Isotopic, fluid inclusion and LA-ICP-MS studies on the worldclassCuiabá gold deposit, Rio das Velhas greenstone belt,Quadrilátero Ferrífero, MG: implications for the mineralizing fluidreservoirs

Abstract

The world-class Cuiabá orogenic Au deposit in the Quadrilátero Ferrífero is hosted at the base of the Archean Rio dasVelhas greenstone belt (RVGB) that contains an extensive volume of sedimentary rocks, being the largest undergroundmine in Brazil (5.78 Moz resources of 10.26 g/t). The main host to gold is a carbonaceous, carbonate banded iron formation(BIF), underlain by andesite and overlain by carbonaceous pelite (CP), along the reclined, isoclinal Cuiabá fold, with anaxis having a 116° azimuth dip direction and plunging between 35° and 12°. Andesite-hosted quartz vein mineralizationconstitutes recently discovered orebodies at the mine, and fluid inclusion data (microthermometry, Laser-raman, LA-ICPMS,ion chromatography) of the VQZ orebody have constrained the physico-chemical nature of the ore-forming fluid. Thedata were obtained on quartz types (Qz1, Qz2, Qz3, Qz5) from veins classified in four main generations, V1 shear vein, andV2, V3, and V4 extensional veins (V1 and V2 mineralized), providing a chronological framework for the fluid evolutionaryhistory. Three fluid types are distinguished: i) aqueous of low salinity (2-4 wt % NaCl equiv), mean homogenizationtemperature at 220°C; ii) aqueous of moderate salinity (6-12 wt % NaCl equiv), and mean homogenization temperature at~260°C; and iii) aqueous-carbonic of moderate salinity (6-15 wt % NaCl equiv), with 30-91.4 mol. % CO2 and 8.6-41.0mol. % CH4, and up 28 mol. % N2 and mean decrepitation temperature at 300°C. Since only one fluid inclusion assemblageshows homogenization into vapour, and aqueous as well as aqueous-carbonic fluid inclusion assemblages are estimated todecrepitate into the vapor phase, supporting arguments for phase immiscibility are insignificant. As a result, temperaturecorrection is applied, and calculated mean minimum trapping temperatures are 360°C in V1 shear veins, 330°C in V2extensional veins, 300°C in V3 extensional array veins, and up to 270°C in late-stage V4 veins. All these temperatures arein the range of the arsenopyrite geothermometer calculated at 300 - 390°C. Ion chromatography analyses on Qz1-V1 veinsreveal a Br/Cl range between 0.71 to 0.74 x 10-3, 1.40 to 1.51 x 10-3 in Qz2-V2, 0.31 to 0.39 x 10-3 in Qz3-V3, and 0.73 to0.85 x 10-3 in Qz5-V4 veins. Base metals (Zn, Pb and Cu) are relatively enriched in the order of 100 to 1,000 ppm inaqueous and aqueous-carbonic fluid inclusion assemblages trapped in all vein and quartz types at Cuiabá, similar to otherorogenic gold deposits hosted in the Rio das Velhas greenstone belt. This sediment-rich greenstone belt may have acted as apotential base-metal source, explaining the elevated Zn and Pb signatures. Consideration of these physio-chemical fluidinclusion data indicates the compatibility with metamorphic fluids. Although geochemical and isotope analyses mayimplicate multiple fluid sources associated with the development of the Cuiabá deposit, the fluid inclusion data set does notdifferentiate them in an absolute sense. A two-step model of hydrothermal fluid flow and gold precipitation at the Cuiabádeposit is suggested to involve the development of the mineralised mineralized V1 shear veins up to V3 extensional arrayveins by an early- stage, aqueous-carbonic, high temperature fluid. It encompasses a minimum ThTRAP at 290°C (Qz2-V3) tomaximum ThTRAP at 360°C (Qz1-V1), and the V4 extensional veins associated with an evolved, aqueous-carbonic, lowertemperature (mean ThTRAP at 260°C), later-stage fluid, all developed during the Archaean D1 shear event. Fluid