The
King Island scheelite mine was located on the 65 x 22 km King Island in
Bass Strait between Tasmania and the Australian mainland, ~200 km
north-west of Burnie on the Tasmanian mainland and 240 km SSW of
Melbourne..
Scheelite mineralisation was first located on the island by a Tasmanian
prospector, Tom Farrell who discovered scheelite bearing float on the
beach near the site of the present main open cut in 1911. Follow up
prospecting revealed significant scheelite mineralisation under some 15 m
of sand cover.
Following the incorporation of the King Island Scheelite Development
Company NL in 1917, a 200 tonne per week plant was erected and mining
operations commenced.
The mine was forced to close in 1920 due to the
decline in the price of tungsten. During 1937, with rising prices, the
mine was revived and King Island Scheelite NL was floated to operate it.
A 500 tonne per week plant was installed and production commenced in
1938. In 1943, with compulsory Federal Government financing, the mine
was further mechanised and by 1944 a new mill had been established
allowing the quadrupling of production in 1946. The Company was
voluntarily wound up in 1947 and reconstructed as King Island Scheelite
(1947) Limited. The Mine experienced a series of boom years during the
early 1950's due to the high tungsten prices brought about by the Korean
War. However, in 1958, as prices slumped, the Mine was put on a
care-and-maintenance basis until 1960 when it reopened. Production
increased during the 1960's, and in 1969, following it's takeover by the
Peko Wallsend Group, a 300 000 tonnes per annum capacity was installed.
Since then the mill has been expanded to handle up to 400 000 tonnes
per year.
Until 1972 all production was from the main open pit. However,
following a concentrated exploration effort in the late 1960's and
early 1970's, two new orebodies were delineated and underground mining
commenced at Bold Head in September 1972 and Dolphin in June 1973.
Operations ceased in the main open cut in October 1974. An artificial
scheelite plant is due to come on-stream in 1978. However, following a
prolonged period of depressed prices in the 1980s, operations closed in
1990.
The deposit, which is located on the south-eastern coast of the Island,
is hosted by a thin (±200m thick) late Neoproterozoic to lower Cambrian
unit of dolomite, shale and tillite overlying a +7000 m thick
Neoproterozoic pelitic sequence. This sequence, which forms a narrow
strip along the coast, is overlain to the south-east by >2500m of
Cambrian picritic-spilitic lavas tuffs and agglomerates, and rests upon
Neoproterozoic quartzites, siltstones, shales and mudstones to the west.
Further west a more intensely deformed sequence of metamorphics cut by
Proterozoic granites form the basement. The whole sequence is intruded
by a series of late Devonian to lower Carboniferous granites, while
extensive Quaternary dune sands derived from beach deposits dredged up
by long shore drift cover much of the Island. A high proportion of the
bedrock below these sands, particularly in the central section of the
Island, is below sea level.
The stratigraphy encountered on King Island
is as follows, from the top:
Quaternary - Fine to medium grained quartz sand dunes.
Lower Carboniferous to Late Devonian - Three coarse granitic stocks
outcrop along the east coast of the Island. The southern pair are
probably fault separated segments of the same body. The southernmost
stock has a granodioritic composition, the central is an adamellite
while the northern body is granitic.
Cambrian - The Cambrian is principally represented by a probable middle
Cambrian, >2500 m sequence of massive picritic and spilitic lavas,
tuffs and agglomerates. These form a strip along the south eastern coast
and are interpreted as reflecting a volcanic centre located nearby.
Basal Cambrian to Late Neoproterozoic, which comprises:
Grassy Group - This is taken to be the stratigraphic equivalent of the
mineralised Mine Series host rocks. Both sequences are sandwiched
between the middle Cambrian basic volcanics and the Neoproterozoic
sandstone siltstone unit. In general the Grassy Group is only around 200
m thick, and comprises a lower tilloid (or breccia-conglomerate) with
intercalated basic tuff bed. This has a transgressive boundary with the
Neoproterozoic sediments in places while appearing conformable in
others. It is overlain by dolomite and dolomitic siltstone. These are
overlain in turn by siltstone and shale, occasionally with discontinuous
bands of dolomitic siltstone.
