A licence area located at the eastern end of the Deering Hills in South Australia's Far North-west Aboriginal Lands, approximately 150 km south-west of Ayers Rock (Uluru), is being explored by Rio Tinto Exploration (RTE) and Delta Gold, as part of...
A licence area located at the eastern end of the Deering Hills in South Australia's Far North-west Aboriginal Lands, approximately 150 km south-west of Ayers Rock (Uluru), is being explored by Rio Tinto Exploration (RTE) and Delta Gold, as part of work planned for a larger set of project exploration licences currently held or under application for grant within the western Musgrave Province. Much of the licence is covered by a veneer of transported sand cover, so knowledge of the local basement geology is scant. However, the area is considered likely to host layered mafic to ultramafic intrusive igneous bodies belonging to the Giles Complex. The primary focus of exploration has been on locating economic deposits of massive magmatic Ni- Cu sulphides and platinoid metals, although the potential for finding occurrences of stratiform Broken Hill - style base metals, diamonds and precious metal mineralisation is also recognised. Exploration activity undertaken by project manager RTE during Year 1 of tenure consisted of: - an office-based review of previous exploration activities, which revealed that there was a limited previous exploration history of airborne magnetics, air photography and outdated geological mapping; - requesting and facilitating a blanket-style Work Area Clearance Survey to be undertaken by the Pitjantjatjara Council, AP and traditional owners, which on completion allowed RTE to have on-ground access to 450 square km (80%) of the tenement for exploration; - commissioning of a consultant's regional geological interpretation of the Musgrave Block and EL 2907 at 1:50,000 scale, using all available airborne magnetic data; - interpretation of ~2300 line km of airborne magnetic and radiometric data, flown at 250 m line spacing by Fugro Airborne Surveys, which had been acquired by RTE during 1999; - interpretation of data from a regional 'Tempest' time-domain AEM survey covering much of the Central Giles Project area (7313 line km in total; ~1300 line km flown over EL 2907, along 300 m spaced lines), which had been acquired by RTE between December 1999 and March 2000. Twelve AEM conductors were selected for further investigation as possible prospective basal portions of ultramafic intrusions. In addition, other conductive zones thought to reflect shallowly buried geomorphic features were recognised. Drainage patterns could be traced that outline two north-west to south-east trending palaeochannels that are several km wide, which appear to be controlled by splays from the Mann-Hinckley Fault zone; - performing high-power ground TEM surveys across 8 of the chosen AEM anomalies during April-May 2002 (31 profiles totalling 50 line km of 100 m interval moving loop coverage). Strong to moderate bedrock conductors were recorded at the K1, K2, K4, K6, K7 and K10 anomalies; - conducting geological mapping during May-June 2002 across prospects located adjacent to the Deering Hills, variously at 1:5000, 1:10,000 and 1:20,000 scales. The objective was to check airborne and ground EM anomalies and place them into geological context. However, restricted work access into portions of the prospect areas required some of the geology to be interpreted from air photographs and aeromagnetic data, and therefore parts of the prepared geological maps remain unverified. No Giles Complex intrusive rocks were found during the prospect mapping, and consequently the nickel-copper potential at Mount Hardy and along the north and north-eastern sides of the Deering Hills was downgraded. The bedrock exposures visited and mapped in the field comprise a suite of Birksgate Group metasediments displaying similar stratal characteristics to the Willyama Supergroup at Broken Hill. The mapping and later surface geochemistry results identified evidence of weak BHT style alteration, but no regional mineralised horizon was located. The previously unrecognised clastic and chemical sediments can be traced on magnetic images as continuous folded packages running along the northern side of the Deering Hills. RTE hopes that these units may form a recognisable subdivision to the poorly defined Birksgate Complex, and that they will be found to persist elsewhere across the Musgrave Province; - collecting 60 partially weathered rock chip samples from bedrock outcrops, for multi-element geochemical analysis. Base metal values obtained from these rocks were generally low. Some samples did return anomalous results, with maximum values of 1100 ppm Zn, 360 ppm Cu, 115 ppm Pb and 3.5 ppm Ag. Several of the samples from the K1 Kurpu prospect returned elevated Zn (300-400 ppm), showing an occasional weak Pb-Ag association. The best result of 1100 ppm Zn, 360 ppm Cu, 19.5 ppm Pb and 1.1 ppm Ag was returned from a sulphide-bearing calcsilicate. 