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The Mount Kare gold deposit is located at an elevation of 3000 m asl, and is some 18 km south-west of the Porgera gold mine and 250 km east of the Indonesian border, in the Papuan Fold Belt, and the highlands that form the central spine of Papua New Guinea (#Location: 5° 30'S, 142° 58'E).
The deposit is spatially associated with a late Miocene alkalic intrusive emplaced into Mesozoic to Tertiary shelf sediments (mainly carbonates) near the edge of the Australian Plate. Both the Mt Kare and Porgera gold deposits lie within an interpreted major NE-SW trending transfer structure that cuts across PNG.
Both the Mt Kare and Porgera intrusive complexes are defined by discrete magnetic highs (2.5 km and 5 km diameter respectively), reflecting the mafic intrusives.
Fault movement associated with the Porgera Transfer Zone is interpreted to have essentially pre-dated emplacement of the Mt Kare and Porgera intrusives and the associated mineralisation. Similarly folding of the sediments is interpreted to pre-date and control emplacement of the mafic intrusives. NE and NW trending faults that cross the deposit have localised higher grade mineralisation, with later north-south faults showing minor displacement.
Host rocks within the Mt Kare-Porgera district comprise a Jurassic to Cretaceous sedimentary sequence of mudstone, calcareous mudstone, sandstone and limestone, which have been folded about a NNE trending axis. The Tertiary Darai Limestone immediately overlies these sedimentary rocks. The 6.0±0.1 Ma Mt Kare Intrusive Complex consists of a hypabyssal suite of comagmatic, volatile rich mafic intrusions, the most common of which are melagabbros and leucogabbros. These intrusives occur as dykes and sills up to 50 m wide in the prospect area, with associated larger (1 to 2 sq. km) gabbroic bodies within a few kilometres.
The immediate host stratigraphy, is a sequence of sandstone, bioturbated calcareous sandstone to siltstone, local conglomerate, siltstone and limestone. Mineralisation is best developed within hydrothermal breccia zones above a listric fault (the Brown Mudstone Fault) and along the sandstone-limestone contact where mafic intrusives are emplaced.
Fault-controlled fluidised breccias with a milled matrix have been emplaced into a NNE trending structure and are overprinted by mineralisation. The ductility of the host rocks governs their ability to fracture and to enable mineralisation localisation. The competent altered sediments and intrusions make the best hosts due to their brittleness, followed by the black shale and sandstone, with the least receptive unit being the incompetent fissile brown mudstone. Mineralisation is developed as sheeted vein sets on the margins of intrusives and in surrounding sediments and as overprinting quartz vein breccias.
Intrusion-related low sulphidation gold mineralisation at Mt Kare has been divided into three stages by Corbett and Leach, (1998). Stage 1 consists of pyrite veins with variable quartz and carbonate contents and halos of sericite-illite alteration. Stage 2 carbonate-base metal-gold mineralisation cuts the earlier formed pyrite veins and comprises a pyrite-galena-sphalerite assemblage followed by minor chalcopyrite, tennantite and arsenopyrite. The gold is present as inclusions in pyrite and sphalerite, and displays a highly variable fineness ranging from 720-930. Stage 3 epithermal gold mineralisation is typified by free gold with quartz or roscoelite and is noted for its bonanza grades.
Mineralisation is defined within 11 domains: the Western Roscoelite Zone (WRZ) North and South, the Black Zone (BZ) and BZ North, the Central Zone (CZ) North, Mid and South, the C9 Zone and the Upper Zone, with two higher grade domains within the BZ and WRZ North respectively. There is a distinctive major NE/SW fault that cuts through the WRZ and appears to separate (and possibly offset the CZ and the C9, although they are both essentially breccia pipes).
A weathering horizon in the Mt Kare region resulting in a “blanket” (Upper Zone) of oxidised bedrock with full to partial oxidation extending up to 100 m below the surface. On the steeper slopes this material is subject to failure with the resultant incorporation of colluvium and thickening of the material across the adjacent valley floor.
Production has largely been by artisinal working of secondary deposits of nugget gold in colluvial workings centred on the oxidised blanket, with primary gold mineralisation in the surrounding hills.
Disseminated auriferous pyrite occurs in brecciated and altered margins of intrusions. Typical alteration minerals in both sedimentary and igneous rocks include illite, sericite and carbonate. Native gold (containing Ag) has been worked by local miners within quartz-roscoelite-cemented vuggy sulphide bearing veins and breccias from a steeply dipping north-south shear zone over lengths of several hundred metres.
Published mineral resources (Indochine Mining Ltd ASX Release, July 2013) at a 0.5 g/t Au cut-off, were:
Measured resources - 20.2 Mt @ 1.84 g/t Au 20.9 g/t Ag;
Indicated resources - 8.3 Mt @ 1.29 g/t Au 8.1 g/t Ag;
Measured + indicated resources - 28.4 Mt @ 1.68 g/t Au 17.2 g/t Ag;
Inferred resources - 14.1 Mt @ 1.27 g/t Au 6.0 g/t Ag;
TOTAL mineral resources - 42.5 Mt @ 1.54 g/t Au 13.5 g/t Ag, with 65.5 t of contained gold;
High grade underground zone - 2.3 Mt @ 5.4 g/t Au within the WRZ and BZ zones;
High surface oxidised ore - 10 Mt @ 1.2 g/t Au, transported from up-slope mineralised zones C9, upper WRZ, CZ & BZ.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 22/7/2013
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.
References to this deposit in the PGC Literature Collection:
Brunker R L, Caithness S J 1990 - Mount Kare 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 1755-1758|
Richards J P, Ledlie I 1993 - Alkalic intrusive rocks associated with the Mount Kare Gold deposit, Papua New Guinea: comparison with the Porgera Intrusive complex: in Econ. Geol. v88 pp 755-781|
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