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

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.