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Open File Report 113

Geochemical dispersion in the Olary District, South Australia: Investigations at Faugh-a-Ballagh Prospect, Olary Silver Mine, Wadnaminga Goldfield and Blue Rose Prospect.

Skwarnecki, M.S., Li Shu and Lintern, M.J.

The Olary Regolith Project was established in July 2000 as a jointly funded project between CRC LEME, Lynas Corporation Ltd and the Department of Primary Industries and Resources, South Australia (PIRSA). The principal objective was to conduct a regolith mapping and orientation geochemical sampling program that would provide targets for further exploration and a basis to conduct effective further geochemical exploration in the Olary region. Three principal areas were selected for study; the Faugh-a-Ballagh prospect (Olary Domain); the Great Eastern-New Milo mines in the Wadnaminga Goldfield; and the Blue Rose prospect (Nackara Arc). An orientation soil traverse was investigated at the Olary Silver Mine (Olary Domain).

At Faugh-a-Ballagh (20 km northwest of Olary), Cu mineralisation occurs in magnetite veins (ironstones) and zones of magnetite dissemination in the upper part of the Quartzofeldspathic Suite of the Olary Domain. Most of the area belongs to an erosional regime, the dominant landforms being high hills formed by slightly weathered gneisses and schists. Soils are skeletal to colluvial, and little or no deeply weathered regolith is present. Although mineralised ironstones (magnetite-rich veins) are anomalous in Ag, Au, Cu, In, Zn, Al, Ga, Cs, K, Rb, Mg, Sr, REE, Th, Ti, U and Y, stream sediment sampling (<2 mm fraction) indicates that Cu is the only reliable indicator of mineralisation and that dispersion is very limited. Preliminary studies show that the <75 um fraction is the optimum sample medium and that Cu dispersion is probably greatest in this fraction. Similarly, orientation soil sampling across a mineralised ironstone vein suggests that Cu is the only reliable indicator of mineralisation. The finer fractions (75-180 µm and <75 um) are optimum fractions, although the bulk <6 mm fraction is an acceptable (average) compromise.

Orientation studies on magnetic versus non-magnetic fractions in <2 mm soils, <2 mm stream sediments and 2-6 mm lags suggest that there is no advantage in using the magnetic fraction, since Cu is irregularly distributed between the fractions. Regional <6 mm soil sampling was carried out on a 400 m triangular grid, with infill at 100 m spacing on Faugh-a-Ballagh Hill. Only Cu and Au consistently delineate the mineralised zones. The most prospective areas are on the southern side of Faugh-a-Ballagh Hill, in an area of relatively intense ironstone veining, to the north of an east-west shear, and the shear itself. Copper is dispersed irregularly over an area 750 m by 250 m. The Au dispersion is much narrower and extends for about 1500 m along the shear. Other, less well-defined zones with anomalous Cu occur to the west and southwest, in zones of disseminated magnetite in albitic rocks. The principal targets are the southern side of Faugh-a-Ballagh Hill and the east-west shear.

At the Olary Silver Mine (about 5 km north of Olary), sampling of the dumps suggests that geochemical indicators of the mineralisation are likely to be Ag, As, Au, Bi, Cu. Hg, Mo, Se and Te in fresh rock. The orientation soil survey indicates that Au and Cu are the best indicators of mineralisation and form an anomalous zone 40 m wide. The best response was in the <75 µm fraction but using the bulk <6 mm fraction would be acceptable.

