Numerical modeling of hydrothermal zinc silicate and sulfide mineralization in the Vazante deposit, Brazil

The Vazante zinc deposit in central Brazil is currently the world's largest known example of a hypogene nonsulfide (i.e. willemite-dominant) zinc deposit. The mineralization is hypothesized to have formed as a result of mixing between a hot, acidic, reducing, metal-rich brine and a cool, more basic and dilute, metal-poor meteoric fluid. The present study sought to investigate this scenario by quantifying the individual effects of temperature, pH, salinity, and oxidation state on willemite and sphalerite solubility, and modeling their combined effects during mixing through reaction path and reactive transport modeling. Solubility calculations showed that in an initially hot, moderately acidic, reducing, metal-rich ore fluid saturated with respect to silica, willemite solubility is relatively insensitive to changes in temperature and log(02) , but highly sensitive to changes in pH and salinity. In contrast, sphalerite solubility was highly sensitive to changes in temperature and log(02), as well as salinity, and was less sensitive than willemite to changes in pH. Reaction path models sought to extend these observations by modeling the geochemistry of mixing. The results show that mixing is able to produce most of the major zinc ore and gangue minerals observed in the field, though not necessarily at the same paragenetic stages, and that both compositional and temperature changes from mixing are needed. Reactive transport models were formulated to investigate spatial patterns of mineralization. The results showed that sphalerite deposition was strongly controlled by temperature and concentrated in the regions of greatest temperature change. Willemite deposition was concentrated along the interface between the metal-rich ore fluid and the surrounding meteoric fluid. The more rapid transport of solute than heat, in conjunction with the higher concentration of silica than sulfide in both fluids meant that willemite mineralization developed over a broader region and in greater concentrations compared with sphalerite.9296115American Philosophical SocietyVotorantim Metais mining compan

Hydrothermal alteration, fluid inclusions and stable isotope systematics of the Alvo 118 iron oxide-copper-gold deposit, Carajas Mineral Province (Brazil): Implications for ore genesis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The Alvo 118 iron oxide-copper-gold (IOCG) deposit (170 Mt at 1.0 wt.% Cu, 0.3 g/t Au) lies in the southern sector of the ItacaA(0)nas Shear Belt, Carajas Mineral Province, along a WNW-ESE-striking, 60-km-long shear zone, close to the contact of the similar to 2.76-Ga metavolcano-sedimentary ItacaiA(0)nas Supergroup and the basement (similar to 3.0 Ga Xingu Complex). The Alvo 118 deposit is hosted by mafic and felsic metavolcanic rocks and crosscutting granitoid and gabbro intrusions that have been subjected to the following hydrothermal alteration sequence towards the ore zones: (1) poorly developed sodic alteration (albite and scapolite); (2) potassic alteration (biotite or K-feldspar) accompanied by magnetite formation and silicification; (3) widespread, pervasive chlorite alteration spatially associated with quartz-carbonate-sulphide infill ore breccia and vein stockworks; and (4) local post-ore quartz-sericite alteration. The ore assemblage is dominated by chalcopyrite (similar to 60%), bornite (similar to 10%), hematite (similar to 20%), magnetite (10%) and subordinate chalcocite, native gold, Au-Ag tellurides, galena, cassiterite, F-rich apatite, xenotime, monazite, britholite-(Y) and a gadolinite-group mineral. Fluid inclusion studies in quartz point to a fluid regime composed of two distinct fluid types that may have probably coexisted within the timeframe of the Cu-Au mineralizing episode: a hot (> 200A degrees C) saline (32.8aEuro degrees to 40.6 wt.% NaCl eq.) solution, represented by salt-bearing aqueous inclusions, and a lower temperature (< 200A degrees C), low to intermediate salinity (< 15 wt.% NaCl eq.) aqueous fluid defined by two-phase (L-H2O + V-H2O) fluid inclusions. This trend is very similar to those defined for other IOCG systems of the Carajas Mineral Province. delta O-18(H2O) values in equilibrium with calcite (-1.0aEuro degrees to 7.5aEuro degrees at 277A degrees C to 344A degrees C) overlap the lower range for primary magmatic waters, but the more O-18-depleted values also point to the involvement of externally derived fluids, possibly of meteoric origin. Furthermore, sulphide delta S-34 values (5.1aEuro degrees to 6.3aEuro degrees), together with available boron isotope and Cl/Br-Na/Cl data provide evidence for a significant component of residual evaporative fluids (e.g., bittern fluids generated by seawater evaporation) in this scenario that, together with magma-derived brines, would be the main sources of the highly saline fluids involved in the formation Alvo 118 IOCG deposit. The restricted high temperature sodic alteration, the pervasive overprinting of the potassic alteration minerals by chlorite proximal to the ore zones, ore breccias with open-space filling textures in brittle structures, microthermometric and stable isotope data indicate, collectively, that the Alvo 118 IOCG system developed at structurally high levels and may be considered the shallower representative of the IOCG systems of the CMP.473299323Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Vale CompanyFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2003/09916-6, 2003/09584-3]CNPq [555065/2006-5

High-K calc-alkaline to A-type fissure-controlled volcano-plutonism of the Sao Felix do Xingu region, Amazonian craton, Brazil: Exclusively crustal sources or only mixed Nd model ages?

