Summary:
The Macraes gold deposits are in a major, low-angle (~15-20°) structure known as the Hyde Macraes Shear Zone (HMSZ). This regionally continuous, late metamorphic deformation zone cuts greenschist facies metasedimentary rocks of the Otago Schist.
Mineralisation within the HMSZ is hosted within lower greenschist facies metamorphosed, pelitic to psammitic sediments that are variably altered, deformed, and mineralized. This package of schist, known as the Intrashear Schist, is bounded above by the Hangingwall Shear, and below by the Footwall Fault, and can be up to 150 m thick. The thickest parts of the HMSZ comprise multiple, stacked shears and associated quartz vein arrays. The shears have ductile deformation textures overprinted by cataclasis (Craw et al., 1999). The Hangingwall shear, which is the most continuous and intensely mineralised structure, can be up to 25 m thick and is commonly darker coloured due to fine grained graphite and sheared sulphide minerals (McKeag et al., 1989).
There is a strong empirical correlation between gold, arsenic, scheelite, silicification and deformation intensity within the HMSZ. Gold-scheelite-pyrite-arsenopyrite mineralisation is associated with replacement and fissure quartz veins within post-metamorphic shear zones. Shear-parallel quartz veins and cataclastic shears contain the highest gold and scheelite grades (Lee et al. 1989). Dore is typically comprised of 5% silver.
The Coronation and Coronation North deposits are located 5 to 6 km to the northeast of the processing plant. Coronation consists of a 15 to 20 degrees dipping hangingwall shear that is between 3 and 10 metres thick. Immediately beneath (1 to 10 metres) the hangingwall shear is a thinner lode structure that parallels hangingwall. Unlike deposits to the south, there is very little development of stockwork mineralisation beneath the hangingwall. Mineralogically, the Coronation deposit is very similar to previously mined deposits to the south. Located 1km to the north of Coronation is the Coronation North deposit which was discovered in 2015. Coronation North differs from most of the previously mined ore bodies along the HMSZ. Pit mapping and grade control data have delineated a left-hand lateral bend in the strike of the HWS coincident with a high-grade zone of mineralisation that plunges to the ENE. Traversing along the shear from southeast to northwest the dip of the HWS gradually decreases whereas the strike bends towards the west. This bend coincides with a zone of steeply dipping en-echelon style mineralised splays beneath the HWS, whose dip steepens to near vertical as they approach the FWF. Zones of finely laminated mineralised quartz veins also form beneath this WSW-striking segment of the HWS, perpendicular to its strike. Compared to the other deposits in the goldfield, the WSW-strike of Coronation North, the relatively narrow approximately 100m width of the mineralised zone and its steep dip are currently unique in the goldfield.
The Innes Mills open pit is centred on mining the HWS and subparallel stacked lenses beneath. In outcrop, the shears typically dip 15 to 20 degrees to the east and are approximately five metres thick. Within the open pit, gold mineralization comprises mineralized schist and cataclasite, shear-parallel quartz veins and arrays of sub-vertical quartz veins.
At the Frasers open pit and 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.
Erratic mineralization locally occurs between the base of the HWS and the Footwall Fault. At the resource drilling stage, this mineralization manifests as poorly developed clusters of elevated gold grades, which often appear discontinuous. During mining, however, these typically present as zones of quartz vein arrays and mineralized shears. The Footwall Fault lies between 80 metres and 120 metres below the HWS and is identified as a cataclastic zone up to ten metres thick. To date, no economic mineralization has been located below the Footwall Fault.
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.