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Genesis and structural architecture of the CSA Cu-Ag (P-Zn) Mine, Cobar, New South Wales

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Kyne, R (2014) Genesis and structural architecture of the CSA Cu-Ag (P-Zn) Mine, Cobar, New South Wales. PhD thesis, University of Tasmania.

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Abstract

The CSA Cu-Ag-Pb-Zn deposit, one of the Cobar-style deposits, is located 11km NNW of
Cobar in central New South Wales within an area known as the Cobar Mining Field. Historically
mined for Pb-Zn, CSA currently recovers Cu-Ag and has a resource base (measured, indicated and
inferred) of 11.4 mt at 6% Cu and 22 g/t Ag. While much work has been done on the Cobar-style
deposits, there remains a variety of questions regarding the formation of each deposit and the mining
field as a whole. Historically, structural studies of the CSA deposit have been restricted to the surface
and upper levels of the mine (>9800m RL). Recent mining within the deposit has allowed for further
structural analysis up to 1.3 km. A structural and mineralogical study of the CSA mine was undertaken
to answer a variety of questions concerning the formation of the deposit, its key characteristics and
features (structurally, mineralogically and geochemically) and how this information could be used to
explore for similar styles of mineralization both on site and in the Cobar region.
Hosted in the Devonian CSA Siltstone, the deposit resides within the hanging-wall of the
steep, west-dipping Cobar Fault. Major ore minerals include acanthite, chalcopyrite, cubanite,
pyrrhotite, sphalerite, galena, pyrite, magnetite, and native bismuth. Gangue minerals include quartz,
chlorite, calcite, stilpnomelane, plagioclase, biotite, muscovite and talc. The most common alteration
styles are Fe-rich chlorite, Fe-Mg-rich chlorite and silicification.
Structural measurements and samples were collected from 45 drill holes across four major ore
zones known as the Western, Eastern, QTS North and QTS South systems. Each system is composed
of multiple lenses ranging in strike length (13-200 m), width (5-80 m), and vertical extent (200 m-1.2
km). Structural analysis highlighted the presence of two major cleavage groups, one corresponding to
the regional cleavage (S2) and another with varying orientations found locally deemed SX to avoid
timing implications. A subset of SX was found to represent an early cleavage and was deemed S1. The
orientation of S2 changes from 80°/090° at surface and the upper portions of the mine (>92000mRL)
to 85°/264 (< 9000mRL) with depth and proximity to the Cobar Fault suggesting drag in the hangingwall
of the fault as it rotated in response to EW compression. Stereographic projections of S2 and
S2∧S0 intersection lineations show that S2 transects the regional folding produced early in the EW
compression. The presence of a stretching lineation with down-dip orientation suggests the lineation
was formed during dip-slip regime with very little transpression. Two cross-cutting fault systems, one
sub-parallel with S2 and the other sub-parallel with S1 were identified and suggested the presence of
orthorhombic fault arrays. Dilation occurred along the intersections of these faults allowing
mineralizing ore fluids to be focused into pipe-like ore lenses.
A 3D model of the regional stress state was produced using the boundary elements method
(Poly3D) in order to ascertain the affect the regional Plug Tank and Cobar Faults had on the formation
and spatial location of the CSA deposit. The model showed that, when activated by EW compression,
the geometries of these two faults produced a zone of minimum ơ3 directly beneath the deposit and
caused dilation in the overlying orthorhombic fault arrays. Deep seated ore-forming fluids were driven
along the Plug Tank Fault towards the eastern margin of the basin and up the Cobar Fault into this
zone of dilation during compression. Ore-forming fluids were then focused along the intersections of
the orthorhombic fault arrays producing the pipe-like ore lenses.
Geochemical analysis suggests that enrichments of Se, Cd, Fe, Mn, Sn, Tl, and Ge and
depletions of Ba, K Na, and Rb occur within 100 m of ore lenses and can be used as vectors to ore.
The most useful distal vector to ore was determined to be the depletion in Sr and Na occuring up to
500 m from mineralization. Fe-Mg-rich chlorite alteration is recognizable within 10 m of ore lenses.
Ore forming fluids are most likely sourced from the deepest part of the Cobar Basin during
inversion. The amount of Cu and the presence of abnormally high fluorine within CSA suggest that the source of the ore-forming fluids was most likely a basal unit in the rift package containing basalts
and carbonates and/or rhyolites. Once the fluids sourced from the rift package travelled along the Plug
Tank and Cobar faults into the orthorhombic fault arrays, they mixed with a cooler, locally sourced
fluid already circulating through the faults causing precipitation of the Cu-Ag-Pb-Zn minerals.
Overall, this study concluded that the CSA deposit was formed via the interaction of far field
stresses acting on regional faults to focus ore-forming fluids into the hanging-wall of the Cobar Fault.
This study suggests that exploration done both on the mine site and within the region should include
analysis of regional folding and faulting patterns as well as conventional geophysical and geochemical
methods.

Item Type: Thesis (PhD)
Keywords: Structural geology, geochemistry, CSA mine, copper, Cobar, faulting
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Date Deposited: 17 Nov 2014 01:16
Last Modified: 11 Mar 2016 05:52
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