A Geophysical Signature for tin deposits in south-east Australia ? revisited

Abstract

Webster (1984) showed ``that hardrock tin deposits in south-east Australia have a characteristic magnetic signature in the vicinity of each deposit'. This magnetic signature was concluded to be due to the high temperature of related granitoids imposing a remanent magnetism on rocks within the metamorphic aureole. The paper also discussed the main granitoid classifications and associated mineralisation in relationship to geophysical parameters and the use of the regional data for geological mapping in New South Wales and Tasmania. The case examples utilised in the original paper have been re-examined using modern airborne geophysical data acquired by Mineral Resources Tasmania (MRT) and the NSW Department of Mineral Resources. The magnetic and radiometric character of the granitoids and local lithology patterns are better contrasted in the new data sets, which allows for mapping of geology and structure to finer resolution. A significant feature of the new MRT data is the magnetic and radiometric aureole, up to 2km wide, now observed to encircle S-type granites, in particular the Meredith Granite. A metamorphic aureole was known from earlier geological mapping, however, the geophysical response was only poorly expressed in earlier datasets. The new data in the vicinity of these tin deposits are resented and profile modelling is used to confirm that remanence effects are involved in the source of the anomalies. The radiometric data sets show complimentary patterns in the vicinity of tin deposits that enhance the recognition of the mineralised target zones. In the central NSW tin-belt the S-type granites are intruded into weakly magnetic Ordovician meta-sediments, however, the zero magnetic response of the granites is well contrasted with the north-south linear fabric of the meta-sediments. The tin and gold deposits generally occur in quartz reefs located along sheared contacts of the granite with the meta-sediments and cross cutting east-west structures appear to be related to the source of the tin and gold mineralisation. The application of the radiometric data is restricted by the lack of outcrop.