Mineralogy and petrography of primary copper mineralisation from Moonta Mines

Abstract

Samples of massive bornite- and chalcopyrite ore from the Moonta and Wallaroo Mines, originally collected by Erica Maud McBriar for her M.Sc. thesis in 1962, have been investigated to determine mineralogical and petrographic relationships between co-existing minerals. Much is known about the mining history of Moonta-Wallaroo but few previous studies have attempted to characterise the mineralization itself and compare it with that present elsewhere. Results show that massive copper ores from the Moonta and Wallaroo Mines share a number of mineralogical and textural similarities with other IOCG systems in the Olympic Cu-Au Province. Other mineralogical features, such as the abundant pyrrhotite, appear to be a reflection of unusually reduced conditions, although late, superposed hematite infers a shift towards oxidized conditions in the final stages of mineralization. The replacement of bornite by chalcopyrite and of chalcopyrite by bornite, even in the same sample is suggestive of multiple episodes of ore crystallization, possibly during a single protracted event. The observations do not contradict established models of ore genesis in which the Moonta orebodies are the products of structurally-controlled IOCG-style mineralization. Ore-forming fluids were likely derived from ~1.6 Ga Hiltaba Suite intrusives and driven along shear zones. The study, based around polished sections prepared from precious sample material collected more than 50 years ago contribute to a genetic model that takes account of the diversity of mineralization styles, which can assist with ongoing exploration in the Moonta-Wallaroo area.