Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil

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 (similar to 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 post-metamorphic. Fluid inclusion studies identified early CO2 (CH4-N-2) and CO2 (CH4-N-2)-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 18-33 mol% of CO2, up to 4mol% of N-2, and less than 2 mol% of CH4 and displays moderate to high densities with salinity around 4.5 wt% NaCl equiv. Mineralization occurred around 310 to 335 degrees 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 delta(18)(H2O) and delta D-H2O (310-330 degrees C) values of +6.2 to +8.4 parts per thousand and -19 to -80 parts per thousand, respectively, which might be metamorphic and/or magmatic and/or mantle-derived. The carbon isotope composition (delta C-13) varies from -14.2 to -15.7 parts per thousand in carbonates; it is -17.6 parts per thousand in fluid inclusion CO2 and -23.6 parts per thousand in graphite from the host rock. The delta S-34 values of pyrite are -2.6 to -7.9 parts per thousand. 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.