Fluid inclusion and stable isotope evidence for mixing of magmatically related fluids with ground waters during Keweenawan Cu-Ag fissure-vein mineralization, Mamainse Point, Ontario

Primary chalcocite (Cc) mineralization at the Coppercorp mine occurs with vuggy Qz and minor calcite (Ct) in hydrothermal breccia veins following normal faults which cut Keweenawan plateau basalts and interflow sediments. Cc is the last and predominant sulfide in a paragenetic sequence of Py-Cp-Bn-Cc-Cu/sup 0/. Hematite (Hm) is found in equilibrium with all sulfides except Py. Fluid inclusion studies of vein Qz reveal a simple mixing trend between high temperature, high salinity brines, and lower temperature, more dilute hybrid fluids. The high end-member fluid temperatures strongly suggest a magmatic association, while the low salinity fluids are probably meteoric ground waters. delta/sup 13/C/sub pdb/ and delta/sup 18/O/sub smow/ values for ten Ct samples associated with sulfides average -4.0 per thousands (s=0.7) and 13.3 per thousands (s=2.1) respectively. These results suggest that the Coppercorp high-temperature ore-fluids were in equilibrium with an intrusion similar to the Jogran porphyry at depth, and that mixing with ground waters at higher levels in fissure veins resulted in sulfide precipitation by cooling, dilution and neutralization. Oxidation of this primary fluid at Coopercorp is also reflected by the ubiquitous presence of Hm with sulfide ores; negative delta/sup 34/S ratios from ten samples of Py, Cp or Cc andmore » positive ratios from two rare baryte samples support this interpretation. Two samples of Py from Jogran give ratios typical of porphyry deposits, and may therefore represent the original undisturbed values of the ore-fluid.« less