Summary:
The Hillgrove mineralisation can be classified as orogenic style, antimony – gold deposits, that are hosted in a combination of the Mid Carboniferous Girrakool Sediments and Late Carboniferous – Early Permian Granites. The setting is part of the New England Orogen, one of four which formed most of the east coast of Australia. The mineralised zones are structurally controlled within a NW trending shear corridor, formed from the movement of two regional faults (Hillgrove and Chandler). Multi-phase antimony – gold – tungsten mineralisation has been hydrothermally emplaced into narrow shears (0.1 m – 10 m wide), which have good strike and depth extents. Gold mineralisation is predominantly refractory (associated with arsenopyrite), and also occurs as aurostibite in stibnite, and as particle gold.
Mineralised vein and breccia systems at Hillgrove are hosted in biotite-grade metamorphosed sedimentary rocks of the late Palaeozoic Girrakool Beds (originally shale, siltstone, argillite, greywacke), biotite monzogranite (S-type) of the ~300 Ma Hillgrove Adamellite and granodioritic-dioritic rocks of the early Permian Bakers Creek Diorite Complex. The structures and mineralisation post-date, and are unrelated to any of the host rocks. Syn-mineralisation lamprophyre dykes that are both cut by mineralisation as well as having intruded mineralised structures, are dated at 247-255 Ma (Ashley et al., 1994), thus bracketing the potential age of mineralisation to approximately the Permian-Triassic boundary. The dykes are closely related spatially to mineralised structures, are up to a few metres wide and include minette and vogesite types. Geochemically, the lamprophyres are related to the high-K I-type granitoids of the Permo-Triassic Moonbi Plutonic Suite (Ashley et al., 1994).
Mineralisation
Mineralisation is developed in veins, vein breccias, sheeted veins, network stockworks and as alteration selvages of disseminated and veinlet arsenopyrite and pyrite adjacent to the main structures. Although mineralised structures are commonly <1 m wide, zones of stockworking and brecciation, plus their accompanying sulphidic halo zone may attain widths of up to 20m in places. Paragenetic studies (Boyle, 1990; Ashley and Craw, 2004) have previously indicated that the earliest mineralising event was a scheelite-bearing phase of quartz veining. Subsequent phases of arsenopyrite–pyrite–quartz–carbonate veining were accompanied by gold and minor base metal sulphides. Alteration is typically sericite–ankerite–quartz and this accompanies the disseminated and veinlet arsenopyrite and pyrite zones. Overprinting stibnite–quartz veining with gold-electrum, aurostibite and arsenopyrite form an important subsequent phase. The sulphidic haloes about the mineralised structures vary from being narrow and tight, to up to 20 m wide. In these zones, it has been shown that gold grades (with little or no accompanying Sb) can be significant, with gold hosted “invisibly” in the sulphides. Arsenopyrite is the main host to invisible gold, although a smaller proportion is also hosted in pyrite that tends to be of arsenical composition (Ashley et al., 2000). The disseminated gold halo about mineralised structures is being sampled and drilled in detail to define the grade. Mineral concentrates derived from the sulphidic halo material have proven to yield good gold recoveries from pressure oxidation treatment, followed by conventional cyanidation.
The presence of the sulphidic halo about mineralised structures and the other changes to wallrock mineralogy have led to the development of geochemical alteration haloes that can extend for up to tens of metres. The alteration-mineralisation process has led to addition of Au, Sb, As, S, CO2, K and Rb, with depletion of Na and Sr (Ashley and Craw, 2004). Fluid inclusion studies have indicated homogenisation temperatures in gangue quartz in the range 100°-250°C and that fluids were of low salinity (Comsti and Taylor, 1984). Metal zonation within the field can be inferred on the basis of past production to be from Au-As at depth to Sb-Au-As at shallower levels, with minor scheelite occurring throughout the production interval. Structural, alteration and mineralisation characteristics of Hillgrove accord with many other orogenic gold deposits, although Hillgrove is unusual in potentially having formed progressively during orogenic uplift leading to a telescoped array of vein systems with overprinting of earlier mineralisation by later (e.g. W by As-Au by Sb-Au).
Dimensions
Brackin’s Spur:
Maximum 1.2 km along strike by 520 m below surface to lower limit. Upper limit approximately 20 m below surface. The oxidized zones have been excluded from estimates. Nominal 4 m width.
Clark’s Gully:
550 m along strike and 270 m below surface, upper limit approximately 20 m below surface. Nominal 7 m width.
Syndicate:
600 m along strike, 820 m below surface to lower limit, 276 m to upper limit. Nominal 1.6 m width.
Sunlight:
690 m along strike, lower limit 550 m below surface and upper limit 40 m below surface. The width of the breccias is 5m from hanging wall to footwall but the individual breccias are a nominal 1.5 m to 2 m width.