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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 in 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 sedimentologic, palaeo-oceanographic
and palaeo-botanic evidence.

Item Type:	Thesis - PhD
Authors/Creators:	Yim, WWS
Keywords:	Tin ores, Cassiterite
Copyright Holders:	The Author
Copyright Information:	Copyright 1990 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1991. Includes bibliographical references
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