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Simberi, Tabar - Sorowar, Pigibo, Pigiput, Botlu, Samat |
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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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The Simberi gold mine, which includes from north to south the Sorowar, Pigibo, Pigiput, Botlu and Samat pits and orebodies, are located on Simberi Island, the northern most of the Tabar Group of islands. These orebodies occur as a number of near surface pods which are distributed within an arcuate >0.2 g/t Au geochemical anomaly that is over 4 km long by up to 2 km in width in the eastern third of the 30 sq km island. The Tabar Group, which include the for main islands of Simberi, Tabar, Tatau and Mapua, are in Papua New Guinea (PNG) approximately 150 km north of Rabaul on New Britain, and 80 km northwest of Lihir Island. The Tabar Group are found towards the north-western end of the 250 km long Tabar-Feni island chain which is approximately 40 km to the north-east of, and parallel to, New Ireland. The Tabar-Feni island chain comprise from NW to SE, the Tabar, Lihir, Tanga and Feni island groups.
The Tabar-Feni island chain comprises a series of Pliocene to Recent volcanoes that occupy a fore-arc position in the New Ireland Basin, part of the Bismarck archipelago. Several of these volcanoes are still active, with the most recent eruption 2300 years ago. They are the are host to large porphyry stocks, active geothermal systems and epithermal gold deposits, including the Ladolam gold deposit on Lihir Island. Volcanism on New Ireland and on most of the islands in the Bismarck archipelago is dominantly calc-alkaline to high K calc-alkaline generated as a result of the subduction of the Pacific plate beneath the Indo-Australian plate along the Manus-Kilinailau trench. The Tabar-Feni island chain parallels the presently inactive Manus-Kilinailau Trench, with the main island groups being spaced at around 75 km intervals. This subduction ceased at approximately 10 Ma when the Ontong Java plateau collided with the subduction zone, followed by a reversal of subduction direction, formation of the north to NE facing New Britain-San Cristobal trench, commencement of back-arc spreading in the Manus Basin at about 3.5 Ma and development of a number of microplates (Kamenov, 2004).
The current New Ireland Basin is characterised by sediment filled horst-graben structures indicating extension and crustal thinning. Volcanism within the Tabar-Feni island chain is largely of alkaline and undersaturated volcanics, lavas, and intrusives, which began on northern-most island, Simberi Island, in the Miocene at 3.7 Ma, coeval with the initiation of back-arc spreading in the Manus Basin, and migrated southward to the southern-most Feni Island by the Holocene in 2.3 Ma (Kamenov, 2004). In the Tabar Islands, raised Quaternary coral reef platforms surround steep sided volcanic plateaus, particularly on Simberi and Tatau Islands. These plateaus are generally 120 to 300 m above sea level and are often fringed by Miocene to Pliocene raised limestone reefs.
The epithermal gold mineralisation is structurally controlled and developed in altered volcanoclastics and lavas. Gold occurs in both the 15 to 45 m thick oxide cap, which is typically found at the top of, or running along ridges, and in the underlying sulphide zone. On Simberi Island the gold mineralisation is associated with irregularly distributed breccia zones while on Tatau and Tabar Islands, it is associated with structurally controlled quartz veining and is often accompanied by anomalous base metals. The grade of the mineralisation is related to the degree of fracturing of the host rocks, which are generally altered alkaline lava flows or intrusives, or volcaniclastics and tuffs.
In the oxide zone, gold is associated and correlates with anomalous As and high Ag grades, while the base metals Zn, Pb and Cu have been depleted. In general the base metals form a broad halo surrounding the higher-grade Au mineralisation. All the Au occurrences on Simberi are are associated with widespread disseminated pyrite mineralisation which occurs as two separate phases, an early, more pervasive, pre-gold barren phase and a second stage which is associated with the gold mineralisation. The arcuate, >0.2 g/t Au, geochemically anomalous corridor which embraces the main deposits is coincident with a strong airborne geophysical radiometric and magnetic anomaly.
Pronounced leaching is associated with an early K-feldspar flooding event, followed by the introduction of pyrite to arsenean pyrite and arsenopyrite, in which gold is encapsulated within sulphides. A subsequent high-grade gold phase is associated with sphalerite-pyrite-carbonate (Corbett and Leach, 1995). Auriferous quartz veins are found on the periphery of the main deposits, forming a zonation pattern from a core of early K-feldspar flooding and an absence of silica, with mineralisation grading from (quartz)-sulphide-gold, to carbonate-base metal-gold and marginal epithermal quartz-Au-Ag. Corbett, et al., 2005 regard the alteration and deposits at Simberi as being typical of low sulphidation, intrusion-related mineralisation formed peripheral to an alkaline magmatic source at depth.
The total inferred + indicated + measured resource on Simberi Island in June 2009 at a cut-off of 0.5 g/t Au - 127.19 Mt @ 1.15 g/t Au for 146 t Au (Allied Gold Ltd, 2009).
Comprising:
Oxides - 35.07 Mt @ 1.14 g/t Au (Measured - 13.83 Mt @ 1.24 g/t Au; Indicated - 13.63 Mt @ 1.06 g/t Au; Inferred - 7.62 Mt @ 1.07 g/t Au)
Transitional - 4.36 Mt @ 1.03 g/t Au (Measured - 0.60 Mt @ 1.18 g/t Au; Indicated - 1.71 Mt @ 1.15 g/t Au; Inferred - 2.05 Mt @ 0.89 g/t Au)
Sulphide - 87.76 Mt @ 1.16 g/t Au (Measured - 1.30 Mt @ 0.93 g/t Au; Indicated - 16.92 Mt @ 1.36 g/t Au; Inferred - 69.54 Mt @ 1.12 g/t Au)
In the Sorowar Pit, the largest of the deposits, the total inferred + indicated + measured resource in June 2008, at a cut-off of 0.5 g/t Au - 59.3 Mt @ 0.98 g/t Au for 58 t Au (Allied Gold Ltd, 2008).
Comprising:
Oxides ore - 20.32 Mt @ 1.15 g/t Au
Transitional ore - 2.51 Mt @ 1.16 g/t Au
Sulphide ore - 36.46 Mt @ 0.87 g/t Au
Earlier estimates in 2005 included:
Measured + indicated resource at a cut-off of 0.5g/t Au - 10.69 Mt at 1.25g/t Au
Inferred resource at a cut-off of 0.5g/t Au - 9.04 Mt at 1.07g/t Au (Allied Gold Ltd, 2005).
The most recent source geological information used to prepare this decription was dated: 2008.
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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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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