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The Panguna porphyry copper-gold deposit is located in the Crown Prince Range of central-south Bougainville Island in eastern Papua New Guinea, at an altitude of 500 to 1200 m (#Location: 6° 19' 0"S, 155° 29' 36"E).
Gold was initially discovered by prospectors at Kupei in 1930, and shortly after at nearby Panguna, some 5.5 km to the sout-west. Cu-Au bearing quartz veins and associated alluvials were mined on a small scale at both Kupei and Panguna, from the 1930s until the Japanese occupation in 1941. In the early 1960s, J E Thompson, Government Geologist in Port Moresby, recognised the porphyry association from a 1936 government report. During the same period, a technical visit to porphyry Cu-Au deposits in the Philippines by K Phillips of CRA Exploration led him to consider searching for an analogue in the New Guinea islands. After discussions at the Geological Survey in Port Moresby Thompson advised Phillips of is conclusions and showed him the reports of Cu-Au mineralisation associated with porphyry intrusions and agglomerates in the Crown Prince Range on Bougainville Island. Phillips then made a field inspection in May 1964, confirmed the similarities, and following stream sediment and ridge and spur soil sampling program, his exploration team delineated a 13 sq km copper anomaly, focused on a 300 m diameter core. By 1969, over 80 000 m of diamond drilling had been completed. The mine commenced stripping in 1969 and full commercial production in April 1972, but was closed due to civil unrest in May 1989.
Bougainville is part of the Melanesian arc, built up from calc-alkaline island arc subduction-related magmatism from the Eocene, and interrupted by deposition of the Miocene Keriaka Limestone on Bougainville. The volcanic sequence, the Kieta Volcanics, includes mostly andesite with lesser basalt and dacites with associated volcanogenic sediments, including the Late Miocene wall rock host, the Panguna Andesite. These volcanics were deposited on a basement of Eocene to Oligocene agglomerates and volcano-sediments A wide variety of intrusions of late Eocene to Recent age intrudes the volcanic pile, including diorite, granodiorite, monzonite and locally more alkaline compositions were emplaced into the volcanic pile over a protracted period of time. Continuing active volcanism at the Bagana Volcano, 32 km north of the mine, has resulted in a blanket of recent ash to several metres thick obscuring much of the island, including the Panguna area.
The Panguna ore deposit and hydrothermal system is related to an oval shaped, NW-SE elongated, 1500 x 1000 m, multiphase apophysis developed on the southeastern margin of the larger, approximately 4 km diameter mass of the 5 to 4 Ma Kawerong Quartz Diorite. The ore is developed where this multiphase apophysis intrudes the Late Miocene Panguna Andesite member of the late Eocene to early Pliocene Kieta Volcanics. The main lithologies of the mineralised complex in the mine area (Fig. 17) include:
Panguna Andesite - the main wall-rock host is a member of the Kieta Volcanics and occurs as a shallow SE dipping hornblende microdiorite lava, agglomerate, lapilli tuff and local pyroclastic bands from 1220 to about 450 m asl. It becomes less agglomeratic with depth, but more welded, fractured and propylitic alteration. It has been contact metamorphosed to a hornblende hornfels for up to 500 m outward from the contact, grading to epidote-chlorite-albite-K feldspar-calcite-pyrite to the limit of exposure, some 1200 m from the contact.
Kawerong Quartz Diorite - the bulk of the intrusion is un-mineralised and occurs to the north-west of the Panguna mine. Within the mine apophysis, there are a number of phases and variants, characterised by gradational, crosscutting and overprinting relationships. These phases are represented by a series of lithologies which include:
Biotite Diorite which comprises the main mass of the apophysis and largely surrounds the Biotite Granodiorite and "Feldspar Porphyry" and is a more potassic altered variety of the main Kawerong Quartz Diorite. It carries much of the mineralisation, and is increasingly brecciated with depth.
Leucocratic Quartz Diorite is a later intrusive phase which occurs on the southern margin of the main intrusive mass. It contains more intense quartz veining and is more siliceous than the Biotite Diorite.
Biotite Granodiorite and "Feldspar Porphyry", which occupy the low grade central core of the orebody. Both post-date the Leucocratic Quartz Diorite and differ only in degree of alteration and texture;
Breccias occur as intrusive, collapse and tectonic breccias and cut most of the main intrusive phases described above. Intrusive breccias were formed by the emplacement of the biotite diorite into the Panguna Andesite and contain angular andesite fragments set in a matrix of biotite, chalcopyrite, bornite and local free gold. Consequently, they are associated with higher ore grades. Metal grades within the breccia bodies decline with depth, and they are cut by dykes of Biuro Granodiorite;
Pebble dykes, including one that can be traced laterally in the open pit for 1900 x 50 m, exhibit fragment milling and consistent fragment and matrix compositions over considerable distances;
Biuro Granodiorite, (3.4 Ma) which is only weakly mineralised and occurs as dykes and as a mass on the western side of the deposit, where it dilutes the ore;
Feldspar porphyries occur as post-mineral intrusions;
Nautango Andesite (1.6Ma) is a the barren post ore intrusive.
