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The geology of the Mount Lyell mines area, Tasmania : a re-interpretation based on studies at Lyell Comstock, North Lyell and the Iron Blow area

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Corbett, Keith D.(Keith Douglas) (2001) The geology of the Mount Lyell mines area, Tasmania : a re-interpretation based on studies at Lyell Comstock, North Lyell and the Iron Blow area. Research Master thesis, University of Tasmania.

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Abstract

This study is based mainly on detailed mapping of the Mt Lyell mine lease, particularly the
Lyell Comstock, North Lyell and Iron Blow areas, with the aim of clarifying the relationship
between the volcanic schists and the Owen Group conglomerate sequence, determining the
general nature of the alteration zone and the setting of the various orebodies within the zone,
and reconstructing the geological history of the area.
The upper part of the Mt Lyell system is preserved at Lyell Comstock, where the alteration
zone cross-cuts the upper andesitic unit of the Central Volcanic Complex and culminates in the
basal unit of the overlying Tyndall Group. An exhalative zone is present in the Lower Tyndall,
consisting of small lenses of massive lead-zinc ore associated with breccias containing clasts of
chert and sulphide, and showing strong sericite-pyrite alteration at the base. Also present are
lenses of limestone, many containing abundant fossil fragments of late Middle Cambrian age.
Alteration and mineralisation die out rapidly up section, and the volcaniclastic rocks of the
Middle and Upper Tyndall Group clearly post-date the alteration, providing unequivocal
evidence of a Cambrian age for the system.
Within the upper part of the alteration zone at Comstock are numerous bodies of pale cherty
silica ('silica heads'), wrapped around by sericite-pyrite schist. The chert bodies culminate in
the 300 m wide mass of the Comstock chert body which caps the system. This huge mass of
chert, up to 200 m thick and extending at least 600 m down dip, has discordant lateral
margins, and, like the other chert bodies, is largely of replacement origin. Numerous veins and
masses of bright red hematite- jasper- barite material cut the chert body, indicating a major
period of oxidation. The presence of clasts of this distinctive red vein material in the Middle
Owen Conglomerate along the Great Lyell Fault scarp, together with abundant chert detritus,
provides clear evidence that the chert-bearing part of the alteration zone had been uplifted,
veined with hematite, and exposed to erosion by Middle Owen time in the Late Cambrian. The
Comstock ore lenses of pyrite-chalcopyrite with minor bornite are located in the footwall
position of the Comstock chert body.
At North Lye!!, a large displaced mass of schists, with many chert bodies, extends some 500 m
into the margin of the Owen basin, and appears to have collapsed eastwards from the scarp of
the Great Lyell Fault. Other masses of less altered schist, partly connected to the main schist
belt to the west, and to the North Lyell mass, lie within the `Tharsis Trough' along the basin
margin between North Lyell and the Iron Blow mine, obscuring the surface expression of the
Great Lyell Fault. Several remnants of the sole of these collapsed masses are exposed resting
directly on upturned beds of conglomerate and sandstone along the Tharsis Ridge- Razorback
Ridge 'shoulder' structure. A large mass of pale chert, identical to the Comstock chert, lies at
the schist-Owen contact at the eastern margin of the schist mass at North Lyell, and appears to
represent part of the cap-like chert zone at the top of the alteration system. It has been
misinterpreted, however, as silicified Owen beds, leading to a widely-held misconception of
post-Owen (i.e. Devonian) silicification and associated bornite mineralisation, since the
bornite-rich North Lyell orebody lies along the footwall of the chert. This study clarifies that
critical relationship.
Bodies of semi-massive hematite, closely associated with hematite-chert-rich breccias, are
developed along the schist-Owen contact from North Lyell to the Iron Blow. These bodies,
and the associated breccias, interdigitate with the Owen sediments, indicating that they were
formed during deposition of the Middle and Upper Owen beds. The hematite alteration
extends below the bodies into the underlying schist, indicating that the schist mass was
exposed at the surface, and that it was oxidised and eroded as it collapsed or rolled into place.
The abrupt change from the coarse fluvial Lower Owen to the red, hematite-rich, shallow
marine-deltaic facies of the Middle Owen, with its abundant volcanic-derived chert and
hematite clasts, appears to coincide with the exposure and erosion of the schist mass. This
mass contained abundant pyrite and other sulphides, and its rapid exposure at surface (possibly
while still hot) appears to have resulted in intense oxidation to produce large quantities of
hematite and barite, much of which was deposited as clasts in the Owen sediments. Similarly,
much of the cherty alteration material now appears as a widespread and abundant elastic
component in the Middle and Upper Owen beds.
A 100 m-wide zone of upturning and folding of Upper Owen sandstone beds occurs along the
schist-Upper Owen contact at the eastern margin of the schist masses. The Haulage
Unconfonnity is developed where the younger Pioneer Sandstone, of probable Middle
Ordovician age, truncates these folded beds as it transgresses across them to rest, in a few
places, on the schist mass. The folds lack cleavage and were apparently formed when the beds
were only semi-consolidated. The folded zone can be attributed to a further advance of the
schist mass against the Owen contact some time in the early Ordovician.
Recognition of the North Lyell schist mass as a section from the chert-rich upper part of the
alteration zone, closer to the geographic centre of the overall system than Comstock (which is
at the northern margin), allows some reconstruction of the major elements of the Mt Lyell
system. Within the overall alteration zone was a core zone, 200-500 m wide, of pyrite-rich
sericite-chlorite schists, with an upper section of 500 m or so dominated by cherty silica bodies
up to 50 m across. This culminated near the top in a cap-like zone of chert masses up to 200 m
thick, with bornite-rich orebodies located just beneath the cap over the central part of the
system. The presence of pyrophyllite and other indicator minerals in this upper zone indicates
that 'high-sulphidation' conditions may have applied during bornite deposition.
Small lead-zinc massive sulphide bodies were formed in the exhalative zone towards the lateral
margins of the system, as at Comstock, but in the central parts of the system it appears that
massive pyrite-chalcopyrite bodies were more typical, as exemplified by the Iron Blow
sulphide body. This body also lies against the Upper Owen contact in a schist collapse zone,
and has a gossan-like Cambrian hematite mass developed on it, indicating its exposure at
surface during Owen time. Slightly deeper in the system, within the zone of silica heads, are
deposits dominated by disseminated pyrite-chalcopyrite (e.g. Crown Lyell 3) or with a small
amount of bornite (e.g. Western Tharsis, Comstock), and deeper still, below the chert-rich
zone, are the large, low-grade, disseminated orebodies such as Prince Lyell.
There is clear evidence at Mt Lyell that a large part of the alteration system was uplifted,
exposed, and eroded during Owen Group deposition, including much of the chert-rich (and
ore-rich) upper part. The cover of Tyndall Group rocks must have been stripped off during
deposition of the thick Lower Owen sequence, and the alteration zone was exposed, oxidised,
and eroded, and large sections of it collapsed into the Owen basin during Middle and Upper
Owen time. A possible explanation for the cupwelling' of the schist mass could be that there
was significant volume increase associated with the large-scale hydrothermal alteration,
particularly the hydration of feldspars to sericite and of ferromagnesian minerals to chlorite, as
noted by Edwards (1939). Further chemical and mass balance studies are required to test this,
however.

Item Type: Thesis (Research Master)
Keywords: Geology
Copyright Holders: The Author
Copyright Information:

Copyright 2001 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:

Includes chart in back pocket. Thesis (M.Sc.)--University of Tasmania, 2001. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:50
Last Modified: 11 Mar 2016 05:54
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