Summary:
The gold deposits at Higginsville are consistent with the greenstone-hosted quartz-carbonate vein (mesothermal) gold deposit model.
Higginsville Gold Operations can be subdivided into seven major geological domains: Trident Line-of-Lode; Chalice; Lake Cowan; Southern Palaeochannels; Mt Henry; Polar Bear Group; and Spargos Project Area.
Trident Line-of-Lode
The majority of mineralisation projects along the Trident Line-of-Lode are hosted within the Poseidon Gabbro and high-MgO dyke complexes in the south. The Poseidon Gabbro is a thick, weakly-differentiated gabbroic sill, which strikes north-south and dips 60° to the east, is over 500 m thick and 2.5 km long.
The mineralisation is hosted within or marginal to quartz veining and is structurally and lithologically controlled. Veins occur on and adjacent to the thrust contact and may be up to 3 m in width and lie preferentially in the basalt host. Alteration consists of silica flooding which has obscured older textures; locally intense biotite alteration within the basalts closely associated with the silicification and arsenopyrite alteration is common and locally intensified with the quartz veining and silica-biotite alteration. Laterisation and erosion have resulted in supergene enrichment within the transitional layer following downward surface water leaching of the upper saprolite.
Chalice
The dominant unit that hosts gold mineralisation is a fine grained, weak to strongly foliated amphibole-plagioclase amphibolite, with a typically lepidoblastic (mineralogically aligned and banded) texture. It is west-dipping and generally steep, approximately 60° to 75°. It is typically more competent than the ultramafic unit. The amphibolite is of basaltic derivation, with alteration and the metamorphic grade generally increasing markedly towards the main mineralisation zone.
Lake Cowan
The area is situated near the centre of a regional anticline between the Zuleika and Lefroy faults, with the local geology of the area made more complex by the intrusion of the massive Proterozoic Binneringie dyke. The anticlinal system is in a rift-phase portion of the greenstone belt, comprising a complex succession of mafics and ultramafics, sulphidic carbonaceous shales, felsic volcanics and volcaniclastic sediments. These have been intruded by several younger felsic granitoids.
The area is interpreted to have undergone intense intraformational folding and transposition, and has a metamorphic grade estimated to be upper greenschist facies with local hornfelsing proximal to the Binneringie dyke.
The Binneringie dyke varies locally from a hornblende dominated dolerite to a feldspar dominated granodiorite, is medium to coarse grained, and is complexly interrelated to the mineralised structures in the Lake Cowan area. In a break of form for these generally east- northeast – west-southwest trending dyke systems, at Lake Cowan the Binneringie dyke follows the deep seated crustal weaknesses north and south for some distance, in the process interfering with the pre-existing mineralisation on a large scale. The majority of mineralisation at the Lake Cowan Mining Centre is hosted within an enclave of Archaean material surrounded by the Binneringie dyke.
Southern Palaeochannels
Throughout the Higginsville Gold Operations, a significant proportion of gold deposits are hosted by sediments within the Southern Palaeochannel network. Mineralised zones comprise both placer gold, normally near the base of the channel-fill sequences, and chemicallyprecipitated secondary gold within the channel-fill materials and underlying saprolite. These gold concentrations commonly overlie, or are adjacent to, primary mineralised zones within Archaean bedrock.
Outcrop is generally poor, due to extensive ferruginisation, calcareous soils, aeolian sands and extensive areas of remnant lacustrine and fluvial sediments. The result is a complex, layered regolith, with considerable chemical re-mobilisation and re-deposition (Lintern et al., 2001).
The regional palaeodrainage system has incised several fault-bounded greenstone sequences, which comprise high-Mg basalt, komatiite and minor interflow sedimentary rocks, intruded by dolerite and gabbro. The orientation of palaeochannels is largely controlled by major faults and shear-zones, that trend north-northwest, parallel to lithological contacts (Swager, 1989; Griffin, 1990).
The Cowan palaeodrainage system that includes the Challenge/Swordsman and Mitchell palaeochannels comprises up to 100 m of Cainozoic sediment overlying Precambrian basement. Clarke (1993) divided the sedimentary sequence into the Eundynie Group, comprising a succession of Eocene sedimentary rocks, and the overlying Redmine Group, comprising Oligocene to Recent deposits.
Two main palaeochannel systems exist at Higginsville:
• Mitchell palaeochannel system includes the existing pits of Graveyard North, Graveyard, Aphrodities and Mitchell. Mitchell 3 and 4 remain unmined and are located to the south of the existing Mitchell pit.
• Challenge/Swordsman palaeochannel system includes the existing pits of Bullseye, Venus, Jupiter, Saturn, Neptune and Pluto. The areas of Mars (south of the Venus Pit) and the southern extension of Pluto remain in situ (unmined). Both palaeochannel systems get deeper towards the south (which suggest the flow direction of the ancient rivers) and flow into Lake Cowan.
Mt Henry
The Archean rocks in the Norseman area have historically been classified into a series of formations, with the following stratigraphic sequence.
The Penneshaw Formation forms the greenstone sequence on the eastern side of the belt. It consists of predominantly mafic volcanic rocks with inter layered units of felsic volcaniclastic and sedimentary rocks, and is intruded by dolerite sills and dykes. Units of the formation host the gold mineralisation at Everlasting and Mildura prospects.
The Noganyer Formation forms a distinct sedimentary sequence of siliclastic rocks, principally silicate facies BIF, chert, sandstones and shales. Intrusions of dolerite dykes and sills are common throughout.
The Woolyeenyer Formation hosts the Norseman style quartz reef gold mineralisation, as well as the Abbotshall gold deposit which is hosted in a regionally extensive porphyry/siliceous sedimentary unit of the Woolyeenyer Formation. The Woolyeenyer Formation is unconformably overlain by the Mount Kirk Formation.
Mount Kirk Formation consists of felsic volcanic and sedimentary rocks which are intruded by large, thick mafic sills. No gold deposits are known to exist within the Mount Kirk Formation.
Polar Bear Group
Lake Cowan covers most of the project area with a 1 m to 3 m thick layer of gypferrous mud and clay with a poorly developed halite crust. The deeper channels within the lake contain thick sequences of transported clays, with intercalated lignite beds from 1 m to 20 m, which overlie laterally extensive fossiliferous sandstone, locally known as the Norseman Formation of probable Eocene age. Locally running sands and gravels are present at the base of the channels, which can be in excess of 90 m thick. Some of these channels are mineralised with gold and have been mined historically, including the Challenge-Swordsman palaeochannel on the Eundynie Peninsula. The entire project area is intruded by numerous Proterozoic dolerite dykes.
Spargos Project Area
Gold mineralisation at Spargos Reward is hosted by a coarse-grained pyrite-arsenopyrite lode in quartz-sericite schists, between strongly biotitic altered greywacke to the east and quartzsericite-fuchsite-pyrite altered felsic tuff to the west. Gold mineralisation is associated with very little quartz veining which is atypical for many deposits in region. Sporadic lower-grade gold mineralisation (1–2 g/t Au) occurs within the footwall tuffs.