ELs 6155 to 6157 and 6250, period September-October 1988 to September 1989

Abstract

Earlier work on detrital heavy mineral grains from catchments draining areas of Seigal Volcanics, indicate that there are minerals within the Seigal Volcanics which have a source with ultrabasic affinities rather than usual basaltic affinities. Whether this 'ultrabasic' material is present as discrete bodies or as xenoliths within the bulk of the Seigal Volcanics has not yet been ascertained. Examination of geophysical data in the area by Starkey (1984) has highlighted a number of features within the Seigal Volcanics in the area. One or more of these features may be related to the source of the 'ultrabasic' derived material. This 'ultrabasic' material is a target in the search for potential gold/platinum group metals (PGM) mineralisation (and diamonds) in the Pandanus Creek region. To date, there is no evidence to suggest the presence of layered intrusions of Bushveld type in the Seigal Volcanics. Thus, it seems more likely that 'ultrabasic' material, if it is present as discrete bodies, will be found in intrusive pipe-like bodies, or associated extrusive tuff, or as xenoliths in the lava flows. 'Hydrothermal alteration zone'-type PGM deposits have only recently been recognised. These, of which Coronation Hill is a probable example, occur in a broader range of host lithologies, eg Edith River Volcanics, than those usually associated with PGM occurrences. It is understood that one subtype of this group, which has recently been recognised in the Pifield district of New South Wales, is associated with concentrations of secondary magnetite in basic to ultrabasic lithologies. This magnetite, which has a recognisable magnetic signature, is not due to magmatic differentiation but to hydrothermal processes which have also been responsible for concentration of PGM minerals. Examination of the detailed airborne geophysical data from the Uranerz 1983 survey of the Pandanus Creek area by Starkey (op cit) highlighted a number of magnetically low zones which it was suggested could be due to the presence of zones of moderate to strong hydrothermal alteration, or to the effects of remanent magnetisation. PGM are closely associated with gold in the Alligator Rivers area. This is to be expected as the conditions of uptake, transport and deposition of PGM, viz acid, oxidising, saline water followed by reduction, are equally effective in the case of gold. As gold was already known in the Pandanus Creek area at grades of economic interest, and geochemically elevated platinum and palladium values had been recorded in the reconnaissance sampling programmes carried out in 1987 and 1988, systematic, comprehensive testing for this style of mineralisation was considered warranted. The results of both the Phase 1 and Phase 2 BLEG stream sediment sampling programmes clearly focus attention to three prospective areas. Each of the areas have long been known for the presence of uranium mineralisation; however, the current programmes now highlight the potential for the occurrence of polymetallic mineralisation in each area. Thus future exploration should be planned bearing this broader potential in mind, particularly the potential for proximal deposits rather than coincident deposits. As noted previously, during his appraisal of the airborne geophysical data, Starkey noted three zones of low residual magnetic response in the processed data. Closer examination of the data in the areas of the ELs (presented in Starkey 1984) indicates that there are a large number of low residual magnetic responses in the area. Although either of smaller areal extent or of lesser intensity, these features warrant examination. The contact between the Seigal Volcanics and Westinoreland Conglomerate outcrops is apparent in the data as a change from 'noisy' residual response associated with Seigal outcrop to relatively 'quiet' residual response associated with Westmoreland outcrop. Within the area of Seigal outcrop a number of low residual features of similar magnitude to the 'Corio' feature of Starkey are apparent. The areal extent of each feature is in general smaller than that of the 'Corio' feature. Nevertheless, they appear to be closely related spatially to areas of known uranium (+- other element) enrichment. Within the area of Westmoreland outcrop, a number of low residual features are also apparent. These features are of lesser magnitude than the 'Corio' feature but are similarly related spatially to known mineralised areas. To the southeast of the 'Corio' feature, a broad low extends over the area of Westmoreland outcrop. This zone terminates at an apparent NE/SW feature extending between Hidden Valley and El Hussen. Two elongated lobes to this low zone extend to the south-east from both the northern and southern extremities of this zone. These lobes are somewhat irregular but roughly parallel known structures in these areas. The southern lobe in particular coincides with the known mineralised El Hussen Fault. Secondary uranium mineralisation is known to occur at surface for in excess of 1,600 m along this structure in this vicinity. Another discrete low is situated approximately 500m to the south, along strike from the extremity of this lobe. The significance of these various features is not known, but should be assessed during any follow-up work, particularly in view of the results of the current sampling programmes. Rock chip sampling and analysis for gold and PGN has previously been undertaken within six prospect areas on reconnaissance grids in the Pandanus Creek project area. Although a number of elevated values were noted, the majority of the samples assayed below the lower limit of detection (Au 10 ppb, Pt 5 ppb and Pd 5 ppb) for the assay methods employed. These results were therefore less than satisfactory from the point of view of determining the background concentrations, and thus anomalous threshold levels, for each of these elements. In order to further assess the background levels of these elements in fresh rock samples, drill core samples were taken for analysis from available cores. Samples were in general taken from all cored intervals through Seigal Volcanics into the uppermost Westmoreland Conglomerate lithologies. Condition of the core was generally good, however, due to partial exposure to the elements, the majority of the core markers were in poor condition. In most cases, hole depths were able to be determined within each core tray by reference to the start and finish depths marked on the trays, which were in most cases still legible and by reference to specific geological features recorded in the original drill logs. For uniformity, sampling was undertaken over 2 m intervals, or as close as practicable thereto. In general, the sample comprised a one quarter split of the core obtained by using an anvil splitter in competent sections of the core and a teaspoon and fork in the incompetent, clayey/shaley/fractured sections of the core. As with the stream sediment samples, the drill core samples were analysed by AAL Townsville. The samples were crushed, pulverised and homogenised by mat rolling and riffling prior to splitting out the assay charge. Analysis was then carried out by fire assay (AAL code FAR) for Au, Pt and Pd with lower limits of detection of 1 ppb in each case. As a matter of routine, AAL undertake and present the results of duplicate assays within each batch. Samples were selected from intervals in 2 diamond holes in the area of the ELs, viz CDD-i at Cobar II and EDD-l at El Hussen .In general the only elevated results correlate with areas noted for increased radiometric response and elevated uranium concentration. There are no significant enrichments in Au, Pt or Pd in any of the sections of Seigal Volcanics sampled. This is not totally unexpected as none of the holes was specifically targeted toward the detection of mineralisation within Seigal Volcanics units.