Many porphyry deposits have potassic and phyllic alteration
Potassic alteration consists of biotite, magnetite and K-feldspar and is indicative of an early, high temperature phase. The high temperature sets up convective cells of water in the surrounding rock leading to a zonation of various minerals which can act as a vector when trying to locate the mineralised porphyry. Cu-Au mineralisation occurs during this phase and also during later phases.
Phyllic alteration consists of sericite, pyrite and silica and indicative a much later cooling event when the convective cell collapses and surrounding ground water encroaches on the intrusive.
The depth of the mineralisation indicates that open pit mining is not an option – it would have to be mined via block cave mining (assuming it is geotechnically suitable). This method has high capital costs (>$500m), about a 15% dilution and long lead times, but has very low operating costs.
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