Summary:
The gold deposits, E42, E41, E46 and GRE46 are structurally hosted, epithermal to mesothermal gold deposits occurring within and marginal to a >500m thick dioritic to gabbroic intrusion (Muddy Lake Diorite) doming the stratigraphically upright andesitic and dacitic volcaniclastic rocks and lavas.
Gold mineralisation is associated predominantly with extensional, dilatant and shear quartz-carbonate veining with pyrite and a later base metal assemblage consisting of sphalerite, chalcopyrite, and galena. Visible gold is often associated with this base metal event.
Geology of the GRE46 prospect is controlled by the north to south striking Glenfiddich Fault which separates the moderate north-west dipping Cowal mine sequence of trachydacitic lavas, volcaniclastic mudstones and diorite on the western side from andesitic lava, various mass flow conglomerate units, mudstones and diorite on the East. Bedding changes sharply on the East of the Glenfiddich with GRE46 units steeply dipping to the west. Bedding remains upright younging to the west.
Mineralisation at the GRE46 underground deposit is structurally controlled and is intimately associated with logged or mapped structures. A complex geometry of E-W striking, sub-horizontal extensional veins, moderate south dipping shear extensions and steep south dipping colloform and laminated veins are present at the GRE46 deposit. The three main structures affecting mineralisation at GRE46 are the Glenfiddich Fault, the Galway Fault and the Manna Fault. Whilst not influencing mineralisation, the Cowal Shear Zone also requires significant consideration given the potential geotechnical hazards it presents to mine infrastructure.
All Cowal deposits with significant primary mineralisation have overlying flat blankets of supergene gold mineralisation within the oxide zones. The weathering profile is divided into three units. Transported (lacustrine clays and gravels- 5-40m thick), Soft Oxide (5-90m thick) and Hard Oxide (5-60m thick). The contact between the ‘Soft oxide’ and the ‘Hard oxide’ (HOX) is irregular and gradational. The Base of Complete Oxidation (BOCOS) is logged as the last point in which oxidation of sulphide minerals is present in the hole. The Top of Fresh Rock (TOFR) is determined as the point in which there is no more weathering present in lithological logging. Logged downhole point locations for the BOCOS & TOFR are recorded and are the basis of modelled surfaces within Leapfrog software. Final validated weathering surfaces are used to allocate metallurgical recoveries based on historical metallurgical performance and metallurgical testwork. The GRE46 underground deposit is hosted entirely within unweathered primary material. Metallurgical testwork of core samples from all domains in the GRE46 deposit has highlighted gold recovery of 87%.
Gold mineralisation at GRE46 is both lithologically and structurally controlled. The gold mineralisation is typically found in dispersed dilatant and shear style quartz-sulphide+/-carbonate +/- ankerite+/-chlorite veins ranging from sub millimetre stringers to veins 20 cm to 40 cm in width. Pyrite is the principal sulphide in the veins with lesser sphalerite, chalcopyrite and rare galena.
Mineralisation
- Gold mineralisation at Cowal is most concentrated to a north-south orientated corridor hosted in second and third order structures marginal to and parallel to the Gilmore Suture. The gold deposits are hosted by a shallowing-upwards sequence of semi-conformable sedimentary, volcaniclastic, and volcanic rocks of trachydacitic and trachyandesitic composition that have been intruded by a diorite sill, andesite dome, and various dykes. The sequence strikes northeast–southwest and dips moderately 30° to 40° to the northwest.
- The mineralisation at CGO comprises four deposits: E41, E42, E46 and GRE46. GRE46 will be discussed exclusively in this section.
- The GRE46 zone trends north-south, dips vertical to -70° west, and extends approximately 2km along strike, 200m across strike and at least 1km down dip. Individual lenses in the GRE46 mineralised zone are 1-15m wide, 25-250m long, and extend 50-200m down dip. Lenses consist of narrow high-grade quartz carbonate, pyrite and base metal veins controlled within a structural north-south corridor, broad zones of alteration around lithology contacts and occasional zones of grade enrichment occur in dilatant structures within the deposit known as Quartz Sulphide Breccias. Host lithology varies from poorly mineralised massive intrusive diorite and fine volcaniclastic sediments through to the preferential mineralised trachydacitic lava in the north, lenses of coarse to conglomeritic volcaniclastic sediments and the andesitic Dalwhinnie lava unit to the east. Lithological contacts with strong competency contrasts also provide broad areas of mineralisation. The trachydacite is brittle with common hyaloclastite and peperitic textures, commonly brecciated to peppertic and is both a good geochemical and rheological host for Au mineralisation.
- Vein orientation at the GRE46 deposit displays a distinct change in orientation on a nominal northing of 37,000mN. Vein in the south of the deposit are generally orientated at 50°/120°. Veins in the north of the deposit rotate clockwise and steep to a general dip direction of 70°/180°.