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 main geological units; a northern monzogranite (Hillgrove Monzogranite), an early-stage metasediment (Girrakool Metasediment) and a late I-type Bakers Creek Diorite in the south.
Geology and Mineralisation Styles
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.
Gold and antimony mineralisation at Hillgrove is structurally controlled as anastomosing sets of sinistraloffset 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.
The deposits display varied hydrothermal styles, from single veins to stingers, stockworks, and breccias. Shear zones range from millimetres to metres. Vein splits and splay veins create high-grade zones where tension-gash stringers crosscut enclosed rocks. Repeated sealing and overpressure produced cyclic mineralisation. Mineralisation shows steeply plunging ore shoots, likely formed in dilational jogs, with good lateral and vertical continuity despite local pinching and swelling.
Highest grades occur in steep, dilatational shoots that may occupy up to 60% of a structure. Stibnite is most abundant in the upper 400 m, transitioning downward from stibnite-dominant to stibnite-gold and finally gold-dominant zones. Some structures retain consistent mineralogy with depth.
All mineralisation phases lie within or adjacent to these structures, with early phases sealing granite-hosted conduits. The arsenopyrite phase forms fine parallel veins and metre-scale silica-sericite alteration halos, and hosts most refractory gold. Free gold is linked to a later quartz-stibnite-gold phase.
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 (2) 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. This combined with the extension drilling to the south of the Garibaldi area has defined the reported Garibaldi Mineral Resource. This mineralisation extends from surface to a depth of 315m over a strike of 350m. The upper portion of the reported Eleanora Mineral Resource contains remnant mineralisation associated with historic mining north of the Garibaldi shaft and the continuation of the mineralisation to 220m below the lowest mining level and 540m below surface.
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. A generally barren, syn-post-mineral lamprophyre dyke of around 1m width has intruded along the mineralised structure and often divides the mineralisation into parallel lodes, each generally of 0.5m to 3m width.
Metz Mining Area
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.
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. Multiple phases of hydrothermal fluids within the Brackins Spur structure have occurred.
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.