The role of hydrocarbons in ore formation at the Pillara Mississippi Valley-type Zn-Pb deposit, Canning Basin, Western Australia

Abstract Hydrocarbons have been suggested previously to play a role as reducing agents for in-situ sulfate reduction — one of several key components of the model proposed for ore formation in Mississippi-Valley-Type (MVT) deposits. Here, we provide direct evidence for hydrocarbon-brine interaction occurring synchronous to ore formation at the Pillara MVT deposit, through the analysis of brine and composite brine-hydrocarbon fluid inclusions by LA-ICP-MS combined with thermodynamic and reactive transport modeling. The occurrence of coeval primary hydrocarbon and brine inclusions in ore-stage calcite and sphalerite implicates the co-existence of hydrocarbons and brine at the time of ore formation. Elevated average concentrations (± 1σ) of 147 ± 141 ppm Zn, 53 ± 64 ppm Pb, 38 ± 36 ppm Cu and 11 ± 9 ppm Ag were measured in brine end member fluid inclusions. Hydrocarbon-bearing fluid inclusions are enriched several orders of magnitude in metal concentrations compared to the brine end member fluid inclusions. The analyzed fluid inclusion assemblages combined with numerical reactive transport simulations indicate that an oxidized acid-sulfate brine, which leached its metals from feldspathic sandstones in the deeper basin, represents a thermodynamically stable ore fluid. Interaction with feldspathic sandstones, granitic basement rocks and/or carbonates, and neutralization of pH, did not affect the Pb and Zn transport capabilities of the fluid. These findings indicate that Pb and Zn can be transported over long distances through carbonate-rich rocks, even though the original ore fluid may have been an acidic brine. The controlling factor for metal deposition at Pillara and similar MVT deposits can be related to the presence of hydrocarbons, permitting in-situ sulfate reduction and precipitation of sphalerite, galena, Ag and Cu sulfides.