SUPERGENE IRON PHOSPHATE MINERALS IN PROTEROZOIC IRONSTONES FROM THE OLARY BLOCK, SOUTH AUSTRALIA

Iron phosphate minerals have been found as apparent supergene alterati on encrustations and cavity fillings in pyrite-bearing, magnetite-hema tite-quartz ironstones of the Proterozoic Willyama Supergroup in the O lary Block, South Australia. The minerals are generally microcrystalli ne, with natrodufrenite and rockbridgeite being the most common. In pl aces, they are accompanied by strengite and chalcosiderite, and are ty pically found in association with jarosite-type phases and goethite. A lthough crystallisation sequences may be reversed, there is an overall paragenesis of strengite altering to natrodufrenite, thence to rockbr idgeite and goethite, implying increasing Fe3+ (+Al)/PO43- as crystall isation of the phosphates proceeded. The surficial occurrence of the p hosphates and their association with jarosite and goethite indicate ox idising, low-pH conditions for their formation. An origin for the phos phate minerals from the dissolution of pyrite in the ironstones and re action of Fe-bearing, acid sulphate groundwaters with hypogene apatite , or with phosphatic guano deposits on the ironstone outcrops is favou red.