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Woodlark Island - Kulumadau, Busai Field - Murua United |
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Papua New Guinea |
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Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All available as eBOOKS
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The gold deposits on Woodlark Island are low sulphidation carbonate-gold lodes mined by both open pit and underground methods and from alluvial/placer accumulations. The 30 x 10 km island is located in the Solomon Sea, some 600 km east of Port Moresby on the mainland and 160 km north of Misima Island, all of which are within Papua New Guinea.
Most of Woodlark Island is covered by Pleistocene coralgal limestone which surrounds a NE-SW aligned, 12 km wide basement horst block. It is a remnant of a Neogene island arc, built on a basement of Cretaceous to Eocene low potassium tholeiitic ocean ridge volcanics and deep water sediments, the Loluai Volcanics which represents an oceanic crust floor. These rocks are unconformably overlain by a Neogene sequence commencing with limestone rich sediments of the Early Miocene Nasai Limestone, overlain by volcanolithic sediments, agglomerate and andesite of the Early to Mid Miocene Wonai Hill Beds, and then by the Early to Mid Miocene Okiduse Volcanics. The Okiduse Volcanics, hosts the bulk of the lode gold and comprises a distinctive suite of probable early to mid Miocene high potassium, calc-alkaline basal epiclastics overlain by predominantly subaerial andesite, trachyandesite, rhyodacite and dacite flows, breccias and tuffs. All of these volcanic rocks are intruded by comagmatic porphyritic microdiorite and monzonite dykes and sills dated at 16.5 Ma and 13±0.4 Ma These rocks are overlain and largely masked on the island by Pliocene to upper Pleistocene reef lagoonal sequence of limestone, clay, sandstones and conglomerates.
Three main lode systems are described, as follows:
Kulumadau - is the other significant deposit, developed within an outlier of Okiduse Volcanics, 5 km WNW of Busai. It includes the old Ivanhoe and Great Northern workings. The main host is a massive, fine too medium grained andesite. Gold mineralisation occurs as a NNE striking tabular body of altered, phreatic explosion breccia and fractured andesite, bounded to the west by an 80°E dipping fault. At surface the body has a 200 m strike length, maximum width of 110 m and down dip entent of at least 150 m. It is in turn composed of a series of 2 to 12 m thick linear breccias separated by volumes of less brecciated/fractured rock. The main breccias are polymict rubble breccia with high to medium matrix content enclosing sub-angular to angular chaotically distributed elongate clasts up to 200 mm long. The matrix is a puggy clay and rock flour with mineralisation in matrix voids. The mineralisation comprises aggregates of calcite and sulphides distributed through out the breccia with minor quartz veins and adularia. Sulphides are about 3 to 10% of the breccia, and include pyrite, galena, sphalerite, minor chalcopyrite and trace tetrahedrite. Gold is present as up to 0.1 mm inclusions in sulphides and carbonates. Alteration is a phyllic assemblage of illite-chlorite or mixed illite/smectite-pyrite-carbonate, overprinting regional propylitisation. The Kulumadau mineralisation has been interpreted to occur marginal to a concealed porphyry intrusion. Intense aeromagnetic highs correlate with areas of fracture-controlled magnetite that are suggested to represent porphyry style potassic-propylitic alteration, which is overprinted by sericite-clay alteration. Anomalous Cu, Pb and Zn display a concentric zonation about the inferred porphyry centre.
