Summary:
Steeply inclined north-northwest, northwest, west-northwest mineralised structures dominate the 10km strike of the Hillgrove mineral field.
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.
Eleanora and Garibaldi Mining Centre
The Eleanora and Garibaldi Lode System is located adjacent to the Hillgrove Processing Plant and 1.5km to the east of the Metz Mining Centre (including Syndicate, Blacklode and Sunlight).
The Eleanora and Garibaldi Lode System is defined over a 1.3km NNW striking shear structure. The mineralisation is generally contained within a narrow shear/breccia that displays multiple hydrothermal fluid events and structural reactivation. The structure and mineralisation are near continuous and contain steeply south plunging shoots of higher-grade Sb-Au mineralisation.
Metz Mining Centre
The Metz Mining Centre is located east of Bakers Creek and 1.5km east of the Hillgrove processing plant. It includes Mineral Resources from the Blacklode, Sunlight and Syndicate lode systems hosted withing the Girrakool metasediments. Blacklode contains east plunging shoots of high antimony and gold mineralisation. Ten lesser subparallel or splaying lodes adjacent to the main shear are included in the Blacklode Mineral Resource.
The Sunlight Lode occurs as a major splay away from the Blacklode structure. Sunlight splays to the south-east as generally two parallel shear/breccia lodes. The structure has been subjected to multiple hydrothermal fluid events and structural reactivation. The intersection of Blacklode and Sunlight Lodes contains high antimony in a limited area. The remainder of the Sunlight Lode is gold dominated with low levels of antimony and tungsten and is more analogous to the Bakers Creek style of mineralisation to the east.
Brackins Spur
The Brackins Spur deposit is located on the southern end of the Central Eleanora Structure, a significant NW trending shear zone that can be traced through several workings for approximately 4km. Hosted in the Bakers Creek diorite, it includes a range of rock types including tonalites, granodiorites and diorites. Strong to intense hydrothermal alteration (predominately sericite) occurs in visibly deformed, veined and mineralised diorite.
Clarks Gully
The Clarks Gully deposit is an advanced antimony project located in the northern most mining lease. The deposit is adjacent to the broad confines of the Hillgrove Fault within the monzogranite, and its large width (up to 10m) is due to the intersection of two structural trends. A pre-existing, ENE trending mylonite zone associated with the Hillgrove Fault is cut by an array of NW striking veins, resulting in a significant dilation zone hosting a mineralised structural breccia. Mineralisation is associated with a network of quartz stringer veins, stockwork and sulphide matrix breccias with intense sericitic alteration of the monzogranite.