The Wirlong prospect is located on the limb of a broad sandstone-dominated anticlinorium. In outcrop there is a well-developed axial plane foliation, evidence of shearing and chlorite alteration, N-S trending massive bucky quartz veins and quartz stock work veining. These characteristics are consistent with a high strain, structurally complicated environment believed to be consistent with a heterogeneous strain regime suitable for creating dilational sites for mineral precipitation.
The constituent formations which construct the Wirlong prospect include the Shume and Shuttleton Volcanic Formations:
The Shume Formation (Upper Amphitheatre Group)
The Shume Formation at Wirlong is characterised by massive to parallel-laminated, lesser ripple cross-laminated and minor graded, moderate to well sorted, medium to fine-grained, lithic-quartz to quartz-rich sandstone interbedded with siltstone; minor rhyolite and rhyodacite. The Fmn does not represent “classic” turbidite facies, it is more analogous to a high-density turbidity current (Lowe sequence) depositional setting.
The Shuttleton Volcanic Formation
The Shuttleton Volcanic Formation at Wirlong is defined by massive to flow-banded, spherulitic, vesicular and mottled, rhyolite and rhyodacite lavas with breccias (lava clasts), marine ‘igniturbidite’ ashflow tuffs, volcanic and lithic sandstone, conglomerate, siltstone and shales. The Fmn is approximately 300m thick and overlain by the Shume Fmn. It accommodates abundant discontinuous lenses of sedimentary rocks occur throughout the volcaniclastics, and sparse, discontinuous lenses of volcaniclastics occur within the Shume Formation. The units represent an intermixing of high energy turbidites derived from the east of the basin with repeated volcaniclastic-rich mass flow units from the west of the basin.
It has since proven to represent a very large hydrothermal system hosting significant copper mineralisation along more than 2.5km strike length and to depths of up to 950m. To date some of the better copper intercepts returned from the Wirlong prospect include: • 9m @ 3.29% Cu, 18 g/t Ag from 70m in WLRC035 • 27m @ 5.3% Cu, 23 g/t Ag from 286m in WLRC026 • 31m @ 3.19% Cu, 11 g/t Ag from 299m in WLRC052 • 9m @ 8% Cu, 17g/t Ag, 0.21 g/t Au from 616m in WLDD001 • 17m @ 4.59% Cu, 8 g/t Ag from 738m in WLRCDD043
Wirlong is considered a volcanic-hosted massive sulphide (VHMS) / volcanic-associated massive sulphide (VAMS) deposit, which has undergone massive high-strain deformation, resulting in a fertile, structurally controlled dilational environment, effectuating the generation of the multiple, stacked, NW-SE orientated, high grade copper lenses recognised (“Cobar style” mineralisation).
Mineralisation occurs as chalcopyrite dominant, lesser pyrite and localised sphalerite and galena within stringer veins. In 2018, Sunao Mochizuki from JOGMEC, conducted a structural analysis of the Copper mineralisation from core samples originating out of Wirlong. It was concluded that the main copper mineralised zone is strongly controlled to NWW-SEE sinistral shears (R-shear), and NE-SW conjugate dextral shear (R’).