Opaque mineral assemblage formed during serpentinization in the Coolac ultramafic belt, New South Wales

Abstract

Abstract The Coolac ultramafic belt in southern New South Wales consists largely of partly to completely serpentinized harzburgite. The rocks contain a diverse assemblage of sparsely disseminated oxide, sulphide, and metallic minerals, of which magnetite, ‘ferritchromit’, heazlewoodite, pentlandite, and awaruite predominate. Other than ‘ferritchromit’, which occurs as rims on primary chromian spinel grains, the opaque minerals are enclosed in serpentine minerals and minor brucite, talc, chlorite, and tremolite, and do not occur in contact with primary olivine, orthopyroxene, clinopyroxene, and chromian spinel. The least serpentinized harzburgite contains only trace amounts of opaque minerals, whereas in almost completely serpentinized rocks, disseminated opaque minerals are common. Textural and chemical data are interpreted as indicating that the opaque mineral assemblage formed during the serpentinization processs, the metals being derived from the primary minerals, and oxygen and sulphur from the fluids responsible for serpentinization. Differing mineral assemblages occurring in the variably serpentinized rocks are interpreted as resulting from rising, although possibly fluctuating, gradients of oxygen and sulphur fugacity during progressive serpentinization.