[ Note - here is only Part 1 of an abstract relating to 2 separately indexed parts ]. Exploration carried out over the Peake and Denison Inliers by the joint venturers since mid-2000 has targeted possible large-scale Proterozoic ironstone-hosted...
[ Note - here is only Part 1 of an abstract relating to 2 separately indexed parts ]. Exploration carried out over the Peake and Denison Inliers by the joint venturers since mid-2000 has targeted possible large-scale Proterozoic ironstone-hosted Cu-Au deposits within a highly magnetic basement terrain having geophysical features similar in size and intensity to those existing on the Stuart Shelf around the Olympic Dam deposit. At the outset the joint venture partners targeted potential IOCG deposits with attributes akin to Olympic Dam, Ernest Henry and Candelaria, viz. size (>500Mt), grade (>2% Cu equivalent) and depth of cover (<200 m). Work commenced with a review of all available previous geophysical and drillhole data, the compilation of this data into proprietary systems, and image processing and interpretation of the magnetic and gravity data. Next, four selected coincident or semi-coincident gravity and magnetic anomalies were gridded (340 line km), 26 line km of trial dipole-dipole IP surveying was carried out at three of these prospects, and another 287 line km of exploratory dipole-dipole IP surveying was then done on all four prospects to define drilling targets. Following an interpretation of the IP results, Rio Tinto drilled 6 angled rotary mud/diamond holes during November-December 2000 at the Davenport Creek prospect on Mount Charles EL 2620, to test a narrow, 3.4 km long IP lineament within an elongate, 7 km long coincident gravity/magnetic feature, looking for structurally controlled IOCG mineralisation. The drillholes were sited along two traverses spaced 3.4 km apart, and comprised totals of 1079.1 m of rotary mud precollars, plus five diamond core tails aggregating 1358 m of basement penetration. The drilling at Davenport Creek showed that a 154-196 m thick veneer of unconsolidated Quaternary and variably indurated Mesozoic Great Artesian Basin sediments unconformably overlies the basement. Multi-element chemical analysis of the drillhole basement core samples gave disappointing results, while geological logging of the drill cores established that the intensely Fe-metasomatised, metasedimentary, metavolcanic (gneiss) and granite rocks encountered by the drilling are non- to weakly copper mineralised, with rare native copper visible in tight, near-vertical fractures (best assays 0.1-0.27% Cu from hole DD00MC003). Drillholes DD00MC001 and 002 met with moderately dipping zones of massive magnetite-actinolite-albite alteration, containing massive coarse magnetite over wide intervals between depths of 163-204 m in hole 001 and 244-402 m in hole 002. The presence of the massive magnetite and of highly variable iron oxide mineralisation is thought to reflect initial high temperature Fe-K alteration, while the presence in the other drillholes of alteration assemblages containing only trace to minor iron plus rare copper sulphides suggests that a later, low to moderate temperature Fe metasomatic event occurred for which the fluids were of low sulphur content. For this drilling target, the observed geophysical response was attributed to a series of metasomatic magnetite (low P) accumulations lacking a significant sulphide overprint. Ground checking of eight total count radiometric anomalies recognised in data from the 1996 airborne survey flown by Renison Gold Corp. over the Spring Hill prospect found that the majority correspond to exposures of Cretaceous Bulldog Shale and artesian mound spring deposits. Two small, unmapped Proterozoic basement inliers were located approximately 7.5 km south-east of Spring Hill, within EL 2620. The next stage of work, carried out during 2001, comprised detailed geological mapping of structures and alteration exposed in the Peake, Denison, Kingston North and Algebuckina Inliers, concentrating on areas of known mineralisation in order to determine its controls. Two differently-orientated groups of veins associated with different alteration assemblages were identified : the NNW trending haematite and haematite-quartz veins have proximal potash feldspar alteration, whereas the NE striking copper-mineralised quartz-only veins have a chlorite-epidote-magnetite alteration. However, Rio Tinto was unable to find any surface evidence of a large IOCG alteration system. Concurrently collected rock chip samples (58) failed to reveal any new geochemical anomalies. In the sediment-covered region flanking these inliers to the west, aeromagnetic data was used to prioritise areas for conducting gravity surveys aimed at detecting any concealed massive iron-rich alteration within the basement there. A 1 km x 1 km gravity survey of 954 stations covering 1500 square km was completed in July 2001. This disclosed six prospective gravity features, three of which were subsequently chosen as drill targets, after their evaluation with dipole-dipole IP traverses to establish the existence of chargeable and conductive sources that could represent buried sulphides. During September 2001, Rio Tinto drilled 9 angled exploratory RC holes for a total penetration of 908 m at these prospects located within EL 2549 Mount Denison. Two of the drillholes reached Palaeoproterozoic basement, and five others were ended in Neoproterozoic sediments. No significant drill sample assay values were returned, other than some elevated Cu-Au in the Wirriecurrie Granite drilled by holes RC00PS001 and 002 on the Peake Siding prospect, which is a 3 mGal gravity high with a chargeability anomaly on its southern margin. The tenor of the assay results plus the weak haematite alteration implies a distal IOCG mineralising event, which is interpreted to have been focussed at the nearby Peake Ruins. An additional fence of three angled holes (RC01PS003, RC01PS004 and RC01PS005) was drilled to test a robust hargeability feature. RC01PS004 encountered weakly anomalous Zn-Pb-Ag hosted by Neoproterzoic carbonaceous and graphitic siltstones. The northern of two weak chargeability anomalies coinciding with a 2.5 km x 4 km, 6 mGal gravity high at Big Blyth West prospect was tested with one RC drillhole (RC01BB001) to TD 136 m. This hole revealed weak Cu anomalism hosted by shale. A 2 km x 3 km area of elevated gravity and chargeability at Rockwater prospect was tested with one RC drillhole (RC01RW001) to 160 m depth. Nothing of interest eventuated. The gravity anomalies for this western region were considered by Rio Tinto to have been adequately resolved, and no further work there was recommended. Furthermore, the company considered that no immediate opportunities remained within the project licences for it to discover IOCG deposits that would meet its open pit mineable criteria, although it still believed that discoveries might be made of mineralisation that diverged from such constraints. The area of large-scale iron-altered basement in the eastern part of the project acreage remained mostly untested, and thus clearly retained prospectivity for having economic IOCG mineralisation. Therefore Rio Tinto and BHP-Billiton decided to seek a farm-in partner to advance the project aims, who would commercially entertain chasing smaller size targets under increased depth of cover. For the following 25 months no ground-based exploration was performed, while Rio Tinto reviewed the project prospectivity against its other mineral venture commitments on a global basis, and then successively presented a project farm-out package to various interested parties.
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