PALABORA CU AND SERRA PELADA AU-PGE: END MEMBERS OF THE FE-OXIDE CU-AU DEPOSIT GROUP

Even if Au- and Cu poor Fe-oxide-REE-P deposits are excluded, the Fe-oxide (Cu-Au) group contains a variety of deposit styles in diverse tectonic settings. A potentially coherent sub-group is represented by late-Archean (?)/Palaeoproterozoic to Mesoproterozoic Fe-oxide Cu-Au-REE (± Ag-Co-Mo-P-U) deposits formed in extensional or transitional tectonic environments adjacent to lithospheric discontinuities in association with broadly coeval A-type granitoids and/or alkaline intrusions. These include the Olympic Dam, Cloncurry and Carajas deposits, which arguably form a broad vertical continuum, from Fe-silicate and/or magnetite- and Cu-rich at depth, to hematite- and relatively Au-rich at shallow levels.

It is argued that the Palabora carbonatite-hosted magnetite-Cu-P-REE deposits, with Au and PGE, is a proximal example of this deposit sub-group, and may be broadly equivalent to the dense magnetic intrusion postulated to underlie Olympic Dam. It has an appropriate age, is close to an Archean craton (lithosphere) edge, has appropriate mineralogy, element association and REE geochemistry, and represents an alkaline ore-fluid source as postulated by many researchers for the deposit group.

The world-class Serra Pelada Au-PGE deposit is sited in the Carajas region, in broadly the same province as the numerous >200 mt Fe-oxide Cu-Au deposits (e.g. Salobo, Igarape Bahia, Alemao, Sossego), close to a craton margin in a terrane with both ca 2.5 and 1.9 Ga A-type granitoids. Serra Pelada, although highly weathered, appears to be a S-poor, hematite-bearing deposit with similar metal association and LREE to the Fe-oxide Cu-Au deposits at Carajas and elsewhere. Significantly, the PGE ratios, particularly high Pd, are consistent with metal transport in a similar acid, oxidizing hydrothermal fluid to that depositing Fe-oxide Cu-Au ores.

The Palabora Cu (Au-PGE) and Serra Pelada Au-PGE deposits are postulated to be the most proximal, high-T and distal, low-T deposits, respectively, so far recognised in the Fe-oxide Cu-Au group. The minor, but significant, PGE contents of the Palabora carbonatite, combined with the high Pd contents of some more-alkaline porphyry Au-Cu systems, should alert explorationists to the possibility of Au ± PGE deposits as distal targets in Fe-oxide Cu-Au provinces.