Lefroy and Beaconsfield gold mines, Tamar region, Tasmania

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Lefroy and Beaconsfi eld
© CRC LEME 2003 Page 1
LEFROY AND BEACONSFIELD GOLD MINES, TAMAR REGION, TASMANIA
J.C. van Moort and D.W. Russell
University of Tasmania, Hobart Campus, University of Tasmania, Burnie Campus
LOCATION
The Lefroy Goldfi eld is located 15 km E of George Town on the E
bank of the mouth of the Tamar River, at 41°06′54″S, 146°59′20″E.
The Beaconsfi eld Mine is 11 km S of George Town at 41°12′06″S,
146°46′20″E, on the W bank. Respective maps are Bell Bay 4844 and
Beaconsfi eld 4843, Tasmania 1: 25 000 series.
DISCOVERY
Alluvial gold led to the discovery, in 1867, of reef gold at Specimen
Hill, 1.5 km SE of Lefroy. Some 30 lines of reefs were discovered with
production mainly restricted to the New Pinafore, Chum, Volunteer
and Native Youth Reefs (Figure 1). Mining ceased in 1911. The 0.47
m wide outcrop of the isolated Tasmania Reef at Beaconsfi eld was
discovered in 1877, again in the search for the source of alluvial gold.
Mining ceased at Beaconsfi eld in 1914 but resumed in 1999.
Figure 1. Lefroy goldfi eld. Location of reefs, simplifi ed geology and Au
anomalies in soil B horizon (Keele 1996, Purvis 1997).
Over the past 30 years, exploration in the Beaconsfi eld area has mainly
used structural control and drilling. In contrast, in the Lefroy area,
stream sediment and soil geochemical surveys were used in addition to
those techniques. No new mineable reefs were found in either region.
In retrospect, as the wall rocks are altered around the reefs, it would
have been benefi cial to i) analyse elements other than Au and As, ii)
sample saprolite or soil C horizon instead of the soil B-horizon and iii)
adopt a sample spacing suffi ciently close to identify targets less than 10
m across.
PHYSICAL FEATURES AND ENVIRONMENT
The Lefroy Hills form a topographically low, rounded basement rise
and reach 200 m. Beaconsfi eld is located on the partially dissected
West Tamar Plain, reaching 80 m on the exhumed structural ridge. The
climate is temperate to Mediterranean, with an average annual rainfall
of 950 mm. Thin secondary growth forest covers the areas that are not
cleared for farming.
GEOLOGICAL SETTING
The Tamar River follows a NNW lineament. Considerable thrusting
occurred during the middle Tabberabberan Orogeny, when the entire
region was metamorphosed to lower greenschist facies (Reed, 2001).
The turbidites in the Lefroy area are among the oldest sediments of the
5-7 km folded and faulted Cambro-Ordovician to Devonian Mathinna
Group. The Au deposits coincide with a W to E transition from
sandstone to a shale-rich succession. In the Beaconsfi eld area, a
series of NNW-striking, E-dipping, imbricate thrust slices incorporates
1 km of Cambrian to Devonian stratigraphy. The sediments change
from shallow-water clastic sediments to deep-water limestones that
are conformably overlain by turbidites equivalent to those at Lefroy.
The Beaconsfi eld deposit is in the Cabbage Tree thrust slice, where
Ordovician grits fi ne upwards to shales and limestones.
REGOLITH
Yellow podzols 1-2 m deep occur in the Lefroy area; the soils in the
Beaconsfi eld area are <0.6 m deep. Vegetation and soil originally
obscured the quartz reefs in both areas. In the Lefroy area, several
N-draining leads, up to 80 m deep, pass beneath Middle Eocene to
Middle Pliocene basalts. A N-draining lead, running along the fl ank of
Cabbage Tree Hill at Beaconsfi eld, is about 140 m deep; the sediments
include locally-derived eluvial detritus.
MINERALIZATION
The Lefroy auriferous quartz-carbonate-sulphide reefs are hosted by
large, E striking, steeply dipping faults that cross-cut recumbent folds,
especially on the location of superimposed, gentle, minor, open folds.
Some of these structures, such as the Chum, Pinafore and Volunteer
Reefs, are 1-2 km long, with crush zones 10-20 m wide and at least
400 m deep. Gold occurs in high-grade shoots (up to 30 g/t) within
the reefs. The ore is concentrated in pipelike bodies at the intersection
of the faults and a deformed earlier thrust, separating shale from
micaceous sandstone. Mine dumps indicate that quartz-veined and
altered sandstone, contain 0.5-2.0 g/t Au around some of the reefs. In
several cases, the anticlinal fold axes have been exploited by late-stage
quartz-vein stockworks. The Pinafore, Chum and Volunteer mines
together produced up to 4.3 t Au.
