McArthur Basin Mineral System, NT

Exploration for Zn-Pb-Ag mineral deposits in northern Australia is hindered by cover, remoteness, and complex geology. Integration of different geoscientific data is providing new information on factors and processes controlling where and how these mineral deposits form, paving the way for future discoveries.

Between 2016 and 2019, CSIRO in collaboration with the Northern Territory Geological Survey conducted a detailed structural and stratigraphic study of the southern McArthur Basin, which hosts the world-class McArthur River and Teena Zn-Pb deposits. This project involved the interpretation and modelling geophysical data to reconstruct the structural evolution of the basin. These results were further integrated with sedimentological, sequence stratigraphic, and chemostratigraphic data to gain a detailed understanding of the Zn-Pb target unit.

Economic Zn-Pb mineralization in the southern McArthur Basin is hosted by sulfidic and organic carbon-rich mudrocks of the ca. 1640 million-year-old Barney Creek Formation, which was deposited in structurally controlled sub-basins. Recognition of structures controlling these sub-basins has traditionally been challenging because overprinting relationships from several extension and crustal shortening events created a complex fault and depositional architecture. This challenge was addressed by integrating the interpretation and modelling results of all available geophysical data with detailed sedimentological logging of 18 drill cores located across the prospective southern McArthur Basin.

Our work demonstrates that the prospective lower Barney Creek Formation was deposited during intermittent, broadly north-south directed extension. This caused strike-slip movement along major north-northwest-trending faults, and normal movement along east–west to east-northeast-trending faults. Faulting resulted in significant sub-basin deepening in some areas, and uplift and erosion in others. These sub-basins developed in transtensional segments of strike-slip faults, and adjacent to normal faults. Geophysical modelling also identified an anomalously thick pile of mafic volcanics within the early basin fill represented by the Tawallah Group. This is significant because mafic volcanics have previously been interpreted as the source of base metals within the region, and the thick volcanic pile we identified occurs in close proximity to the McArthur River and Teena deposits. Sub-basin bounding faults that were active during mineralization tap into this anomalously thick pile of volcanics, suggesting that they likely represented fluid pathways for ascending metalliferous brines.

The prospective lower part of the Barney Creek Formation was deposited in offshore environments (i.e., below storm-wave-base). The mineralization at McArthur River and Teena is associated with a maximum flooding surface (a surface formed at the same time everywhere in the basin). This flooding surface can be developed as sulfidic and highly organic carbon-rich black shale in sub-basin depocenters. Therefore, it represents the most suitable chemical trap in syngenetic models for clastic-type Zn-Pb deposits. However, the maximum flooding surface, where developed as black shale again, would rather form a physical trap (seal) to ascending base metal brines in diagenetic sub-seafloor replacement models. Regardless of the preferred genetic model, Zn-Pb mineralization can be targeted by integrating maps of the metal source distribution, fault network (fluid pathways); as well as sequence stratigraphy to predict the occurrence of the target horizon, coupled with facies maps to understand where the maximum flooding surface is developed as black shale (i.e., ideal chemical trap or seal facies).

geophysics

Publications/reports:

Kunzmann, M, Crombez, V., Blaikie, T.N., Catuneanu, O., King, R., Halverson, G.P., Schmid, S., Spinks, S.C., 2022. Sequence stratigraphy of the ca 1640 Ma Barney Creek Formation, McArthur Basin, Australia. Australian Journal of Earth Sciences

Blaikie, T.N., Kunzmann, M., 2020. Geophysical interpretation and tectonic synthesis of the Proterozoic southern McArthur Basin, northern Australia. Precambrian Research 343, 105728.

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.

Kunzmann, M., Schmid, S., Blaikie, T.N., Halverson, G.P., 2019. Facies analysis, sequence stratigraphy, and carbon isotope chemostratigraphy of a classic Zn-Pb host succession: The Proterozoic middle McArthur Group, McArthur Basin, Australia. Ore Geology Reviews 106, 150-175.

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.