Mineralisation and retrograde overprint of the MEM2 orogenic gold deposit along the Antimony Line, Murchison Greenstone Belt, South Africa

The Antimony Line is a regional structural feature in the central parts of the Murchison Greenstone Belt (MGB) in South Africa which exhibits a complex tectono-metamorphic evolution and renowned antimony-gold (Sb)-(Au) deposits. The MEM2 deposit in the northeastern parts of the Antimony Line exhibits disseminated Au-only mineralisation hosted by highly-altered, lens-like intercalations of quartz-carbonate schist contained in mafic- to ultramafic talc-chlorite schist. Economic Au mineralisation is confined to mm-scale, closely-spaced, discontinuous quartz-carbonate-sulphide stringers. Mineral textures and parageneses suggest the mineralisation formed at amphibolite-facies conditions (T ca. 500 to 630°C). The mineralised quartz-carbonate-sulphide stringers are dismembered along pervasive, closely spaced quartz-muscovite-chlorite shear bands that formed during retrograde shearing at lower- to mid-greenschist facies (T ~350 to 400°C) conditions. The intensity of the retrograde structural overprint is underlined by the attenuation and steep plunge of modelled grade shells, forming distinct ore shoots parallel to the steep stretching lineation that formed during regional sinistral transpression. Preservation of the earlier, higher-temperature Au mineralisation is primarily attributed to the position of the MEM2 deposit in a restraining bend resulting in limited fluid influx during the near-pervasive retrogression of its wallrocks. The MEM2 deposit represents a case of earlier vein-hosted Au mineralisation that has been overprinted and dismembered during later retrograde deformation resulting in a disseminated nature of mineralisation. On a regional scale, this may also point to successive mineralisation episodes along the Antimony Line with an earlier Au mineralisation formed at close to peak-metamorphic conditions followed by a lower-temperature Sb mineralisation during retrogression.