A study of the 'greenstone' basement at Mount Isa Mines, Queensland

Abstract

The "greenstone" basement at Mount Isa Mines consists of basic volcanics, with minor interbedded sediments, and it appears to be a block of Eastern Creek Volcanics which have undergone intense alteration. Four basic rock types have been recognised - Altered Volcanics, Sediments, High Carbonate Rocks and Sheared "Greenstone", each with a fairly distinctive petrography and chemistry. It is considered that the Altered Volcanics. like the basalts of the Eastern Creek Volcanics, are tholeiitic in origin. The alteration present is predominantly chloritization, accompanying the introduction of silica (+/- carbonate) into the "greenstone". This alteration and introduction of silica (+/- carbonate) is thought to represent the passage of mineralizing fluids through the "greenstone", during which copper was removed from the "greenstone". This copper-rich fluid then passed into the more permeable "silica-dolomite", where the copper, silica and carbonate were deposited in bulk, forming the rich and unique copper ores of Mount Isa.