Geochemistry of the mammoth copper deposit' northwest Queensland, Australia

Abstract

The stratabound Mammoth Cu deposit, situated north of Mount Isa, northwest Queensland, Australia, consists of a number of orebodies in the Proterozoic (Carpentarian) Myally Subgroup. The sulphides are predominantly chalcocite/bornite, chalcopyrite and pyrite. Only No. 1 and No. 2 orebodies are being mined and the majority of the data presented is obtained from core samples from them. Each sample has been analyzed for 10 major elements, 27 trace elements, C, S and H 2O, and the major mineralogical components have been determined by X-ray diffraction. Immediately above the middle levels of No. 2 orebody is a bleached zone with depleted Fe, Mn and Mg. A similar bleached zone is also present above the No. 3 orebody but there is no conspicuous bleaching in the upper levels of No. 2 orebody or associated with No. 1. Significant geochemical variations between the hanging and the footwalls have been detected. K, Na and Ba are more abundant in the hanging walls while Co, Ni, Fe, Mn, H 2O and, in the case of No. 2 orebody, Mg are concentrated in the footwalls. Trace elements associated with the sulphides are Ag, As, Bi, Co, Mo, Ni, Pb, Sb and Tl. Of these Tl and As are concentrated in pyrite, whilst Ag and Bi are associated with Cu minerals. Deep weathering has caused oxidation of the ore in the upper levels of the mine and some chalcocite in No. 1 orebody may be the result of supergene enrichment. Gossans and oxidized ore were removed during open-cut operations. However, at the nearby Esperanza/Pluto prospect where similar mineralization is known to occur, anomalous concentrations of As, Sb, Mo and Pb associated with Cu in the ironstone capping and oxidized zone appear to be significant for future exploration.