Progress report No 1 Mount Bonnie Prospect GML 127B and EL 615

Abstract

A programme of detailed geological mapping and diamond drilling over the, old Mount Bonnie Mine, 130 miles south of Darwin, NT was accomplished by Horizon Explorations Ltd on behalf of Horizon Ventures NL and Jingellic Minerals NL in the period April to October 1973. The Mount Bonnie lode was discovered in the late 1890's and actively worked from 1912 to 1917. Gold and lead appear to have been the metals sought and mining activities were restricted to the o:xidized portion of the lode. The prospect area lies in the central portion of the lower Proterezoic, Pine Creek Geosyncline and includes rocks of the Golden Dyke Formation (which consists of calcareous and carbonaceous siltstones and shales, sedimentary breccia, and dolomite) and the Masson Formation (which consists of turbidite-forined greywackes and siltstones). The prospect area has been intruded by amphibolite dykes and sills. Structurally the Mount Bonnie lode lies within sedimentary rocks of the Golden Dyke Formation and an amphibolite sill. The sedimentary rocks at this locality have attiudes ranging from 020 to 040 strike and 30 W to 80 W dip. Several periods of faulting are represented which have affected the lode to varying degrees. Thrust faulting parallel to the lode has caused pinching and swelling; high-angle cross faults and shears have displaced portions of the lode; and secondary dip-slip faults have affected its hanging wall. Despite the effects of faulting the top of the lode has a relatively uniform 020, 40 W attitude, but pinching and swelling have caused major variations in its thickness which ranges from 0.03rn to 16 m. Mineralogically the lode consists of pyrrhotite, pyrite, sphalerite, galena, arsenopyrite, chalcopyrite and minor silver and antimony suiphides which occur in a gangue of dolomite, chlorite, talc, actinolite, and quartz. The first stage diamond drilling programme consisting of 6 holes outlined a possible 480,000 tonnes of sulphide mineralisation averaging 7.67% Zn, 4 % Cu, 1.48% Pb, 0.91% As, 6.0 oz Ag/tonne, and 1 dwt Au/tonne. Data obtained from geology, geophysics, and drilling suggest that the lode may be continuous both laterally and down dip from the area drilled, giving a potential of at least 1,500,000 tonnes. Further work, particularly metallurgical testing and diamond drilling, is recommended.