The Union Reefs prospect, N.T.: Structure and geochemistry of a turbidite hosted gold deposit

The Union Reefs gold prospect is located approximately 15.5 km north-west of Pine Creek, in the Northern Territory. It is hosted within the Burrell Creek Formation, a Palaeoproterozoic turbidite sequence of the Pine Creek Geosyncline. Two deformations have been recognised at the prospect. The first deformation produced tight folds and a continuous slaty cleavage. The second deformation produced long wavelength folds, mesoscopic folds, a discontinuous spaced and crenulation cleavage, and conjugate kinks. A domal structure was produced from fold interference. The mineralisation event occurred during the second deformation. It is associated with breccia zones and quartz sulphide veins, and crosscuts buck quartz veins and sheeted quartz veins. Mineralisation consists of pyrrhotite, arsenopyrite, marcasite, pyrite, chalcopyrite, sphalerite, galena, gold, bismuth, bismuthinite and an unidentified phase. Based on data derived from field observations, petrographic studies and sulphur isotope analyses, a genetic model for gold mineralisation at the prospect is proposed. The genetic model suggests that sulphur was derived from a mixed igneous and sedimentary source. Gold was sourced from the sedimentary pile by fluids convecting under elevated temperature in response to emplacement of the Allamber Springs Granite. The convecting fluids were focussed into dilatant structures within uplifted, fault bounded blocks and corridors. Deposition of gold is thought to be a result of a decrease in the oxygen fugacity of the gold bearing fluid.