A magnetic signature for tin deposits in south-east Australia

Abstract

The major hardrock tin deposits in south-east Australia have a characteristic magnetic signature in the vicinity of each deposit. This magnetic signature is due to the high temperatures of intrusion, being close to the Curie point of magnetite, imposing a remanent effect within the metamorphic aureole. The tin deposits are intimately related to the ilmenite series granitoids of lshihara (1978) which have been linked with the S-type granitoids of Chappell and White (1974). These granites are usually non-magnetic and circumscribed by magnetic aureoles in the country rock. These magnetic parameters can be explained by the temperature - oxygen fugacity characteristics of the parent melt. The non-tin-mineralized granitoid suite, labelled magnetite series by lshihara and I-type by Chappell and White, are moderately to strongly magnetic in south-east Australia. Non-magnetic I-type granitoids are common elsewhere, and can be caused by the replacement of Ti-magnetite by secondary sphene. The I-type granitoids (magnetite + sphene series) have a Cu - Mo - W mineral association. The relationship of tin mineralization to the S-type granitoids is that the deposits are found peripheral to the granitoids, in close association with their late and fine grained derivatives. The S-type granitoids may themselves be quite barren (Strong 1981). The mineralized late phase intrusions of north-west Tasmania exhibit a similar magnetic aureole to that surrounding the granitoids, as evidenced near the carbonate-replacement deposits at Mt Lindsay, Cleveland and Severn at Zeehan. Examples are given from north-west Tasmania to illustrate the signature of the Renison-Bell, Cleveland, Mt Lindsay and Severn deposits. Regional and local geophysical data from central New South Wales are presented to further illustrate the model for granites and mineralization at Ardlethan, Kikoira and Tallebung.