The Spar Lake strata-bound Cu-Ag deposit formed across a mixing zone between trapped natural gas and metals-bearing brine

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 (sub 2) S-bearing) natural gas trapped in the host sandstones. Fluid inclusion volatile analyses have very high CH (sub 4) concentrations (> or =1 mol % in most samples), and a sample from the fringe of the deposit has between 18 and 36 mol % CH (sub 4) . The ratio of CH (sub 4) /CO (sub 2) in fluid inclusions appears to vary regularly across the deposit, with the lowest CH (sub 4) /CO (sub 2) ratios from high-grade chalcocite-bearing ore, and the highest from the chalcopyrite-bearing fringe. The helium R/R (sub a) isotope ratios (0.23-0.98) and concentrations define a mixture between crustal and atmospheric helium. The volatiles in fluid inclusions (CH (sub 4) , CO (sub 2) , H (sub 2) S, SO (sub 2) , H (sub 2) , H (sub 2) O, and other organic gases) and values of fO (sub 2) 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, (super 3) He, (super 4) He, N (sub 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. Abundant ore-stage Fe and Mn calcite cements from the reduced fringe have delta (super 13) C values as low as -18.4 per mil, and many values less than -10 per mil, which indicate that significant carbonate was generated by oxidation of organic carbon from the natural gas. The zone of calcite cements with very low delta (super 13) C values approximately envelopes chalcocite-bearing ore. Sulfur isotope data of Cu, Pb, and Fe sulfides and barite indicate derivation of roughly half of the orebody sulfide directly from sour gas H (sub 2) S. That sour gas H (sub 2) S had developed in steps known from other sedimentary basins, starting with (1) bacterial sulfate reduction (BSR) of seawater sulfate having delta (super 34) S of about 20 per mil and sequestering of the sulfide in organic matter in source rocks stratigraphically below the deposit host rocks, followed by (2) maturation of the sulfide-bearing organic matter into liquid petroleum with relatively homogeneous sulfide having delta (super 34) S of 5 + or - 5 per mil, then by (3) thermal cracking of the oil to CH (sub 4) and H (sub 2) S with relatively homogeneous sulfide having delta (super 34) S closely distributed, about 6 per mil. The CH (sub 4) and H (sub 2) S migrated and were trapped in sandstones of the upper member of the Revett Formation, where they were later met by the 200 degrees C metals-transporting brine. There was additional contribution of sulfide to ore from later thermochemical sulfate reduction (TSR) operating on sulfate delta (super 34) S of 20 to 29 per mil in both formation waters and metals-transporting solutions. A large range of delta (super 34) S in sulfides resulted as the 6 per mil sour gas sulfide was supplemented with varying proportions of 20 to 29 per mil sulfide from TSR.