Carson, Chris1, Henson, Paul1, Huston, David1, Jarrett, Amber1, Champion, David1 and Boreham, Chris1.
1Geoscience Australia, Canberra, Australia
Proterozoic rocks of the South Nicholson region, north-eastern Northern Territory, 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 region, encouraging greenfield resource exploration.
Here we discuss preliminary findings on an unreported manganese oxide (MnO) occurrence in the Carrara Range in the South Nicholson region. The occurrence is hosted by north-dipping quartz sandstones of the ca. 1640 Ma 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 MnO body is almost entirely comprised of pyrolusite (MnO2) and cryptomelane (KMn8O16), surrounded by a halo of partially MnO altered host sandstone, crosscut by numerous 1‒5mm wide zoned ‘feeder’ veinlets. Veinlets consist of quartz, pyrolusite and cryptomelane with euhedral wall quartz projecting into the veinlets and 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. Coexisting 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 13C of the released CO2 is -22.96 ‰, consistent with a biogenic source, possibly from hydrocarbon oxidation.
The mineralising fluids were high salinity and low temperature (ca. 120°C) brines, typical of mineralising fluids for Mississippi-Valley and/or Mount Isa style base-metal deposits. The host Plain Creek Formation is broadly 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 and the discovery of the Carrara Range Mn occurrence may help stimulate regional base-metal exploration.
Chris has worked in Antarctica, Canadian Arctic, Alaska, New Caledonia and northern and central Australia, specialising in metamorphic petrology, geochronology and structural geology. Joining Geoscience Australia in 2006 he dabbled in SHRIMP geochronology and, in 2017, joined the Onshore Energy program, working in the South Nicholson region of the NT.