9 additional rock chip and grab samples were collected from the K7 Fir Valley prospect, for analysis of 30 trace elements by ICP-OES. Maximum base metal values returned from this batch were 165 ppm Cu, 145 ppm Zn and 60 ppm Pb; - collecting 45 x 200g stream sediment samples from creeks draining the Deering Hills, to submit for geochemical analyses for 40 elements using ICP-OES or ICP-MS and for 3 elements using fire assay techniques. RTE's earlier geochemical orientation work done in the West Musgrave region had established that the optimal size of detrital sample grains for giving the best discrimination from the stream sediment medium is the minus20 to plus40 mesh fraction. Two adjacent such samples (5304882 and 5304883) returned elevated lead (320 ppm and 220 ppm respectively) from minor gutters draining metasediments at the K1 Kurpu prospect. Traces of of galena had been seen in strata adjacent to these sample sites, so the stream sediment sampling results were able to confirm these observations and demonstrate dispersion anomalism associated with this package of rocks; - collecting 418 x minus80 mesh soil samples along traverses laid out across some of the geophysical targets, the traverse positions being based on location and quality of bedrock conductors, outcrop features and soil geomorphology. Samples were generally taken from the B or C soil horizon at nominal 50 m intervals. Across the more interesting geological or geophysical features, sampling was closed to 25 m intervals. Traverses across the K7 Fir Valley prospect returned a prominent lead anomaly (maximum 650 ppm Pb) with a coincident well-defined first order Ag anomaly (maximum 1.3 ppm) and elevated Zn-Cu (maximum 210ppm Zn, 110ppm Cu). This Pb-Ag-Zn±Cu elemental association appeared to be stratigraphically controlled, and was thought to represent the best geochemical target in the tenement; - collecting 9 x 30 kg, minus2 mm samples (6 of loam and 3 of drainage gravels) for heavy mineral analysis, as part of an investigation into the abundance and distribution of heavy mineral populations across the Central Giles Project region. Large amounts of heavies were separated from each sample, comprising mostly garnet, ilmenite, amphibole, orthopyroxene and rock fragments. The separates were processed to maximise the recovery of a gahnite fraction. All of the samples except for 5575788 (from near East Deering Hills) returned gahnite with abundances that ranged from a few grains to hundreds. Large numbers of gahnite grains were returned from the K1 Kurpu, K2, Kunal (K4/K6) and K7 Fir Valley prospects, where Broken Hill style mineralisation was suspected. Three chromite grains were recovered from two samples along the Tankaanu – Longwater Camp access trail. However, neither of these grains was considered to have a kimberlite origin. 26 outcrop samples of a broad range of granulite facies rocks including garnet felsic gneiss, mafic granulites, quartz mafic gneiss and meta-ironstones, collected during mapping of the K1 Kurpu, Kunal (K4/K6) and K7 Fir Valley prospects, were sent to Stafford McKnight at Ballarat University for petrological determinations. He noted that mineral assemblages present within the samples are characteristic of high grade granulite facies metamorphism. Many of the samples were interpreted to reflect metasedimentary protoliths, although some ultramafic and mafic protoliths were also indicated. His rock studies suggested that some of the quartz-rich rocks identified as quartzite when mapped may instead reflect a range of different lithotypes, including interpreted rhyodacite protoliths. Garnet is a common constituent of the felsic and intermediate granulites. They are dominantly pyrope-almandine, although some spessartine-almandine is present. Magnetite iron formation was considered to have formed from a chemical sediment protolith. Its high apatite content, and associated spessartine-almandine garnets, suggested an exhalative depositional environment. Trace amounts of graphite and sulphides were observed. Sulphide minerals in some samples include pyrite, chalcopyrite, sphalerite and galena. It is likely that much of the sulphide is diagenetic in origin, although some is remobilised along later-stage fractures. One sample (6075594) of ferruginous garnetiferous felsic granulite from K7 Fir Valley prospect contains abundant jarosite, probably after sulphide, and trace graphite. The three preferred targets at the K1 Kurpu, K2 and K7 Fir Valley prospects were drill tested by RTE during September-October 2002 with three inclined RC/HQ-NQ diamond cored holes (DD02KAN001-003) having a total penetration of 758.8 m, including 102 m of RC precollars. Another 3 vertical RC holes (RC02KAN001-003) totalling 154 m were drilled in search of ground water. Graphite and sulphide accumulations ranging up to 40% were observed within several thick metapelite beds that occur within a broader stratigraphic sequence containing felsic gneiss, calcsilicate, metabasite and marbles. The sulphides are disseminated or blebby, and consist mostly of pyrrhotite and trace chalcopyrite. The conductivity anomalies targeted are clearly caused by the existence of these graphitic and sulphidic units. Geochemical analyses of drill cuttings and core samples returned some elevated copper and zinc values plus minor lead and gold enrichments, that contrast with more subdued background trace metal contents in the remainder of the granulite rock sequence, but they are regarded as immaterial to the search for a Broken Hill type lode. The drilling programme results were deemed by RTE to have downgraded the licence's perceived potential for containing BHT style mineralisation. No further field work took place on EL 2907 during the remainder of its term. Ongoing office-based reviews of the project exploration data are in progress.