Figure 1. Domains of Eastern Goldfields Super Terrane (Swager, 1989).
The Bullabulling Project is comprised of 26 wholly owned tenements covering an area of approx. 50km2 to the west of Coolgardie in the Eastern Goldfields of Western Australia.
The Project area is located in the Bullabulling Goldfield, which is part of the Coolgardie Goldfield (Figure 1). The Coolgardie Goldfield covers approximately 900km2 from Mt Burgess in the north to the Londonderry Pegmatite in the south and is part of the Coolgardie Geological Domain of the Kalgoorlie Geological Terrane (Figure 1). The first discovery of gold in the Coolgardie Goldfield was in 1892, and it has since produced more than 3 million ounces of gold.
The broader region is also emerging as an area of significant lithium prospectivity with recent exploration success including Future Battery Minerals Limited (ASX:FBM) [formerly Auroch Minerals Ltd (ASX:AOU) Kangaroo Hill Lithium Project and BMG Resources Limited (ASX:BMG) Ubini and Red Panda prospects both located within the same structural trend as Bullabulling Project, and these prospects are considered analogous to the pegmatites recently identified in the Bullabulling Project area.
Figure 2. Local geology surrounding the BelaraRox WA tenements.
Red = basalt | Green = dolerite | Blue = felsic volcanics | Orange = granite
Pink = sediments | Light green = ultramafics (Kenex Ltd, 2018b).
Dots show locations of mineral occurrences (GSWA, 2018).
Archaean Orogenic gold deposits in the Bullabulling Goldfield occur in all lithologies, but with a spatial preference for mafic and iron rich lithologies (Figure 2). Gold mineralisation appears to be broadly synchronous with peak metamorphism, the main phase of granitoid emplacement and regional deformation. The most distinctive feature of the Bullabulling Goldfield compared to the greenstone sequences further to the east is the increase in regional metamorphic grade from greenschist facies in the east to mid to upper amphibolite facies in the west where the Bullabulling Project is located. Several different structural mineralisation styles are present, including gold in alteration halos along sheared ultramafic rock contacts, gold in gabbro-hosted quartz-vein sets, gold in fault-bounded quartz vein sets, and gold in laminated quartz reefs sited in brittle ductile shear zones.
Mineralisation in the Bullabulling Project area is controlled by D1, D3, D4, and D5 structures (Partington et. al., 2017). Gold was deposited during D1-D4, with the geometry of the deposits being modified during D5 (Partington et. al., 2017). Gold is primarily hosted in D4 high strain zones, as well as adjacent D1 and D3 structures. The highest gold grades are associated with D5 high strain zones, particularly along the margin of the Bali Monzogranite, where D4 structures are locally steepened (Figure 2 and Figure 3). Gold mineralisation in the Bullabulling Goldfield is not confined to a major regional shear zone as previously interpreted, and that the distribution of gold mineralisation is more extensive than originally thought. Consequently, the prospectivity of the goldfield has been underestimated and exploration not effectively targeted.
Figure 3. Geological features interpreted from seismic data. Note anticlinal and dome-like features defined by the ultramafic amphibolite marker horizon in green mapped by the seismic associated with the main zones of gold mineralisation (in gold) and distinct discontinuity at depth that marks the top of the Bali Monzogranite (in purple) against which the folds terminate. The Bullabulling Monzogranite (in red) appears late and cuts the earlier structures and lithological contacts.
The ultramafic units are well defined reflectors in seismic data that can be traced from the contact of the Bali Monzogranite, where ultramafic amphibolite facies lithologies crop out with shallow dips, through to the Bullabulling Trend (Figure 3). The seismic maps a sequence of sub horizontal units that form a broad basin structure with several minor anticlines that taper out towards the Bali Monzogranite contact at depth. There is a clear spatial association between gold mineralisation with folds interpreted on the seismic sections, which have not been targeted to date by exploration (Figure 2 and figure 3). The largest gold resources along the Bullabulling Trend and at Gibraltar are spatially associated with the 100m scale folds. Additionally, folds tend to be located where the contact of the Bali Monzogranite steepens to more than 40o. The Bullabulling Project tenements that cover the contact of the Bali Monzogranite cover similar prospective structures that host mineralisation at the Bullabulling gold mine.
More than 3,000 soil samples have been collected and 71 RAB holes have been drilled to sample the bed rock in the Bullabulling Project tenement area.
The RAB drilling is mostly shallow, with total depths of between 3 and 60m, suggesting the depth potential has not been adequately tested when the 3D prospectivity targets are considered. Several anomalous gold intersections in the RAB drilling have also not been followed up. The Bullabulling Project tenements are at an early stage of exploration, with only soil sampling and shallow RAB drilling having been conducted on the ground. There is therefore the potential for the discovery of gold and, or nickel mineralisation at depth.
Figure 4. Lithium deposits and projects located within the Coolgardie region.
The Bullabulling Project is also host to several potential Lithium-Caesium-Tantalum pegmatite systems associated with highly fractionated granites, including the Bali Monzograntite in the east, and the Bullabulling Granite in the west. These granites are associated with pervasive post-gold pegmatites and quartz veining, with most of the regional Lithium projects located within a structural corridor adjacent to these fractionated granitic pegmatite source rocks.
Rock chip assay results from recently mapped pegmatites have returned elevated lithium (Li) and Rb values. Lithium (Li) values were converted to lithium oxide (Li2O) using the standard conversion ratio of 2.153. Rubidium in particular shows widespread anomalism in these pegmatites and is considered a good pathfinder element for the identification of fertile LCT pegmatites. Elevated lithium values are more localised, which is attributed to the highly weathered nature of the rock chip samples and the corresponding higher mobility of lithium in this type of material.
Figure 5. Bullabulling Project showing rock chip results with the Belararox tenements and proximity of the existing mineral occurrences in adjacent tenures.
The most significant rock chip assay results include:
- BBGS029 ʹ 196ppm Cs, 2,444 ppm Li2O, and 982 ppm Rb located in the south of the project adjacent to the Bali Monzogranite.
- Anomalous Rb values in an extensive pegmatite system along trend to the south of Ubini with several values approximating or exceeding 500 ppm, including: BBGS004 @ 577 ppm Rb, BBGS006 @ 496 ppm Rb, and BBGS010 @ 573 ppm Rb.
- Several discrete pegmatites located in the western portion of the project, more distal to the Bali Monzogranite and in favourable mafic metamorphic rocks, showing anomalous Rb values including: BBGS061 @ 839 ppm Rb, BBGS059 @ 676 ppm Rb, BBGS037 @ 750 ppm Rb.
- There is a general correlation between anomalous rubidium and elevated tantalum values. The widespread distribution of these pegmatites, with both anomalous Li and Rb, is considered very encouraging and suggests the broader Bullabulling Project area is host to a potentially significant LCT pegmatite district. The first assessment to date, pending further technical evaluation, is that the sampled pegmatites on the eastern side of the Bullabulling project are likely to be staged first for the ground follow-up soil sampling.
Based on the recent success of geological mapping and rock chip sampling, the future proposed exploration program will continue to assess the potential of both LCT pegmatites and gold across the Bullabulling tenement package.