Abstract

Silver in the Cobalt mining camp occurs in carbonate-quartz-arsenide veins that crosscut Proterozoic metasediments, Archean metavolcanic rocks, and diabase sills. Pre-, syn-, and postore fluid inclusions are found in hydrothermal axinite, calcite, and quartz. Pre- and post- ore inclusions contain liquid + vapor + halite at room temperature and occur in assemblages of uniform phase ratios. These inclusions homogenize on the halite liquidus at 230 degrees to 250 degrees C, at pressures well above the liquid + vapor field. Secondary ion mass spectrometry (SIMS) analysis yields Ca/Na/K ratios of approximately 3:2:1, in agreement with microthermometry and qualitative SEM-EDS measurements. Although the fluid was originally in equilibrium with calcite, CO 2 in the fluid inclusions is below Raman microprobe detection levels. The inclusions, with a constant bulk composition of approximately 8 mole percent CaCl 2 , 5 mole percent NaCl 3 mole percent KCl, and 84 mole percent H 2 O, were trapped from a homogeneous (one-phase) vein fluid. Synore fluid inclusions consist of liquid + vapor at room temperature. Ice melts between -3 degrees and -26 degrees C, and the inclusions homogenize into the liquid over a limited range of temperatures. The intersection of fluid inclusion isochores with the stability bounds for the vein mineral assemblages indicates vein and ore formation between 290 degrees and 350 degrees C and 480 and 1,350 bars.Previous fluid inclusion studies of the Ag-transporting fluids at Cobalt have identified boiling as a major control on ore precipitation. This study presents fluid inclusion evidence from the Siscoe, Beaver-Temiskaming, and Hellens Eplett deposits that contradicts the interpretation of boiling. These data suggest that ore precipitation occurred when the highly saline, Ag-bearing, preore fluid mixed with a second, genetically unrelated, weakly saline fluid. Variable dilution upon mixing formed the synore fluid, decreasing the solubility of the Ag chloride complexes, resulting in precipitation of native silver.

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