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Morobe Goldfield - Wau, Edie Creek, Hidden Valley, Kerimenge, Bulolo |
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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 Wau gold deposits, including the Golden Peaks, Golden Ridges and Upper Ridges mines, and the Kerimenge (9 km south of Wau) and Hidden Valley (16 km SW of Wau) gold deposits are all located within the Morobe Goldfield, near Wau in Morobe Province, Papua New Guinea. The historic and geologically similar Edie Creek mines are some 5 km SW of Wau, while the abondoned Bulolo dredging operations are 10 km to the NW. All fall within the 15 km wide, NNW trending Wau Graben. The total production from the Morobe Goldfield to 1975 amounted to 115 tonnes of Au, of which approximately 15% was from lodes, the remainder being from alluvial operations (Rebek, 1975).
Wau
The Morobe Goldfield is within the Highlands Fold Belt of the New Guinea Orogenic Province, situated between the overthrust Papuan Ultramafic Belt to the east and the Neogene sediment filled Aure Trough to the west. The basement in the district is the Cretaceous Owen Stanley Metamorphics (known locally as the Kaindi metamorphics which includes blue-grey graphitic slate, chloritic and sericitic schist and phyllites with minor quartzite and marble) which have been intruded by the Miocene Morobe Granodiorite Batholith.
These are overlain by a Pliocene sequence that includes, the Bulolo Ignimbrite which is several hundred metres thick and dips at 40 to 65°E., the Otibanda Formation comprising 700 m of poorly sorted auriferous conglomerate and sandstone with minor reworked tuff and ignimbrite near the base, and the Pliocene (3.8 to 3.4 Ma) dacite to andesite porphyry which has strongly altered (silica-sericite-pyrite) the surrounding metamorphics. These are all cut by the Namie Breccia of the Wau Maar which is generally coarse and poorly sorted and is believed to be of subaerial phreatomagmatic origin. The Wau Maar is 1.4 km in diameter, is centred about 500 m south of Golden Peaks and is outlined by a 50 to 70° inward dipping ring fault cutting the Kaindi metamorphics, the Bulolo Ignimbrite and the Otibanda Formation. Intra-maar volcanics and sediments are highly altered and soft sediment deformed pyroclastics and epiclastics. Some dacite porphyries in the district are present as domes emplaced during the formation of the maar and generally found along the ring fault. All of these are overlain in part by Pleistocene fluvial and fluvioglacial sediments. A major NNW trending east dipping fault zone, the Escarpment Fault cuts most rocks up to the beginning of Maar Formation.
Primary gold occurs in a number of forms in the district. These include i). Lodes and Veins - stringers and veins of quartz with gold on the margin of the Morobe Granodiorite, mainly in the Kaindi metamorphics, ii). mesothermal gold associated with the Pliocene dacite-andesite porphyries and iii). the bulk of the gold which is epithermal and hosted by rocks related to the Wau Maar.
The epithermal gold at Wau occurs as:
* Lodes and Veins - including the Upper Ridges, Golden Peaks and Golden Ridges deposits. At Upper Ridges, there are at least seven lodes in an en echelon pattern striking 300 to 320° and dipping 30 to 50°SW, almost normal to, and possibly cut by, the Escarpment Fault. Individual lodes are lenticular, up to 15 m thick, 150 m long and persist for around 60 m down dip. Other parallel striking smaller lodes dip west. Most are open space fissure fillings with crustiform banding. Primary vein mineralogy includes quartz, calcite, manganocalcite and pyrite, with subordinate manganite, rhodochrosite, siderite and minor sulphides (chalcopyrite, galena, sphalerite, tetrahedrite, argentite and silver sulphosalts). Gold occurs as electrum, generally at grades of 5 to 30 g/t in the primary lodes, but locally up to 300 g/t Au. Supergene oxidation is to 50 m depths.
* Stockworks - veins of calcite and drusy quartz are up to a few cms wide and 10 m long, usually above lodes, but are only of limited extent.
* Disseminations - fine grained disseminated gold occurs in the alteration haloes of porphyry dykes intruding the maar breccia with associated intense alteration to quartz, sericite and pyrite with anomalous and occassionally high grade gold. These alteration zones are usually steep, have diffuse margins and are associated with brecciation.
Production and reserve figures are as follows:
Golden Ridges (Homestead Lode - underground production) 1929-1960 - 0.210 Mt @ 21.1 g/t Au.
Golden Ridges (Demitrius & Davidson open pits production) 1961-1989 - 0.580 Mt @ 1.9 g/t Au.
Golden Ridges (open pit reserve), 1989 - 0.138 Mt @ 2.14 g/t Au.
Golden Peaks (production)1953-1977 - 1.5 Mt @ 4.8 g/t Au.
Upper Ridges (underground production) 1933-1962 - 0.249 Mt @ 13.8 g/t Au.
Upper Ridges (open pit production) 1962-1986 - 0.822 Mt @ 1.97 g/t Au.
Upper Ridges (open pit production) 1986-1989 - 1.38 Mt @ 1.74 g/t Au.
Upper Ridges (in situ resource) 1983 - 2.6 Mt @ 2.7 g/t Au.
Koranga Creek alluvials (production)1939-1967 - 11.5 M cub m for 2 t Au.
