Nature and conditions of gold mineralization at the Tau deposit, Mupane mine, Tati Greenstone Belt, Botswana: Evidence from fluid inclusion and arsenopyrite geothermometer

The gold mineralization at the Tau deposit in northeastern Botswana can be distinguished into two stages. The first stage of mineralization consists of invisible gold associated with early, sulfur-rich arsenopyrite (Apy1). The second stage is characterized by native gold associated with late, sulfur-poor arsenopyrite (Apy2). This study aims to constrain the nature of the ore-forming fluid at the Tau deposit on the basis of fluid inclusion microthermometry and arsenopyrite geothermometry. Quartz crystals closely associated with gold-bearing sulfides (Apy1) from the host rock host three types of fluid inclusions occurring in the same assemblages: Type I, two-phase aqueous fluid inclusions, Type II three-phase aqueous-carbonic fluid inclusions and Type III one- or two-phase CO2-rich fluid inclusions. Microthermometric and Raman spectroscopic studies revealed the dominance of coexisting H2O+NaCl (Type I) and CO2-N-2-CH4 (Type III) fluid inclusions. The coexisting Type I and Type III fluid inclusions could represent products of the immiscibility of a homogenous H2O-CO2-NaCl-N-2-CH4 primary ore fluid. The salinity of Type I (calculated from melting temperature of ice) and Type II (calculated from dissolution temperature of clathrate) fluid inclusions vary from 3.4 to 9.2 wt% NaCl equiv. and 5.8 to 9.8 wt% NaCl equiv., respectively. The overall fluid inclusion salinity range (<10 wt% NaCl equiv.) and gas compositions of fluids suggest that the primary ore-forming fluid at the Tau deposit was of low salinity and H2O-CO2-N-2-CH4-NaCl composition. Integration of fluid inclusion microthermometry and arsenopyrite geothermometer results suggest that the first stage and second stages of gold mineralization and the associated alteration at the Tau deposit occurred at pressure and temperature conditions of 75-145 MPa and 290-370 & DEG;C and 85-160 MPa and 330-370 & DEG;C, respectively. These results suggest that there was an increase in temperature from the first stage to the second stage of mineralization, resulting in the recrystallization of invisible gold-bearing arsenopyrite in the first stage to form native gold-bearing arsenopyrite in the second stage. Fluid composition and the estimated physico-chemical conditions at the Tau deposit confirm that gold was transported as reduced bisulfide complexes and initial gold deposition was likely caused by a reduction in sulfur contents as a result of sulfidation reactions and pressure-induced phase separation (fluid immiscibility).