By Don McColl
The Mount Elliott Mine was discovered around the turn of the century. It is in the Selwyn Range about a hundred kilometres south of Cloncurry in northwest Queensland. Mining there commenced in earnest with the formation of a company, Mount Elliott Limited, which appointed the almost legendary W.H. (Jimmy) Corbould as mine manager in 1909. He was to guide mining and production successfully throughout the years of the first world war. Operations in those days were limited to the oxidised ore in the upper 100 metres or so of the lode, which was amenable to treatment by the elementary furnaces of that time. By 1919 copper prices were falling all over the world, and when this was coupled with industrial unrest in that remote region, the mine was closed down in 1922. Since 1992, Australian Resources Pty Ltd (ARIMCO) has operated the Mount Elliott Mine, where they have extracting sulphide ores, for production of a chalcopyrite concentrate with appreciable gold content. They are currently at a depth of 290 metres, and expect to continue operations for about five more years.
The Mount Elliott copper orebody has strong similarities to a traditional contact metamorphic deposit, but produced in this case within a grossly brecciated lime-rich skarn host rock during an epoch of moderate metamorphism. An adjacent granite could have been involved in its formation. Considerable hydrothermal activity and lime metasomatism has produced a suite of distinctive minerals in close association with the orebody. Calcite is consequently a major component of the primary ore, and includes masses of chalcopyrite which may occasionally be several metres in width. The calcite is a bright pink colour from minute inclusions of haematite, and is usually very coarsely crystalline. The wallrock is metamorphosed in many places to a fringe of blocky well formed crystals of the pyroxene, diopside. These crystals are of exceptional quality for what is in reality a very rarely crystallised mineral even by international standards. They are frequently of very considerable size, but difficult to collect as they occur in clusters which are most commonly enclosed within the pink calcite. Apatite and scapolite are also commonly well crystallised components, apparently produced by metamorphism in conjunction with the diopside and calcite. The chalcopyrite seems to have been concentrated into voids in the core of the ore zones in a late more hydrothermal spasm of mineralisation, which frequently developed vughs lined with crystals of quartz and calcite. The quartz is frequently a delicately tinted amethyst variety producing geode linings of great beauty. These later generations of calcite show an incredible range of colours and forms, and in some instances are crystallised in geodes with the amethyst.
Copper mines in the Mount Isa-Cloncurry region are all notable for the development of quite deep zones of oxidation and supergene enrichment. Mount Elliott is no exception, cuprite, malachite, azurite and chrysocolla, were originally common in the oxidised zone, with an unknown assortment of other species. Virtually all the oxidised mineralisation was removed during the mining early this century, however the mine has continued to be famous as the source of the largest crystals of gypsum recorded from anywhere in Australia. Selenite prisms up to 30 centimetres in diameter and up to ten metres in length were found crisscrossing huge crystal lined cavities in the secondary ore zone early this century. Even more amazing was the plentiful occurrence of native copper in small arborescent clusters. These were often enclosed within the great prisms of transparent glassy gypsum, and so preserved in bright untarnished condition. Corbould, who was most interested in mineralogy, comments on these and other wonderfully crystallised oxidised copper minerals that he saw in the unusually cavernous ore material.
Mount Elliott is remarkable among Australian copper mines as an exceptionally lime-rich contact metamorphic orebody, which has produced superb crystallisation of the minerals of both the primary and oxidised zones. It is to be regretted that so little from the oxidised ore zone was preserved, with the exception of the huge gypsum crystals, which are represented in most major Australian museums.