Koongie Park Geology
The massive sulphide deposits of Koongie Park have been traditionally classified as volcanogenic massive sulphide (VMS) deposits. The Onedin deposit has been referred to as a carbonate associated VMS. Based on the assumption there are no exhalative rocks at Onedin, it is proposed that the best model for the base metal occurrence is as a sub-horizontal basin floor replacement VMS. Other genetic models proposed include: distal skarn and structurally controlled replacement during deformation within high strain zones. CSA Global considers the weight of evidence supports their interpretation as VMS deposits. As a result, the deposits are interpreted to have been formed around the time of deposition of the host volcanic and sedimentary strata in which they are bound and generally in bedding parallel lenses. Hydrothermal fluids associated with volcanic activity is interpreted to have been the source of the metals and other constituents of the mineralisation.
CSA Global place the Koongie Park deposits in the bimodal-felsic sub-type of VMS deposit - in the host-rock lithology classification of Barrie and Hannington (1999). The bimodal-felsic types are common in Phanerozoic, typically occur in sequences with >50% felsic volcanic rocks, <35% mafic volcanic/intrusive rocks, and <15% siliciclastic rocks. Examples include the VMS deposits in the Honolulu District of Japan, and the Rosebery district in Tasmania.
The mineralogy of the primary mineralisation at Sandiego is pyrite-sphalerite-pyrrhotite-chalcopyrite +/- galena which is largely hosted in the magnetite-rich exhalative suite of rocks where it occurs as a massive conformable wedge-shaped lens 200 m in length with a maximum thickness of 75 m. In general, the sulphides exhibit replacement textures and show evidence of mobilisation, which is a result of deformation and metamorphism subsequent to initial formation. Weak to moderate sulphide vein and stringer mineralisation occur at the base of the exhalite package in the underlying tuffs. Mineralisation is relatively rare in the carbonate zone but may extend into the talc-chlorite schists. Overall, there is poor spatial correlation between copper and zinc mineralisation at Sandiego. Moreover, discrete zinc-rich and copper-rich zones have been identified from core logging and assay results in the vertical dimension.
At Onedin sphalerite is the main sulphide in the primary mineralisation with subordinate pyrrhotite-pyritechalcopyrite-galena. Sphalerite chiefly occurs as fine-grained masses. In general, the sulphides exhibit replacement textures and show evidence of mobilisation, which is a result of deformation and metamorphism subsequent to initial formation.
The Koongie Park Formation exhibits a deep weathered profile at Sandiego and particularly Onedin resulting in three weathering domains – oxidised zone at surface, primary zone at depth, and the transition zone in between. Each zone has very different mineral assemblages and consequent very different metallurgical properties.
The oxidised zone consists of completely oxidized material, the base (BOCO) base of complete oxidation. This surface is on average about 100 m below ground level. It is undulating and deepens significantly in the vicinity of steeply dipping faults. The Transition Zone consists of partially oxidized material and is located between BOCO and the top of fresh rock (TOFR). Supergene mineralisation is comprised of secondary mineralisation hosted in the oxidised and transition Zones. Gossans are developed at surface above the mineral deposits.
Supergene mineralisation at Onedin is well developed as the bulk of the deposit is located in the oxidised and transition zones. Significant supergene enrichment of copper has occurred with a range of secondary copper minerals present: malachite, chrysocolla, bornite, covellite, chalcocite, cuprite, digenite and native copper. Moreover, a major sub-horizontal torpedo-shaped lens of supergene copper 200 m in length straddles the oxide and transition zone. Lead is also relatively enriched in gossans above the TOFR surface where it occurs as the minerals pyromorphite and cerussite. Localised occurrences of secondary zinc are also present in the form of smithsonite, however zinc is generally depleted in the oxide zone.
