Geology and mineralization, United North Australian Mine, Watsonville, North Queensland

The United North Australian Mine lies within a roof pendant of sedimentary rocks of Silurian- Devonian age. These sediments have been intruded by the Upper Carboniferous Elizabeth Creek Granite to the south and the Watsonville Granite to the north. High level sub-volcanic porphyries of the Slaughter Yard Creek Volcanics intrude the eastern part of the area.\ud \ud A mineralised, partially cone shaped, porphyry which was derived from the Elizabeth Creek Granite has been recognised in the area. This body has been named the UNA Porphyry. This porphyry was intruded with a dissolved aqueous phase which separated during emplacement and crystallisation. The resultant hydrothermal fluid produced extensive sericitization and some silicification. Alteration and mineralization proceeded contemporaneously. The metals in the aqueous phase were deposited as disseminated and semi-massive mineralization throughout the porphyry. Massive sulphide bodies are localized within thicker portions of the porphyry such as the Two Treys Mine area.\ud \ud The polymetallic sulphide assemblage in the Two Treys ore body formed between 334° and 235°C, at pressures possibly as low as 500 bars, and at depths as shallow as 2,000 feet. All the ore minerals apparently formed simultaneously. Metal zoning in the ore body reflects gradients in oxygen and sulphur fugacities present within the hydrothermal fluid at the time of formation rather than paragenesis.\ud \ud The sulphides have suffered deformation and annealing effects subsequent to their formation. A later stage of mineralization has been superimposed over this earlier stage producing indistinct zoning in the area