Reassessment of thermal preservation of organic matter in the Paleoproterozoic McArthur River (HYC) Zn-Pb ore deposit, Australia

The 1.64 Ga old McArthur River (HYC) Zn-Pb deposit in northern Australia is one of the best preserved clastic-dominated, stratiform base-metal deposits. However, the mechanisms of ore formation are still controversial. To characterize the influence of metal-bearing fluids on organic matter in the McArthur River mine, we used lipid biomarkers in combination with compound-specific isotope analysis and Rock-Eval pyrolysis on a succession covering the hanging wall, orebody, inter-ore layers and foot wall. Aromatic and methyldiamantane maturity parameters indicate that bitumens through the entire succession, including the ore zone, fall into the beginning of the gas window (~160 °C). The thermal maturity of kerogen in the hanging and foot wall is consistent with this degree of thermal alteration. By contrast, kerogen in the ore zone possesses extremely high thermal maturities that are inconsistent with the bitumen. This temperature anomaly shows that the extractable hydrocarbons migrated into the orebody after mineralization, most probably from surrounding sediments of the Barney Creek Formation. Polycyclic aromatic hydrocarbons (PAH) found in the McArthur River orebody are detected in similar distributions in the unmineralized hanging wall and foot wall and are thus not the products of hydrothermal activity, as previously assumed, but formed through burial-temperature processes. Therefore, the hydrocarbons in the McArthur River ore zone do not yield information about the temperature of mineralizing fluids, negating earlier suggestions for temperatures of 200 °C, or 250 to 400 °C. Yet, based on kerogen maturity, the organic matter in ore lenses and inter-ore breccias is still severely altered. The data is most consistent with the flow of metal-bearing fluids with temperatures in the range of thermochemical sulphate reduction but inconsistent with the activity of bacterial sulphate reducers.