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)In the Sao Felix do Xingu region, southern portion of Amazonian craton, voluminous well-preserved Late Paleoproterozoic volcanic and plutonic rocks are grouped in the Sobreiro and Santa Rosa formations. The basal Sobreiro Formation (1.88 Ga) is composed of massive andesitic to rhyodacitic flows and volcaniclastic fades that are high-K and metaluminous, geochemically similar to calc-alkaline granitoids of volcanic arcs. Fractional crystallization of magnetite + augite + magnesiohastingsite (up to 20 vol. %) and zircon + plagioclase in most evolved rocks controlled the magmatic evolution of this unit, under oxidizing condition. The Santa Rosa Formation (1.87 Ga) comprises: 1) massive, bedded, and foliated rhyolitic lava flows, and large dikes of banded rhyolite and ignimbrite; 2) felsic ignimbrite associated with thin unwelded ash-fall tuff and highly rheomorphic felsic ignimbrite; 3) felsic crystal tuff, lapilli-tuff, and breccia; and 4) stocks and dikes of granitic porphyry, and subordinate equigranular granite intrusions. This unit was in part erupted along major fissures and has A-type intraplate geochemical affinity, peraluminous composition, and transitional subalkaline to alkaline characteristics. The magmatic evolution was predominantly controlled by fractional crystallization of zircon + apatite + Fe-Ti oxides + feldspars (up to 30 vol. %) with subordinate biotite, probably under intermediate oxidizing conditions. Nd model ages (3.0-2.49 Ga) for the Sobreiro Formation strongly suggest its generation by mixing of mantle-derived and anatectic melts of Archean rocks beneath the volcanic sequences in the Sao Felix do Xingu region. The Santa Rosa Formation could have been originated from several Archean crustal sources (T(DM) 3.12-2.56 Ga), involving assimilation/contribution of juvenile mantle components. The integrated data point to a narrow transition from Andean-type subduction to a dominantly extensional tectonic setting for the volcanic and plutonic event of the Sao Felix do Xingu region. A stage of flattening in the subduction angle of the subducted plate during the Paleoproterozoic ocean-continent orogenesis in the Tapajos Gold Province might explain the volcanism and plutonism in the Sao Felix do Xingu region as consequence of the arc migration, resulting in the similar to 2.0-1.87 Ga geochronological and metallogenetic zoning observed in southern Amazonian craton. (C) 2011 Elsevier Ltd. All rights reserved.324SI351368Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [103/98 Proc. 66.2103/1998-0]CAPES [0096/05-9]CNPq [CT-Mineral 555066/2006-1]FAPESP [MCT/CNPq/FAPESPA 573733/2008-2

Geochemistry and spatial distribution of heavy metals in Oxisols in a mineralized region of the Brazilian Central Plateau

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)The majority of the soils of the Central Plateau of Brazil are product of long time development over stable surfaces, usually associated with erosion and re-deposition cycles. In this context, to successfully study the geochemistry and spatial distribution of heavy metals, it is essential to understand the geology as well as the geomorphology of the landscape and the properties of the metals related to their mobility. The main objective of this work was to evaluate contents of naturally occurring heavy metals (Co, Cu, Mn, Ni, Pb and Zn) in some highly weathered tropical soils as related to their geochemistry, geology and geomorphology. Additionally, it was aimed to interpret the spatial distribution patterns of these metals and of major elements (Al, Fe, Ti and Mg), evaluating the association and taking the influence of the geology and geomorphology into consideration. In geo-referenced sites, ranging from 480 to 1040 m in altitude, samples were collected at two depths (0-20 and 60-80 cm), ground, sieved, and prepared for analysis. Heavy metals and major element contents were determined by extraction with aqua regia. Descriptive statistics, Pearson correlation coefficients, Principal Components Analysis (PCA) and spatial variability analyses were conducted and soil metal concentrations for the whole area were predicted by kriging (interpolation). A close association between Pb, Zn, and Mn was observed using PCA and Pearson's correlation. Highest concentrations of Pb, Zn, and Mn were determined in soils between 650 and 550 m altitude surrounding the dolomite massif hills and Ambrosia and Fagundes mineral deposits. Copper and Fe were found to be geochemically associated with highest concentrations observed in soils formed from carbonaceous phyllite with quartzite layers. Cobalt and Ni concentrations were associated with areas characterized geomorphologically as alluvial-colluvial deposits at the lowest altitudes within the region. Although there is a marked dispersive action of tropical weathering on metal distribution in the region, Cu, Pb, and Zn were found spatially associated with their geological source. This probably happens because of greater affinity of those metals to Fe and Mn oxides formed predominantly under tropical conditions and present in higher concentrations near to the sources of these heavy metals. On the other hand, both the low affinity of Ni and Co for these oxide minerals and the region's geomorphology allowed for their migration towards the drainage network to the regions of sediment accumulation at lower altitudes during the landscape's evolution. (C) 2010 Elsevier B.V. All rights reserved.1602131142Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)project EMBRAPA Cerrados/IRD [0203205]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)CNPq [475623/2003-7]CAPES [514-05]project EMBRAPA Cerrados/IRD [0203205

