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Misima |
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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 papers now Open Access.
Available as Full Text for direct download or on request. |
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The Umuna low sulphidation adularia-sericite epithermal gold deposit is located on the 25 x 8 km Misima Island in the Louisiade Archipelago, which is within the Milne Bay province of Papua New Guinea, some 240 km ESE of the New Guinea mainland and 670 km ESE of Port Moresby.
Misima Island is part of a near-linear, NW-SE trending chain in the Solomon Sea that includes the islands of the D'Entrecasteaux Group to the north-west. This chain is at the western end of the Woodlark Basin rift system, an active crustal extensional feature that developed at around 5 Ma in response to subduction and plate rotation along the irregular boundary between the Pacific and Indo-Australian plates. The Woodlark Basin is opening at about 7 cm/yr and propagating westerly at around 12 cm/yr. Differential crustal expansion related to this translation has resulted in extensional tectonics and rapid updoming to expose domal, high-grade metamorphic core complexes, within the ophiolitic assemblages in the Cretaceous Eastern Fold Belt of the Papuan Islands.
The oldest rocks on Misima are Cretaceous to Paleogene metamorphic rocks, which can be subdivided into two main geological/structural units, namely:
1). the lower core complex Awaibi Association in the western end of the island, composed of medium grade metamorphics (after gabbro, pyroxenite, trondjhemite, amphibolite and various gneisses), and
2). the overthrust, the less metamorphosed Eocene Sisa Association composed of metasediments and occupying the eastern end of the island, that is host to the gold and copper mineralisation
Both associations are intruded by sheet-like bodies and stocks of undeformed Boiou microgranodiorite intrusions, which have yielded an 11.3±0.6 Ma 40Ar/39Ar age, and 8.1±0.4 Ma zircon Miocene age microgranodiorite and other intrusives and are overlain by younger tuffs and sediments, including reef limestones, which are in turn intruded by lamprophyre dykes.
All of the gold is within the Sisa Association which has been divided into:
i). Umana schist - up to 400 m of micaceous and graphitic schist at the top of the association,
ii). Halibu calc-silicates - a discontinuous 30 m thick marble unit and laterally equivalent calc-silicates and graphitic schists - where calc-silicates are absent the position is occupied by a limonite enriched breccia,
iii). Ara greenschist - conformably underlies the Halibu calc-silicates and comprises 90 m of massive, weakly foliated metabasaltic volcanics composed of a fine assemblage of amphibole, albite, quartz, chlorite, clinozoisite, biotite and magnetite,
iv). Bulpat schist - which conformably underlies the Ara greenschist and forms the basal unit of the association. It commences with a lower unit of interbedded meta-pelites and meta-psammite grading up into carbonaceous and aluminous schist with minor calc-silicates, which in turn grades into the Ara greenschist.
The Sisa Association meta-sediments have a gentle outwards dip away from a domal core in the centre of the eastern half of the island, and are regionally gently folded about NW trending fold axes. Faulting is common, as at least three pulses, starting with an early E to SE trending shallow thrust phase, followed by steeper dipping NW and NNW faults.
Two phases of magmatic arc mineralisation have been recognised on Misima Island:
i). Early porphyry copper skarn occurrences are related to the generally sheeted porphyry intrusions of the Miocene Boiou Microgranodiorite. Endoskarns have been produced at the contact between these intrusions and carbonate (marble) units within the Sisa association host rocks. Typical skarn assemblages and paragenetic sequences and zonations include wollastonite-grossularite proximal to the intrusion, passing out into more distal andradite-diopside dominated mineral associations, overprinted by magnetite-pyrite-chlorite, with later retrograde chalcopyrite-calcite-chlorite-haematite-epidote-sphalerite. Grades are generally sub-economic (<0.3% Cu). This porphyry style mineralisation has been dated at 11 to 8 Ma, before the Misima Basin began to spread, possibly related to subduction below the Trobriand Trough.
