafic dikes and sills. The latter crosscut the metasedimentary sequence, are extensional and display no preferential orientation. The most common ore minerals in all vein types are arsenopyrite, pyrite, berthierite, and pyrrhotite. Microprobe analyses reveal the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena, cobaltite, which commonly overgrow the sulfides. Fluid inclusion studies trapped in quartz from V1, V2 and V4 veins have identified a H2O-CO2 ± CH4-NaCl mineralizing fluid that may contain KCl, MgCl2 e FeCl2. The presence of CH4 in fluid inclusions of the V1 and V2 veins reflect interaction of the fluid with the Córrego do Sítio and Santa Quitéria carbon-bearing metapelitic host units.Based on the microthermometric data, the hydrothermal fluid is interpreted to have evolved in at least two stages: i) an early fluid stage, trapped in smoky quartz, of moderate salinity (~ 8.5 eq. wt% NaCl), and minimal trapping temperature of 330 ± 17 °C; and ii) a late-stage fluid trapped in recrystallized quartz with lower salinity (~ 4.6 eq. wt% NaCl), and a minimal trapping temperature of 365 ± 33 °C. Isotopic values of δ18Ofluid (+ 7.9 and + 13.0‰), Δ33S (+ 0.3 to + 3.5‰) and δ34S (− 2.9 to + 6.1‰) suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the Carvoaria Velha deposit must have had a metamorphic origin, and interacted with metasedimentary sequences during its ascending path. The addition of CH4 during fluid-rock interaction may have caused some decrease in ƒO2 of the fluid which, as a consequence, destabilized gold-bearing sulfur complexes, liberating S− 2 for the formation of Fe sulfides and sulfosalts (arsenopyrite, pyrite, pyrrothite and berthierite, etc.), and outcome gold deposition.