DATE: Mid 1991 sometime...., 5.15 for 5.30pm
VENUE: Rugby Club, Crane Place, Circular Quay.
SPEAKER: Peter Rea
TOPIC: "BALCOOMA BASE METAL PROJECT"
ABSTRACT
The Balcooma Project is located 230 km NW of Townsville and 35 km NW of Greenvale, Qld, at lat 18046'S, long 144043' on the Einasleigh (SE 55-9) 1:250 000 scale and the Conjuboy (7860) 1:100 000 scale map sheets. Four volcanic-hosted massive sulphide (VHMS) deposits occur within an 8 km long belt at the northern end of the Balcooma Metavolcanics (Withnall, Black and Harvey, 1991), metamorphosed to middle amphibolite facies. These have been correlated with the Cambro-Ordovician Mount Windsor Volcanics (Henderson, 1986) which host similar VHMS deposits in the Charters Towers region 200 km to the SE. The Balcooma district has about 33 km strike length of prospective metavolcanic rocks and associated metasediments on which exploration is still in progress. Title to all deposits is held by Lachlan Resources NL (Lachlan).
The earliest work in the area was at the nearby Balcooma Goldfield, a small field that achieved its heyday between 1890 and 1896. Modern exploration commenced in 1960, but no discoveries were recorded until October 1975, when Geopeko geologist Warwick Maehl mapped and sampled the Surveyor 1 gossans with results to 0.21% copper, 0.58% lead, 0.14% zinc, 28 ppm silver, and 0.4 ppm gold. Mining Lease 1393 was pegged on 1 November 1975 (Fig 2). In 1978, Carpentaria Exploration Company Pty Limited (CEC) took up Authority to Prospect 2036 M surrounding the Surveyor 1 lease, and in late 1978 and discovered the Balcooma deposit.
Withnall, Black and Harvey (1991) dated an intrusive quartz-feldspar porphyry dyke from the Balcooma deposit using SHRIMP ion probe analysis. They obtained two different ages using different isotopic ratios; an age of 471+/-4 Myr using Pb207/U238 ratios, and an age of 507+/-22 Myr using Pb207/Pb206 ratios. Both ages are consistent with the correlation of the Balcooma metamorphic belt with the Mount Windsor Subprovince to the south.
Massive sulphide mineralisation occurs at a regional scale hiatus in volcanic activity marked by the contact between felsic Dry River volcanics and metagreywacke of the Clayhole Creek beds. The polymetallic massive sulphides are coarsely recrystallised and consist of euhedral sphalerite with up to 15% interstitial galena, 'buckshot' pyrite and 1-5% chalcopyrite. Massive sulphides in the Copper rich lenses, consist of interlocking pyrite and chalcopyrite. Resources currently total 6.2 Mt: with 3.9 Mt of polymetallic resources @ 1.1% Cu, 3.8% Pb, 8.94% Zn, 81 g/t Ag, and 0.74 g/t Au; and 2.3 Mt of copper resources @ 3.44% Cu.
P.S. Rea and R.J. Close
Note the Balcooma Project has recently been bought by Kagara Zinc - Nov 2000.
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