Craton-scale hydrothermal alteration in the Yilgarn, Western Australia

A combination of petrography, and mineral and whole-rock chemistries from various deposits show that two major metasomatic events are spatially and/or temporally related to the introduction of gold across the breadth of the Archaean Yilgarn Craton, Western Australia. Alteration was characterized within the maficultramafic sequences that commonly host Archaean lode gold deposits. Type 1 metasomatism is characterized by hornblende+biotite+oligoclase/andesine±albite±titanite±fluor-apatite±quartz±garnet. Mineral compositions and Back Scatter Electron (BSE) images indicate that metamorphic actinolite was replaced by a tschermakitic hornblende during type 1 metasomatism. Type 2 metasomatism is characterized by alkali feldspar+albite+chlorite±clinozoisite±actinolite/tremolite±pyrrhotite±prehnite± quartz±carbonate±serpentine/talc. The overlap of, and paragenetic sequence of metasomatic events is vital to the formation and tenor of gold mineralisation. Feenrichment during type 1 metasomatism was particularly favourable for the subsequent precipitation of goldhosting Fe-sulphide from excess Fe generated from the hornblende-clinozoisite reaction during type 2 metasomatism. Whole-rock geochemistry displays the role of lithology in the development of alteration minerals and how it can be used to complement textural and compositional studies in mineral exploration.