Mineralogical investigation of silver and cobalt trace metals in the Mount Isa deposit

Abstract

The world class Mount Isa deposit, located within the Mount Isa Inlier, North West Queensland, contains deposits of copper, lead and zinc with ore genesis still contested. The copper orebodies are characterised by a relatively simple mineralogy of chalcopyrite, pyrite and pyrrhotite. Previous work on the sulphide assemblage of the Mount Isa copper ore bodies has identified significant accessory silver and cobalt. The mineralogy, paragenetic timing and mineral associations of these trace metals is still poorly defined. Samples from the 3500, 3000, 650 and 500 copper orebodies were investigated using optical petrology, SEM-EDS and MLA to identify and compare silver and cobalt-bearing minerals and textures. Monazites were also located for their potential to be analysed by U-Pb geochronology. LA-ICP MS was used to ascertain the relative abundances of trace elements within selected mineral species. A total of two cobalt-bearing minerals and four silver-bearing minerals were identified, including argentian chalcopyrite. Paragenetic observation results were consistent with findings from the lead-zinc orebodies at Mount Isa Mines. A monazite sample which had been previously found at Mount Isa Mines was dated to constrain paragenesis of mineralisation fluids and gave an age of 1614± 52 Ma, which correlates with the D1 event in the Mount Isa inlier. Through the aforementioned methods, distribution of silver and cobalt trace metals throughout the Mount Isa copper system was determined to be heterogeneous and appears to be paragenetically comparable to the Pb-Zn lodes.