DENUDATION HISTORY IN THE FORMATION OF ALLUVIAL TIN PLACERS IN TASMANIA, AUSTRALIA

A heavy mineral provenance study of alluvial tin placers in northeastern Tasmania indicated that episodic recycling dating back to at least the Permian was involved in their formation. These placers are associated with deeply buried high-energy alluvial channel deposits referred to as deep leads. Age constraints of the deep leads are provided by Tertiary basalts overlying the deposits which can be K-Ar dated; alluvial zircons within the deposits which can be fission track dated; palynoflora associated with the deposits, and, the occurrence of overlying post-Middle Miocene duricrusts. The sequence of events forming the placers are: (1) Pre-Permo-Triassic erosion and unroofing of mineralised Devonian to Carboniferous granitic batholiths; (2) Intrusion of Jurassic dolerite sheets followed by extensive erosion; (3) Middle Eocene basaltic volcanic activity; (4) Post-Middle Eocene period of rapid erosion; (5) Post-Oligocene lateritisation; (6) Middle Miocene basaltic volcanic activity; (7) The capture of streams draining from the Blue Tier Massif by the present day Ringarooma River; (8) The capture of the tributary of the Great Mussel Roe River by the Ringarooma River, and (9) Reworking of alluvial deposits along the course of the Ringarooma River associated with periods of high discharge in the Quaternary. The post-Middle Eocene period of rapid erosion is in agreement with the lowering of sea level associated with the development of the Antarctica ice cap indicated by DSDP sites 277, 279 and 281. In contrast to this, Southeast Asian tin placers in Malaysia and Indonesia have poorer age constraints but may also be similar in age in spite of age diferences in bedrock mineralisation.