Abstract
The Caxias deposit is situated in the Gurupi Auriferous Province of the Paleoproterozoic Sao Luis Craton, northern Brazil. It is a lode-type gold mineralization associated with a narrow, steeply dipping, NE-trending, shear zone crosscutting a hydrothermalized microtonalite (northern sector) and schists (southern sector). Fluid inclusion studies on vein quartz crosscutting the microtonalite (northern sector) have identified early carbonic and aqueous-carbonic inclusions and late aqueous inclusions, unrelated to the mineralizing event. The CO 2 / H 2 O ratio and the other microthermometric properties show a wide range of values, which are interpreted as product of heterogeneous trapping of two (partially) immiscible fluids and to deformation-related post-formational processes. The resulting mineralizing fluid has XCO 2 - 6-45 mol %; XN : < 2.5 mol %; XH 2 O: 55-95 mol %; mean salinity of 4.5 wt % NaCl equiv., and moderate density (0.7-1.0 g/cm 3 ). Bulk isochores coupled with chlorite geothermometry constrained the P-T entrapment conditions between 262-307°C and 1.6-3.7 kb. Log |O 2 for this P-T-X range was estimated between -29.8 and -34.2. Geological characteristics and fluid properties found in the northern sector of Caxias gold mineralization are similar to those described for metamorphic fluids of mesothermal gold deposits.
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