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 Late-Cretaceous dioritic intrusions.
Gold-rich skarn mineralisation is hosted within carbonate-rich sandstones and conglomerates, located on the hanging wall of a sill-like body and abutting a monzonite intrusive body to the west. The mineralisation forms a shallow-dipping tabular mineralised body located between 250 m and 450 m below surface, measuring 650 m long, up to 350 m wide, and with variable thickness from less than 20 m in the margins to more than 100 m in the core of the mineralised zone. Coarse gold is often observed in areas of intense retrograde skarn alternation and is found mainly in proximity to syn-mineral diorites within the higher-grade core of the deposit. The current MRE has been prepared on the portion of the Project where gold-rich skarn mineralisation occurs.
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 early mineral porphyry (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
At Coka Rakita, exoskarn formation in the S1/S2 calcareous clastic sedimentary rock sequence on the hanging-wall of the EMP intrusive is the principal mineralised horizon in terms of gold endowment.
The mineralisation is located between 250 m and 600 m below surface and has been traced over a footprint of 650 m x 350 m. It has variable thickness, from less than 20 m in the margins to more than 100 m in the core of the mineralised zone. The mineralisation forms a lens-like shape that dips between -40° to -50° to the east. Mineralisation is primarily stratigraphically controlled, with the lower boundary of mineralisation closely following the EMP sill contact. Endoskarn formation typically persists for a short distance within the EMP. As a second order control, steeper north-south striking structural trend can also be determined, which is evidenced by the north-south elongation of the high- grade mineralised zones and with the occurrence of mineralised subvertical phreatic breccia zones.
Gold mineralisation is located within the andradite-grossular garnet skarn and is dominantly associated with a retrograde assemblage that comprises a quartz, K-feldspar, epidote, biotite, chlorite, albite, calcite, and apatite paragenesis. Gold is present in its native form and thought to have precipitated in a wide range of hydrothermal phases with the main ones being: (1) native gold and pyrite-dominant mineralisation, with minor chalcopyrite-bornite-chalcocite±molybdenite; and (2) native gold and pyrrhotite-magnetite mineralisation with minor chalcopyrite-sphalerite-pyrite-galenabismuth sulfosalts- tellurides. A petrographic analysis of 48 samples from across the Coka Rakita deposit (Pacevski, 2023) determined that although gold occurs in its native form, it almost always contains silver in different concentrations, preliminary SEM-EDS analysis indicating up to 10 wt% Ag content.
Gold appears as disseminations and often as visible aggregates that reach up to a few centimetres in size. These gold grain aggregates frequently occupy interstitial position between the garnet and pyroxene grains affected by the retrograde alteration. Gold grade continuity is variable; high levels of grade continuity are observed in the core of the system but this gradually decays moving outwards.
PORPHYRY GOLD-COPPER±MOLYBDENUM MINERALISATION
Porphyry gold-copper mineralisation occurs at two stratigraphic levels, including: 1) stockwork quartz veinlet related mineralisation in the potassic altered EMP, and 2) epiclastic-hosted gold mineralisation with a quartz-biotite-epidote-sericite-pyrite footprint controlled by structural and
intrusion contacts.
A pervasive secondary biotite±magnetite-K-feldspar assemblage formed in the EMP intrusion is associated with the low-grade disseminated and porphyry quartz-veinlet hosted copper±gold-molybdenum mineralisation. The mineralisation is represented by chalcopyrite, magnetite, pyrite and molybdenite. Although, the continuity of this mineralisation can be mapped and correlated across the entire EMP intrusion, the contained gold and copper grades are low and currently subeconomic.
STRATABOUND COPPER-(GOLD) MINERALISATION
Stratabound copper-gold-polymetallic mineralisation is located on the footwall of the EMP intrusive, approximately 550 m below surface, and is hosted by basal recrystallised siliciclastic conglomerates and sandstones, mainly in skarn altered carbonate fragments in the basal part of the unit, or in structurally predisposed zones. The mineralisation type is characterised by an assemblage dominated by pyrite and chalcopyrite, with molybdenite, sphalerite, galena, bornite, chalcocite, pyrrhotite in subordinate quantities.
At deeper levels, stratabound copper-gold-polymetallic mineralisation can also be found in marbleised limestones and on the limestone-sandstone contact. It is mainly associated with skarn alteration and intensive iron-oxides replacements in paleokarst and structurally predisposed zones. The garnet- dominated skarn-altered limestone also contains pyroxene, actinolite, secondary calcite and silica. Chalcopyrite and pyrite are major sulphide minerals, with minor bornite, chalcocite, molybdenite, sphalerite and galena. Sulphide minerals occur in the form of disseminations, mottled aggregations, or veinlets.
Long intercepts of continuous copper-gold stratabound, limestone-hosted mineralisation had been encountered during initial scout drilling within the north of Coka Rakita Project. So far this mineralisation type has been found between 650 m and 1,000 m below surface. Due to the depth of the prospective formation, systematic drilling has yet to be completed and the continuity of mineralisation yet to be established.