Decoupled lamprophyric magmatism and gold mineralization at the Archean Darlot lode gold deposit, Western Australia

Abstract

A close spatial association is observed between lamprophyric rocks and Au mineralization at the Archean Darlot lode gold deposit. Field observations indicate that both lamprophyric magmas and gold mineralizing fluids have exploited moderately east- and west-dipping reverse-(oblique) fault zones and sub-horizontal fractures but consistent crosscutting relationships indicate that lamprophyric rocks predate Au-bearing veins and fault zones. Therefore, lamprophyric magmatism and gold mineralizing fluids may have been active during the same deformation event but at different stages. Disseminations of garnet porphyroblasts that are spatially and it is inferred temporally associated with lamprophyric rocks indicate that hydrothermal fluid circulation accompanied lamprophyric magmatism.

Xenoliths in lamprophyric rocks, which contain veins that are similar in appearance to those associated with gold mineralization, provide an equivocal argument for the temporal overlap of magmatic-hydrothermal systems and circulation of gold-mineralizing fluids at Darlot. However, the low intrinsic Au abundances (<3 ppb) of lamprophyric rocks suggest they are not a particularly favorable source for Au. Geochemical indices such as a correlation of MgO-Ni suggest significant evolution of the lamprophyric magmas but the lack of any MgO-Au correlation argues against separation of auriferous volatiles from the evolving primitive source of the Darlot lamprophyric rocks. It appears more likely that Au within lodes at Darlot was derived from structurally focussed, deep-seated (metamorphic/mantle) fluids probably similar to those in other Archean orogenic lode gold systems.