inclusiontrapping mechanism and interpreted gold precipitation processes may include 1) intermittent phase immiscibilty, and 2)intermittent partial mixing of low and moderate saline, two or more fluids of aqueous and aqueous-carbonic nature.Textural, chemical and multiple sulfur isotope analyses were conducted in sulfides from the BIF-hosted Fonte Grande Sulorebody to track distinct trace element (TE) signatures from syngenetic to epigenetic pyrite types in different host units, andto deduce the nature and source of the mineralizing fluids. Five pyrite types are classified based on textural relations asspongy, syngenetic (Py1, detected only in CP), porous early-(Py2, all 3 lithotypes), smooth main-(Py3, all 3 lithotypes) andsmooth isolated and overgrown late-stage (Py4 and Py5, only BIF) variably present in metamorphosed CP, BIF andandesite. The TE abundance maps and LA-ICP-MS analyses display that Py1 yields high As, Co, Ni, Pb and Ag values,whereas Py2 (formed by agglomeration of Py1) maintains high TE with slightly less of those elements. The TEincorporation in Py3 is similar to Py1 in CP. The Py2 and Py3 in BIF and andesite have increased Co and Ni, but have lessAu and As. BIF-hosted late-stage Py4 is characterized by lower TE concentrations, whereas Py5 is further enriched in As,Bi, Co, Ni and Pb. The study shows that CP is pre-enriched in Co, Ni and Pb, whereas certain elements like Ag, Au, Bi andAs are only hydrothermally concentrated during advanced stages in BIF and andesite, supporting a syngenetic versushydrothermal origin of distinct elements. Multiple sulfur isotopes suggest that mineralizing fluids reflect a complex mixingof sulfur evolved from three possible sources: seawater, mantle, and reduced elemental sulfur. Syngenetic Py1 yields 33Svalues ranging from 2.28 to 0.25 , separated into two ranges, (i) 2.28 to 1.97 , (ii) 0.96 to 0.25 , whereby (i)suggests the deposition in seawater environment. These pyrites probably mixed with later mantle sulfur or could have mixedwith fluids sourced from sedimentary rocks at depth carrying a positive 33S signature (ii). The second scenario is morecommon in sedimentary/diagenetic pyrite. Early-, main- and late-stage pyrites in CP, BIF and andesite present a continuousprocess of crustal assimilation towards positive 34S and 33S values. This confirms evidence for a sedimentary-derivedsulfur source as also indicated by FI data of the andesite-hosted VQZ orebody.Finally we attempt to integrate the fluid-inclusion-based, two-step hydrothermal fluid evolution with that of the pyriteevolution at Cuiabá. Both Py2 and Py3 represent the peak of Archaean gold mineralization event, and considered part of thefirst hydrothermal pulse having evolved during the development of V2 and V3 veins, respectively. This takes intoconsideration: (i) higher As and Au concentrations in Py2 than in Py3; ii) lower As and Au in FIAs of V2 veins compared toV1 and V4 veins; and (iii) Ag values that increase in FIAs from V1/V2 to V3 veins, while Ag concentrations indicate adecreasing tendency from Py2 to Py3. Both Py4 and Py5 are attributed to the second hydrothermal pulse, and the nonIIImineralized late-stage V4 veins by comparing the reverse trend of lower (decreasing) Pb and Mn concentrations in FIAstrapped in V4 and higher (increasing) Pb and Mn in Py5. Gold in FIAs in V4, and the lack of Au in BIF-hosted Py4 and Py5suggest that Au remained in the fluid and its precipitation conditions were not attained at this stage.Both studies show clear geochemical influences of the ore-forming fluid by the sedimentary units of the Archaean RVGB,indicated by 1) FIAs containing significant amounts of Pb and Zn; and 2) pyrite types that clearly demonstrate asedimentary-derived sulfur signature.