The Cobar Basin developed as four deep-water troughs bordered by shallow-water shelves. Wagga Tank lies within the Mt Hope Trough, which is interpreted as a volcano-tectonic rift associated with a submarine cauldron. During the rift-phase, sub-aerial volcanism was followed by submarine, felsic volcanism. A-type volcanics occur along the major N-S structures and are overlain by S-type volcanics. Large accumulations of sediments and volcanics up to 2.5km thick were deposited (Scheibner, 1987).
The stratigraphic units which form the Wagga Tank / Southern Nights prospect are steeply west dipping and generally upright, they are made up of:
1) The Upper Amphitheatre Group / Mt Kennan Volcanics (the Wagga Tank formation)
Informally, known as the Wagga Tank Formation, which overlays the following units. It is comprised of shales, siltstones and less commonly sandstone (a turbidite facies). The Vivigani Formation, occurs underneath, often with a transitional contact and was formed prior to the Wagga Tank Fmn. The formation is a complexed volcanic package of coarse polymictic volcaniclastics, sandstones, breccias, mass debris flow breccias, rhyolitic/rhyodacitic lavas, peperites with minor shales/siltstone beds.
2) The Lower Amphitheatre Group
Informally called the Eastern Formation, it underlies the Vivigani Fmn with a sharp contact. This formation it is also made up of turbiditic facies of shales, silts and sandstones, making it very similar to the Wagga Tank Fmn in composition, however the Eastern formation can commonly be distinguished by the higher occurrence of sandstone beds, likely suggesting that it was formed in a higher energy / shallower marine environment than the more recent Wagga Tank Fmn.
Mineralisation occurs in the form of Pyrite, Sphalerite, Galena and Chalcopyrite, commonly developing in veins (Stringer veins > Colloform veins), which frequently exhibit sulphide zonation, and Massive/Laminated sulphides.
The veins exclusively form in the footwall (within the Vivigani more than Eastern formation, this is surmised to have occurred due to the porosity and abundance of accommodation space in the unit).
The contact between the Wagga Tank and the Vivigani Fmn predominately hosts the bulk of mineralisation in for of massive/laminated sulphides. It is theorised that volcanic-hosted massive sulphides (VHMS) / volcanic-associated massive sulphides (VAMS) originate in a unit underlying the Eastern Fmn and percolate their way up through the Eastern and Vivigani Fmns, until they encounter the ocean floor (syngenetic with the formation of the Wagga Tank Fmn) and deposit in “pools” from volcanic vents (possible evidence seen within drill core), where they form massive/laminated sulphides.