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