Busai Field - spread over an area of 800 x 450 m, mainly within north to north-west trending, near vertical quartz lodes, mostly within a leucocratic to mesocratic feldspar porphyritic microdiorite, and a >150 m thickness of mesocratic hornblende andesite and volcaniclastics that are probably comagmatic with the microdiorite. In addition to the steep, shear controlled lodes there are broader, thicker zones of flat-dipping fracture-controlled mineralisation in both the porphyry and Okiduse Volcanics, constrained between the lodes, and interpreted to have formed in dilatant settings created by reverse fault movement on the steep structures. The sequence of alteration and mineralisation includes initial phreatomagmatic fluidised breccias, overprinted by banded chalcedonic quartz-pyrite with local jasper. Mineralisation in the largest of the two mines, the Murua United mine is controlled by a NW trending dextral dilational shear containing an ellipsoidal 220 x 100 m zone made up of numerous steep fractures. This zone is limited on the eastern side by a 6 to 15 m wide, steeply W dipping, 260 m long footwall zone of intense fracturing. The fracture zones contain single and multiple quartz veins and stockworks persisting to depths of 100 m or more. Individual lodes are 0.1 to 4 m wide with variable trends rtending towards an average of NW parallel to the shear. Gold occurs in two assemblages, namely a dominant quartz-sulphide and a less common calcite-dolomite-sulphide. Carbonates may be up to 15% while quartz is up to 20% of the mineralised zone, with 1 to 8% sulphides, comprising pyrite, lesser galena and sphalerite, minor chalcopyrite and trace tennantite. Busai mineralisation is dated from illite wall rock alteration adjacent to mineralised veins as 12.2±0.1 and 12.4±1.6 Ma.
Woodlark King - is located ~7 km SSE of Kulumadau and 5 km SSW of Busai.
Historic production totalled: 5.6 tonnes of Au and 1.2 t Ag from a total of 0.20 Mt of ore and from alluvial workings, mainly from 1899 to 1918 and 1930 to 1939.
Subsequent testing has indicated an in situ geological resource (2002) of:
Busai Deposit - 0.53 Mt @ 4.9 g/t Au (at a 2.0 g/t cut-off)
Kulumadau Deposit - 0.8 Mt @ 5.09 g/t Au (at a 2.0 g/t cut-off)
At a 1.0 g/t Au cut-off, these and other smaller deposits total - 2.55 Mt @ 3.7 g/t Au (1990).
Reserves and resources as at June, 2014 (Kula Gold webpage, July 2014) were:
Kulumadau Deposit
Measured resources - 5 Mt @ 1.78 g/t Au
Indicated resources - 4.4 Mt @ 1.75 g/t Au
Inferred resources - 8.6 Mt @ 1.4 g/t Au
TOTAL resources - 18.0 Mt @ 1.6 g/t Au
Proved reserves - 3.283 Mt @ 2.2 g/t Au
Probable reserves - 2.811 Mt @ 1.9 g/t Au
TOTAL reserves - 6.094 Mt @ 2.1 g/t Au
Kulumadau East Deposit
Probable reserves - 0.33 Mt @ 3.7 g/t Au
Busai Deposit
Measured resources - 3.9 Mt @ 1.54 g/t Au
Indicated resources - 10.4 Mt @ 1.4 g/t Au
Inferred resources - 8.8 Mt @ 1.3 g/t Au
TOTAL resources - 23.1 Mt @ 1.4 g/t Au
Proved reserves - 3.144 Mt @ 2.2 g/t Au
Probable reserves - 0.751 Mt @ 2.4 g/t Au
TOTAL reserves - 3.863 Mt @ 2.3 g/t Au
Woodlark King
Indicated resources - 3.0 Mt @ 1.2 g/t Au
Inferred resources - 1.0 Mt @ 1.8 g/t Au
TOTAL resources - 4.0 Mt @ 1.4 g/t Au
TOTAL (Probable) reserves - 0.704 Mt @ 2.4 g/t Au.
TOTAL Resource at a 0.5 g/t Au cut-off - 45.1 Mt @ 1.5 g/t Au (as tabulated above).
TOTAL Resource at a 1.0 g/t Au cut-off - 19.7 Mt @ 2.45 g/t Au.
TOTAL Reserve at a 1.0 g/t Au cut-off - 10.991 Mt @ 2.2 g/t Au (as tabulated above).
The most recent source geological information used to prepare this decription was dated: 1990.
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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Russell P J 1990 - Woodlark Island Gold deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1735-1739
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White, N.C., Leake, M.J., McCaughey, S.N. andd Parris, B.W., 1995 - Epithermal gold deposits of the southwest Pacific: in J. of Geochemical Exploration v.54, pp. 87-136.
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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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