Neoproterozoic (possibly to Lower Cambrian) - More than 7000 m of
interbedded fine, clean to dirty, quartz-sandstone, siltstone, shale and
mudstone are found immediately below the Grassy Group. This sequence
dips consistently to the east, and in the mine area, comprises massive
fine grained highly siliceous quartzite. In some areas, carbonaceous
shales carrying disseminated magnetite are found within the sequence.
Carbonates are absent from the succession.
Palaeoproterozoic - Two thirds of the Island's surface comprises
probable Lower Proterozoic metamorphics which may have formed a basement
high to the west during the Neoproterozoic. An inferred, but not
observed, major unconformity is believed to occur at the base of the
Neoproterozoic. This sequence of metamorphics is +6000 m thick and dips
consistently to the west. It comprises a lower succession of quartzites
and quartz-muscovite-staurolite schists, overlain to the west by
quartzites and quartz-biotite-andalusite schists. These sequences carry
some tourmaline-quartz rocks. Along the north western and western margin
of the Island a major development of gneissose granite of Precambrian
age is exposed. Alternatively, these may be metamorphosed Upper
Proterozoic sediments, rather than Lower Proterozoic.
The mineralisation at the No. 1 open cut, and the Dolphin and Bold Head
underground mines is hosted by the "Mine Series" which are cut in the
immediate orebody area by the Lower Carboniferous to Late Devonian
granites.
The Mine Series is in general 150 to 200 m thick and is discordantly
overlain in the mine area by middle Cambrian mafic volcanics. It
comprises an easily distinguishable sequence readily recognisable in all
three mines as follows, in descending order:
B lens hanging wall unit, 10 to 20 m thick - strongly jointed
actinolite-biotite and fine biotite rock;
B lens ore horizon, 25 to 30 m thick - this is a banded sequence of fine
biotite-pyroxene rock, marble and grossularite garnet with minor
pyrrhotite and variable amounts of scheelite;
Hanging wall unit, 5 to 50 m thick - strongly jointed, fine,
actinolite-biotite and biotite rock;
Pyroxene garnet unit, 2 to 15 m thick - this is a blotchy green
(diopside) and pink (grossularite) rock containing calcite ovoids up to
15 cm in diameter and variable scheelite;
Upper C-lens horizon, 0 to 12 m thick - this is the principal ore
horizon and comprises coarse andradite "tactite" with interbedded marble
and minor banded pyroxene-grossularite rock;
Marble marker, 1 to 5 m thick - a barren, or weakly mineralised marble
and pyroxene-grossularite-biotite rock;
Lower C-lens horizon, 6 to 15 m - a banded, alternating
andradite-tactite and pyroxene rock;
Banded footwall beds, 7 to 30 m thick - these comprise interbedded (1 to
5 cm) marble and pyroxene-biotite-grossularite rocks, with variable
quantities of scheelite;
Biotite-pyroxene unit, 20 to 30 m thick - thinly banded (0.5 to 1 cm)
biotite-pyroxene-actinolite rock;
Lower metavolcanics, 5 to 8 m thick - this is now a
tremolite-phlogopite-chlorite-magnetite rock with textures reminiscent
of a basic tuff.
The Mine Series concordantly(?) overlies a massive, fine grained, highly
siliceous quartzite.
The pyroxene-garnet unit has a texture reminiscent of a breccia
conglomerate and has been correlated with the so called "tilloid" at the
base of the Grassy Group. If this is the case, it means that the main
ore zone does not have stratigraphic equivalents along strike and
corresponds to the inferred hiatus observable locally as a transgressive
contact, at the base of the Grassy Group.
The sequence containing the Dolphin orebody is the same as that carrying
the No. 1 orebodies. The Dolphin sequence is offset downwards and to
the south-east along the No. 3 Fault. Both sequences dip at around 30 to
40Ëš, with the Dolphin dipping to the south east and No. 1 to the
south.
The Mine Series is sharply limited to the east of the Dolphin Mine by
the Grassy River Fault. This is a large normal fault which displaces the
surface expression of the Mine Series by some 3 km to the north. The
Bold Head orebody is located at the intersection of the Mine Series with
the Grassy River Fault to the north. It is confined between the Grassy
River Fault to the west, a second small fault, the Boundary Fault to the
east and the Bold Head adamellite to the north. In this region the dip
is generally 20 to 30° to the south, although in the orebody area it
averages only 15°.
