The McArthur River (HYC) Zn-Pb Deposit, NT
The McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy.
In this study we present evidence for a subsurface carbonate dissolution-style process for the HYC deposit by employing new state-of-the-art analytical techniques and, critically, by quantitatively examining mineralized samples from different portions of the deposit through a range of scales from the submeter to the nanometer scale. We focus on aspects that have been previously inadequately explored, such as the distribution of sphalerite in relationship to sedimentary carbonate, preservation of original carbonate in the sphalerite zones, the distribution of trace Mn and Tl and their occurrence in the ore paragenetic sequence, and thermodynamic modeling of the hydrothermal ore fluid.
We utilized world-class analytical facilities as part of this study including the Maia Mapper, state of the art electron backscatter diffraction, and Focused ion beam-time of flight-secondary ionization mass spectrometry.
Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid.
We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.
Publications/reports:
Spinks, S., Pearce, M., Ryan, C., Moorhead, G., Kirkham, R., Sheldon, H., Kunzmann, M., Liu, W., Blaikie, T., Schaubs, P., 2019. Ultra-high resolution trace element mapping provides new clues on the origin of the McArthur River (HYC) sediment-hosted Zn-Pb-Ag deposit. AGES 2019 Extended Abstract, https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/88375.
Spinks, S., Pearce, M., Ryan, C., Moorhead, G., Kirkham, R., Sheldon, H., Kunzmann, M., Liu, W., Blaikie, T., Schaubs, P., 2020. Carbonate Replacement as the Principal Ore Formation Process in the Proterozoic McArthur River (HYC) Sediment-Hosted Zn-Pb Deposit, Australia. Economic Geology, https://doi.org/10.5382/econgeo.4793.