The geology and geochemistry of the Mount Elliott copper-gold deposit, northwest Queensland

A new resource of approximately two million tonnes at 4.2% Cu equivalent has been discovered beneath old workings of the Mount Elliott mine in northwest Queensland. Potentially economic copper and gold mineralisation occurs as massive chalcopyrite with pyrite and pyrrhotite hosted in breccia matrix and shallow-dipping calc-silicate+calcite veins which cross-cut the local stratigraphy. The en-echelon series of mineralised veins are constrained between two northwest-trending, steeply-dipping reverse faults. The deposit occurs within a unit of brecciated and pervasively skarn-altered shales (\Elliott Beds\") near to the top of the Kuridala Formation. Bleaching of the shales followed by Ca-F metasomatism and albite-hematite (red-rock) alteration formed an early ground-preparation event that enhanced brittle fracturing and brecciation of the shale. The advancing red-rock alteration is geochemically characterised by increasing Na20 and Al2O3 contents and decreasing SiO2 content (quartz dissolution). Brecciation was accompanied by pervasive prograde skarn alteration consisting of coarse-grained diopside-hedenbergite scapolite sphene apatite and calcite. Wholerock geochemical analysis of calc-silicate altered rocks reveals severe depletions in SiO2 Na20 and Al2O3 contents accompanied by markedly increased Fe2O3 content. Magnetite plus chalcopyrite pyrrhotite and pyrite replaces the calc-silicate assemblage preferentially along the contact of calcite and clinopyroxene grains. Late anhydrous retrograde alteration of the skarn comprises minor chlorite and epidote. Fracturing is the primary control on localisation of alteration in which hydrothermal fluids repeatedly utilised the same pathways during progressive stages of alteration."