The sequence at Bold Head varies slightly from the sequence of No. 1 and
Dolphin in the following respects: i). At Bold Head, a 15 to 40 m thick
basic meta-volcanic lens is developed above the B lens horizon,
separating it from, and inter-fingering in part with, the B lens hanging
wall unit. ii). Also at Bold Head, the B lens hangingwall unit
increases in thickness to 50 to 100 m (in comparison to 10 to 20 m at
Dolphin)
The northern margin of the Grassy Granodiorite dips shallowly to the
north at ~20°, below the No. 1 and Dolphin ore zones, before plunging
steeply below Grassy township. This contact cuts the Mine Series at a
high angle. The southern margin of the Bold Head adamellite dips
shallowly to the south east cutting the host sequence in the western
sections of, and below, the main Bold Head orebody.
Mineralisation
The mineralisation occurs as a series of pods and lenses in a skarn
composed of outer actinolite-biotite and actinolite hornfels,
biotite-pyroxene hornfels, marble, blotchy and banded
diopside-grossularite hornfels and massive andradite skarn (with the
main ore). The overlying mafic volcanics are altered to
tremolite-phlogopite-chlorite-magnetite rock. Individual ore lenses
were of the order of 20 to 25 m thick and mined both by open pit and
underground room and pillar.
Ore mineralisation is found at a number of horizons within the Mine
Series, as outlined below.
The ore lenses are usually developed at the
gradational contact between pure marble and pure pelitic sediments
within a mixed carbonate-sediment facies host. Scheelite occurs as
finely disseminated grains averaging 0.05 to 0.20 mm, and almost always
occur in bands parallel to bedding within the host. The scheelite is
usually found within "tactite"* developments, with the highest grades
accompanying andradite garnet. Grossularite rich lenses usually only
carry low grade scheelite although actinolite is often accompanied by
high grade mineralisation. In general the scheelite occurs on the
margins of the andradite garnet grains, occasionally being found at the
contact between andradite and calcite crystals, and very rarely is
enclosed by calcite. In places scheelite occurs as coarse well formed
crystals from 2 to 5 cm across in quartz gash veins and within the
quartz-calcite pods of the pyroxene-garnet unit.
The majority of the scheelite is powellite, a molybdenum rich variety of
wolframite, which fluoresces yellow under UV light. Minor blue
fluorescing scheelite is found within the quartz-calcite pods of the
pyroxene-garnet unit and in the banded footwall beds. No wolframite or
cassiterite has been detected although the final 75% WO3 concentrate
carries 0.01% Sn.
Trace amounts of sulphide, usually <0.5%, are found within the ore
zones comprising molybdenite, pyrite, arsenopyrite, pyrrhotite and
chalcopyrite. Of these, only molybdenite approaches commercial
quantities, while the chalcopyrite accounts for 100 to 200 ppm average
Cu grade.
A few massive pods up to a few metres across of pyrrhotite
have been observed, mainly at Bold Head adjacent to faults and within
the pyroxene-garnet rock. The principal gangue minerals are garnet,
carbonate, quartz, pyroxene, apatite, pyrite and pyrrhotite.
The principal mineralised horizons are as follows:
A-lens - This is a small lens of scheelite bearing pyroxene-andradite
tactite found at Bold Head. It was believed to represent a separate lens
but is now known to represent a fault offset section of B-lens.
B-lens - This lens is present in all three mines, although it is only
economically significant at Bold Head. At No. 1 and Dolphin, the ore is
very patchy with some small very high grade patches. However, overall
the average grade in these two mines is nearer 0.2 to 0.3% WO3 over
thicknesses of around 5 m, with restricted lateral extents. At Bold Head
the grade varies from 0.6 to 0.8% WO3 over an average thickness of near
5 m.
Pyroxene-Garnet unit - In the No. 1 open cut, small tonnages of economic
mineralised rock have been extracted from this unit.
C-lens - C-lens, which is the principal ore lens in all three mines, can
be divided into an upper and lower lens, separated by a 1 to 5 m bed of
barren or weakly mineralised marble with pyroxene-grossularite-biotite
lenses and patches.
