Pre-requisites for sulphide-poor PGE and sulphide-rich Cu-Ni-PGE mineralization in Alaskan-type complexes

Alaskan-type ultramafic complexes are known as low-sulphide systems where PGE enrichment may occur in chromite-rich layers and concentrations of economic grade may be found in spatially associated placer deposits. The primary reason that many Alaskan-type complexes are sulphide-deficient appears to be related to the elevated fO₂ conditions under which parental magmas are generated in the supra-subduction zone environment. At fO₂, conditions in excess of QFM+2, sulphate is the predominant S species rather than sulphide. During fractional crystallization basaltic and andesitic liquids are produced that have relatively high S concentrations in the form of sulphate. Local enrichment in PGE alloys may occur due to the production of micro-reducing environments in areas of chromite crystallization. The sulphate which is transferred to derivative magmas may be strongly partitioned into a fluid/vapour phase as either SO₂ or H₂S, depending on prevailing fO₂ and fH₂O. Oxidized magmas may also play a role in the generation of porphyry-Cu style hydrothermal ore deposits. Because Alaskan-type complexes are parts of conduit systems through which large volumes of metal-bearing mafic magmas have passed, they have the potential to host significant concentrations of sulphide-rich Cu-Ni-PGE mineralization. The key ore forming process appears to be magma-country rock interaction. Assimilation of organic carbon bearing sedimentary country rocks acts to reduce the fO₂ of the magma and promotes sulphide stability. Incorporation of sulphur from country rocks is essential to produce large masses of metal-rich sulphides. Recent studies of Cu-Ni-PGE sulphide mineralization in the Duke Island Complex provide compelling isotopic evidence for the involvement of externally derived components in the mineralized system. Relatively early assimilation of S is critical in the generation of Ni-rich sulphides; Cu- and PGE-rich sulphides may be produced if sulphide saturation is delayed. The potential for the occurrence of world-class Cu-Ni-PGE sulphide deposits in Alaskan-type complexes should not be overlooked, particularly if evidence exists for the presence of low-fO₂ magmas and extensive magma-country rock interaction.