Reports on AP 2580, Oonagalabi copper-zinc prospect, Harts Range area, Alice Springs, NT

Abstract

Russgar Minerals Harts Range Area situated some 65 miles northeast of Alice Springs, Northern Territory, consists of 934 square miles held under 6 Prospecting Authorities or, as they are being renewed, Exploration Licences, licences which are partly in the name of the Company and partly under option to the Company. The rocks of the area are almost entirely Archaean (Nullaginian) highly metamorphosed sedimentary and basic, often concordant, intrusive rocks belonging to the Arunta Complex. Until recently the known mineralisations - consisted of mica mines, minor narrow copper occurrences and pegmatitic mineralisations, especially beryllium, and exploration had been limited to little prospecting and gouger mining and geological work largely to the survey carried out by GK. Joklik (Bulletin 26, 1955). Russgar Minerals, specifically formed to explore the Harts Range Area, arranged for an aeromagnetic survey of the whole area to be flown in late 1970 and, as the geology is extremely complex and the topography rather varied, the aeromagnetic data is complex and a regional survey of combined visual (surface mineralisation and favourable geological environments) and rock geochemical survey and integrated geological aerial photography interpretation and geological mapping was therefore commenced in March-April 1971, a survey which should provide adequate coverage to locate any significant ore body with some surface expression and at the same time provide the data needed for the interpretation of the aeromagnetic data, which in turn may lead to the discovery of possible ore body(ies) with little or no surface expression. Late in April 1971 it was decided that it would be advantageous in terms of company finance policy to carry out a detailed appraisal of a virgin copper prospect with considerable potential, the Oonagalabi Copper Zinc Prospect, situated in the eastern part of the Harts Range Area. Consequently, the regional programme was postponed and a programme of road upgrading, plane table and alidade geological mapping, sampling for assaying and microscopic work, VLF (radio) electromagnetic and ground magnetic surveying, minor resistivity traversing, down-hole-hammer drilling, costeaning, aerial photograph interpretation and general geological traversing undertaken and carried out by Russgar Minerals staff. In all 2,103 feet were drilled in 14 vertical holes, the deepest hole being 295 feet, plus one hole to 135 feet when a drill rod was broken, and a total of 842 samples were collected of which 43 samples were for petrological examination and/or ore microscopy. The mineralisation at Oonagalabi occurs in Mg and Ca amphiboles quartz diopside schist and forsterite marble, and has very tentatively been classified as a contact metamorphic or metasomatic mineralisation with sphalerite, chalcopyrite, galena (minor) and their oxidation products and less tungsten, bismuth, silver and gold. The economically significant mineralisation is copper, gold and zinc and by far the best intersection obtained in the drilling is a 130 feet intersection of forsterite marble with 0.96% Cu, 1.61% Zn and 1 ppm Au (1.52 ppm is equivalent to 1 dwt per long ton - the gold value is based on three samples) - while a good Mg and Ca amphiboles quartz diopside intersection would be exemplified by the 35 feet intersection in an adjacent hole, 0.68% Cu and 0.53% Zn over 25 feet and 0.39% Cu and 0.67% Zn over 10 feet. The largest proportion of host rock is Mg and Ca amphiboles quartz diopside and it is suggested that the reason for the rather consistently lower values in this rock type may well be that very few samples were collected of fresh or unweathered rock even in the drilling and the mineralisation would therefore have been subject to leaching while the forsterite marble is a highly weathered rock at the surface but fresh with sulphide niineralisation 2-4 feet below the surface. The Oonagalabi Formation, to which the two host rocks mentioned above belong, occurs partly as an open synform, plunge 20 degrees, and partly as limb dipping to the northwest at 45 degrees or less, where this structure is part of a regional almost isoclinal fold structure, also plunging 20 degrees. On the surface the mineralised Oonagalabi Formation rocks extend over a strike length of 8,000 feet discontinuously and over 5,400 feet continuously with a maximum width of 750 feet (not orthogonal thickness). As the copper mineralisation is leached in the shallow synform, the northwest dipping limb becomes the focus of interest. The major part of this structure represents some 8,000,000 tons over 1,700 feet of strike length, 85 to 165 (conservative) feet orthogonal width or thickness and taken to 500 feet down dip depth. The geophysical data suggests that the structureextends another 1,600 feet but with a smaller width. The possibility of further ore reserves in the synform area at a greater depth than the one reached in the drilling is a question of whether the fold structures are penetrative in accordance with the plunge and it is not possible to answer that question on the basis of the information available. To the south of the part of the Oonagalabi Prospect covered in the detailed mapping, further and sii.tlar mineralisation is encountered. Based partly on similarity in magnetic expression between the area mapped in detail and this area in the aeromagnetic survey and partly on the very encouraging results obtained in a limited VLF electromagnetic survey, it appears that this southern occurrence may possibly be a partly buried occurrence as extensive as the one examined in detail. Further afield, the interpreted large regional fold structure on which the Oonagalabi Prospect is located affords a promising exploration target and the whole Harts Range Area is considered an area endowed with a strong potential as so far three other copper occurrences, of which two are known to be of very limited size and the third not investigated, lithologically similar to Oonagalabi, have been located 1.5, 14 and 18 miles respectively and in different directions from Oonagalabi. It should be remembered that the area has never been subjected to intense exploration regionally or otherwise. The forward exploration programme must necessarily consist of two approaches or programme components which should preferably be carried out concurrently: (A) Regional exploration in the whole Harts Range Area as is presently in progress (B) Exploration in the Oonagalabi Area; the large fold structure, the area to the south and the drilling of two diamond drill holes at Oonagalabi. The cost of the recommended forward exploration programme is substantial but it is suggested that a substantial expenditure is fully justified by the potential of the area.