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Wapolu ( Fergusson Island ) |
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Papua New Guinea |
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Au
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Super Porphyry Cu and Au
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The Wapolu low sulphidation epithermal gold deposit is located on the north-west point of the 55x25 km Fergusson Island in the D'Entrecasteaux Group, which is within the Milne Bay province of Papua New Guinea, approximately 400 km east of Port Moresby.
The D'Entrecasteaux Group of islands are dominated by four large ovate metamorphic domes, similar to the metamorphic core complexes of the North American Cordillera. The Wapolu deposits are on the northern edge of the Mailolo Dome, cut by the north-south Gewata Fault in an area where amphibolite facies metamorphic footwall rocks of amphibolite, gneissic amphibolite and quartz-feldspar gneiss are exposed. These are overlain by the hangingwall ultramafics. Uplift and doming has led to decollement structures and gravity sliding between the footwall metamorphics and the overlying ultramafics (originally dunite and peridotite).
The Wapolu deposit is located where two of the faults bounding the Mailolo Dome intersect and coalesce, and are folded into an anticline dividing the deposit into a western and eastern sector. The eastern sector comprises footwall metamorphics dipping at 25 to 30°NE, overlain by a transition zone of mixed gneisses or amphibolites and ultramafics with local tremolite-chlorite schist, overlain by the variably leached, altered and brecciated ultramafics, which are in turn overlain by large gravity slide blocks of brecciated metamorphics and talus breccias. The western sector is similar without the talus breccias.
Alteration is controlled by lithology. The ultramafics may be intensely altered to serpentine, talc, chlorite and anthophyllite, while hydrothermal carbonate, quartz and chalcedonic or opaline silica over print and replace original minerals and textures. Higher grade gold tends to be associated with silicified (opal or chalcedonic quartz) zones and occassionaly contain adularia. Lower grade mineralisation is in zones that have been carbonated, but with little chalcedonic quartz, while weak mineralisation accompanies carbonate, clays, chlorite and talc.
Three zones of mineralisation had been outlined in 1989, each in a different setting, as follows: i). Didigayagaya Zone - lenticular development of ore within and adjacent to the decollement structure at the contact between the ultramafic and underlying tansition lithologies. ii). Dagwalala Zone - which is a mineralised, chaotic talus breccia in gneisses above the ultramafics. It contains clasts of ultramafic, leucogneiss, amphibolite, reticulated network silica and plagiogranite in a matrix of talc-chlorite-smectite. iii). Luke Zone - of less significance, hosted by a high angle cross fault.
The in situ geological resource quoted in 1988 was 5.3 Mt @ 1.9 g/t Au at a cut-off of 1.0 g/t Au.
For detail consult the reference(s) listed below.
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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McNeil 1990 - Wapolu gold deposit, Fergusson Island: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1783-1788
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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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