The Benambra volcanic hosted massive sulphide deposits

Abstract

Benambra volcanic hosted massive sulphide (VHMS) zinc-copper deposits, Wilga and Currawong, lie in the Limestone Creek Graben in northeastern Victoria. Early stream sediment sampling indicated the prospectivity of the region for base metals. Detailed soil geochemistry helped establish several prospects. However drillholes based on geochemical and induced polarization anomalies intersected disseminated pyrite and, in some cases, stringer chalcopyrite, but no massive These disappointing results with induced polarization prompted the use of time-domain electromagnetics (TEM). A 50 m moving loop MPPO-1 TEM survey revealed an elevated value at the last station on the last line. The survey was extended, revealing a strong anomaly, which was subsequently drilled. The discovery drillhole, BEND17, drilled 25.5 m of massive sulphides (4.10% Cu, 0.46% Pb, 7.28% Zn, 31.5 g/t Ag). A 100 m loop MPPO-1 TEM survey of Currawong Prospect detected southern and northern anomalies. BEND24, drilled into the northern TEM anomaly, intersected massive pyrite-sphalerite-chalcopyrite between 126.48 and 140.68 m (1.98% Cu, 0.50% Pb, 3.06% Znand 42 g/t Ag). Gold values within the massive sulphides ranged from 0.05 g/t to 0.40 g/t. A second lens was intersected when the southern anomaly was drilled. The Wilga deposit is a single ellipsoidal lens with a strike length of 300 m, extending 240 m down dip, and with a maximum thickness of 37 m, giving a reserve of nearly 4 Mt grading 3% copper, 6.2% zinc and 23 glt silver. The Currawong deposit, about 4 km to the northeast, consists of several lenses giving a resource of 9.5 Mt grading 1.7% copper, 4.3% zinc, 0.9% lead, 38 g/t silver and 1.3 g/t gold. The two deposits are both pyrite-sphalerite-chalcopyrite massive They occur as concordant to subconcordant bodies in interbedded sediments and dacitic-andesitic volcanics of the Middle to Upper Silurian Enano Group. The volcanics and sediments have undergone substantial folding and faulting, giving steep to moderate west-dips, and northeast-strikes. Petrophysical measurements on barren and mineralised core indicate that the host volcanic and sedimentary rocks are resistive and weakly to moderately magnetic, the disseminated sulphides are polarizable but resistive, and the massive sulphides are conductive and moderately to intensely magnetic. At both Wilga and Currawong, the massive sulphides are associated with conductivity anomalies (from the TEM surveys) and with discrete magnetic responses (evident in aeromagnetic data). In 1986 the deposits were sold to Macquarie Resources who went into a joint venture partnership with Denehurst Limited in 1992. Mining at Wilga commenced in September 1992, and to June 1995, 727 000 tonnes of copper-rich ore and 237 765 tonnes of zinc-rich ore was mined. Both Wilga and Currawong are genuine geophysical discoveries. The prospective area was defined by geochemistry but both deposits were found by following up TEM anomalies after numerous drillholes based on geochemical and induced polarization anomalies failed to intersect massive sulphides.