Assessment of exploration and drilling results, EL 5858, Curtain Springs, NT.

Abstract

Exploration Licence EL 5858 was granted in May 1988 and at the commencement of the fourth year of tenure in May 1991 comprised a reduced area of 100 blocks covering approximately 287km Currently the tenement is held by a consortium headed by Stirling Resources N.L. with 80% equity and by Kingsgate Consolidated Limited and Jerele Mining Pty Limited each of whom own 10% equity. Prior to the granting of EL 5858, Weeks Australia Limited carried out an aeromagnetic survey utilising the services of Data Science Pty Limited. The objective of this survey was to assist in the elucidation of the underlying structure of the area aimed at petroleum exploration. This survey was flown in 1982 and included the area of EL 5858 and surrounds to a total area of 1600 km The main magnetic sources located were interpreted to be a layer of basic volcanics near the top of the Upper Proterozoic Bitter Springs Formation. On the basis of the above aeromagnetic data, some of the more significant discrete magnetic anomalies were considered to offer targets for base metal mineralization associated with igneous or volcanic centres. The lack of previous sub-surface exploration for base-metal mineralization in the area and the relatively poor geological knowledge of the area because of rare rock exposure meant that progress in defining exploration targets was limited to remote sensing techniques in the form of the interpretation of satellite imagery and geophysical investigation prior to exploratory drilling. Exploration investigation of EL 5858 since the granting of the Exploration Licence in 1988 is summarised below: Year 1 The tenement was examined using computer enhanced Landsat TM satellite imagery. This remote sensing interpretation was carried out in order to identify structures which were interpreted to be related to the discrete circular aeromagnetic anomalies and to identify linear features which reflected the presence of faults and/or major lineaments beneath superficial cover. The most significant aeromagnetic anomaly, Si which is over 2km in diameter, was interpreted to lie within an ENE trending graben structure. Radiating linears emanating from Si and attributed to tensional fault structures were identified trending in a southeasterly and southwesterly direction. A prominent NNE linear associated with the eastern margin of Si was attributed to a deep fault possibly related to faulting in underlying basement, and a north trending linear was detected on the western margin of S1. The study concluded that 13 discrete aeromagnetic anomalies were present within the tenement holding and that anomalies Si, S2. S3 and N4 were related to linears interpreted from the Landsat study. A programme of ground geophysical surveying followed by drilling was proposed and costed (Ellyard, 1988). Year 2 Ground geophysical surveys were undertaken on the tenement during late 1989 as part of the Year Two exploration programme. Ground magnetometer, induced polarisation, electromagnetic (SIROTEM) and gravity surveys were conducted variously on aeromagnetic anomalies, numbered 51, S2, S3 and N4. The results from these surveys were subsequently modelled using Curtin University computer programmes. The objective of the modelling was to estimate depth, size and intensity of the causative bodies, and in the case of interpreting the electromagnetic data, the intensity and depth of conductors present. The induced polarisation surveys yielded poor data for chargeability and these data were therefore not assessed in detail. In summary, the results of the geophysical surveys provided the following useful data for the aeromagnetic anomalies surveyed by ground geophysics. This report presents the results obtained from an exploratory diamond drilling programme carried out during August and September 1991 over EL 5858, Curtin Springs, Northern Territory. The objective of the drilling programme was to investigate a variety of gravimetric and electro-magnetic geophysical anomalies associated with discrete aeromagnetic targets in Upper Proterozoic sediments on the southern margin of the Amadeus Basin. This exploratory drilling was primarily aimed at testing for the occurrence of base metal mineralization which was considered to be possibly associated with the sedimentary pile above the source of the discrete aeromagnetic targets. The exploration philosophy was that the aeromagnetic targets were possibly igneous or volcanic centres from which sulphide mineralization may have emanated to mineralize the surrounding sedimentary pile either as an epithermal event or in a post-eruptive sediment-hosted stratiform copper environment. The search parameters of the Olympic Dam deposit (copper-uranium-gold-silver) and dominantly copper deposits of the Stuart Shelf of South Australia were borne in mind in the assessment of the Curtin Springs area. However, because little was known of the geological setting of the area of specific interest, other possible sources for the discrete aeromagnetic anomalies were considered including carbonatite and lamproite intrusives. The results obtained from the exploratory drilling of targets S2 and S3 show that the causative rock-type for the magnetic and gravimetric anomalies can be attributed to olivine basalt or olivine dolerite stratigraphically associated with the base of the Inindia Beds and top of the Bitter