A spessartite-rich alteration assemblage from the Bega Granite,N.S.W., Australia

Pipe deposits of quartz-bismuthinite-molybdenite in the roof zone of a granitic pluton metasomatised to a leucocratic soda syenite are surrounded by concentric spessartitic and sericitic alteration zones. Chemical and petrographic data on the alteration zones are presented. It is suggested that the alteration has formed from laterally moving acidic hydrothermal solutions which changed in composition as a result of reaction. The lateral change in MoS₂ from the 2H polytype of the pipes to mixtures of the 2H and 3R polytypes in the surrounding altered rocks may reflect the lateral changes of solution chemistry.