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Contrasting mineralisation and alteration styles and a possible alkalic lithocap at the Cadia East porphyry Au-Cu deposit, NSW, Australia


Fox, N and Harris, A and Cooke, D and Collett, D, Contrasting mineralisation and alteration styles and a possible alkalic lithocap at the Cadia East porphyry Au-Cu deposit, NSW, Australia, 33rd International Geological Congress Programme, 6-14 August, Oslo, Norway (2008) [Conference Extract]

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Au-Cu mineralisation at Cadia East is directly associated with alkalic porphyritic monzonite dykes intruded into the Forest Reefs Volcanics, an Ordovician volcanic and volcaniclastic succession, within the Lachlan Fold Belt of Eastern Australia. Resource development indicates that over 20 million ounces of gold is contained in two mineralised domains - a high-grade, sheeted vein array at depth and a shallow disseminated mineralisation zone at shallower levels.

Multiple sheeted quartz-carbonate-bornite-chalcopyrite molybdenum vein arrays characterise the high grade zone (720 Mt @ 0.7g/t Au), with associated biotite-magnetite-orthoclase alteration assemblages occurring dominantly as vein selvages. The broadly continuous NW-SE to E-W strike of these sheeted veins across the deposit suggests a tectonic influence on vein development. Quartz monzonite dikes intruding the Ordovician Forest Reefs Volcanics have a similar trend to these sheeted vein arrays and appear to be concurrent with mineralisation.

Disseminated low grade mineralisation (430Mt @ 0.4 g/t Au) is preferentially developed in volcanic conglomerates of the Forest Reefs Volcanics in the upper sections of the deposit. This mineralisation is associated with a high level, broadly stratabound, texturally destructive quartz - K-feldspar - albite - tourmaline - sericite alteration. This alteration has been suggested to represent the alkalic equivalent of lithocaps which overlie some calc-alkaline porphyry Cu deposits.

Previous isotopic studies at Cadia East have precluded externally derived fluids in the hydrothermal system, solely implicating magmatic fluids for the observed mineralisation and alteration zonation. However, externally derived brines have been shown to have been involved in the formation of sodic alteration in the porphyry environment at the Yerington system, Nevada, and a comparison between the two systems suggests that there may have also been an external fluid involved in the formation of the upper alteration domain at Cadia East. This distinctive, broadly stratabound alteration zone has not been previously documented in deposits of this type and may represent a potential exploration vector for this mineralisation style.

Item Details

Item Type:Conference Extract
Keywords:porphyry, alkalic, Cadia, mineralisation
Research Division:Earth Sciences
Research Group:Geology
Research Field:Resource geoscience
Objective Division:Mineral Resources (Excl. Energy Resources)
Objective Group:Mineral exploration
Objective Field:Precious (noble) metal ore exploration
UTAS Author:Fox, N (Dr Nathan Fox)
UTAS Author:Harris, A (Dr Anthony Harris)
UTAS Author:Cooke, D (Professor David Cooke)
ID Code:100615
Year Published:2008
Deposited By:Centre for Ore Deposit Research - CODES CoE
Deposited On:2015-05-22
Last Modified:2015-05-22
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