The Geophysical Signature of Mt Isa Cu, Zn-Pb-Ag Ore Bodies

At a regional scale the Mt Isa Cu and Zn-Pb-Ag deposits do not have a noticeable gravity or magnetic response. In addition there is little recent literature on the geophysical signature of the deposits and the data which is available is dated.  Fallon and Busuttil 1992 and Valenta 2020 provide summaries of the available geophysical data. Given that pyrite extends up to 10 km north of economic grade mineralization within the Urquhart Shale and across a width of >1km, pristine mineralization would have  strong IP, EM and MT responses however this data is not readily available.

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The Eastern Creek Volcanics: Source for the 8 Mt Copper Deposits at Mt Isa

Discovery of the Mt Isa Copper Deposits

Silver-lead ore was discovered at Mt Isa in 1923 but it was not until 1927 that a surface drill hole aimed to test silver-lead bodies at depth intersected 15 m of oxide copper and chalcocite grading 17% copper in the Black Rock area. This secondary mineralization was explored underground in 1937, mined intermittently for flux between 1941 and 1962, and by open cut from 1957.  Combined oxide and chalcocite ore mined from the Black Rock open cut between 1963 and 1967 totaled 2.26 Mt at an average grade of 3.9% copper.  It was not until 1930  while drilling lead-silver mineralization at depth that primary chalcopyrite mineralization was encountered reporting a best result of 8.8m at 8.5% Cu. Follow-up drilling in 1953 reported 17m at 2.0% Cu and in 1954 a most respectable 202m at 2.2% Cu (Perkins, 1999). 

Deposit Geology

The copper mineralization is hosted entirely within a broad zone of intense silica-dolomite alteration developed above the Paroo Fault within the Urquhart Shale (~1650-1955MA).   Economic copper ore bodies extend across a combined width of more than 1,000 metres and along a strike of 4,200 metres and entirely within the Urquhart Shale.

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