The Spar Lake Strata-Bound Cu-Ag Deposit Formed Across a Mixing Zone Between Trapped Natural Gas and Metals-Bearing Brine

摘要

Ore formation at the Spar Lake red bed-associated strata-bound Cu deposit took place across a mixing and reaction zone between a hot oxidized metals-transporting brine and a reservoir of "sour" (H_2S-bearing) natural gas trapped in the host sandstones. Fluid inclusion volatile analyses have very high CH_4 concentrations (≥1 mol % in most samples), and a sample from the fringe of the deposit has between 18 and 36 mol % CH_4. The ratio of CH_4/CO_2 in fluid inclusions appears to vary regularly across the deposit, with the lowest CH_4/CO_2 ratios from high-grade chalcocite-bearing ore, and the highest from the chalcopyrite-bearing fringe. The helium R/R_a isotope ratios (0.23-0.98) and concentrations define a mixture between crustal and atmospheric helium. The volatiles in fluid inclusions (CH_4, CO_2, H_2S, SO2, H_2, H_2O, and other organic gases) and values of f_(o2) and temperature calculated from the volatiles data all show gradations across the deposit that are completely consistent with such a mixing and reaction zone. Other volatiles from the fluid inclusions (HCl, HF, ~3He, ~4He, N_2, Ar) characterize the brine and give evidence for only shallow crustal fluids with no magmatic influences. The brine entered the gas reservoir from below and along the axis of the deposit and migrated out along bedding to the southwest, northeast, and northwest. Metals-transporting brines may have been fed into the host sandstones from the East Fault, but that remains uncertain.