Proterozoic rocks of the South Nicholson region, straddling the north-eastern Northern Territory and north-western Queensland, are juxtaposed between the Mount Isa Province and the McArthur Basin. Whereas the latter two provinces are well-studied and highly prospective for energy and mineral resources, the geological evolution and resource potential of the South Nicholson region, until recently, remained largely unevaluated. Geoscience Australia, under the Exploring for the Future (EFTF) initiative (2016–2020), in collaboration with State and Territory Geological Surveys, conducted a range of regional and targeted geoscience investigations across the South Nicholson region to better understand the resource potential, and to encourage greenfield resource exploration. Poster presented at the 2021 Australian Earth Sciences Convention (AESC)

Proterozoic rocks of the South Nicholson region are juxtaposed between the Mount Isa Province and the McArthur Basin. Whereas the latter two provinces are well-studied and prospective for energy and mineral resources, the geological evolution and resource potential of the South Nicholson region is not well understood. Geoscience Australia, under the Exploring for the Future (EFTF) initiative, in collaboration with State and Territory Geological Surveys, conducted a range of regional geoscience investigations to better understand the resource potential across the South Nicholson region to encourage greenfield resource exploration. Here we discuss preliminary findings on an unreported massive manganese oxide (MnO) occurrence in the Carrara Range in the South Nicholson region, north-eastern Northern Territory. The occurrence is hosted by a north-dipping quartz sandstone unit of the ca. 1640 Ma) sandstones of the Plain Creek Formation (McNamara Group), in the hanging wall of the south-verging, east-west trending Wild Cow Fault zone. The Plain Creek Formation conformably overlies the Shady Bore Quartzite, and conformably underlies shales and carbonaceous siltstones of the Lawn Hill Formation. The Plain Creek Formation is stratigraphically equivalent to the Riversleigh Siltstone in the Lawn Hill Platform. The massive MnO body is comprised of pyrolusite (MnO2) and cryptomelane (KMn8O16), surrounded by a halo of partially MnO altered host sandstone, crosscut by numerous 1‒5mm wide ‘feeder’ veinlets. These zoned veinlets consist of quartz, pyrolusite and cryptomelane with wall quartz projecting into the veinlets with Mn minerals infilling the centre of the veins. The MnO body is ~20 m wide across strike. The lateral and depth extent of the occurrence is unknown, but satellite imagery indicates that MnO mineralisation is visible, along strike, for at least several hundred metres. These observations suggest that the Carrara Range MnO occurrence is likely an epigenetic replacement stratiform body. Geochemistry on the MnO body return 49.8 wt% MnO with appreciable (ca. 450 ppm) Zn; the host sandstone return 10.8 wt% MnO and ca. 25 ppm Zn. Reconnaissance fluid inclusion analysis on quartz-MnO veinlets reveals both brine+vapour aqueous inclusions and hydrocarbon+vapour inclusions. Co-existing aqueous and hydrocarbon were not observed. Homogenisation temperatures are 90‒180°C for aqueous inclusions and 60‒140°C for hydrocarbon inclusions. Fluid salinities are 10‒23 wt% (NaCl equivalent), which may suggest interaction with evaporites. Decrepitation of the fluid inclusions yielded CO2 with no accompanying hydrocarbon gases, suggesting an oxidising fluid. The 𝛿13C CO2 of -22 ‰ is consistent with an organic source, possibly from oil oxidation. The mineralising fluids were high salinity, low temperature (ca. 120°C) fluids, typical of fluids for Mississippi-Valley and/or Mount Isa style base-metal deposits. The host Plain Creek Formation is stratigraphically equivalent to units that host world-class regional Pb-Zn deposits such as Century, McArthur River (HYC) and Lady Loretta and others of north-western Queensland and north-eastern Northern Territory. This correlation, together with the knowledge that many Pb-Zn deposits across the region are associated with manganese enrichment, increases the potential of a base metal resource in the South Nicholson region. Discovery of the Carrara Range Mn occurrence may stimulate regional base metal exploration. Abstract presented at the 2021 Australian Earth Sciences Convention (AESC)