Springs Formation, both of Upper Proterozoic age. The drilling results obtained from both the S2 and S3 targets give no encouragement for the occurrence of economic concentrations of base or precious metals and therefore the programme of work over these anomalies was successful in identifying the cause of the geophysical anomalies but failed to yield evidence for economic levels of mineralization. The drilling of the S1 target similarly yielded low tenor assay values for the base and precious metal values sought and failed to detect evidence which supports the hypothesis that the magnetic and associated gravimetric and electromagnetic anomalies at this target have a possible link with a potentially mineralized environment of economic significance. The drilling of this target failed to locate the source of the magnetic anomaly despite the depth of some 271m reached by borehole CS 1. The nearby borehole CS2 for technical reasons also failed to penetrate the 150m to 200m target depth to test for possible discrete conductors. However, since these two boreholes were in close proximity (c.200m) it is considered that if significant mineralization associated with the electromagnetic anomalies did exist at C52 then some indication of a mineralised environment of significance would have been detected in borehole CS1. In general terms however, the sequence of pyrite mudstones, siltstones, carbonaceous calcilutites, limestones and calcareous siltstones encountered by borehole CS2 are of interest in considering the possibility of stratiform sulphide occurrence of marine association. For pyritic sediments barren of significant concentration of lead and zinc do occur stratigraphically above significant base metal mineralization in deposits of the stratiform association. Unfortunately, positive evidence which would justify sustained exploration to pursue such a model at Curtin Springs is weak on present data. Drilling encountered high rates of flow particularly at anomaly S1 rendering compressed air RC percussion ineffective. Borehole logs for boreholes CS1 CS2 CS3 and CS4 are provided but location data is appoximate.Based on these considerations, the exploratory drilling at Curtin Springs generally supports the interpretation that the geophysical anomalies associated with the Upper Proterozoic succession are attributable to mafic flows and intrusive centres at the top of the Bitter Springs Formation. On the basis of the work conducted so far there is little encouragement to support the view that base and/or precious metal mineralization is associated directly with this igneous event. The exploratory drilling programme over EL5858 was based on modelled geophysical anomalies and the development of an imaginative model for base-metal exploration. Drill testing of two out of the three target areas proved the causative bodies for the geophysical anomalies to be olivine basalt and olivine dolerite and no economic mineralization was encountered.The third target, Si, over a really large (>2km) circular magnetic anomaly with coincident gravimetric anomaly was also drill tested but the source of this anomaly was not satisfactorily explained by the drill programme which tested up to 271m of Upper Proterozoic sediments without penetrating mineralization of potentially economic significance. On the basis of the results obtained and the deposit model adopted there is no encouragement for further work for base metal exploration. However, the inconclusive testing of target Si remains to be resolved. it is worth noting that in the event that it is decided to carry out further work over the S1 target, possibly using a modified deposit model, then this will require considerably more electro magnetic geophysical surveying in order to delineate potential drill target zones. In considering the search area for future copper exploration, it is possible that the basaltic rocks proved to exist at the top of the Bitter Springs Formation may offer a highly dispersed copper source from which, through oxidation and alteration, copper may have been released in solution to surrounding country rock. If this process has occurred then a search for sediment-hosted copper deposits in the search area would be concentrated in favourable stratigraphic positions adjacent to major faults or basin margins. Again testing of these possibilities will require a considerable effort in terms of further geophysical surveying, interpretation of the sedimentary environment and exploratory drilling since there is no near surface evidence to show that base-metal mineralization exists in the area. A further future exploration possibility is to consider the area for Pb-Zn stratiform sulphide occurrence of marine association using the search techniques mentioned above. In general terms, the future for new base metal discoveries will probably lie in the application of geophysical techniques over blind deposits. However, the evidence so far obtained from Curtin Springs does not encourage the view that the amount of additional work that would be necessary to test the area more thoroughly is justified on the assessment of the present data base. It is recommended however that any stratigraphic information provided by test drilling and released by the oil exploration investigation currently being conducted in the area is sought and reviewed before a final decision is made on whether or not to proceed with further exploration for base metals.