Summary:
At the Frasers open pit and Frasers underground mine (FRUG), deposits are centred on mining the hangingwall shear. In outcrop, the shear typically dips at 15 to 20 degrees to the east and is approximately 5 metres thick. At depth, the dip of the shear flattens to approximately 5 to 10 degrees and develops into an approximately 20 to 30 metres thick mineralised high-grade zone of quartz cataclasite, and mineralised schist. Within the open pit, gold mineralisation comprises mineralised schist and cataclasite, shear-parallel quartz veins and arrays of sub-vertical quartz veins. hangingwall shear and arrays of subvertical quartz veins account for most of the mineralisation within the open pit, although there are a few shear-parallel quartz veins. These veins typically splay off the base of the hangingwall Shear and dip at between 5 and 10 degrees to the west.
A large amount of erratic mineralisation occurs between the base of the hangingwall shear and the footwall fault. At the resource drilling stage, this mineralisation manifests as poorly developed clusters of elevated gold grades, which often appear discontinuous. During mining however, these typically present as extensive zones of quartz vein arrays and mineralised shears. The footwall fault lies between 80 metres and 120 metres below the hangingwall shear and is identified as a cataclastic zone up to 10 metres thick. To date, no economic mineralisation has been located below the footwall fault.
FRUG encompasses the down-dip continuation of the hangingwall shear mined in the Frasers open pit, which is known to extend approximately 600 metres beyond the limit of the open pit design. The thickest, most mineralised part trends approximately northeast and tapers in length from approximately 350 metres at its western end to approximately 150 metres at the eastern limit of drilling, where it abuts the Macraes Fault zone. Mineralisation is contained within the intrashear schist which is generally 80 metres to 100 metres thick, with the higher gold grades confined to the upper part, which is dominated by cataclasite, lode schist and local stockwork pelite lithologies. Numerous drill holes have penetrated through the intrashear schist into the Footwall Psammite, particularly at the western end where the Footwall Fault is relatively shallow, at depths of less than 500 metres. Mineralisation is consistent with the ore delineated in the Frasers open pit. The highest gold grades are contained within the strongly developed and visually distinguishable zone within the upper hangingwall, characterised by quartz cataclasite and silicified breccias. This typically forms a well mineralised, continuous zone up to 15 metres thick, with a grade of approximately 3 g/t Au. Less intensely mineralised lode schist is typically developed lower in the hangingwall package.
Golden Point Underground (GPUG) encompasses the down-dip continuation of the HWS mined in the Round Hill and Golden Point open pits. Current drilling has shown this to extend more than 700 metres beyond the limit of the open pit design. The thickest, most mineralized part is a series of stacked lodes proximal to the Golden Point open pit. Mineralization continues as a single higher-grade lode down-dip to the north-northeast. Mineralization is contained within the intrashear schist, which is generally 80 metres to 100 metres thick, with the higher gold grades confined to the upper part, which is dominated by cataclasite, lode schist and local stockwork pelite lithologies. Numerous drill holes have penetrated through the intrashear schist into the Footwall Psammite. Mineralization is consistent with the ore delineated in the Golden Point and Round Hill open pits, however down-dip of Golden Point this is constrained to a single lode. The highest gold grades are contained within the strongly developed and visually distinguishable zone within the upper hangingwall, characterized by quartz cataclasite, and mineralized schist. This typically forms a well mineralized, continuous zone up to five metres to ten metres thick, with a gold grade of approximately 3 g/t.
Mineralisation
The Macraes deposit is a classic example of an orogenic style gold deposit, with mineralisation broadly synchronous with deformation, metamorphism, and magmatism during lithospheric-scale continental-margin orogeny. Most orogenic gold deposits like Macraes occur in greenschist facies rocks. Orogenic deposits typically formed on retrograde portions of pressure-temperature time paths during the last increments of crustal shortening, and thus postdate regional metamorphism of the host rocks. The following four types of mineralisation occur within the HMSZ at Macraes:
(a) Mineralised schist. This style of mineralisation involves hydrothermal replacement of schist minerals with sulphides and microcrystalline quartz. Mineralisation is accompanied by only minor deformation;
(b) Black sheared schist. This type of schist is pervaded by small scale anastomosing fine graphite, and sulphide bearing microshears. This type of mineralisation is typically proximal to the hangingwall shear;
(c) Shear-parallel quartz veins. These veins lie within, and/or, adjacent to the black sheared schist and have generally been deformed with the associated shears. The veins locally crosscut the foliation in the host schist at low to moderate angles. Veins are mainly massive quartz, with some internal lamination and localised brecciation. Sulphide minerals are scattered through the quartz, aligned along laminae and stylolitic seams. These veins range from 1 centimetre to more than 2 metres; and
(d) Stockworks. These veins occur in localised swarms that are confined to the intrashear schist. Individual swarms are up to 2,000 square metres in area and consist of numerous subparallel veins. Most of these veins formed sub perpendicular to the shallow east dipping shear fabric of the intrashear schist. Stockwork veins are typically traceable for 1 metre to 5 metres vertically with most filling fractures that are 5 centimetres to 10 centimetres thick but can be up to 1 metre thick.