The fracture pattern exhibits a concentric form, modified by the regional NE structural grain, which is exploited by intrusive features such as pebble dykes (Clark, 1990).
Alteration was developed as follows:
Potassic alteration is closely associated with the best mineralisation and is found in areas of >0.5% Cu, particularly in the Panguna Andesite, Biotite Diorite and Leucocratic Quartz Diorite.
Propylitic alteration is dominant in areas of <0.3% Cu, where chlorite is > biotite and pyrite is > chalcopyrite.
Argillic alteration is widespread but weakly developed, overprints potassic and propylitic phases and is characterised by clay development and disseminated pyrite.
Phyllic alteration is irregularly developed with sericite, silica and pyrite and overprints other alteration in all rock types.
Gypsum and minor anhydrite occurs in joints and microfractures below about 150 m from the uppermost benches.
Copper and gold mineralisation forms an annular zone of higher grade surrounding a low grade core, and weakens outwards in all directions.
Mineralisation was developed as follows:
i). Early pyrite in the Kawerong Quartz Diorite,
ii). Chalcopyrite and gold associated with hydrothermal biotite in the Biotite Diorite and Panguna Andesite,
iii). Intense quartz veining with chalcopyrite and bornite accompanied the subsequently intruded Leucocratic Quartz Diorite to form a stronger zone of Cu-Au mineralisation in the south of the ore zone annulus,
iv). Chalcopyrite and pyrite, with small areas of high grade Cu (disseminated chalcopyrite and bornite) accompanied the Biotite Granodiorite which intruded and partly replaced the mineralised Biotite Diorite,
v). Pyrite on joints with phyllic selvages occurs irregularly through all rocks in the system, except late andesite dykes.
While copper occurs within chalcopyrite and bornite, chalcocite and other secondary copper minerals are recognised but were not of economic importance
The original pre-mining ore reserve in 1969 was - 994 Mt @ 0.48% Cu, 0.56 g/t Au, 3 g/t Ag (Clark, 1990).
Production over this period amounted to 710 Mt of ore @ 0.53% Cu, 0.63 g/t Au for 3 Mt of contained Cu metal, 306 tonnes of gold and 784 tonnes of silver (Clark, 1990, Bougainville Copper Ltd., Annual Report, 1992), plus 630 Mt of waste (Bougainville Copper Ltd., Annual Report, 1999).
Remaining reserves in 1999 were officially listed at 496 Mt @ 0.45% Cu, 0.55 g/t Au. In addition, mineralisation amenable to upgrading by screening would add a further 520 Mt @ 0.22% Cu, 0.18 g/t Au to the reserves to produce a mill feed of 195 Mt @ 0.34% Cu, 0.28 g/t Au. This would result in a total combined mill feed from both remaining ore groups of 691 Mt @ 0.40% Cu, 0.47 g/t Au (Bougainville Copper Ltd., Annual Report, 1999).
Remaining Mineral Resources (Bougainville Copper Limited release to the ASX, March 2016) were:
Indicated Resource - 1.538 Gt @ 0.3% Cu, 0.33 g/t Au,
Inferred Resource - 300 Mt @ 0.3% Cu, 0.4 g/t Au,
TOTAL Resource - 1.838 Gt @ 0.3% Cu, 0.34 g/t Au.
The most recent source geological information used to prepare this summary was dated: 1998.
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.
Agnew, M., 2018 - Return to Bougainville - Reassessing the Mineral Potential of a Long-Forgotten Island: in SEG Newsletter, April, 2018, No.113, p. 1, pp. 18-24.|
Baldwin J T, Swain H D and Clark G H, 1978 - Geology and grade distribution of the Panguna porphyry copper deposit, Bougainville, Papua New Guinea: in Econ. Geol. v73 pp 690-702|
Baumer A, Fraser R B 1975 - Panguna porphyry copper deposit, Bougainville: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 855-866|
Clark G H 1990 - Panguna Copper-Gold deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1807-1816|
Eastoe C J, 1978 - A fluid inclusion study of the Panguna porphyry copper deposit, Bougainville, Papua New Guinea: in Econ. Geol. v73 pp 721-748|
Eastoe C J, Eadington P J 1986 - High-temperature fluid inclusions and the role of the biotite granodiorite in mineralization at the Panguna Porphyry Copper deposit, Bougainville, Papua New Guinea: in Econ. Geol. v81 pp 478-483|
Ford J H, 1978 - A chemical study of alteration at the Panguna porphyry copper deposit, Bougainville, Papua New Guinea: in Econ. Geol. v73 pp703-720|
Macnamara P M, 1968 - Rock types and mineralization at Panguna: in Stephenson H H, 1973 Bougainville, The Establishment of a Copper Mine Construction, Mining, Engineering Publications, Melbourne, Australia pp 36-43|
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