Abstract

The Serrinha gold deposit in the Juruena-Teles Pires gold province is spatially and genetically related to the hydrothermal alteration of the I-type calc-alkaline 1872 ± 12 Ma Matupá monzogranite in the south-central Amazonian craton. The mineralized areas are characterized by intense hydrothermal alteration of the monzogranite, comprising incipient autometasomatism, K silicate, sodic, Mn chlorite, phyllic, carbonate, and microcline alteration, with partial overprinting of the early alteration by later stages. A distal barren propylitic zone is interpreted from drill core. The gold mineralization is disseminated in the most altered samples and genetically related to Mn chlorite, K silicate, and phyllic hydrothermal alteration types. Hydrothermal magnetite and rutile are ubiquitous within pyrite.

The early Serrinha gold mineralization stage is characterized by Ag-poor gold included in pyrite (Au/Ag ~7–15), associated with pyrrhotite, cubanite, and galena. The fluids interpreted to be associated with this early auriferous stage were found only in quartz from the Mn chlorite alteration and are typically saline (45–57 wt % NaCl equiv) and high-temperature (375° and 480°C) H2O-NaCl-KCl fluids entrapped at pressures of at least 1.3 kbars. The late-stage gold mineralization, which is coeval with the phyllic alteration, occurs either as isolated gold grains or as fracture-fillings and/or inclusions within pyrite associated with tetradymite, galena, tsumoite (BiTe), hessite (Ag2Te), and aikinite (PbCuBiS3). Gold grains associated with this late stage are richer in Ag (Au/Ag ~2–5). Data from chlorite geothermometry and aqueous-carbonic and saline (NaCl-KCl) fluid inclusions associated with this second auriferous event indicate P-T conditions of 1.5 to 2.4 kbars and 293° to 365°C. The occurrence of lower temperature coeval aqueous, aqueous-carbonic, and carbonic fluid inclusions with no postentrapment modification in the K silicate assemblage, partially overprinted by phyllic alteration and in phyllic assemblages, suggests that fluid immiscibility and/or mixing with meteoric water occurred in the hydrothermal system. H2O-NaCl-CaCl2 fluids contemporaneous with carbonate and late microcline alteration, which postdated gold mineralization, were entrapped at lower temperatures (172°–200°C). Calculated Δ18O and ΔD values are 8.2 and −37 per mil, respectively, for the early saline fluids in equilibrium with the Mn chlorite assemblage and 1.7 to 4.7 and −20 to −15 per mil for the late fluids in equilibrium with the phyllic assemblage. These data in conjunction with fluid inclusion results suggest that early fluids exsolved from granitic melts and later mixed with meteoric water. Gold is interpreted to have been initially transported from the crystallizing magma as chlorine complexes in a hot, saline, acidic, and oxidized fluid. Decrease in temperature during fluid ascent, immiscibility, or pH increase is interpreted to have caused gold precipitation. Subsequent dilution of the saline fluid could have been responsible for the deposition of late gold in pyrite fractures.

Based on field, petrological, mineralogical, fluid inclusion, and isotopic evidence, we propose that Serrinha is a typical proximal intrusion-related gold deposit, similar to porphyry-style gold deposits.

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