The Tasmania Reef at Beaconsfi eld is a single, quartz-carbonate-
sulphide dilational vein 400 m long with minor splays and bifurcations
and is at least 1200 m deep. Overall, the NE-striking, SE-dipping reef
averages 2.7 m in width, ranging from 8 to 0.5 m. From 1877 until
its closure in 1914, the reef produced 26.6 t Au at a recovered grade
of 24.5 g/t to a depth of 455 m below surface. The mine produced
more than half of all the Au from NE Tasmania. The present combined
proven and probable reserve is 1 143 000 t at 17.4 g/t.
Exploration at Lefroy has outlined alluvial gold reserves of 200 000 m3
at an average grade of 0.5 g/t in long sinuous gullies averaging 30-40
m wide and 0.8 m thick under 1-2 m of overburden. The gullies lead N
from the cluster of reefs (Murdoch, 1985).
REGOLITH EXPRESSION
Active stream sediment
Keele (1996) carried out a BLEG (bulk leach cyanide extractable gold;
detection limit 0.05 ppb) survey of 3 kg composite samples in an area
of 210 km2 surrounding Lefroy and in the Back Creek area immediately
to the E. Of fi ve strongly anomalous samples with >100 ppb Au and
a further nine anomalous samples with 10-100 ppb Au, all but two
correspond to known mineralization.

Lefroy and Beaconsfi eld
© CRC LEME 2003 Page 2
Figure 2. Soil B horizon soil Au anomaly over the Volunteer and Land
O’Cakes Lodes, at Lefroy (Keele 1996). For cross section MMI see
Figure 3. For cross section AA see Figure 4. For line BB results see
text.
Figure 3. Match of Au and Mo in MMI analysis in soil C horizon across
the Volunteer Reef, line 499900E, Lefroy. See Figure 2.
Figure 4. Match of Au by fi re assay in the soil B horizon and Ca in the
acid insoluble residue of the same sample, across the Volunteer Reef
(section AA in Figure 2), line 500050E, Lefroy. Na and Ca enrichment
and K and Rb depletion over the reefs can be used as reef indicators,
enhanced as the index (Na x Ca / K x Rb)1000. The smaller peak at
5448500N indicates the Land O’Cakes Reef. See Figure 2.
Soil
Fifteen hundred B-horizon soil samples (from a depth of 200-300
mm) were collected at 20 m intervals (and at 10 m intervals at reef
intersections) over the Lefroy area. These clayey soils with minor
quartz were dried, pulverized and analyzed for Au (dl 5 ppb) by fi re
assay and As (dl 1 ppm) with an AAS fi nish. Those with >40 ppb Au
and >50 ppm As were considered to be strongly anomalous. The results
represent the major reefs well and, in particular, the major producing
reefs at Volunteer, Land O'Cakes, Pinafore and Chum (Keele, 1996)
(Figures 1 and 2). The 20 m spacing of the samples outside the areas of
known reefs was, in retrospect, too wide in relation to the target sizes.
There has been no systematic soil sampling in the Beaconsfi eld area.
Channel and chip materials
Sixty-fi ve channel samples over 4 m and 31 chip samples over 2 m
were taken from bedrock along road cuttings in the Land O'Cakes
– Volunteer area and analyzed for Au and As. Overall, there is an
excellent correlation between the channel and chip Au results and those
from the soil. The bedrock Au abundances are two to three times
greater than that in the overlying soil (Purvis 1997).
Mobile elements in soils
The Volunteer soil anomaly was followed up by Purvis (1997) by a
Mobile Metals Ions analysis (weak cyanide leach, ICP-MS, dl 1 ppb
and for Au 0.01 ppb). Twenty samples were taken at a depth of 0.7 m at
10 m intervals along line 499 900E across the western end of the reef.
This gave a strong Au and Mo expression of the reef (Figure 3), a less
strong positive As anomaly and a negative Ba anomaly over the reef.
Acid insoluble residue of soils
Forty six samples of the B-horizon soil at 20 m intervals were selected
from line A-A 500 050E across the Volunteer Reef at Lefroy, line B-B
48 400N parallel to a barren section of the Volunteer Reef and line
499 550E across the non-productive Windermere Reef to the N (Figure
1). The samples were digested overnight in hot aqua regia, to remove
carbonates and Fe-Mn oxides, and then washed thoroughly. The
acid insoluble residue consists mainly of quartz with minor sericite.
Thirty four elements were determined by simultaneous PIXE/PIGME
spectroscopy (see Stott et al., 1997). The results show a positive
correlation between Au, As, Na, Ca and Cl. There is a strong positive
correlation between elements comprising phyllosilicates (Al, F, Fe, Ga,
K, Li, Mg, Rb, Sr and V), but these generally correlate negatively with
Au.
