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Hematite-barite alteration in the Owen Conglomerate, North Lyell, Tasmania

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Hart, Ian M (1993) Hematite-barite alteration in the Owen Conglomerate, North Lyell, Tasmania. Unspecified thesis, University of Tasmania.

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Abstract

The North Lyell mine area is located within the ML Read volcanics adjacent to the north-south
trending Great Lyell Fault (GLF). It was one of the more copper rich deposits within
the Mt. Lyell mining field having produced 5mt of ore with 5.3% Cu, 33 g/t Ag and 0.4 g/t
Au. The main zone of mineralisation at North Lyell is located near the intersection of the
GLF with the WNW trending North Lyell Fault. Although most of the bornite
mineralisation at North Lyell occurred within the volcanics, there was bornite recognised
within the Cambrian-Ordovician siliciclastic conglomerate which is also adjacent to the
GLF.

The timing of alteration/mineralisation at North Lyell is reflected in the extent and style of
alteration observed within the overlying conglomerates, sandstones and limestones.
Extensive hematisation with associated barite occurs along the GLF between the volcanics
and sediments. The hydrothermal hematisation associated with the mineralisation at North
Lyell and Lyell Tharsis extends for some distance into the sediments. As the fluids
progressed through the North Lyell system their compositions were modified by their
interaction with the various host lithologies, producing distinct alteration assemblages
within the surrounding units. Three main geochemical associations related to the alteration
can be recognised in the North Lyell area; phosphate-hematite (Fe2O3+P2O5+La+Sb),
sericite (K20+Al203+Cr+Rb) and barite (Ba+Sr+Sb). The barite assemblage is the least
dispersed, being confined to the main fluid conduits and wallrocks.

The change in alteration styles from the weakly acidic, reduced sulphide rich volcanic
environment, to the highly oxidised conditions within the conglomerate is reflected in the
'dumping' of hematite and pyrite on the GLF boundary. Computer modelling of the fluids
show that this redox front alone cannot account for the observed mineralisation at North
Lyell. The most successful mechanism for metal deposition was a thermal gradient coupled
with the redox boundary. A fluid dominated system with input from the oxidised brines of
the Owen Conglomerate produced an assemblage representative of the North Lyell
mineralisation.

The recognition of detrital hematite clasts adjacent to the Haulage Unconforrnity, coupled
with the concept of metal deposition in a relatively shallow environment, suggests the
hydrothermal enrichment at North Lyell took place during Late Cambrian-Early Ordovician
sedimentation.

Item Type: Thesis (Unspecified)
Keywords: Sedimentation and deposition, Conglomerate, Hematite, Barite
Copyright Holders: The Author
Copyright Information:

Copyright 1993 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 (M.Econ.Geol.)--University of Tasmania, 1994. Includes bibliographical references

Date Deposited: 08 Dec 2014 23:59
Last Modified: 18 Nov 2016 00:06
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