A replacement origin for the BIF-hosted gold deposit at Mt. Morgans, Yilgarn Block, W.A.

Abstract

The BIF-hosted gold deposit at Mt. Morgans, with resources of 6.2 M tonnes at 3.4 g/t gold, is situated in an Archaean greenstone succession in the Eastern Goldfields Province, Yilgarn Block, some 3 km east of the Celia Lineament, a major deformation zone. Tholeiitic basalts, lenticular BIF horizons intruded by concordant and subconcordant quartz-feldspar porphyries, and ultramafic rocks are the dominant wallrocks. The deposit is sited in a dilational jog, at the intersection of a major NNW-trending sinistral shear zone and later, also sinistral, NNE-striking shear and fault zones. Moderate to steeply SE-plunging high-grade ore shoots occur in the BIF unit at these structural intersections. Despite the lack of macroscopic quartz veining in the orebody, there is a close spatial relationship between gold mineralisation. Fe-sulphides and quartz- and Fe-carbonate veins on a microscopic scale. Pyrite occurs as anhedral to euhedral grains and aggregates parallel to layering, in discordant fractures and associated alteration haloes, and as massive sulphide-quartz veins. Together with chalcopyrite and rare sphalerite, pyrite contains gold grains up to 220 μm in diameter. Chemically, gold is characterized by an average fineness of 866 and by a limited fineness range, typical for Archaean mesothermal gold deposits. Both ore textures and structures at a variety of scales strongly implicate an epigenetic, replacement style of mineralisation, initiated via local sulphidation of the geochemically favourable Fe-rich wallrocks which were sited in a structurally heterogeneous zone comprising rocks of contrasting competency at a relatively high crustal level.