Summary:
Steeply inclined north-northwest, northwest, west-northwest mineralised structures dominate the 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 predominantly NW-striking dilation zones along shear structures between these bounding faults. Of this set of NW-striking structures, the major structure identified runs through the centre of the field from Brackins Spur in the south, through the Garibaldi and Eleanora mines, to the Cosmopolitan deposit in the north, and can be traced over a strike length of 4km. The Metz Mining Area is located to the west of this structure and is a combination of NW-striking structures and an almost E-W (~100°) major shear zone. The mineralisation occurred late in orogenic development and has characteristics of most structurally controlled mesothermal deposits.
With Gold and antimony mineralisation at Hillgrove is structurally controlled as anastomosing sets of sinistral offset fracture zones, which pinch and swell along-strike. Local dilational 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 taken advantage of the mineralised zones of weakness and have themselves been subsequently variably altered and mineralised, indicating the multiple episodes of mineralisation within the system.
Within mineralised structures, the highest mineralisation 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 mineralised in the upper zones of the system, usually within 400m of the surface. At around this 400m vertical depth, dominant mineral abundance transitions from stibnite-dominant, through a stibnite-gold zone, to a gold-dominant system with depth. Individual mineralised structures are occasionally observed which have a consistent mineralogical character with comparatively uniform proportions with depth.
Eleanora-Garibaldi Mining Area
The Eleanora-Garibaldi mineralisation is defined over a 1.3km strike and contained within a NW striking shear/breccia structure. The mineralisation is generally contained within this structure and adjacent selvedge and 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.
The two areas are essentially the one system, with the Garibaldi area located directly along strike, south of Eleanora. Here, the main mineralised structure is present as two parallel mineralised zones.
The mineral resources are hosted within the Girrakool metasediment package. The main structure and mineralisation extend north into the monzogranite, but no resources have been reported into this area and further exploration and drilling is required. Although the mineralisation is generally strongest on the main structure and splays, parallel structures and network veining host hanging wall and footwall mineralisation of varying width and tenor.
Metz Mining Area
The Metz Mining Area is located west of Bakers Creek and 1.5km west of the Hillgrove processing plant. It includes mineral resources for the Blacklode, Sunlight and Syndicate mineralised systems.
The Metz Mining Area is entirely hosted within the Girrakool metasediments. The Blacklode deposit -is defined over a 1km ~E-W striking shear structure. It occurs as a cross-linking, ductile shear in an area of predominately NW extensional shears (Syndicate, Coxes Lode and Bakers Creek). The Blacklode mineralisation contains easterly plunging shoots of high-grade antimony and gold mineralisation. Ten lesser sub-parallel or splaying lodes adjacent to the main shear are also included in the Blacklode Mineral Resource.
The Sunlight deposit occurs as a major splay away from the Blacklode structure, which 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. An initial phase of pervasive sericite-silica alteration has been overprinted with a broader ductile event consistent with the quartz-arsenopyritepyrite-gold phase. This has resulted in a wider selvedge zone of quartz stringer/individual veining to quartz breccias with disseminated refractory gold. Later reactivation resulted in a narrow zone (up to 2m wide) of brittle deformation, resulting in distinct hanging wall and footwall breccias with high grade free gold. These breccias are continuous along strike and depth, potentially joining in a combined breccia zone on the western end of the lode.
The intersection of Blacklode and Sunlight lodes contains a small area of elevated antimony mineralisation. The remainder of the Sunlight lode is gold dominated with lower grades of antimony and tungsten, which is more analogous to the Bakers Creek style of mineralisation to the east.
The Syndicate lode system is defined over a 600m SSE striking extensional shear structure which intersects the western end of the E-W striking ductile Blacklode shear. Syndicate contains narrow steep south plunging shoots of high-grade antimony and gold mineralisation, contained within a broader gold mineralised shear structure. Two minor adjacent structures run parallel to the main Syndicate lode. An initial phase of quartz-scheelite mineralisation has resulted in weak tungsten grades (~0.3% W) occurring sporadically as small clasts and veinlets, proximal to the peripheries of the shear. An arsenopyrite phase forms a broad halo of fine parallel stringer veins in siliceous-sericitic altered rocks within the shear and is responsible for much of the refractory gold in the deposit.
Brackins Spur
The Brackins Spur deposit is located on the southern continuation of the Eleanora-Garabaldi structure (hosted in the Bakers Creek diorite), it includes a range of other 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 located adjacent to the interpreted location of the Hillgrove Fault and the deposit is hosted entirely within the monzogranite. An ENE trending mylonite zone associated with the Hillgrove Fault is cut by an array of NW striking veins, with significant dilation and brecciation.
Mineralisation is associated with a network of quartz stringer veins, stockwork and sulphide matrix breccias with intense sericitic alteration of the monzogranite. Gold-rich arsenopyrite-pyrite-quartz-carbonate veins are overprinted with quartz-stibnite veins in a NW orientation. Low-grade refractory gold and the absence of free gold at Clarks Gully indicate low saturation levels in the arsenopyrite. Low-grade tungsten, in the form of scheelite veins, occurs on the periphery of the main structure. The deposit is open along strike and at depth, with the current drilling having only tested the mineralisation to a depth of 300m below surface. The position of the Hillgrove Fault and its effect on the mineralisation on the northern end of the deposit is untested and is a highpriority exploration target.