A novel application of image analysis to interpret trace element distributions in magmatic sulphides

Abstract

The Albany–Fraser Orogen in Western Australia hosts significant magmatic sulphide mineralisation but the impacts of its extensive metamorphic history on this mineralisation are poorly understood. Post-formation modification, including fluid-facilitated alteration, can upgrade or destroy mineralisation, with significant consequences for economic ore deposits. We present laser ablation trace element data from samples of variably altered sulphide breccia from the mineralised Octagonal prospect located in the Fraser Zone of the Albany–Fraser Orogen. Mineral compositions are combined with laser ablation mapping and novel co-localisation analysis to characterise the magmatic and post magmatic processes that modified element distributions within magmatic sulphide ore. Higher concentrations of Re, Co, Ni, Os, Ir, and lower concentrations of Cu, Zn and Ag in pyrrhotite and pentlandite relative to chalcopyrite record sulphide crystallisation from a sulphide liquid. Textural evidence and the compositions of pyrrhotite and pyrite are consistent with replacement of pyrrhotite by pyrite and associated magnetite. Element maps reveal intergranular variations in pentlandite Ag content that are systematically linked to the sulphide distribution. Co-localisation analysis shows that the associations among Mn, Pb, Ag and the sample fracture networks are statistically significant; the association is interpreted as a consequence of fluid-assisted element remobilisation. Post-formation alteration processes at Octagonal were accompanied by variable remobilisation and depletion of metal concentrations associated with mineralisation; the co-localisation analysis provides an invaluable tool that enabled quantitative assessments of these relationships.