The Ernest Henry copper-gold mine, located around 38 kilometres NE of Cloncurry in north-west Queensland, was officially opened in October 1997. Commercial production commenced in May 1998. The mine is named after the explorer/prospector/miner Ernest Henry, born in England in 1837, who founded Cloncurry and explored the district in 1866. Ernest Henry subsequently discovered numerous copper deposits in the district, including the Great Australia and Mount Oxide.
The Ernest Henry copper-gold deposit was discovered in 1991 by WMC and lies within a SE-plunging breccia system developed in a sequence of altered, porphyritic, intermediate volcanic rocks. The breccia body dips 30°-50° degrees SSE and lies between a hanging-wall sequence of variably altered felsic volcanic rocks and a footwall sequence of carbonate altered mafic volcanic rocks and siltstone. Economic mineralisation is hosted by a breccia comprising strongly altered and replaced felsic volcanic fragments in a matrix assemblage of predominantly magnetite, chalcopyrite and carbonate.
The magnetite, copper and gold minerals are thought to have been introduced to these rocks around 1500 million years ago as a result of the nearby intrusion of a large granite pluton. Fluids from the granite passed through fractures, altering and replacing some minerals and forming the minerals present in the ore body - magnetite, chalcopyrite, pyrite, gold, cobalt molybdenum, rare earth elements and low levels of uranium. Mineralisation boundaries are commonly sharp and conform to the limits of brecciation and fracturing.
The combined thickness of the mineralised sequence is around 250 metres, the width averages 300 m. The ore body is open at depth and to the south west. The upper portion of the mineralisation is weathered and variably oxidised and is classified as supergene ore. This zone extends, in places, to 150 m below surface and comprises about 12% of the entire resource. Supergene mineralogy is complex with copper occurring as native copper, bornite and chalcopyrite. Due to localised enrichment some of the highest copper and gold grades are present in this zone. In the primary ore, copper occurs as chalcopyrite and gold occurs mainly in the molecular framework of the chalcopyrite. Copper to gold ratios stay consistent throughout at around 1% copper to 0.5 grams/tonne gold. The total reserve + resource prior to commencement of mining in 1998 was 166 Mt @ 1.1% Cu, 0.54 g/t Au. As of June 2003 the remaining resource totalled 117.9 Mt @ 1.13% Cu, 0.52 g/t Au. Conventional open-pit mining of about 60 million tonnes of material per year utilises a strip ratio of 5:1.
( Source: Xstrata Copper Ltd 2003 & 2006)