Edie Creek
The historic Edie Creek mine is 5 km south-west of Wau and geologically similar, except that the main hosts are sediments of the Kaindi metamorphics. Gold occurs in fissure veins and veinlets within the Kaindi metamorphics and the Pliocene quartz-biotite porphyries (the Edie Porphyry), mainly in disrupted or shattered metamorphics adjacent to the porphyry stocks. Individual veins, narrow zones of veinlets and a few individual thin rich veins were exploited. The largest production was from the Edie Lodes which extend over a NW strike trend of 4 km as a single to multiple vein set dipping steeply south-west. Individual veins ranged from 0.3 to 7 m, averaging 1 m in thickness, with lengths of around 450 m. In the primary zone carbonate dominates over quartz. Gold grades are around 10 to 20 g/t Au in both the primary and weathered secondary zone, while Ag grades are substantially higher grade in the oxidised interval. The largest single mine, Edie Creek-Karuka produced 7.8 tonnes of Au from 0.2 Mt of ore to 1941.
Bulolo Alluvials
Alluvial gold has been dredged from the flats of the Bulolo River over an interval of around 18 km. These workings are downstream of the Wau and Edie Creek lode deposits where rich alluvial gold has also been worked in smaller tributaries. It is estimated that around 65 tonnes of gold were won from the dredging, while 34 tonnes have come from alluvial workings in the Edie Creek and Wau districts, and on the Upper Watut River, a downstream tributary of the Bulolo River.
Kerimenge
In the Kerimenge deposit is 9 km south of Wau. Metasediments of the Owen Stanley Metamorphics country rock are intruded by the main host to mineralisation, the Kerimenge porphyry, a sub-volcanic sill at least 300 m thick with a planar basal contact with the Owen Stanley Metamorphics. In the deposit area tis intrusive is a medium grained dacitic, to locally rhyodacitic, porphyry with plagioclase phenocrusts. Both of the above units/intrusives are cut by a fine grained, massive tuffaceous lithology that forms an irregular ovoid body covering an area of 1.5 sq. km and consisting of around 20% variably rounded clasts of a few cms or less in diameter in the fine tuffaceous matrix. This is the Kerimenge Diatreme.
The main control on mineralisation is the complex north-south trending Kerimenge Fault which is a narrow (up to 10 m wide), well defined, foliated and brecciated zone (with a fine grey rock flour matrix surrounding subrounded clasts of porphyry) with a sinistral displacement of around 500 m. Zones of intense stockworking, replacement and vein breccias are found adjacent to the fault associated with splays of the fault. Northwards the fault zone broadens into clay altered multiple fracture zones with widths of up to 80 m. In this latter zone the mineralisation is in and to the east of the fault, while further south where the fault narrows, mineralisation is restricted to the western side within the porphyry.
Mineralisation is present as stockwork, vein and breccias associated with alteration within the Kerimenge porphyry and exploiting the faulting and fracturing associated with and splaying from the Kerimenge Fault, extending outwards from the fault for up to 200 m and over a length of around 600 m. The veining is composed of silica and/or manganocalcite and/or pyrite, with accessory arsenopyrite (to 1%), sphalerite and galena (to 0.1%), adularia (to 1%) and trace marcasite, chalcopyrite, tetrahedrite and gold. There are four vein assemblages from oldest to youngest of i). Quartz ±pyrite, ii). Quartz-pyrite-adularia-sphalerite-chalcopyrite-galena, iii). Quartz-calcite/manganocalcite-pyrite-arsenopyrite-marcasite ±galena ±barite, and iv). Manganocalcite/calcite-quartz ±pyrite ±galena-silver. Very fine free primary gold is in all of these assemblages, although the better grades are in the second. The dominant alteration is a vein related quartz-sericite-pyrite which overprints pervasive propylitisation and extends from a few cms to 20 to 30 m from vein systems. Localised argillic alteration (clay-pyrite) overprints the quartz-sericite, associated with breccia zones.
A geological resource has been delineated at Kerimenge of (Hutton, et al., 1990):
55 Mt @ 1.0 g/t Au using a 0.5 g/t Au cutoff.
The total resource quoted in the June 2003 Abelle Limited Quarterly report is:
14.641 Mt @ 1.6 g/t Au.
Hidden Valley
See the separate Hidden Valley record.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2003.
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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Carswell J T 1990 - Wau 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 1763-1767
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Denwer K P, Leach T M and Mowat B A, 1995 - Mineralisation of the Morobe Goldfield, Morobe Province, Papua New Guinea: in Mauk J L and St George J D, (Eds.), Pacrim 95 congress, Exploring the Rim, Auckland, New Zealand, 19-22 November 1995, The AusIMM, Melbourne, Proceedings volume pp. 181-185
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Fisher N H 1975 - Alluvial gold of the Morobe goldfield, P.N.G.: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 1045-1049
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Hutton M J, Akiro A K, Cannard C J, Syka M C 1990 - Kerimenge 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 1769-1772
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Nelson R W, Bartram J A, Christie M H 1990 - Hidden Valley Gold-Silver deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1773-1776
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Rebek R J 1975 - Edie Creek and Wau gold lodes: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 867-872
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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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