Summary:
The Onedin and Sandiego deposits are recognized as volcanogenic massive sulphide (VMS) style deposits.
Massive sulphide deposits at Sandiego and Onedin are hosted by the Koongie Park Formation, which is composed of mafic and felsic volcanics, associated sediments including sandstone, mudstone, carbonate, chert and ironstone, and is intruded by rhyolitic to rhyodacitic sills, dolerite bodies and basalt dykes. Massive sulphide mineralisation is stratabound, with disseminated sulphides overlaying the massive sulphides. Both deposits are interpreted to occur within the limbs of intensely folded, higher order, double-plunging anticlinal structures.
The massive sulphide deposits of Koongie Park are classified as Volcanogenic Massive Sulphide (VMS) deposits. The mineralogy of the primary mineralisation at Sandiego is pyritesphalerite-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. At Onedin, sphalerite is the main sulphide in the primary mineralisation with subordinate pyrrhotite-pyrite-chalcopyrite-galena. Onedin comprises numerous stacked lenses of mineralisation with a folded and faulted geometry over a vertical zone of 400 m.
Both deposits have a deep weathering profile (up to 250 m below the surface), resulting in three weathering domains: an oxidised zone at the surface, a primary zone at depth, and a transition zone in between.
Dimensions
The Onedin Mineral Resource extends along strike 300 m, across strike by 200 m and has a depth extent below surface of 400 m.
The Sandiego Mineral Resource extends along strike 300 m, across strike by 200 m and has a depth extent below surface of 600 m.
Structure
Onedin Deposit
The Onedin deposit is situated on the southern limb of a regional NE–SW-trending, double-plunging antiform. Stratigraphy is overturned and complexly folded, with units dipping generally to the west. Across strike, from northwest to southeast, the stratigraphic sequence includes the Coolibah Tuff, Mimosa Sub-Member, Camp Shale Member, and Weldons Creek Tuff. The Camp Shale Member in this area is notably more deformed than at Sandiego, hosting abundant phyllitic and schistose units. At the deposit scale, the dominant structural feature is a southwest-plunging isoclinal fold developed within the Camp Shale Member.
Sandiego Deposit
The Sandiego deposit is hosted within a sheared antiformal structure that plunges to the southwest. Local stratigraphy trends NNE–SSW and dips steeply (80–85°) to the east. From east to west, the stratigraphic sequence comprises the Weldons Creek Tuff, Camp Shale Member, Mimosa Sub-Member, and Coolibah Tuff. A weak penetrative fabric is developed throughout the deposit, indicative of relatively lower strain compared to the Onedin deposit.
These structural features have influenced both the geometry and localisation of sulphide mineralisation.
Mineralisation
Base metal sulphide mineralisation is primarily hosted within the thicker portions of the Mimosa Sub-Member, located at the base of the Camp Shale Member. The mineralised horizon is a mixed chemical sediment composed of silicate, oxide, and sulphide facies, featuring sphalerite, galena, chalcopyrite, pyrrhotite, and minor tetrahedrite.
The distribution of massive sulphide mineralisation exhibits a strong structural control, typically localised near major fault structures and within tight isoclinal folds, often parallel to the plunge of fold axes. Lead isotope analyses suggest a single hydrothermal system as the mineralising source, with model ages of approximately 1,825 Ma, consistent with the age of the host stratigraphy.
Supergene Mineralisation
A deeply weathered profile is observed in the Koongie Park Formation at both Onedin and Sandiego, comprising two distinct zones:
• Oxidised Zone – entirely oxidised material, bounded at depth by the Base of Complete Oxidation ('BOCO'), generally located ~100 m below surface and deepening near steeply dipping faults.
• Transition Zone – partially oxidised material, occurring between the BOCO and the Top of Fresh Rock ('TOFR'). Supergene mineralisation is developed in both zones.
Onedin Deposit
Onedin exhibits well-developed supergene enrichment due to overturned stratigraphy, placing primary sulphide mineralisation within the Oxide and Transition Zones. Copper shows strong supergene enrichment, evidenced by the presence of malachite, chrysocolla, bornite, covellite, chalcocite, cuprite, digenite, and native copper. A prominent sub horizontal, torpedo-shaped supergene copper lens (~200 m long) straddles the BOCO-TOFR interface.
Lead is enriched in gossans above TOFR, occurring as pyromorphite and cerussite. Secondary zinc minerals, including smithsonite and rare willemite, are also present. Smithsonite is the dominant zinc mineral in the upper transition zone, although underreported due to the reliance on percussion drilling, which limits visual discrimination from siderite.
Primary Mineralisation
At Onedin, primary mineralisation is mainly hosted in the carbonate zone, with sparse exhalites. Additional mineralisation is observed in chloritic schists between two major carbonate lenses. Mineralisation is structurally controlled, concentrated in fold cores and limbs, with evidence of sulphide remobilisation.
Sphalerite is the dominant sulphide, occurring as fine-grained replacement textures within carbonates. Galena is more abundant at Onedin than Sandiego and shows a strong spatial association with sphalerite, as does chalcopyrite. Notably, massive sphalerite also fills open-space textures in collapse and tectonic breccias.
Copper-rich zones are rare and limited to oxidised material and talc–chlorite schists, possibly related to shearing. The copper tenor is generally lower than at Sandiego.
Sandiego Deposit
In contrast, supergene mineralisation at Sandiego is limited. Most mineralisation is found in the primary zone, with minor enrichment observed in the transition zone, particularly along subvertical shear zones, where remobilised chalcopyrite occurs as chalcocite. Occasional gossanous zones along faults also contain minor supergene sulphides.
Primary Mineralisation
Primary sulphide mineralisation is hosted within the magnetiterich exhalative package, forming a massive, wedge-shaped lens approximately 200 m long and up to 75 m thick. The lens strikes NNE–SSW and dips steeply (80–85°) to the east. Stringer and vein-style sulphides extend into underlying tuffs. Mineralisation is uncommon in the carbonate zone but may extend into talc–chlorite schists.
Copper and zinc occur in spatially distinct zones:
• Zinc-rich zones: Dominated by sphalerite, pyrrhotite, galena, pyrite, minor chalcopyrite, and trace argentite and arsenopyrite. Sphalerite commonly replaces magnetite and pyrite. Hemimorphite (likely supergene) appears as botryoidal forms in vuggy cross-cutting veins (5–15 mm wide).
• Copper-rich zones: Characterised by chalcopyrite, pyrite, chalcocite, covellite, marcasite, bornite, and minor sphalerite. Chalcopyrite occurs as space-filling veins and stringers, particularly in cherty exhalite, often associated with pyrrhotite and magnetite. Copper mineralisation is frequently associated with fault/shear zones and talc–chlorite schists, suggesting remobilisation and later emplacement.
Zinc mineralisation is closely associated with magnetite and pyrite. Sphalerite is often visually obscured by martitised hematite derived from magnetite oxidation.