The Bartons, Crossing, Genie, Hopetoun and Red Ensign deposits are situated within tenements forming part of the greater Nullagine Gold Project, and are held by Millenium Minerals Pty Ltd; a wholly owned subsidiary of Calidus Resources Ltd.
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Summary:
PROPERTY GEOLOGY
Diamond drilling during the 2018 field season confirmed the Nullagine sub-basin subdivision of the Hardey Formation by Blake (2001). Mineralisation is restricted to a ~200 m sequence of poorlystratified, poorly-sorted, polymictic, pebble-to-boulder ferruginous conglomerate sequence (P4), which is restricted to an area within a few kilometres of Nullagine (Figure 7.4). The underlying sequence (P3) is of similar composition, but generally finer and including sandstone beds and minor tuffs. The overlying sequence was not separated by Blake and resembles the lower sequence (P3). These beds are characterised by a more regular sediment input and range between sandstone to pebble-conglomerates with a number of extensive tuff horizons.
PROPERTY MINERALISATION
Gold mineralisation within the Beatons Creek conglomerates occurs as fine grains, larger flakes, and rounded particles up to several millimetres across, but rarely exceeding 2 mm. Coarse and fine gold is spatially related to higher concentrations of pyrite, and there seems to be a correlation with gold and the ‘buckshot pyrite’ clast size. Coarse gold particles are regularly visible, and fine gold can readily be panned from crushed matrix material with large pyrite concentrations.
Mineralisation is restricted to fluvial type channel conglomerates or marine lag reworked conglomerates and readily recognisable from outcrop and drill core. The wider Beatons Mineralised unit and Beatons Middle unit contain minor disseminated pyrite, but the background mineralisation is generally no more than 0.1 g/t gold.
DEPOSIT TYPE
Both fluvial and marine lag-type conglomerates are interstratified, indicating the depositional facies in which they formed were laterally proximal. The depositional environment for these conglomerates is interpreted to have been a river fan delta along a coastline as shown in Figure 8.1. During periods of low-stand, a braided river delta prograde seaward, depositing channelised fluvial type conglomerates.
As sea levels rose, wave action winnowed out fine, light sediment leaving behind a transgressive armoured lag deposit of large siliceous boulders and heavy minerals including gold. It is in this environment that the economic conglomerates at the Beatons Creek Project likely formed. This process repeated several times to create the interbedded conglomerates exposed currently.
The Palaeoplacer deposition model employed by Novo for the Beatons Creek Project is based predominantly on detrital gold sourced from the nearby Mosquito Creek Formation and deposited locally. Mineralisation is further concentrated by reworking an already endowed sequence of conglomerates by marine processes as described above.
Similarities with other conglomerate hosted deposits of similar age lends credence to the mineralisation model used. The presence of significant concentrations of rounded detrital pyrite was also a factor in reef and model identification, with the best exploration success primarily driven by understanding the sedimentary processes and their effect on concentrating gold. A clear correlation between high depositional energy (in channels) and amount of reworking (for marine lags) and gold content allows for a fairly straightforward depositional model to be successfully employed.
Some comparable conglomerate hosted deposits debate around potential hydrothermal mineralisation either as the sole mineralising event or as an overprint (Phillips and Meyers, 1989; Phillips and Law, 1994; Barnicoat et al., 1997). Despite local remobilisation of pyrite (and potentially gold) within the matrix, possibly due to dewatering during burial or low level metamorphism, no evidence of hydrothermal overprinting has been documented at Beatons Creek or elsewhere in the Pilbara.
Other debate around organic or microbially-mediated syn-sedimentary gold precipitation (or entrapment) (Hallbauer, 1975a,b; Mossman et al., 2008) is likely of less relevance at the Beatons Creek Project due to the limited amount of organic carbon (kerogen or stromatolites) in the system, but may play an important part with other conglomerate hosted gold targets in the wider Pilbara region (e.g., Virgin Creek).
Exploration by Novo has been successful in delineating the extent of marine lag mineralisation in areas beyond 100 m below surface. High density costean sampling across the full mineralised sequence has subsequently better defined the domains where channel mineralisation is common, with most dominant channels now well defined by sampling and drilling.