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Geochemical indications of concealed copper mineralization in an area northwest of Mount Isa, Queensland, Australia

Journal of Geochemical Exploration
Publication Date
DOI: 10.1016/0375-6742(77)90050-4
  • Chemistry
  • Earth Science


Abstract Ferruginous outcrops (ironstones) are a common feature of deeply oxidized dolomite members of the Middle Proterozoic dolomitic and carbonaceous/siliceous sequence in northwestern Queensland, Australia. The textural and geochemical interpretation of the ironstones is important in regional and detailed exploration for Cu, Ag, Pb and Zn. In the Mount Kelly area, primary Cu mineralization occurs in breccias, veins and disseminations in dolomitic-rich bands. Because of structural complexities, the moderate- to high-grade (>2% Cu) zones do not outcrop. The geochemistry of this primary mineralization is described, no element other than Cu being detected characteristic of the chalcopyrite. Surface indications of mineralization are Cu anomalous ironstones in the dolomitic units and some secondary Cu mineralization. The ironstones are classified as (1) gossans forming directly from in-situ oxidation of primary mineralization; (2) fault ironstones; (3) “stratigraphic” ironstones which are broadly distributed along particular units but which transgress lithological boundaries in detail. Whole rock geochemistry of the fault and “stratigraphic” ironstones reveals a significant positive correlation of Cu with Ag, a weak positive correlation with Ni and a strong negative correlation of Cu with Co. There is no statistical or spatial evidence of Cu being related to Mn. Co and As anomalies can reflect the presence of pyrite. The Fe in the ironstones is derived from dolomite and sulphides in the sequence. Cu in the ironstones is derived from three sources, namely: disseminated chalcopyrite in the sequence, Cu in pyrite, and specific higher-grade Cu concentrations. These three sources cannot readily be differentiated. The magnitude of a Cu anomaly is no indication of Cu values in the primary zone. Exploration should be aimed at locating ironstones and prospective stratigraphic horizons which are anomalous in Cu, followed by subsurface testing, but not necessarily with the subsurface continuation of ironstones as drilling targets. Where the primary dolomitic rocks average over 1500 ppm Cu over significant thickness, the sequence should be considered prospective.

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