Report on rotary percussion drilling programme, EL 78 Victoria River Downs, Northern Territory

Abstract

Four rotary percussion holes were drilled at Victoria River Downs, NT between September 6 and 13, 1972. These were located by an induced polarization survey, three holes being sunk on Target Area 2 and one on the Colt Prospect. Both these areas lie between VRD Station and the Fitzgerald Ranges. The contractors were Gorey & Cole, Drillers of 56 Todd Street, Alice Springs, NT. An Ingersoll-Rand Drillmaster TRUCM-3 Drill was employed, using a 6 1/2' bit, although original specifications required only a 5 1/2' bit. No extra expense was incurred by AMI for the use of a larger bit. The drilling was supervised by RGB Allen and NJ Cranley of AMI. The upper part of the Skull Creek Formation, which forms part of the Bullita Group, is exposed in the two areas drilled. This formation consists of interbedded dolomite and dolomitic siltstone, minor dolomite and chert. It dips slightly to the east and has been relatively unaltered since deposition. Sporadic mineralization, consisting of galena, sphalerite and chalcopyrite grains and blebs occur throughout the sequence but is best developed in the Colt Prospect. No massive deposits or mineralized veins, joints or faults are exposed in these areas. Preliminary work in this area started in September, 1970, when a regional stream sampling programme, in conjunction with prospecting, was initiated. Airborne geophysical surveys were flown in May 1971, from which target areas were delineated. Detailed geological mapping was carried out in May 1972 outlining suitable areas for further work. An induced polarization survey then commenced in three areas. This work, carried out by LJ Starkey & Associates, involved the use of a Geoscience 8kw requency Induced Polarization Unit and a Geomite receiver. The electrode spacing was 60 metres and levels 1, 2, 3 and 4 were read. Detailed work was instigated after favorable results were gained from the initial survey. For this a 30 meter dipole spacing was selected, the survey being carried out on Lines 1 to 5 of Target Area 2 and at the Colt Prospect. Supplementing this, a second analysis of the I.P. data was carried out by SS Webster of McPhar Geophysics. This report was made available after 2 drill holes had been sunk. He indicates that 'the data of the survey contains a high degree of 'noise' that reduces confidence in the anomalies encountered.' Further drilling in the area he concluded 'cannot be recommended from the results of the IP survey.' He did, however, recommend one further site as a possible target. This was tested by the fourth drill hole. Rotary Percussion Drilling - Three 400 foot holes and one 210 foot hole were drilled to test the anomalies, sampling occurring every 5 feet. No economic mineralization was encountered. Scarce grains of chalcopyrite, galena, sphalerite and fluorite were observed. The samples were analyzed by Geochemical and Mineralogical Laboratories (NT) Pty Ltd of Darwin.

Fine to medium grained dolomitic shales contain the highest grades of mineralization, galena being the dominant mineral present. No anomalous copper results were obtained. High zinc values were recorded between 100 feet and 200 feet below the surface on holes 1, 2 and 3. It is believed that geophysical anomalies are due to the interaction of a fluctuating water table and nearly horizontal shale bands. Due to the sporadic nature of mineralization encountered and lack of any mineralized horizons as deduced from the drill hole stratigraphy, no further work is justified in this area.