The Cobar Superbasin is one of several intracratonic basins developed within the Lachlan Orogen during the Silurian/Devonian; it is the richest polymetallic basin in the Lachlan Orogen as evidenced by estimated pre-mining metal inventories: >2.5 million tonnes copper, >200 tonnes of gold, >4.8 million tonnes of zinc, >2.8 million tonnes of lead, and >4,000 tonnes of silver2.
Peel believes that the prospectivity of the southern portion of the Cobar Superbasin (the area covered by Peel Mining’s tenements) is extremely high, factoring in the presence of metal-bearing fluids and high strain domains which favour mineral deposits and occurrences; this is supported by the presence of major deposits/mines in the area such as Nymagee, Hera, Federation, Wirlong, May Day, Mallee Bull, Mt Hope and Southern Nights-Wagga Tank.
Wirlong is located within Peel’s 100%-owned EL8307 and EL8126. Peel was initially drawn to the area by the presence of historic copper workings, a topographic high, a >2km multi-element surface geochemical anomaly and coincident or semi-coincident magnetic (>3km), radiometric, gravity, IP and electromagnetic anomalies. Exploratory drilling at Wirlong in early 2015 identified significant primary copper mineralisation with follow-up drilling in late 2015 yielding strong copper mineralisation and confirming the potential of the prospect.
The deposit is located approximately 1.5km north of the historic Wirlong workings and is hosted in a N-S striking, moderate to steeply west dipping (~65 degrees) sequence of interfingering sediments and volcanics that exhibit open to locally tight parasitic folds. The sediments comprise massive to locally laminated quartz-rich sandstones and interbedded shale, siltstone and sandstone turbidites of the Shume Formation. The volcanics comprise massive, porphyritic, flow-banded and auto-brecciated rhyolite of the Shuttleton Rhyolite Member. Soft sediment deformation, intense hydrothermal alteration, local hyaloclastites and peperitic margins are indicative of the rhyolite being emplaced as a very high-level sill into wet, semi-consolidated sediments. Age dating of the Shuttleton Rhyolite Member by Walternberg et al (2016) suggests an age of 421.9 +/- 2.7Ma for the rhyolite which is consistent with the sequence forming during late Silurian to Early Devonian rifting related to the formation of the Cobar Super Basin. Field observations and younging data from drill core have delineated a westward-younging sequence that is bound to the east by the major N-S striking Woorara Fault.
The sequence at Wirlong is overprinted by a strong, consistently N-S striking foliation that dominantly dips steeply to the east, rotating to steeply west where proximal to the Woorara Fault. Evidence from bedding and younging data shows that the west dipping sequence of sediments and volcanics is parasitically folded, dominated by open to locally tight asymmetric style folds.
Peel believes the deposit is hosted within a NW-SE striking fault known locally as the John Owen Fault. The John Owen Fault is observed at the surface by local gossanous float and as a series of isolated, massive quartz veins that range in thickness from <1m to over 10m. The fault has been mapped at the surface for a current strike length of over 1.3km and forms a conjugate structure set with a NE-SW striking fault that passes through the historic Wirlong shafts. Peel hypothesizes the faults formed during near E-W compression during basin inversion that gave rise to conjugate fault sets that are consistent with a Riedel shear model.
Mineralisation at Wirlong is hosted in all lithologies and comprises massive to semi-massive breccia-fill and vein hosted chalcopyrite-pyrrhotite-pyrite (+/- arsenopyrite, sphalerite, galena). In the oxide zone azurite, malachite, chalcocite and smithsonite are locally abundant. Intense Fe-Mg chlorite alteration is spatially associated with chalcopyrite-rich mineralisation. Structural analysis indicates that movement along the John Owen Fault created local zones of dilation that facilitated the movement of mineralising fluids into pre-existing structures and fabrics such as faults, fractures and foliation.
Mineralisation forms a series of stacked, en-echelon style lenses that are hosted within and proximal to the John Owen Fault. The en-echelon style lenses increase in grade towards the east of the deposit which is theorized to reflect an increase in the interaction between the NW-SE striking John Owen Fault and the regional N-S striking Woorara Fault to the east. Mineralization remains open in all directions with WLDD040 (205m @ 1.4% Cu) drilled on the eastern-most section.
The style of mineralisation, alteration and structural setting are considered analogous to that observed at the world class CSA Mine in Cobar.
2 – See Vladimir David – “Cobar Deposits – Structural Control” (2018)Other References: Waltenberg, K., Blevin, P. L., Bull, K. F., Cronin, D. E., and Armistead, S. E. 2016. New SHRIMP U-Pb Zircon ages from the Lachlan Orogen and the New England Orogen, New South Wales