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King Island, Bold Head, Dolphin |
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Tasmania, Tas, Australia |
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W
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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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.
Geological Setting
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 40˚, 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.
The most recent source geological information used to prepare this decription was dated: 1990.
Record last updated: 4/12/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Brown S G 1990 - King Island Scheelite deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1175-1180
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Danielson M J 1975 - King Island scheelite deposits: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 592-597
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Kwak T A P, Tan T H 1981 - The geochemistry of zoning in skarn minerals at the King Island (Dolphin) mine: in Econ. Geol. v76 pp 468-497
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Solomon M 1981 - An introduction to the geology and metallic ore deposits of Tasmania: in Econ. Geol. v76 pp 194-208
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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