Symplectitic augite from the Platreef -textural evidence for fluid/rock interaction in the Northern Sector of the Northern Limb of the Bushveld Complex?

Abstract

This paper describes the occurrence of a rare form of exsolution texture known as symplectitic augite in a melagabbronorite from the lower portion of the Platreef in the Northern Sector of the Northern Limb as intersected by the Moordkopje (MO-1) drill hole. The texture is thought to have formed at around 1100°C, a temperature almost indistinguishable from the crystallization temperature of pyroxene in the sample, by discontinuous precipitation induced by increased grain boundary mobility of augite as a result of the presence of a water-rich fluid phase within the Platreef magma. This would suggest that fluid/rock interaction may also have been of importance where the footwall to the Platreef is composed of granite and gneiss, as opposed to where the footwall is composed of dolomite and where fluid/rock interaction is known to have been of importance to mineralization.