The maxima in the cross sections over the Volunteer and Windermere
Reefs (1245 ppm Na and 520 ppm Ca respectively) clearly show the
position of the reef (background values one third of peak). At the
Volunteer reef there is a distinct K, Rb and Al depletion over the reef.
Both the EW section along the barren part of the Volunteer Reef and
the NS section along the weakly mineralized Windermere Reef have
much less Na and Ca than the section across the mineralized part of
the Volunteer Reef, which has no K and Rb depletion. The product
ratios Na x Ca/K x Rb or Na x Ca x Cl/K x Rb may be used as reef
indicators.
Wallrock alteration
Widespread sericite-chlorite-pyrite alteration and quartz veining in the
Lefroy area have been described by Purvis (1997). On a smaller scale,
sericite destruction appears to have taken place around the reefs, as
described above. In the Beaconsfi eld area, where the reefs contain
more carbonate, ankerite spots occur up to 15 m from the Tasmania
Reef. The EPR signals in small quartz veins indicate a 40 m wide halo
below the Tasmania Reef in DDH 11 (Russell, 1995).
Quartz
A study of 327 core samples shows that the auriferous parts of the
Beaconsfi eld and Lefroy reefs consist of microcrystalline quartz. This
quartz has mean contents of 21 ppm As, 3 ppm Ge and 14 ppm Li,
whereas barren parts of the reefs contain <2 ppm As, <1 ppm Ge and
<3 pm Li. Statistical screening of quartz samples of Al, Ge, Li, Na,
Rb, Ca and Cl contents is a useful indicator of gold mineralization in
quartz (Russell, 1995; Russell and van Moort, 1999). Barren samples
invariably have weak electron paramagnetic resonance (EPR) (Russell

Lefroy and Beaconsfi eld
© CRC LEME 2003 Page 3
SAMPLE MEDIA SUMMARY TABLE
Sample medium Indicator element Analytical method Detection limits
Quartz Au
As, Ge, Li fire assay
PIXE/PIGME 5 ppb
3, 1, 1 ppm
Stream sediments Au BLEG 0.05 ppb
Soil, channel and
chips Au
As fire assay
hydride AAS 5 ppb
1 ppm
Soil, mobile metal ions Au
Mo MMI
MMI 0.01 ppb
1 ppb
Acid insoluble residue
of soils Au
Na, Ca and K
Rb
fire assay
PIXE/PIGME
PIXE
5 ppb
40 ppm
3 ppm
and van Moort, 1997).
ACKNOWLEDGEMENTS
P.B. Hills of Allstate Explorations NL at Beaconsfi eld and Dr D.
Duncan provided much appreciated technical information. AINSE
sponsored the multi-element analyses of the acid insoluble soil residue
as part of the project “Chemical composition and paramagnetic defects
of quartz”, Grant 97/079.
REFERENCES
Keele, R.A. 1996. Annual report for gold exploration over EL 1/95,
Lefroy, NE Tasmania. Open File Report 96-3852, Mineral
Resources, Tasmania.
Murdoch, R. 1985 A report on the third stage of alluvial exploration
at Lefroy, Tasmania. Open File Report 85-2405, Mineral
Resources, Tasmania.
Purvis, J.G. 1997. Annual Report Lefroy ,EL 1/95, NE Tasmania.
Open File Report 97-4005, Mineral Resources, Tasmania.
Reed, A.R. 2001. Structure and setting of Proterozoic and Palaeozoic
rocks in the Tamar region, northern Tasmania. Geological
Society.Australia. Specialist Group in Tectonics and
Structural Geology, Field Guide no. 9, 101pp.
Russell, D.W. 1995. The characterisation of quartz and gold in the
Beaconsfi eld and Lefroy areas, northern Tasmania.. Ph.D.
Thesis, University of Tasmania. Vol. 1, 230pp and Vol. 2,
data. (Unpublished).
Russell, D.W. and van Moort, J.C. 1999 The chemical characterisation
of auriferous vein quartz, Beaconsfi eld gold mine, Tasmania.
Journal Geochemical Exploration 67: 191-200.
Russell, D.W. and van Moort, J.C. 1997 EPR as an exploration tool
to assess quartz reefs, Beaconsfi eld Gold Mine, Tasmania.
In: C.Z. Rudowicz (Editor), First Asia-Pacifi c EPR/ESR
Symposium, Hong Kong. Springer, Singapore, pp 295-303.
Stott, C.L., Xu Li, Butt, C.R.M., Bailey, G.M. and van
Moort, J.C. 1997. Gold and associated elements in
lateritic regolith at Jim’s Find South, Tanami Desert,
Northern Territory, Australia. In: Xie Xuejin (Editor),
Geochemistry. Proceedings 30th International Geological
Congress 30:241-256.