Geology and Mineralisation
The regional geology consists of early – mid Archean greenstone belts, intruded by late Archean granitoids and overlain by sporadic Proterozoic metasediments. Vast areas of Cainozoic sediments and transported regolith cover the region, showing a close spatial relationship with the underlying bedrock.
Research conducted by Tingey (1985), subdivided the Gum Creek Greenstone Belt into three geological domains. These comprise the Lake Mason Zone and Montague Range Zone, which are dominated by basalt and BIF, and the Intermediate Zone, which sits between the Lake Mason Zone and Montague Range Zone. The Intermediate Zone is poorly exposed, but where outcropping, is dominated by felsic sediments and volcaniclastics. The Gum Creek Greenstone Belt was inferred to contain an overall synclinal structure with a low-grade, greenschist facies metamorphism (Elias et al. 1982; Tingey 1985).
Local Geology and Mineralisation
The areas of interest are centred on the Montague Granodiorite Dome, an elliptical pluton of enriched dioritic to granodioritic composition which forms the core of an open north-plunging anticline. The granodiorite has dimensions of approximately 8.5km x 2.6km and has intruded into a sequence of metamorphosed basalts and volcano-sedimentary rocks. Steeply east dipping, the granodiorite contacts are discordant with the immediate surrounding basalt stratigraphy which on western side is shallow west dipping between 30-45 degrees and in the east, steeply east dipping.
A mafic intrusion occurs along the western margin of the granodiorite and is locally fractionated from Olivine Gabbro to Dolerite and has intruded along the contact zone after the emplacement of the granodiorite. This unit is generally <60m wide but is likely to have been structurally duplicated by shearing along the western margin of the granodiorite.
Mineralisation at the Gidgee Gold Project shares a strong spatial relationship with the margin of the Montague Granodiorite and occurs predominantly as NNW striking lodes within moderate dipping shear zones laterally continuous (Montague-Boulder/Evermore) as well as steep faulting and veining (Whistler) within both basalt and granitoid lithologies. Transported regolith and surficial cover mask a significant portion of the region, with outcrops limited to low relief slopes of metabasalt and sub-cropping granodiorite.
The Montague-Boulder deposit is located at the north-western contact of the Montague Granodiorite with the adjacent shallow dipping basalt stratigraphy and was previously mined as an open cut pit during the 1990s. The deposit is interpreted to comprise two domains of mineralisation, the eastern granodiorite hosted lodes and a western domain of WSW dipping mafic hosted lodes which predominantly comprise the resource area.
Mineralisation within the resource area is associated with the laterally extensive and well-developed Montague-Boulder shear structure, one of several parallel shallow west dipping shears (interpreted as thrust faults) that extend along the western margin of the Montague Granodiorite occurring between basalt flow boundaries .
Shearing and alteration is often strongly developed and is in places several 10’s of metres thick and continuous for several kilometres along the margin. Alteration is typically zoned from outer chlorite to inner biotite-carbonate + quartz veining which is most pronounced within the basalt stratigraphy. Mineralisation is present both within quartz veining and within shear zone alteration locally concentrated within the Montague-Boulder Resource area in response to interpreted NE cross cutting structures and variation in geometry of the primary Montague-Boulder shear zone.
The Evermore deposit is located approximately 800m along strike of the Montague-Boulder Resource and hosted in shearing within the western mafic stratigraphy, understood to be the continuation of structures hosting the Montague-Boulder resource.
The distribution of gold mineralisation within the shearing is interpreted to be related to the combination of the varying dip of the primary shear and subsidiary splay shears, host lithology and proximity of cross cutting NE trending structures. Mineralisation is similar to that at Montague-Boulder, exhibiting a zoned outer chlorite and inner biotite-carbonate alteration and associated quartz veining withing shearing. The majority of Evermore mineralisation is associated with a primary shallow dipping structure and flat subsidiary shear which splays from the main primary shear at a consistent RL ~400-420mRL and continuous for over 1km. The highest tenor mineralisation has so far been identified where this flat structure passes through a gabbro unit (altered to talc-carbonate) adjacent to the Montague Granodiorite. The intersection of the flat structure and gabbro unit creates a linear NNE trending, shallow plunging lode geometry which persists over 1km at an RL of between 430m in the south and 400m in the north. NNE trending faults with minor offset occur north and south of the resource area which may also have had an effect in mineralisation distribution in the lode.
The Achilles North deposit is located north of the historic Rosie open pit and includes direct extensions to existing mineralised zones along a strike distance of 500m. The mineralisation at Achilles/Rosie is broadly associated with the sheared western margin of the Montague Granodiorite which forms a NNE trending structural corridor also hosting the Airport, LA international and several other historical gold prospects. Predominantly shallow oxide and supergene in nature, the mineralisation is associated with a series of moderately (55-60°) east dipping shear structures and quartz veining which host primary mineralisation and occur within the contact zone between granodiorite, dolerite and basalt lithologies. Mineralisation extends to the near surface and in places, directly beneath the base of transported cover.
The Airport deposit is hosted entirely within the Montague Granodiorite within the same NNW trending structural corridor as the Achilles/Rosie deposits and located approximately 500m south along strike. North-northeast trending cross cutting faults are believed to have localized mineralisation at Airport within the greater Achilles corridor.
The majority of mineralisation occurs at shallow depths within the oxide zone forming three stacked supergene blankets between 9m and 28m below surface which overprints a primary network of shallow and steep dipping, quartz stringers with associated weak shearing.
The Whistler open pit was the last pit to be discovered and mined in the Gidgee project area due to the lack of geochemical signature at surface. The geology consists of a granodiorite-basalt contact which strikes at ~345°, along the shear zone through the center of the pit. Several gabbroic dykes can be seen intruding the basalt to the east, while thick 1 – 15m thick lamprophyric dykes are seen to intrude into the granodiorite.
A ~1m thick quartz vein is visible on the eastern wall of the Whistler pit with sheared contacts, believed to be an extensional fracture and is not mineralized. At the southern end of the pit, two sets of sheeted quartz veins demonstrate a cross-cutting relationship, suggesting a conjugate vein set with a maximum compressive stress vector at ~30°. Beeson et al. suggest a dextral transcurrent shear model, with parallel stockwork vein sets in an en-echelon arrangement along the basalt-granodiorite as being host to the mineralization at Whistler. At the current water level in the pit, Standing was not able to make the same observations. He suggests that the conjugate vein arrangement could support this model. Drilling in the past few years by Gateway Mining has confirmed the presence of stockwork vein sets within the granodiorite unit near the contact as hosting the majority of the mineralisation at Whistler.