Summary:
The gold and copper deposits on the Peak Gold Mines (PGM) properties in the Cobar Gold Field can be characterized based on their own unique mineralization and alteration signatures. The shear zones which host the mineralization include the Peak-Perseverance Shear, Blue Shear, Lady Greaves Shear, and the GCF (Great Chesney Fault).
The deposit groupings are as follows:
- Group 1 - Peak and Perseverance;
- Group 2 - New Occidental, Chesney, and New Cobar;
- Group 3 - Gladstone and Great Cobar.
THE NEW COBAR
Deposit Setting and Morphology The New Cobar gold deposit is located approximately 2.5 km north of the New Occidental Mine on the GCF. The deposit flanks a pronounced bend in the contact thrust and occurs on the southwestern side of a broad silicified ridge. At its northern end, the New Cobar mineralization is only 20 m from the contact thrust but diverges rapidly to the south and is up to 150 m from the thrust at its southern extent.
Mineralization is characterized by a stockwork of pyrrhotite-chalcopyrite-gold veins, which overprint an older quartz-magnetite vein stockwork. Both stockworks are characterized by gradational margins. The mineralization is developed over a strike length of some 500 m, with a central zone 300 m long by up to 35 m wide that strikes north-south, dips steeply to the east, and plunges steeply to the north, parallel to regional cleavage. Bedding dips steeply to the west.
The New Cobar deposit contains four steep east dipping and steep north plunging lenses of gold-copper mineralization. From north to south they are:
1. The Jubilee Lode is a narrow north-northwest trending lode approximately three to six metres wide and 50 m long at the northern extremity of the deposit. The lode has been traced vertically to a depth of 200 m below surface where it appears to bottom out.
2. The Northern Lode trends north-northwest, is six metres to 12 m thick, and is 60 m to 80 m long. A rapid thinning of the lode below 400 m depth below surface is apparently associated with a steepening in the dip of the lode.
3. The Southern Lode trends north-northwest, is six metres to 12 m thick, and is 60 m to 80 m long. It has been traced to 700 m below surface by drilling and is apparently open ended. The Southern and Northern Lodes were the most productive lodes in the deposit and are collectively referred to as the Main Lode.
4. The Western Lode is located 60 m to the west of the Southern Lode. It trends northwest and is approximately two to five metres wide and 50 m long. The Western Lode does not outcrop and is first discernible at 200 m from surface, has been traced with diamond drilling to 500 m below surface, and is open at depth.
CHESNEY
The Chesney copper-gold deposit is located approximately 1.8 km north of the New Occidental Mine and 600 m south of New Cobar on the GCF. The deposit is localized within the steep east dipping GCF, which juxtaposes the Chesney Formation sandstones with the siltstones and shales of the Great Cobar Slate. The deposit has a pipe-shaped geometry dipping steeply east parallel to the regional cleavage and plunging steeply north parallel to a strong stretching lineation. The deposit is approximately 200 m long, six metres to 12 m wide, and extends over a vertical distance of at least 1,200 m. Near-surface oxidized mineralization is known to exist.
THE NEW OCCIDENTAL
The New Occidental deposit is approximately 200 m long, 10 m to 30 m wide, and extends over a vertical distance of at least 1,200 m. Gold is intimately associated with native bismuth and bismuth sulphides and shows an erratic association with overprinting pyrrhotite, galena, sphalerite, and chalcopyrite.
THE PEAK SHEAR
From north to south along the Peak Shear, the principal deposits of interest to PGM, based on the presence of significant infrastructure and Mineral Resources and Mineral Reserves are the Peak and Perseverance deposits.
Located approximately 200 m east of, but parallel to and part of the Peak Shear is the Blue Shear and associated gold and silver mineral deposits, the Blue Lode and the Silver Peak. Two hundred metres further to the east is another parallel shear zone called the Lady Greaves Shear. The Lady Greaves Shear is host to the Lady Greaves Gold Mine. All the surface deposits are sediment hosted in fault-related veins and breccias. Deposits of the Peak Shear typically have moderate chlorite alteration and variable quantities of pervasive silicification and quartz veining.
The majority of the Peak deposit lies between 300 m and 700 m from surface and extends over a strike length of 300 m along the Peak Shear. A low grade oxide resource exists from surface to the base of oxidation, some 70 m below surface. In a broad sense, the deposit is vertically elongate parallel to the dominant steep west dipping cleavage and the strike of the Peak Shear Zone. The mineralized system is thickest around the volcanics, where it attains a maximum thickness of 150 m, and rapidly tapers upwards into the overlying sediments along the shear zones.
The majority of higher grade mineralization is controlled and localized about the apical portions of a series of flow banded rhyolite and rhyolitic subvolcanic breccia bodies. These rhyolites and rhyolitic breccias do not outcrop and are only known from drill core and underground openings. These bodies are shallowest in the centre of the Peak deposit where they are located 450 m from surface. They are known to extend at least 1,000 m south, 500 m north, and 300 m east of the deposit.
Four styles of gold and associated base metal mineralization occur at Peak:
1. Sediment Hosted – Sheeted veins to 200 mm wide of quartz + sphalerite + galena + chalcopyrite + gold occur in zones of intensely silicified sandstones and lesser siltstones of the Chesney Formation. The vein sets are parallel to the welldeveloped, steep west dipping cleavage. The Western Lead Zinc (WLZ) lens is the largest example of this style of mineralization at Peak.
2. Contact Zone Hosted – The Contact Zone is defined as the zone of altered sediment at the sheared contact with rhyolite and volcanic breccia. These zones are of arcuate geometry and surround the western margin (and lesser eastern margin) of the subvolcanic bodies.
3. Volcanic Hosted – En-echelon quartz veins within riedel shears in the eastern volcanic breccia bodies host pyrrhotite + chalcopyrite + galena + sphalerite + Au mineralization. These lenses lie within the dominant cleavage and vary in thickness from one metre to twenty metres. The Terror Lens is the largest lens of this style.
4. Late Stage Shear Hosted – Small discontinuous pods of banded to semi-massive sphalerite-galena- chalcopyrite-pyrrhotite-pyrite carrying high grade silver and patchy gold mineralisation associated with black chlorite-talc-carbonate alteration are developed in late shears including the Peak/Polaris Shear and the Hecla Shear.
THE PERSEVERANCE
The Perseverance deposits are part of the gold and base metal deposits of the Peak Shear system. They ar located approximately 150 m west of, and parallel to, the Peak Shear. Quartz breccia and vein hosted gold and base metal mineralization are intimately associated either within or adjacent to the Perseverance Shear and several smaller parallel structures to the east.
Five highly mineralized zones have been defined within the global deposit to date, a northern Zone A lens (shear hosted gold), Zone B (vein hosted gold), the southern Zone D lens (mixture of shear and vein hosted), the fault splay related Hulk Lens, and the Rhyolite breccia hosted Hercules lens.
Individual mineralized lenses mimic the orientation of the Perseverance Shear and vary in thickness from four metres to 30 m, averaging eight metres. The most intense gold and, more so, base metal mineralization occurs where the rhyolite is sheared against the more sandstone rich portions of the Chesney Formation.