At Wadnaminga (about 30 km south of Olary), Au mineralisation occurs in sulphidic quartz veins in metasedimentary rocks of the Burra Group, along the northern margin of the Wadnaminga Anticlinorium. High-grade mineralisation at New Milo contains significant concentrations of Ag, As, Au, Bi, Cd, Cu. Hg, Pb, S, Sb, Se and Zn. Most of the area investigated belongs to an erosional regime; dominant landform units are hills formed by weathered siltstones, dolomites and phyllites, with thin soils and abundant lag. Calcrete sampling along the lodes indicates that anomalous Au is associated with As, Cu, Pb and Zn; the narrow hydrothermal alteration halo gives rise to anomalous Ba, Mg, Sr, K, Na, Rb, Tl, U and W. Using calcrete as a sample medium, geochemical dispersion along the lode horizons is very limited. In augered samples, dispersion halos are broader. The respective widths of dispersion are 25-50 m for ore-associated elements (As, Au, Cd, Cu, Pb, and Zn), and of the order of 150 m for elements associated with hydrothermal alteration (Ba, K, Mg, Sr, Th, U and W). Auger drilling and calcrete sampling are effective sampling techniques in this environment, although a sufficiently dense sampling grid (25 m) would be required for prospect-scale calcrete sampling. Other conventional sampling techniques, such as soil sampling, are likely to be valid.

The Blue Rose prospect is located about 40 km south of Olary and 10 km south of the New Milo and Great Eastern mines, and occurs on the southern limb of the Wadnaminga Anticlinorium in metasedimentary units of the Burra Group. Most of the prospect area is covered by alluvium, with transported cover locally 42 m thick. Disseminated Cu mineralisation occurs in dolomitic rocks. The geochemical signature of the Cu mineralisation is Bi, Cs, In, K, Mo, Rb, S, Se and Tl. Copper dispersion haloes in saprock and saprolite are broader than in bedrock and extend into the basal 2 m of transported overburden. There is a vertical zonation in the upper 50 cm of the soil-calcrete profile. Arsenic, Au, Ba, Ca, Mg, S, Se, Sr and Y are most abundant at 20-40 cm depth, whereas Ag, A1, Bi, Co, Cr, Cs, Cu, Ga, Fe, Hf, In, K, Mn, Mo, Na, Nb, Ni, Pb, REE, Rb, Sn, Th, Ti, Tl, U, V, W and Zn are most abundant at 0-10 cm depth. Soil sampling (<2 mm fraction) over the mineralised zone on grid line 428820E failed to detect the mineralisation. Similar disappointing results were achieved by >6mm and <6 mm augered samples, 2-6 mm magnetic lags and partial leach analyses (water, potassium cyanide, sodium pyrophosphate, MMI), even though transported overburden is only 8-9 m thick over part of the mineralised zone. Auger drilling was carried out on six traverses. In general terms, samples with the greatest element abundances in soils and calcrete occur where transported cover is relatively thin (<6 m). A zone anomalous in Au and Cu was outlined by <6 mm augered samples at the southern end of grid line 427250E (to the south-west of the presently known mineralised zone), and later confirmed by <2 mm soil sampling. Two other geochemically anomalous zones were also identified at Blue Rose.

At Wadnaminga, where cover is minimal, both auger drilling and calcrete sampling were effective sampling techniques. The geochemical suite for further exploration should include: As, Au, Ba, Ca, Cu, Fe, K, Mg, Mn, Pb, U, W and Zn. Although not used in this study, soil sampling would also be a valid sampling medium in this environment (erosional regime). At Faugh-a-Ballagh, soil sampling (<6 mm fraction) appears to be the most effective sampling medium in this environment (dominantly erosional regime). Stream-sediment and rock-chip sampling provide only limited Cu dispersion. There is no improvement using magnetic fractions of soil, lag or stream sediments. The geochemical suite for further exploration should include: As, Au, Bi, Ca, Cu, Fe, Mn, Na, U and W. At Blue Rose, which occurs in a dominantly depositional regime, soil sampling (<2 mm fraction) and auger drilling are effective techniques only where transported cover is thin (<6 m). Partial leach analyses failed to detect Cu mineralisation, even where transported cover was 8-9 m thick. This indicates that drilling is the only effective technique where cover is >6 m thick. However, broad Cu dispersion haloes in saprolite and basal parts of transported cover provide larger targets than the primary zones. The geochemical suite for further exploration should include: Au, Bi, Cu, Fe, K, Mg, Mn, Mo, Se and Tl.

Last updated: Friday, January 25, 2002 3:54 PM


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