Heavy mineral provenance and the genesis of stanniferous placers in northeastern Tasmania

Abstract

A heavy mineral provenance study of stanniferous placers in northeastern Tasmania reveals that the placers were formed during 'long' periods of geologic time by episodic recycling dating back at least to the Permian. With the aid of heavy mineral species demonstrably deriving from granitic and basaltic source rocks, it is possible to identify placer formation events. Deep leads such as Briseis and Pioneer were stratigraphically confined between the Middle Eocene (ca. 47 Ma) and Middle Miocene (ca. 16 Ma) episodes of basaltic volcanic activity. This is supported by both fission track dating and electron spin resonance (ESR) studies of alluvial zircons. The latter is a rapid semi -quantitative technique for the age estimation of the host rocks of zircon. In addition to deep leads comparatively shallow leads are also present filling bedrock channels or covering erosion surfaces cut across the rock basement. Because the erosion surfaces represent episodes of sufficient duration to cause heavy mineral enrichment, they are useful for the stratigraphic correlation of placer sequences. Quaternary placers are represented iñ the form of fluvial terrace deposits along the lower Ringarooma valley. Bedrock tin mineralization is widespread in northeastern Tasmania and it is unnecessary to explain the placers by invoking long transport distances for cassiterite. The presence of large cassiterite nuggets and composite grains in most alluvial workings is consistent with local derivation involving short transport distances. This is also supported by geochemically distinctive trace element patterns in Mount Cameron and Blue Tier cassiterites. The former are enriched in niobium, tantalum, and zirconium, but depleted in tungsten in comparison to the latter. The selective removal of light minerals by winnowing through episodic recycling is considered to have been mainly responsible for the genesis of the placers in northeastern Tasmania. Major geomorphological evolution events have been identified in the study area. During the Middle Miocene, basaltic flows along the middle Ringarooma valley caused drainage diversion. The northwesterly flowing streams of the Blue Tier which were formerly connected to a proto- Boobyalla River system were diverted eastwards by these basalts into the South Mount Cameron Basin. An outlet of this basin into the Great Mussel Roe River existed before downcutting and /or capture shifted the river course towards the Great Northern Plains to form the Ringarooma River course of today. At least from the Middle Eocene until the Middle Miocene, the catchment area of the proto- Boobyalla River extended well into the Blue Tier prior to diversion by the younger basaltic flows. A wide range of factors helps to determine the age of placer deposits. In the present study stratigraphic control of placers was achieved through an examination of heavy mineral provenance; volcanic and duricrust formation events, and sedìmentologic, palaeo -oceanographic and palaeo- botanic evidence.