Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil |
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| Authors: | Evandro L Klein Chris Harris Christophe Renac André Giret Candido A V Moura Kazuo Fuzikawa |
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| Institution: | (1) CPRM/Geological Survey of Brazil, Av. Dr. Freitas, 3645, Belém-PA,, CEP: 66095-110, Brazil;(2) Département de Géologie, Université Jean Monnet, 23, rue du Docteur Paul Michelon, Saint Etienne 42000, Cedex 2,, France;(3) Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa;(4) Laboratório de Geologia Isotópica, Pará-Iso, Universidade Federal do Pará, CP 1611, Belém-PA,, CEP: 66075-900, Brazil;(5) CDTN/CNEN, Rua Mario Werneck s/n, Cidade Universitária, Belo Horizonte-MG,, CEP: 30270-010, Brazil |
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| Abstract: | The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW–SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and postmetamorphic. Fluid inclusion studies identified early CO2 (CH4-N2) and CO2 (CH4-N2)-H2O-NaCl inclusions that show highly variable phase ratios, CO2 homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less associated with postentrapment modifications. The ore-bearing fluid typically has 18–33mol% of CO2, up to 4mol% of N2, and less than 2mol% of CH4 and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ18OH2O and δDH2O (310–335°C) values of +6.2 to +8.4‰ and −19 to −80‰, respectively, which might be metamorphic and/or magmatic and/or mantle-derived. The carbon isotope composition (δ13C) varies from −14.2 to −15.7‰ in carbonates; it is −17.6‰ in fluid inclusion CO2 and −23.6‰ in graphite from the host rock. The δ34S values of pyrite are −2.6 to −7.9‰. The strongly to moderately negative carbon isotope composition of the carbonates and inclusion fluid CO2 reflects variable contribution of organic carbon to an originally heavier fluid (magmatic, metamorphic, or mantle-derived) at the site of deposition and sulfur isotopes indicate some oxidation of the originally reduced fluid. The deposition of gold is interpreted to have occurred mainly in response to phase separation and fluid-rock interactions such as CO2 removal and desulfidation reactions that provoked variations in the fluid pH and redox conditions. |
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| Keywords: | Fluid inclusion Stable isotope Gold Gurupi belt Paleoproterozoic |
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