Upper C-lens - This is a massive coarse (2 to 4 mm) andradite
quartz-calcite tactite, with scheelite evenly distributed throughout. It
usually carries around 0.85% WO3 over a thickness which varies from 5
to 15 m, but averages near 12 m at No. 1 and Dolphin. Grades of 3.7% WO3
have been encountered over restricted intervals. At Bold Head similar
grades and thicknesses are encountered.
Lower C-lens - This lens varies from 5 to 40 m in thickness but
averages near 25 m at No. 1 and Dolphin. At Bold Head it is around 12 m
thick. It comprises alternating bands of andradite quartz-calcite
(identical to the Upper C-lens tactite) and biotite
pyroxene-grossularite from 0.5 to 3 cm thick. The andradite rich bands
are mineralised, while the biotite-pyroxene-grossularite bands are
virtually barren. The grade of the unit averages around 0.7% WO3.
D-lens or Banded Footwall Beds - Below Lower C-lens, the banded footwall
beds locally carry economic mineralisation within the banded pyroxene
biotite-grossularite beds. This mineralisation is usually continuous
with the base of C-lens and does not have sharp lithological boundaries
as other lenses do. Ore blocks are outlined on grade boundaries, and
where present are referred to as D-lens. D-lens usually comprises 1 to 3
beds each from 2 to 5 m thick, carrying around 0.6% WO3. This grade and
the outlines however vary with the ruling tungsten price.
The distribution of these ore beds within the individual mines is as
follows:
No. 1 Open Pit - Mineralisation within this orebody was largely confined
to the Upper and Lower C-lens, with minor tonnages being worked from
the pyroxene-garnet unit and the banded footwall beds (D-lens). C-lens
decreases in thickness and grade both down dip and to the west, although
the Grassy Granodiorite limits its low grade fringes in both of these
directions. To the east it was limited by the No. 3 fault. It had an
overall strike length of 550 m and down dip extent of around 250 m.
Dolphin Mine - Ore is extracted mainly from Upper and Lower C-lens, with
lesser contributions from B-lens.
The main orebodies are developed in
the nose zone of a shallowly south east plunging anticline over a strike
length of around 250 m and similar down dip extent.
Bold Head Mine - Production from this mine is from Upper and Lower
C-lens, B-lens and D-lens. The orebodies are truncated to the west and
in part to the north by the Bold Head adamellite, while they lens out
down dip. To the east the Boundary Fault limits, mineralisation,
although a sub-parallel structure, the No. 2 Fault, displaces the ore
horizons upwards by about 70 m on the eastern limit of the Mine.
Adjacent to this fault, and between No. 2 and the Boundary Faults, the
ore zones are far thicker, and mineralisation is well developed in a
broad fringe on either side of the fault. The orebodies are developed
over a strike length of between 100 and 150 m, and for at least 250 m
down dip.
Production and reserve figures are as follows:
Total production from No. 1 open pit until its completion in 1974 -
6.29 Mt @ 0.53% WO3;
Reserves at July 5 1977 (Peko Wallsend Annual Report, 1977) - 6.70
Mt @ 0.80% WO3;
Total production + reserves (Peko Wallsend Annual Report, 1977) -
13.92 Mt @ 0.67% WO3;
Cut off grade in 1977 - 0.3% WO3;
Reserves at the commencement of underground mining:
Bold Head - 5 lenses containing - 2.6 Mt @ 0.8% WO3;
Dolphin - 2 lenses containing - 6.8 Mt @ 1.0% WO3.
Production to 1988 totalled 9.7 Mt @ 0.64% WO3, while at the same time
the remaining in situ resource was 6.82 Mt @ 1.01% WO3 for a total
production + resource of 17.5 Mt @ 0.85% WO3.
JORC compliant resources at 2012 (King Island Scheelite Ltd, 2012) were:
Dolphin (0.70% WO3 cut-off),
Indicated resource - 4.752 Mt @ 1.29% WO3;
Inferred resource - 0.007 Mt @ 0.73% WO3;
Total resource - 4.759 Mt @ 1.29% WO3;
Bold Head (0.50% WO3 cut-off),
Indicated resource - 1.500 Mt @ 0.93% WO3;
Inferred resource - 0.150 Mt @ 1.22% WO3;
Total resource - 1.650 Mt @ 0.96% WO3.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2012)
