Summary:
The Project conforms to an oxidised gold skarn type deposit. Such gold skarn deposits are exploited predominantly for gold, and exhibit calc-silicate alteration, usually dominated by garnet and pyroxene (Einaudi et al., 1981; Meinert et al., 2005). Most gold skarns form in orogenic belts at convergent plate margins. They tend to be associated with syn to late inter-oceanic island arc intrusions emplaced into calcareous sequences in arc or back-arc environments.
On the Project, skarn type mineralisation is primarily stratigraphically controlled and to a lesser extent structurally controlled and is found as massive, manto-like, stratabound lenses in Cretaceous calcareous clastic sedimentary rock sequence, intimately related with the proximity to fertile LateCretaceous dioritic intrusions.
The Coka Rakita deposit exhibits characteristics that align with an oxidised gold skarn type. These qualities include:
- The dominance of garnet over pyroxene is typical characteristic of other oxidised gold skarns (Meinert, 1998). At Coka Rakita, andradite-grossular garnet types are dominant with lesser pyroxene, wollastonite, and quartz in prograde alteration.
- In comparison to reduced gold skarns, oxidised gold skarns exhibit high garnet/pyroxene ratios, relatively poor iron garnet and pyroxene, and low total sulphide content (Brooks et al., 1991; Meinert, 2000). Gold at Coka Rakita is present in sulphide-poor mineralisation assemblage and assumingly precipitated in a wide range of subsequent hydrothermal mineralising phases, the main ones being: (1) native gold and pyrite, with minor chalcopyritebornite-chalcocite-molybdenite; and (2) native gold, pyrrhotite and magnetite with minor chalcopyrite-sphalerite-pyrite-galena-bismuth sulfosalts-tellurides.
- Additionally, the highest gold grades are often associated with later retrograde alteration including abundant K-feldspar and quartz. The most significant gold mineralisation at Coka Rakita, in terms of grade and continuity, is associated with retrograde alteration assemblages, which typically contain quartz, K feldspar, epidote, biotite, chlorite, albite, calcite and apatite. Native gold, in grains up to several millimetres in size, frequently occupies interstitial position between the garnet and pyroxene grains affected by this retrograde alteration.
Gold grades are typically high (average 10 to 30 g/t and up to 1,000 g/t), whereas the contents of copper, zinc, lead, and other metals are very low in most mines (Prodeminca, 2000).
Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer potassium- and sodium-enriched zone in the volcano-sedimentary unit. Gold mineralisation is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulphide-poor vugs and milky quartz veins and veinlets in association with hematite (Vallance et al., 2009).
Mineralisation
The main mineralisation type found within the Coka Rakita Project is the high-grade manto-like skarn gold-copper mineralisation, found as primarily stratigraphic controlled and to a lesser extent as structurally controlled massive stratabound lenses within calcareous S1 and S2 sandstones at the hanging-wall contact of the sill-like EMP intrusion.
The outlined high-grade gold-skarn mineralisation is intimately linked at deposit and project scale to other mineralisation types including:
Porphyry gold-copper-molybdenum mineralisation at two stratigraphic levels, including:
1. Stockwork quartz veinlets and disseminations related mineralisation in the potassic altered EMP, and
2. Epiclastic-hosted gold mineralisation controlled by structural and lithology contacts.
Stratabound copper-gold mineralisation at deeper stratigraphic settings, including:
1. Conglomerate-hosted, copper-gold-polymetallic mineralisation, located on the footwall of the mineralised EMP intrusion, and
2. Marble and skarn altered limestone-hosted copper-gold mineralisation with iron-hydroxides, pyrite, chalcopyrite, bornite and chalcocite.
High-grade Manto-like Gold-copper Skarn Mineralisation
Exoskarn developed within the S1/S2 calcareous clastic sedimentary sequence on the hanging-wall of the EMP intrusive represents the principal gold-bearing horizon at Coka Rakita, occurring between 250m and 600m depth as a lens-shaped body that dips 40–50° east and is primarily stratigraphically controlled, with secondary north–south structural influence. Gold mineralisation is hosted by andradite–grossular garnet skarn and is mainly associated with retrograde alteration, occurring in two dominant assemblages: native gold–pyrite and native gold–pyrrhotite–magnetite, with subordinate sulphides, sulfosalts and tellurides. Gold occurs as native Au with variable Ag content (up to ~10 wt% Ag), forming disseminations and locally centimetre-scale aggregates, with good grade continuity in the core of the system that diminishes toward the margins.
Porphyry Gold-copper±molybdenum Mineralisation
Porphyry gold–copper mineralisation occurs at two stratigraphic levels within the EMP system: as quartz stockwork veinlets in potassic-altered EMP, and as structurally and lithologically controlled epiclastic-hosted gold mineralisation with a quartz–biotite–epidote–sericite–pyrite alteration footprint. Disseminated and veinlet-hosted Cu±Au–Mo mineralisation in the EMP intrusion is associated with secondary biotite ±magnetite–K-feldspar alteration and consists mainly of chalcopyrite, magnetite, pyrite and molybdenite, but forms a laterally continuous yet low-grade and subeconomic system. Epiclastic-hosted gold mineralisation is sporadic, poorly continuous between drillholes, and commonly occurs above underlying skarn-related mineralisation, suggesting formation along leakage structures that enabled upward escape of hydrothermal fluids.
Stratabound Copper-(gold) Mineralisation
Stratabound copper–gold–polymetallic mineralisation occurs on the footwall of the EMP intrusive at ~550m depth and is hosted by basal siliciclastic conglomerates and sandstones (S1Q unit), as well as deeper marbleised limestones, where it is closely associated with skarn alteration and structurally favourable zones. The mineralisation is dominated by pyrite and chalcopyrite, with subordinate molybdenite, sphalerite, galena, bornite, chalcocite and pyrrhotite, occurring as disseminations, mottled aggregates and veinlets within garnet–pyroxene skarn assemblages. Long intercepts of this limestone-hosted stratabound mineralisation have been intersected at depths of 650–1,000m in the northern Coka Rakita Project area, but its overall continuity remains untested due to limited deep drilling.