Superposition of gold mineralisation on pre-existing carbonate alteration:structural evidence from the Mulgarrie gold deposit, Yilgarn Craton

Carbonate alteration at the Mulgarrie gold mine in the Eastern Goldfields o f Western Australia, is represented by porphyroblasts. veins and pervasive, texturally destructive, carbonatisation. Two foliations. S-1M and S-2M. we re produced by two separate deformation events at the mine-scale. D-1M and D-2M D-1M and D-2M both occurred in response to regional D-2 tectonism. Car bonate alteration was the product of two separate episodes of fluid ingress : the earlier produced magnesite and the latter Fe-dolomite. Both periods o f carbonate alteration occurred pre- to early syn-D-2M, when mafic to ultra mafic komatiltic rocks reacted with fluids that moved along regional faults and pre-date the alteration associated with regional peak metamorphism. Go ld at Mulgarrie overprints pre- and late syn-D-2 quartz veins in zones of m assive carbonate alteration, suggesting it has a late- to post-D-2 timing. This late timing agrees with the generally accepted syn-D-3 land younger) a ge for gold mineralisation in the Eastern Goldfields. We suggest that carbo nate alteration at Mulgarrie is not a product of the hydrothermal event res ponsible for the gold mineralisation. Rather, the different relative timing of magnesite. Fe-dolomite and gold indicates there were two carbonate-prod ucing fluid systems and a fluid transporting the gold overprinted these. Si milarly, early carbonate alteration may play a role in localising auriferou s vein deposits throughout the Yilgarn and other Archaean cratons.