An area of high country in the far northern Mount Lofty Ranges was taken under licence to try to find the primary source(s) of widely dispersed detrital gold known from Recent and Tertiary cover sediments in the region, while focussing mainly on...
An area of high country in the far northern Mount Lofty Ranges was taken under licence to try to find the primary source(s) of widely dispersed detrital gold known from Recent and Tertiary cover sediments in the region, while focussing mainly on identifying a source for the large reconstituted composite nuggets found at the Ulooloo alluvial goldfield. The overall aim was to find metasediment-hosted, Carlin type large bulk tonnage low grade gold resources of mineable value. In recent times, regional studies of the Nackara Arc region of South Australia undertaken by the Geological Survey of South Australia and by mineral explorers had demonstrated that it is a tectonic environment likely to have induced structural permeability that has facilitated the migration of mineralising fluids. Initial research performed by Colin Brooks, the subject licensee, and his technical associate Lindsay Curtis indicated that complex landform conditions, plus often concealed bedrock and pervasive palaeoweathering effects on its exposures that hinder recognition of primary alteration, could now be surmounted by the application of high precision geochemical exploration methods; a limited field trial of these demonstrated that significant stratabound gold mineralisation could be present within the Basin Creek catchment area. Evaluation of the trial BLEG sampling assay data revealed that ultra-fine gold (~10+/-5 microns particle size) is present, which may be a primary relict preserved through diagenetic silica encapsulation in this low permeability environment. The bleached condition of the bedrock, existence of the above 'micro-nuggets', and strong relative gold depletion within recognisable positive gold pathfinder element anomalies of stature, collectively were taken to indicate that pallid zone leaching processes had been active. Thus the licensee concluded that good potential existed for finding higher tenor gold mineralisation within the Mount Scrub Range : to pursue this, a refined stratabound target model was proposed. During the first licence year, effort was put into developing a target - befitting and locale - suited geochemical exploration method which would effectively and reliably return critical information about the tenor and continuity of bedrock gold mineralisation. Using Colin Conor's Penny Creek BLEG gold discovery as an orientation test case, it was determined that high and variable regolith dilution factors were affecting gold grain dispersion within the surrounding complicated alluvial environment. Rock chip assay results from exposed bedrock, obtained using a new low (5 ppb Au) analytical detection limit, were also compared with the spatially equivalent historic rock chip geochemical anomalies, and from this it was deduced that the region had multiple sources from which gold was being released into Basin Creek, largely comprising current bedrock exposures (scalds) and slightly older transient alluvial deposits in the catchment. Accordingly, the decision was made to systematically sample bedrock below the exotic soil cover in such a way as to derive a cross-stratal geochemical transect with a high degree of continuity, that would be independent of any later, superficially mobilised gold input influences. Full continuous shallow sampling of the bleached bedrock by trenching was selected to be the most appropriate approach, compared to digging rows of close-spaced auger holes or backhoe pits. During Year 2, a trenching programme was conducted with a contractor's Bobcat-mounted boom digger that used a toothed chain. Consideration was given to avoiding contamination of the bedrock sample with soil cover. This was satisfactorily overcome by making two passes. The first cut was designed to penetrate at least 10 cm below the red-brown soil cover. The Bobcat was then driven over the windrow to tamp it flat, and a second cut made along the same trench slot deposited a tidy windrow of all-bedrock cuttings on top of the former. The uppermost part of the bedrock cuttings material was then sampled in 1.5 m lengths using PVC U-channel receptacles (~16 cm2 cross section) to ensure uniform mass distribution, and to provide a 2.0-2.5 kg individual sample mass. 837 such trench samples taken along 9 field traverses in March 2004 were then analysed using the Instrumental Neutron Activation Analysis (INAA) method by Becquerel Laboratories in Sydney, for preference irradiating 30 g aliquots of the -150 mesh sample fraction. 64% of the trench samples returned gold values in excess of the -5 ppb detection limit, ranging between 5.09 to 611 ppb Au, with a dominant concentration range of 20-40 ppb Au. Much of this gold is present as grains of <5 microns in size. An inferred maximum single gold grain size of ~23 microns was modelled using empirical nomograms: however, the existence post-leaching of residual pockets of larger 'nugget' gold was implied in the assay results returned by 30% of the samples (where perhaps 60-90 such composite gold particles are contained per sample) . A major new conclusion made from the above work was that a significant portion of the near-surface gold has a grainsize of 7 ppb Au occurs in soil vs a <1 ppb background; this area surrounds a small 50+ ppb core zone. The MMI profiles made at Gosse's Ridge and Twigham Nose, where in total a further 192 soil samples were collected, demonstrated that gold and silver enhancement is associated with the Saddleworth Formation along 3 km of strike in each locality. Silver in these areas is relatively high compared to at Penny Creek, and a lithostratigraphic base metal association is inferred. At Twigham Nose, gold enhancement is associated with the overturned thrusted south-eastern limb of the anticline, and for the first time it was thought that this observation might serve to resolve the enigma of the Ulooloo Alluvial Goldfield gold source. Further infill MMI profiling was seen as necessary in both areas to constrain the anomalous patterns already detected. HyLogger SWIR scans of 271 analytical pulps from surface rock samples, shallow trenches and weathering zone sections of the licensee's drillholes identified abundant kaolinite, muscovite and phengite. Phengite is a high silica variety of muscovite which has variable Mg and Fe lattice substitution for K. Its diagnostic hyperspectral signature makes this relatively easy to identify alteration mineral a potential vector to mineralisation. There appeared to be a correlation of phengite / high temperature kaolinite to gold mineralisation found in Wonna's drillhole BDRC07. The tempo of exploration slowed a little during 2010-2011. Extensive negotiations with third parties for attracting interest to fund the ongoing project proved unsuccessful, so a decision was made to persist only with the limited funds still available. Drilling at Penny Creek was committed to proceed, for three steeply inclined RC holes each to ~100 m depth to test unweathered bedrock below the significant 50+ ppb MMI gold anomaly. An additional 48 rock chip samples were collected in the western and south-eastern portions of the tenement and submitted for INAA and SWIR spectral analysis, as prospecting was continued further away from proven areas of interest, with particular attention paid to areas of bleached and degraded bedrock mapped at 61 locations. A significant zone of likely strong hydrothermal alteration was found in an abandoned railway cutting ~5 km north-west of Hallett. After much delay, the three inclined RC drillholes at Penny Creek were completed during mid-July 2012, each to 100 m total depth, but the results were very disappointing, as they indicated that the hoped for primary (sub-weathering) gold sources were not present below the MMI targets [Planned hole BDRC10 was not drilled]. The geology encountered comprised a secondary, weathered zone about 10 to 25 m thick of buff coloured, highly weathered shales grading somewhat abruptly to a parent rock of grey, weakly pyritic Tapley Hill Formation shale/siltstone. The gold content in the weathering zone in each hole frequently exceeded anticipated background (10 ppb Au), including values up to some hundreds of ppb, with one exceptional 1-metre sample interval spike value of 2270 ppb Au. The primary zone rock is very weakly gold-anomalous, with spotty values generally occurring in zones 1 to 2 m thick, of up to 5-20 ppb Au, among lengthy intervals of 50 ppb discovered at Penny Creek is reflecting an accumulation of trace gold within the weathering profile, without any significant underpinning primary mineralisation. By inference, this meant that MMI is unable to distinguish between primary and shallow supergene gold sources in this geological setting. Therefore, without the aid of a supergene-hypogene gold discriminator, it was decided that to continue exploration there at an acceptable risk and cost was no longer feasible.