Appraisal exploration results and permissible expenditures.

Abstract

Available information concerning the degree of success achieved by exploration expenditures for copper and gold in the Tennant Creek Goldfield indicates that 122,000 is a permissible expenditure for a complete search of the primary and secondary zones of an ore target of substantial size. The Angel Group of leases cover one of the largest aeromagnetic anomalies in the Tennant Creek Goldfield. Two main target areas are present, namely the Crusader-Black Angel and White Devil ironstone bodies. A complete search of the primary zone is yet to be carried out in both areas. In the White Devil area gold has been intersected in one drillhole and thus additional drilling is required to determine its continuity and extent. In the Crusader section drilling has shown that copper anomalies persist in depth, that the zone of oxidation is deep and consequently the primary zone should be tested. Surrounding the Angel Group is an area of 48 square miles held as an Authority to Prospect. The aeromagnetic map indicates that extensions of the Explorer 5 group of anomalies may be present. This Authority to Prospect because of its geological setting is worthy of geological and geophysical investigations with follow-up drilling of ore targets as they become delineated. The Tennant Creek mining field occupies an area of some 2,500 square miles and is generally taken to be that area lying between Latitudes 19' 10% and 19' 50 lS and Longitudes 133' 45' E and 138 38l E. This report attempts to value the known deposits of the Field and draw conclusions as to the permissible exploration expenditures for a complete search prior to withdrawal in the Angel Group and the surrounding Authority to Prospect of 48 square miles. The permissible exploration expenditures are based on the estimated chances of success in finding an economic ore deposit in the light of known geological knowledge concerning the Goldfield. The results of exploration and the geology of the Angel Group have been detailed in earlier reports by the writer. The geological setting of the mineralised bodies is briefly reviewed together with the known geological controls of ore deposition. The gold and sulphide ore-bodies of the Tennant Creek Goldfield are in shales, siltstones and graywackes of Lower Proterozoic Age which have undergone moderate to severe folding and shearing. The following igneous rocks are intrusive into the sedimentary rocks: granite, adamellite, quartz-feldspar porphyry, diorite, dolerite and lamprophyre bodies. Most of the known ore-bodies are associated with quartz- u magnetite and quartz-hematite masses colloquially termed ironstones. The ironstones range from a few inches to more than forty feet in width and up to several hundred yards in length. The host of the bodies are tabular or lenticular but some are pipelike. The ironstones occur in fault or shear zones, as replacements of unsheared or slightly sheared favourable beds or at the margins of some of the larger porphyry bodies especially along concordant contacts. Beds favourable to the replacement by ironstone bodies have been purple hematite shale. Slump structures as indicated by masses of breccia are reportedly good hosts to ore but more evidence to confirm such an ore control is needed. The ironstone bodies in the hematite shale and along the concordant porphyry contacts tend to be jasperoidal. Hematite and magnetite masses along the margins of porphyries have produced little gold. Factors Related to Probability of Success and Permissible Exploration I Expenditure. General: Within the Tennant Creek Field 117 mines have produced gold. Two mines (Peko and Orlando) have produced gold and copper. Another three ore deposits (Explorer 5, Explorer 8 and Ivanhoe) are potential gold and copper producers. It has been indicated that in a relatively homogeneous but fractured geological unit in any area of about 50,000 square miles there are 100 mini% regions of all sizes and the pattern for each is roughly the same, namely: - 1 deposit worth $A33,000,000 or more 3 or 4 deposits worth $A3,300,000 - $A33,000,000 or more 8 to 12 deposits worth $A330,000 to $A3,330,000 and a tail of small ones. Broadly speaking the average area of the mining region in the unit area of 50,000 square miles is about 500 square miles. The Tennant Creek 1 mile sheet area is about 560 square miles and in that area have been found: - Peko approximately gross value $30,000,000, Nobels Nob $11,000,000, Ivanhoe $4,000,000, Eldorado $1,600,000, Explorer 8 $1,500,000. For the Tennant Creek Field itself (i.e. an area of 2,500 square miles) possibly more regions could be found by continued exploration but at least one more region is expected to be present. So far only two additional mines have been proven, namely Orlando and Explorer 5 valued at approximately $3,000,000 and $23,000,000 respectively. The figures quoted are based on published ore reserves and production figures.