Summary:
Gold mineralisation within the Cracow field forms along various broadly north-south striking corridors. Historical gold mining focused on the Golden Plateau deposit which yielded over 850 thousand ounces of gold between 1932 to 1992. Current underground mining, referred to as the Western Vein Field, is located immediately west of Golden Plateau.
The Cracow project area gold deposits are in the Lower Permian Camboon Andesite on the south-eastern flank of the Bowen Basin. The regional strike is north-northwest and the dip 20° west-southwest. The Camboon Andesite consists of andesitic and basaltic lava, with agglomerate, tuff and some inter bedded trachytic volcanics. The andesitic lavas are typically porphyritic, with phenocrysts of plagioclase feldspar (oligoclase or andesine) and less commonly augite. To the west, the Camboon Andesite is overlain with an interpreted disconformity by fossiliferous limestone of the Buffel Formation. It is unconformably underlain to the east by the Torsdale Beds, which consist of rhyolitic and dacitic lavas and pyroclastics with inter-bedded trachytic and andesitic volcanics, sandstone, siltstone, and conglomerate.
Gold mineralisation at Cracow is located in shear hosted quartz-carbonate veining, with typically low grade mineralisation in the wall rock. At Cracow, veins are found predominantly in andesitic lavas due to its brittle fracture qualities. Small scale lateral and vertical offsetting by faults has been observed at various locations. Rhyolite (rarely mineralised) and barren mafic dykes were recorded intruding and offsetting the veins.
Gold mineralisation at the Cracow Operation is hosted in the Lower Permian Camboon Volcanics (intermediate volcanics) on the south-eastern flank of the Bowen Basin. The Camboon Volcanics consists of andesitic and basaltic lava, with agglomerate, tuff and inter-bedded trachytic volcanics. Gold mineralisation is hosted in steeply dipping low sulphidation epithermal veins. These veins are found as both discrete structures and as stockwork. They are composed of quartz, carbonate, and adularia, with varying percentages of each mineral. Vein textures vary widely and include banding (coliform, crustiform, cockade, moss), breccia channels and massive quartz. The differing textures indicate depth within the epithermal system. Sulphide percentage in the veins are generally low (<3%) primarily composed of pyrite, with minor occurrences of hessite, sphalerite and galena. Rare chalcopyrite, arsenopyrite and bornite can also be found.
Domaining of the Cracow mineralised lodes is based on a combination of lithological, quartz vein percent and gold grade information. Both discrete “vein/lode” domains, mineralised halo or stockwork domains and waste domains were interpreted. Locally varying anisotropy is used for any non-planar domains to account for orientation changes and improve search and estimation parameters.
Alteration of the country rock can be extensive and zone from the central veined structure. This alteration consists of silicification, phyllic alteration (silica, sericite and other clay minerals) and argillic alteration in the inner zone, grading outwards to potassic (adularia) then an outer propylitic zone. Gold is very fined grained and found predominantly as electrum but less common within clots of pyrite.