ii). Low sulphidation, epithermal Au-Ag mineralisation occuring as two styles, namely, in lodes and in schistose wall rock. The epithermal mineralisation is younger and has been dated at 4 to 3.2 Ma, after the Misima Basin spreading commenced. The majority of the gold mineralisation at Misima is related to the Umuna Fault Zone which occurs as a NNW-trending, west dipping, 3 km long, 50 to 500 m wide fault and breccia zone, including a series of sub-parallel splays, mainly in its footwall. A normal displacement of 200 to 300 m is estimated. It has a configuration similar to a dilational fault jog characterised by down drop within the dilatant segment. The dilation was formed by a component of dextral, transpressional strike-slip movement between the two bounding regional WNW-ESE trending parallel fault systems it connects to the north and south respectively. Brittle deformation on Misima is partly associated with the post-5 Ma extension of the Woodlark Basin rift.
The mineralised section of the fault is some 3 km long and is known down dip for at least 500 m.
The fault zone is silicified, altered and is characterised by well developed extensional brecciation, with the proportion of quartz decreasing with depth and to the south.
Mineralisation within the fault occurs as frequent quartz and quartz-carbonate veining with associated pyrite, galena, sphalerite, barite and minor tetrahedrite. This style of veining is best developed in the upper central part on the hangingwall (western) side of the fault zone over a width of up to 20 m and length of 1500 m, known as the Umuna Lode Zone. This lode in turn encloses higher grade sections carrying 8 to 10 g/t Au. At surface, near the centre of the mineralised system (laterally), massive quartz bodies are exposed. Banded quartz and quartz replacing carbonate textures predominate in the upper portions, passing to breccias at depth. In the hangingwall and footwall of the Umuna Lode Zone the degree of quartz-carbonate and quartz veining also decreases, with correspondingly lower gold grades.
Gold occurs as very fine grains in quartz and carbonate veins and in sulphides, the most abundant of which is pyrite, although most of the gold is associated with acompanying base metal (Cu, Pb, Zn) sulphides. Oxidation extends to 200 m below surface, and locally as far as 300 m. The oxidation has resulted in an upgrading of silver (by more than 8 times), although the amount of gold remains fairly constant with the only upgrading due to the reduction in SG of the host by weathering. There is a poorly developed transition between the oxidised and fresh rock with higher than usual Cu values. The oxidation makes the ore softer and easier to mine and liberates gold making it more amenable to treatment.
The mineable reserve quoted in 1988 was 55.9 Mt @ 1.38 g/t Au, 21 g/t Ag at a cut-off of 0.7 g/t Au
The total ore reserve in 1995 was 36.1 Mt @ 1.1 g/t Au, 9.7 g/t Ag at a 0.7 g/t Au eq. cut-off (Orogen Mineral Prospectus, 1996)
The total measured+indicated resource (includes reserve) in 1995 was 56.7 Mt @ 1.03 g/t Au, 8.6 g/t Ag (Orogen Mineral Prospectus)
Production in 1995 totalled (Orogen Mineral Propsectus, 1996):
Ore + waste - 33.254 Mt Ore treated - 5.731 Mt @ 1.97 g/t Au, 12.65 g/t Ag Mill recovery - 91.2% for Au, 40% for Ag.
When the mine closed in 2004 it had produced 115 t Au, and 572 t Ag.
Further exploration in the Umuna Zone has outlined an NI 43-101 compliant mineral resource (WCB Resources, release, October, 2013):
inferred resource - 43 Mt @ 1.1 g/t Au, 6.1 g/t Ag for 48.8 t of gold and 264 t of silver.
The most recent source geological information used to prepare this decription was dated: 1996.
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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Adshead N and Appleby A K, 1996 - The Umuna Au-Ag deposit, Misima Island, Papua New Guinea: Spatially but not genetically associated with porphyry: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide, pp 14.1 - 14.10
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Anonymous 1996 - Misima Gold Mine: in Orogen Minerals Limited 1996 Prospectus, Orogen Mineral Limited pp 65-72
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Appleyard G R, Gillett L J 1996 - Misima Mines Pty Limited: in Orogen Minerals Limited 1996 Prospectus, Orogen Mineral Limited pp 158-169
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Lewis R W, Wilson G I 1990 - Misima 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 1741-1745
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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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