Mineralogical and fluid characteristics of the fluorite-rich Monakoff and E1 Cu-Au deposits, Cloncurry region, Queensland, Australia: Implications for regional F-Ba-rich IOCG mineralisation

摘要

The Monakoff iron oxide-Cu-Au (IOCG) deposit, located to the north east of Cloncurry within the Eastern Succession of the Mount Isa Inlier, Queensland, Australia, is characterised by high concentrations of F and Ba, with a host of other enriched elements including Co, Ag, Mn, REE, U, Pb, Zn and Sr. This gives the deposit a characteristic gangue assemblage dominated by fluorite, barite and calcite. The nearby El deposit, located 25 km to the NNE of Monakoff, and the large Ernest Henry deposit, 3 km to the west of El, also contain abundant fluorite, barite and calcite in late stage assemblages. The three deposits, therefore, constitute a distinct group of IOCG deposits within the district, based on their F-rich geochemical and mineralogical affinities.The Monakoff ore zone is hosted in dilational openings along a shear zone developed within metasediments and metavolcanic rocks at the boundary between competent hangingwall rocks of the Toole Creek Volcanics and foot-wall rocks of the Mount Noma Quartzites. Four stages of alteration and mineralisation are recognised: Stage 1 garnet-biotite alteration; Stage 2 biotite-magnetite alteration; Stage 3 main F-Ba-ore mineralisation; and a Stage 4 pyrite-alloclasite Au-Co-As overprint. The El deposit has a more complex history, but Stage 5 has veins of fluorite-barite-carbonate that are comparable to Monakoff s main stage. The Stage 3 assemblage at Monakoff comprises a sheared groundmass of fluorite, barite, manganoan calcite, magnetite, chalcopyrite, pyrite, galena and sphalerite, with coarser grained pods of the same mineralogy interpreted to be dilational structures infilled during syn-ore deformation. Accessory minerals include U-Pb-oxides, REE-F-carbonates and Ag-Pb-Bi-sulfosalts, with no discrete Au minerals. The sulfosalts are interpreted to have formed from an immiscible Bi-melt within the mineralising fluid at temperatures higher than the melting point of Bi. The Stage 4 overprint at Monakoff contains pyrite and alloclasite. Laser ablation analyses of the sulphide minerals at Monakoff reveal that Stage 3 sulphides contain only trace amounts of Au (0.04 ppm in pyrite), although galena and chalcopyrite contain significant concentrations of Ag. Stage 4 pyrite and alloclasite, however, contain -1 ppm Au in solid solution and mass balance calculations indicate the majority of bulk rock Au to be present in these minerals, although the majority of bulk Ag is present in Stage 3 sulphides. The Stage 5 veins at El have an identical gangue and accessory mineralogy to Stage 3 at Monakoff and differ in the sulphide mineralogy only in the lack of galena and sphalerite.