Mesoarchean (3.0 and 2.86 Ga) host rocks of the iron oxide-Cu-Au Bacaba deposit, Carajas Mineral Province: U-Pb geochronology and metallogenetic implications

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The Bacaba iron oxide-copper-gold deposit, situated within a WNW-ESE-striking shear zone in the Carajas Domain, Carajas Mineral Province, is hosted by the Serra Dourada Granite, the Bacaba Tonalite, and crosscutting gabbro intrusions, which were intensely affected by sodic (albite-scapolite), potassic, chloritic, and hydrolytic hydrothermal alteration. This deposit is located 7 km northeast of the world-class Sossego iron oxide-copper-gold deposit and might represent a distal and deeper portion of the same or related hydrothermal system. The U-Pb laser ablation inductively coupled plasma-mass spectrometry data for zircon from a sodically altered sample of the Serra Dourada Granite yielded a 2,860 +/- 22 Ma (MSWD=11.5) age. Three samples from the Bacaba Tonalite, including one with potassic alteration and two with Cu-Au mineralization, rendered the 3,001.2 +/- 3.6 Ma (MSWD=1.8), 2,990.9 +/- 5.8 Ma (MSWD=1.9), and 3,004.6 +/- 9 Ma (MSWD=2.2) ages, respectively. The ca. 2.86 and ca. 3.0 Ga ages are interpreted as the timing of the igneous crystallization of the Serra Dourada Granite and the Bacaba Tonalite, respectively, and represent the oldest magmatic events recognized in the Carajas Domain. The Serra Dourada Granite and the Bacaba Tonalite are interpreted to greatly predate the genesis of the Bacaba deposit. A genetic link is improbable in the light of the similarities with the Sossego deposit, which is also hosted by younger ca. 2.76 Ga metavolcano-sedimentary units of the ItacaiA(0)nas Supergroup. In this context, the iron oxide-copper-gold deposits in the southern sector of the Carajas Domain could be mainly controlled by important crustal discontinuities, such as a regional shear zone, rather than be associated with a particular rock type. These results expand the potential for occurrences of iron oxide-copper-gold deposits within the Mesoarchean basement rocks underlying the Carajas Basin, particularly those crosscut by Neoarchean shear zones.467789811Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [555065/2006-5]CNPq [303359/2008-0]FAPESP [MCT/CNPq/Fapespa 573733/2008-2

Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide copper-gold deposits, Carajas Mineral Province (Brazil)

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The Carajas Mineral Province in northern Brazil contains a variety of world-class (>100 Mt ore) iron oxide copper-gold (IOCG) deposits, including the only Archean examples of this deposit class (e.g., the Igarape Bahia/Alemao and Salobo deposits). Tourmaline of schorldravite composition, a common gangue mineral in these deposits, precipitated shortly prior to and after the ore assemblage. A boron isotope study of texturally different tourmaline from three IOCG deposits (Igarape Bahia, Salobo, and Sossego) using secondary ion mass spectrometry (SIMS) provides new evidence in the long-standing debate of magmatic versus non-magmatic sources for the high salinity (tip to 50 wt% NaCl equiv.) of ore fluids in these deposits. Values of delta B-11 front 14 parts per thousand to 26.5 parts per thousand for the Igarape Bahia and Salobo deposits confirm marine evaporite-derived brines in the ore fluids, whereas lower delta B-11 values for the 1garap6 Bahia deposit (5.8 parts per thousand to 8.8 parts per thousand) suggest that these fluids may have mixed with an isotopically different hydrothermall fluid, or one that had a mixed boron source. More variable and isotopically lighter boron in tourmaline front the Sossego deposit (-8 parts per thousand to 11 parts per thousand) is attributed to mixed sources, including light boron leached from felsic intrusive and volcanic host rocks, and heavy boron derived from marine evaporites. The boron isotope data indicate that the characteristic high salinity of the ore fluids in the Carajiis Mineral Province was acquired by the interaction of hydrothermall fluids with marine evaporites. For IOCG deposits that contain tourmaline as a common gangue mineral, boron isotopes offer a valuable tool to constrain the high-salinity source problem, which is a critical issue in metallogenesis of IOCG deposits worldwide.369743746Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [03/09584-3, 04/02278-7]CNPq [450706/2005-2