Summary:
Mining at Cowal currently centres on the E42 deposit, with underground development of the GRE46 deposit underway, and studies ongoing for the E41 and E46 Open Pits. The gold deposits occur within the 40km by 15km wide Lake Cowal Volcanic Complex. The deposits occur within a 5km (n-s) by 2km (e-w) trend known as the Gold Corridor.
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. The CGO tenements also host the copper-gold porphyry prospect at Marsden/E43, and the volcanogenic-massive sulphide prospect at Decoy/E35.
Gold deposits aligned along a north-south orientated corridor within the bounding Booberoi Fault on the western side and the Reflector Fault on the eastern side. The area is commonly referred to as the Gold Corridor. The current architecture and associated mineralisation within the Gold Corridor is related to a succession of intrusions exploiting pre-existing zones of weakness, most likely the Cowal Shear Zone. The intrusions have domed the camp, resulting in a faulted anti-form that plunges shallowly to the north-northeast.
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.
Mineralisation is also associated with the highly fractionated Monzonite core of the Diorite sill. Commonly, a set of shallow dipping veins crosscut a steeper vein set associated with faults and dykes. Occasionally, steep vein orientations produce hydrothermal breccias known as Quartz Sulphide Breccias (QSB) that are often high-grade, short-range blow outs of the dilatant vein set. Alteration typically falls into early chlorite-K-feldspar-hematitecalcite-pyrite (propylitic) or a later sericite ± ankerite (phylic) assemblage associated with the base-metal-gold event.
The E42 deposit is the largest deposit currently defined in the Gold Corridor. E42 lies 1.25km north of E41. GRE46 is located immediately to the east of E42 with E46 lies 1.5km to the north of E42. E41E is adjacent to E41W and is separated by a major structure currently interpreted as the Pullabooka Fault. The eastern portion of the deposit has a major fault running through what is interpreted as the southern extents of the Glenfiddich fault. E41W consists of a large Quartz Monzonite which along with a coarse regional Diorite unit, has intruded along a structural zone of weakness and has likely caused the regional anticline present in the CGO stratigraphy.
E46 is a shallow, largely oxide gold resource. Primary lithology at E46 consists of a diorite sill, two adjacent volcaniclastic sediment units, a barren package of andesite to the south of the deposit and a trachyandesitic lava unit. The units are interpreted to be the same stratigraphy package as the E42 Open Pit.
Galway Regal Open Pit (GR) has formerly been referenced as GRE46 oxide and primary open pit. The deposit directly overlies the GRE46 underground deposit comprising the supergene component of the same system. The GRE46 zone trends north-south, dips vertical to -70° west, and extends approximately 1,650m along strike, 175m across strike and at least 900m down dip. Individual lenses in the GRE46 mineralised zone are 1.0m to 15m wide, 25m to 250m long, and extend 50m to 200m down dip.
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 traditional.
Gold mineralisation within the CGO Open Pit deposits 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 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 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°.
The E46 deposit mineralisation trends north-northeast, dips -40° west to flat-lying, and measures approximately 650m along strike and 170m across strike. Individual zones are approximately 50m wide and extend 200m down dip.
The E41 West mineralisation strikes north-northeast and dips -70° east, and measures 750m along strike and 250 m across strike. Individual mineralised zones are 35 m to 50m wide and extend down dip for 125m. Mineralisation is proximal to the Kilara fault which is interpreted to be the fluid conduit for the deposit.
The E41 East mineralisation strikes east-west and dips -35° to -80° south, and measures 475m along strike and 500 m across strike. Individual mineralised zones are 35m to 50m wide and extend down dip for 225m. Mineralisation is most closely associated with the contact between the hard, brittle diorite and the softer, malleable sediments.
The E42 deposit dips -35° to -45° to the south west with an approximate extent of 850m by 850m and extends 500m down dip. Mineralisation is contained within small discontinuous, dilatant veins contained within larger mineralised envelopes approximately 50m wide. The Muddy Lake Diorite show distinct fractionation with the internal Quartz Monzonite preferentially mineralised. The Trachyandesitic Lava within E42 shows a variety of depositional environments from peperitic to competent porphyritic lava. Peperitic and brecciated lava provides a favourable host for mineralisation within E42.