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Geology and genesis of the Ridgeway porphyry Au-Cu deposit, NSW

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Garcia-Cuison, AL 2010 , 'Geology and genesis of the Ridgeway porphyry Au-Cu deposit, NSW', Research Master thesis, University of Tasmania.

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Abstract

The Ridgeway alkalic porphyry Au–Cu deposit is located in the Molong
Volcanic Belt of the Macquarie Arc, part of the Lachlan Fold Belt in eastern
Australia. Ridgeway is hosted by a Middle Ordovician sequence of volcanosedimentary
rocks that were deposited in an active submarine sedimentary basin. The
volcanic sequence evolved from fine grained distal volcaniclastics (Weemalla
Formation) to coarse proximal breccias and sandstones (Forest Reef Volcanics). The
host sequence was intruded first by pyroxene- and feldspar-phyric dikes and sills,
and then by the Ridgeway intrusive complex, a cluster of subvertical porphyry pipes,
dikes and stocks of monzodiorite (U–Pbzircon age: 448.2 ± 2.1 Ma), mafic monzonite
and quartz monzonite (U–Pbzircon age: 444.2 ± 1.3 Ma). Cross-sectional shapes of the
monzonite intrusions are broadly pipe-like. They swell at the contact between the
Weemalla Formation and Forest Reef Volcanics.
The highest Au–Cu grades at Ridgeway are associated with quartz–
magnetite–bornite vein stockworks and intense K-silicate alteration that formed
during the emplacement of the mafic monzonite. These early stage veins have been
truncated by the quartz monzonite porphyry and its lower grade quartz–chalcopyrite ±
molybdenite veins (Re–Osmolybdenite: 445.7 ± 2.8 Ma, 442.8 ± 2.3 Ma) associated with
less intense K-silicate alteration. A late-stage quartz monzonite cut the earlier phases,
and defines the low-grade core of the deposit.
The mafic monzonite and quartz monzonite contain magnetite- and quartzrich
unidirectional solidification textures (USTs), miarolitic cavities and aplite veindikes.
These textural features imply that magmatic-hydrothermal fluids streamed
through and accumulated within the narrow pipes, which acted as a conduit to supply
fluids from a deeper magma to the site of ore deposition. Mineralizing fluids were
released when the carapace of the crystallizing fluids failed, and were emplaced
preferentially into two subvertical vein systems that formed via hydraulic fracturing. The older veins (set 2) strike N, WNW and NE, whereas the younger mineralized
structures (set 3) strike E, NE and NW.
Cathodoluminescence imaging of quartz shows that most of the quartz (Qz-1)
crystallized early in the history of vein formation. Dissolution of Qz-1 was then
followed by the deposition of a second quartz generation (Qz-2). Cu–Fe sulfides were
then deposited together with a later generation of darkly-luminescent quartz (Qz-3).
Bright luminescence in Qz-1 correlates with elevated Al, Ti and K concentrations,
whereas dull-luminescent Qz-3 is comparatively rich in Fe. High-temperature Qz-1
precipitated during vein stockwork formation at temperatures between 601º and
850ºC in equilibrium with hydrothermal K-feldspar. Changes in pressure and
temperature occurred during mechanical fracturing that created secondary
permeability exploited by Qz-2. Further decrease in pressure and/or temperature
facilitated the precipitation of Qz-3 at temperatures below 589ºC, synchronous with
Fe–Cu sulfides.
Sulfur isotopic compositions of sulfides from Ridgeway show increasing δ34S
depletion in the sequence of pyrite (ave. -1.8 ‰), chalcopyrite (ave. -3.6 ‰) and
bornite (ave. -4.9 ‰). Low δ34Sbornite and δ34Schalcopyrite values occur in the core of the
deposit. Isotopically light δ34Spyrite values are also found in the core of the deposit, but
these become more negative towards the top of the deposit, in the epidote–chlorite–
hematite alteration zone. This is consistent with isotopic fractionation caused by
cooling of the magmatic-hydrothermal fluids under oxidizing conditions during latestage
pyrite deposition.
The Ridgeway deposit was localized at the intersection of NW-trending faults
and a NNW-trending monocline. The pre-existing NW-trending parallel wedge-shape
faults provided the pathways for the deep-seated magma to migrate into the shallow
crust. Roof-lifting within the fault wedge provided space for monzonite emplacement.
At Ridgeway, there was an intimate link between magmatism and a dynamic
structural environment that ultimately controlled the genesis of the high-grade
orebody and provided an excellent focus for fluid flow in a well-developed vein
stockwork.

Item Type: Thesis - Research Master
Authors/Creators:Garcia-Cuison, AL
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