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Structural and geochemical controls on ore formation at the New Occidental gold deposit, Cobar, New South Wales, Australia

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Stegman, CL (2007) Structural and geochemical controls on ore formation at the New Occidental gold deposit, Cobar, New South Wales, Australia. PhD thesis, University of Tasmania.

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Abstract

The 1.5 Moz New Occidental gold deposit is one of a number of Devonian faulthosted
gold-base metal deposits along the eastern margin of the Devonian Cobar
Basin in the central belt of the Lachlan Orogen, western New South Wales. Cobar
deposits do not easily conform to conventional ore deposit classifications, exhibiting
characteristics of orogenic gold, intrusion-related, volcanic-hosted massive sulfide
and epithermal gold deposits. This study has focussed on defining the relationship
between formation of the New Occidental deposit and the structural evolution of the
eastern margin of the Cobar Basin and characterising the composition and physical
characteristics of hydrothermal fluids associated with deposit formation. The research
has resolved many of the enigmatic aspects of Cobar deposits, including (i)
mechanisms of deposit formation, (ii) the origins of their poly-metallic character and
distinctive metal zoning evident at both the district scale and within individual
deposits, and (iii) provides important new constraints on the source(s) of metals and
sulfur in the deposits.
The New Occidental deposit is a steeply dipping pipe-shaped ore body with a strike
length of 150-200m, width of 10-30m and vertical extent of more than 1200m. It is
hosted within the Great Chesney Fault, a major north-northwest trending sub-vertical
fault on the western flank of the south-plunging Chesney-Narri Anticline. Folding
and faulting are interpreted to be the products of a single phase of deformation (D 1)
during the Early Devonian, which was characterised by relatively constant orientation
ofregional stresses. Variation in the plunge and inter-limb angle of parasitic folds in
the vicinity of the deposit (previously interpreted to indicate polyphase deformation),
is here shown to reflect progressive fold development (with greater tightening and
reorientation of earlier formed folds) in Dr high-strain zones.
Analysis of fault zone kinematics and detailed stratigraphic reconstruction of the
eastern Cobar Basin margin has resolved movement vectors on the Great Chesney
Fuult. Reverse dip-slip displacement on the fault (parallel to the prominent sub
vertical stretching lineation on cleavage) is in the order of 1.5-2.5 km (east-block up),
based on stratigraphic offsets across the fault. There is no evidence for significant
syn-mineralisation strike-slip displacement on the fault, although many previous
workers have postulated this as a means of explaining the sub-vertical pipe-like
geometry of the orebody. The steep pitch of both the New Occidental deposit and of
high-grade shoots within it (both parallel to the stretching lineation), are attributed to
large-scale reverse movement on the fault, with vertical extent of the deposit and
shoots a function of the amount ofreverse displacement on the host fault.
Five texturally distinct stages of quartz veins and breccias have been recognised
within the New Occidental deposit (Table 1). During Stages 1-4, successive stages of
quartz veining and brecciation were localised along the footwall contact of the
previous vein stage, resulting in an asymmetric distribution of the zones of maximum
Stage 1 to 4 vein density from hanging wall to footwall across the deposit. This
zonation is consistent over the full strike length (~200m) and known vertical extent
(~1200m) of the deposit, reflecting the tendency for brecciation and dilation to be
localised along rock mass boundaries where there is greatest competency contrast;
principally the contact between the previously deposited vein arrays and more ductile
siltstone/mudstone footwall.
Each of the five vein stages record cyclic episodes of sub-horizontal extensional
:fracturing followed by fault-rupture and slip on steeply dipping fault planes. Variably
deformed quartz fault-fill veins are cut at high angles by arrays of sub-horizontal
extension veins, which are in tum truncated by younger generations of fault-fill veins.
The predominance of open-space filling vein and breccia textures in Stage 1-4 vein
arrays indicates substantial fault-induced dilation accompanied displacement on the
host Great Chesney Fault at the site of the New Occidental deposit. Similar features
are not developed immediately along strike from the deposit, indicating it occupies a
limited strike-length dilational jog formed progressively during the main phase of
reverse displacement on the Great Chesney Fault system. Stage 2-3 veins exhibit the
most intense implosive breccia textures, suggesting the bulk of dilation occurred
during deposition of Stage 2-3 quartz veins. Stage 4 veins record a transition from
predominantly brittle deformation (implosive brecciation and open-space vein fill) to
a mixture of brittle and brittle-ductile deformation (e.g. crack-seal vein growth and
cataclastic shear), indicating less dilation of the fault zone (and/or lower fluid fluxes)
during displacement. A progressive decrease in the intensity of folds overprinting
Stage 1-4 veins indicates significant fault-perpendicular shortening was broadly
contemporaneous with fault displacement and vein development, with the greatest
shortening occurring during deposition of Stage 2-3 veins. Significant displacement
on the fault had largely ceased prior to deposition of Stage 5 veins.
The intimate association of sub-horizontal extension veins with each stage of fault-fill
quartz veins and breccias, the latter characterised by vein textures indicating rapidly
fluctuating physiochemical conditions during vein formation ( colloform- and
crustiform-banding), indicates fluid pressures alternated between supra-lithostatic and
near hydrostatic values (i.e. extreme fault-valve behaviour) during slip on the Great
Chesney Fault. Continued east-west shortening at the eastern margin of the Cobar
Basin is interpreted to have rotated the fault from moderate dips (perhaps <60-70°
initially) to progressively steeper dips. Once oriented at very high angles (>75°) to
the maximum principal stress direction, supra-lithostatic fluid pressures were required
to initiate slip on the fault. Additional criteria for fault-valve behaviour are also
satisfied; specifically (i) the depth of deposit formation is of the order of5-7 km, (ii)
no more favourably oriented faults occur in the vicinity of the Great Chesney Fault,
and (iii) upper basin siltstones and mudstones overlying the deposit appear to have
formed a low permeability seal that limited the discharge of over-pressured fluids
from depth.

Item Type: Thesis (PhD)
Keywords: Gold ores, Gold mines and mining, Geology, Structural, Ore deposits
Copyright Holders: The Author
Copyright Information:

Copyright 2007 the Author

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2007. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:25
Last Modified: 11 Mar 2016 05:54
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