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• W7050690 abstract "The 1.5 Moz New Occidental gold deposit is one of a number of Devonian faulthostedgold-base metal deposits along the eastern margin of the Devonian CobarBasin in the central belt of the Lachlan Orogen, western New South Wales. Cobardeposits do not easily conform to conventional ore deposit classifications, exhibitingcharacteristics of orogenic gold, intrusion-related, volcanic-hosted massive sulfideand epithermal gold deposits. This study has focussed on defining the relationshipbetween formation of the New Occidental deposit and the structural evolution of theeastern margin of the Cobar Basin and characterising the composition and physicalcharacteristics of hydrothermal fluids associated with deposit formation. The researchhas resolved many of the enigmatic aspects of Cobar deposits, including (i)mechanisms of deposit formation, (ii) the origins of their poly-metallic character anddistinctive metal zoning evident at both the district scale and within individualdeposits, and (iii) provides important new constraints on the source(s) of metals andsulfur in the deposits.The New Occidental deposit is a steeply dipping pipe-shaped ore body with a strikelength of 150-200m, width of 10-30m and vertical extent of more than 1200m. It ishosted within the Great Chesney Fault, a major north-northwest trending sub-verticalfault on the western flank of the south-plunging Chesney-Narri Anticline. Foldingand 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 orientationofregional stresses. Variation in the plunge and inter-limb angle of parasitic folds inthe vicinity of the deposit (previously interpreted to indicate polyphase deformation),is here shown to reflect progressive fold development (with greater tightening andreorientation of earlier formed folds) in Dr high-strain zones.Analysis of fault zone kinematics and detailed stratigraphic reconstruction of theeastern Cobar Basin margin has resolved movement vectors on the Great ChesneyFuult. Reverse dip-slip displacement on the fault (parallel to the prominent subvertical 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 significantsyn-mineralisation strike-slip displacement on the fault, although many previousworkers have postulated this as a means of explaining the sub-vertical pipe-likegeometry of the orebody. The steep pitch of both the New Occidental deposit and ofhigh-grade shoots within it (both parallel to the stretching lineation), are attributed tolarge-scale reverse movement on the fault, with vertical extent of the deposit andshoots a function of the amount ofreverse displacement on the host fault.Five texturally distinct stages of quartz veins and breccias have been recognisedwithin the New Occidental deposit (Table 1). During Stages 1-4, successive stages ofquartz veining and brecciation were localised along the footwall contact of theprevious vein stage, resulting in an asymmetric distribution of the zones of maximumStage 1 to 4 vein density from hanging wall to footwall across the deposit. Thiszonation is consistent over the full strike length (~200m) and known vertical extent(~1200m) of the deposit, reflecting the tendency for brecciation and dilation to belocalised along rock mass boundaries where there is greatest competency contrast;principally the contact between the previously deposited vein arrays and more ductilesiltstone/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. Variablydeformed quartz fault-fill veins are cut at high angles by arrays of sub-horizontalextension 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 veinarrays indicates substantial fault-induced dilation accompanied displacement on thehost Great Chesney Fault at the site of the New Occidental deposit. Similar featuresare not developed immediately along strike from the deposit, indicating it occupies alimited strike-length dilational jog formed progressively during the main phase ofreverse displacement on the Great Chesney Fault system. Stage 2-3 veins exhibit themost intense implosive breccia textures, suggesting the bulk of dilation occurredduring deposition of Stage 2-3 quartz veins. Stage 4 veins record a transition frompredominantly brittle deformation (implosive brecciation and open-space vein fill) toa mixture of brittle and brittle-ductile deformation (e.g. crack-seal vein growth andcataclastic shear), indicating less dilation of the fault zone (and/or lower fluid fluxes)during displacement. A progressive decrease in the intensity of folds overprintingStage 1-4 veins indicates significant fault-perpendicular shortening was broadlycontemporaneous with fault displacement and vein development, with the greatestshortening occurring during deposition of Stage 2-3 veins. Significant displacementon 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-fillquartz veins and breccias, the latter characterised by vein textures indicating rapidlyfluctuating physiochemical conditions during vein formation ( colloform- andcrustiform-banding), indicates fluid pressures alternated between supra-lithostatic andnear hydrostatic values (i.e. extreme fault-valve behaviour) during slip on the GreatChesney Fault. Continued east-west shortening at the eastern margin of the CobarBasin 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°) tothe maximum principal stress direction, supra-lithostatic fluid pressures were requiredto initiate slip on the fault. Additional criteria for fault-valve behaviour are alsosatisfied; 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 haveformed a low permeability seal that limited the discharge of over-pressured fluidsfrom depth." @default.
• W7050690 created "2016-06-24" @default.
• W7050690 creator A5059038247 @default.
• W7050690 date "2007-01-01" @default.
• W7050690 modified "2023-09-26" @default.
• W7050690 title "Structural and geochemical controls on ore formation at the New Occidental gold deposit, Cobar, New South Wales, Australia" @default.
• W7050690 hasPublicationYear "2007" @default.
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