Summary:
Steeply inclined north-northwest, northwest, west-northwest mineralised structures dominate the 10km strike of the Hillgrove mineral field. The mineral field spans across three geological units, a northern monzogranite (Hillgrove adamellite), an early-stage metasediment (Girrakool Metasediment) and a late Itype diorite in the south.
The main mineralised structures are composite, occurring as anastomosing sets of fractures, which pinch and swell along-strike. Local dilutional zones host mineralised hydrothermal breccias. The main structures are accompanied by arrays of sub-parallel narrow veins. The northwest striking mineralised structures commonly contain lamprophyre dykes which have been emplaced into mineralised rock and have themselves been variably altered and mineralised.
The mineralisation occurred late in orogenic development and has characteristics of most structurally controlled mesothermal deposits. With metamorphic derived mineralising fluids migrating during uplift and unloading through shear zones to the brittle-ductile transition at which point deposition occurred within high angle faults. Deposition sealed fluid paths and promoted cyclic deposition.
Locally, the mineralisation of the structures occurs as simple single veins, quartz-wallrock breccias, zones of parallel stringer veins and splay structures. Bifurcations in the major structures enclose mineralised zones up to eight metres in width where tension gash type stringer veins cut across the enclosed rocks. Splay structures enclose similar zones that lessen as the structures diverge. Larger splays will separate up to 20m from their parent structure.
Within structures the highest grades occur in vertical to steeply plunging dilatational shoots that can occupy up to 60% of the structure. Zonation of stibnite is recognized in the metasediments and the monzogranite where it is most strongly deposited within 400m of the surface. Otherwise, individual structures have a consistent mineralogical character with phases occurring in comparatively uniform proportions.
Major structures are seen to contain regular mineralisation over strikes of up to 1.2km. These major structures occur within corridors that span up to 10km strike of the Hillgrove Mineral Field.
The Hillgrove Mineral Field is cut by two regional scale faults of east-northeast strike, the Hillgrove Fault on the northern margin and the Chandler Fault on the southern margin. These faults pre-date the mineralisation, with late reactivation opening dilation zones along shear structures between the bounding faults. These dilation zones provide favourable sites for mineralisation. Nearly all the mineralised shears at Hillgrove are associated with a NW trending structural belt between the two faults, with dips commonly 70° to vertical. A major structure running through the centre of the field from Brackins Spur in the south, through the Garibaldi and Eleanora mines, to the Cosmopolitan deposits in the north can be traced over a strike length of 4kms. The Metz Mining Centre is located to the west of this structure.
Gold and antimony mineralisation at Hillgrove are structurally controlled. The deposits exhibit various styles of hydrothermal activity, with veining ranging from simple single veins through parallel stingers to quartz stockwork and wall rock breccias. All major veins have been intruded along shears with left lateral movement. The shears range in width from millimetres to multiple metre widths. Splits in the veins enclose high grade mineralised zones where tension gash type stringer veins cut across the enclosed rocks. Splay veins enclose similar zones that die out as the vein diverges away from the main lode.
All phases occur within ore bearing structures, with the first two phases often sealing structures in the granites resulting from restrictions to later phases. The arsenopyrite phase forms a broad halo of fine parallel veins in a siliceous-sericitic alteration. It appears that all wall rock alteration is associated with this phase, as there is little dispersion of stibnite into surrounding rocks. Alteration effects are commonly on the scale of metres around structures, occurring via pervasive fluid flow, with the more focused quartzstibnite open space filling phase following. The arsenopyrite phase is responsible for most refractory gold in the deposits with the particle free gold associated with the quartz-stibnite-gold phase.
Ore grade material in structures is restricted to vertical or steeply plunging ore shoots, caused by localised flexures forming dilational jogs. The ore shoots generally occupy up to 60% of the structures with good vertical continuity.
Dimensions
The Eleanora / Garibaldi mineralised system is defined over 1.3km along strike to 800 m below surface. The Resource is currently limited to 500m below surface. The width of the mineralisation is generally between 0.3 to 6m. A lamprophyre dyke of generally around 1m width has intruded along the mineralised structure and often divides the mineralisation into parallel lodes.
Although the mineralisation is generally strongest on the main structure; splays, parallel structures and network veining host hanging wall and footwall mineralisation.
In the south, in the Garibaldi area an additional two parallel lodes are defined in the east wall. Of these lodes the eastern lodes become more dominant toward the south. In this area the resource is limited to 300m depth due to the current depth extent of the drilling.
Blacklode is defined over 900 m along strike to 700 m below surface. The width of the mineralisation is generally between 0.3 to 2 m reaching up to 8 m. 10 adjacent sub parallel or splay lodes are included in the Blacklode Resource.
Sunlight is defined over 690 m along strike to 550 m below surface. The Sunlight Resource includes the two main breccias (strike 115), generally 0.2m to 2 m wide, separated by up to 5 of weaker vein mineralisation. 10m to the north a similar sub parallel weaker mineralised lode occurs. Two additional lodes Magazine reef (strike 150) and Gold Zone (strike 100) each of 180m strike, occur south of the Blacklode to Sunlight junction.
Syndicate mineralisation is defined along a 500 m strike and to a depth of 800 m below surface. The width of the mineralisation is generally between 0.3 to 2 m reaching up to 8 m. The current Mineral Resource excludes historically mined areas and is defined between 300m and 800m below surface.
The mineralisation is defined within a shear structure containing stibnite veining and gold mineralisation within quartz – arsenopyrite veining. Minor sub-parallel lodes were also modelled but were not included in the Mineral Resource
Brackins Spur is defined over a 1.2 km strike from 20m to 520 m below surface. The oxidized zones have been excluded from estimates. Nominal 4 m width.
Clarks Gully is defined over a 550 m strike from 20 m to 270 m below surface. Nominal 7 m width.