Papua New Guinea
Cu Au Mo
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The Wafi gold deposits and the related Golpu porphyry copper-gold deposit around 1 km to the north-east, are both parts of a complex hydrothermal system comprising porphyry, as well as high and low sulphidation Cu-Au mineralisation, often with overprinting alteration relationships. These deposits fall within the Morobe Goldfield in Morobe Province, Papua New Guinea, approximately 60 km south-west of the port of Lae and 70 km NNW of the Hidden Valley project. (#Location: 6° 53'S, 146° 27'E).
Gold was originally reported from the Wafi River from the 1930s. A 1977 geochemical sampling program conducted in the area by CRAE (no gold assays), part of a program that led to the discovery of the nearby Wamum porphyry copper, returned low level base metal anomalies which were followed up in 1977. During this follow-up program, a pyritic float boulder was identified that assayed 22 g/t Au, 0.57% Pb, 89 g/t Ag. Upstream follow up led to the recognition of altered pyritic volcanics of the Wafi high sulphidation system. The Wafi gold zones were subsequently delineated, but due to the low grade and refractory nature of the gold, the area was joint ventured the property to Elders in 1988. The JV however required Elders to drill 4 conceptual holes based on a consultants structural and fluid flow model to test the source of the high sulphidation fluid. The third of these holes in 1989, intersected 263 m @ 1.86% Cu, 0.27 g/t Au. Elders were subsequently acquired by another party who sold their mineral holdings, which were bought back by CRA. In 2002 the property was taken over by Aurora Gold, then Abelle Ltd who have subsequently been bought by Harmony, the current holder whose principal interest is gold.
The Wafi project lies within the Wau Basin, the north-south structural corridor that divides the western and eastern segments of the New guinea Orogen. One of the most intense structures that constitute this corridor, the Wafi Structure, localises the hydrothermal system at Wafi and also produces a dextral offset of the Ramu Fault to the north.
The country rock to the mineralised system comprises metasediments of the lower Jurassic to Cretaceous Owen Stanley Metamorphics derived from a sequence of argillite, phyllite, metagreywacke, conglomerate and schist, subsequently metamorphosed to lower greenschist facies and strongly deformed into a series of tight NE trending folds. In the project area, the dominant lithologies are clast supported gabbro-rich conglomerate, sandstone, siltstone and shale, with fine to medium grained sandstones. These have been intruded by the middle Miocene (12.9 ±2Ma) Morobe Granodiorite and the 14 Ma Golpu diorite porphyry, and are unconformably overlain by the Pliocene Babwaf Conglomerate.
The mineralised system is reflected at surface by a zone of disseminated pyrite surrounding areas of higher gold content within an advanced argillic, quartz-alunite lithocap. Mineralisation and alteration are associated with a Miocene to Pliocene intrusive complex whose influence is spread over an area of 9 sq. km, centred on the Miocene Golpu diorite to dacite porphyry plug and a late stage overprinting diatreme breccia.
The 800 x 400 m maar-diatreme breccia pipe has steep-dipping margins and comprises a polymictic milled matrix breccia. Several phases of xenolith bearing Wafi dacite porphyry intrusions cut the milled matrix breccia about the pipe margins, as possible endogenous domes. The late-mineral Heking Andesite cuts the dacite porphyry. Potassic alteration within the concealed Golpu diorite porphyry has been dated at 14 Ma and is cut by the diatreme breccia, which is inferred to be associated with the later intrusion event leading to the epithermal gold mineralisation, dated of 13 Ma from alunite within the high sulphidation alteration
The Golpu porphyry copper-gold deposit is associated with a near vertical diorite stock with a diameter of 300 m in its upper sections, contracting to 150 m some 650 m lower. It has a vertical extent in excess of 1200 m and is bounded in part by two NE striking post mineralisation faults. The porphyry system is vertically zoned, with the upper part, below the pyritic advanced argillic quartz-alunite lithocap, being overprinted by a high sulphidation epithermal event that has produced an enargite-covellite-tennantite assemblage. The high sulphidation zone has a well marked base, below which covellite development is associated with a zone of phyllic (silica-sericite-pyrite) alteration, overprinting a lower mass of chalcopyrite-bornite mineralisation at depth accompanied by a core of potassic alteration (biotite-quartz-magnetite±Kfeldspar). Immediately outside of the porphyry contact a 5 to 30 m wide shell of porphyry related stockwork A and D style veining is developed within the meta-sediments (with grades of up to 1% Cu, 2 g/t Au), grading out into propylitic alteration (inner actinolite-epidote and outer epidote-chlorite) and a molybdenum anomalous outer halo. The upper part of the system, above the advanced argillic layer, has a leached cap overlying a thin, rich (eg., 4 m @ 7% Cu) supergene chalcocite-digenite blanket.
The characteristics of the deposit (from Williamson and Hancock, 2005) are:
i). the lack of any surface geochemical manifestations of the porphyry within the barren quartz-alunite cap;
ii). supergene copper enrichment grades of 2.5 to 3.5% Cu which extend from below the base of oxidation at 100 m below surface to 250 m depth, as a several metre thick high-grade zone of chalcocite-diginite, underlain by transition zone silica-alunite-altered porphyry containing covellite-enargite-pyrite (chalcocite-tennantite). In this zone the porphyry Cu-Au mineralisation is overprinted by high sulphidation alteration and remobilised mineralisation;
iii). the upper portion of hypogene phyllic-argillic altered porphyry extending to a depth of 350 m yields grades of 1 to 2% Cu and 0.3 to 0.8 g/t Au in association with pyrite-covellite;
iv). at the deepest levels, where the potassic alteration is preserved and pyrite-chalcopyrite±gold±bornite±molybdenite ores have grades of 1 to 3% Cu and 1 to 2.5 g/t Au, although much of the pyrite is interpreted to have been introduced during the later high sulphidation event. Native gold occurs as minute inclusions within chalcopyrite and bornite.
In mid 2008 the identified mineral resource at Golpu was quoted at 163.1 Mt @ 1.08% Cu, 0.13% Mo, 0.57 g/t Au for 92.2 tonnes (2.96 Moz) of gold (Harmony Gold, website).
Recent drilling (2009-10) has indicated extensions of the main Golpu deposit to the northwest and at depth with intersections of 379 m @ 0.88g/t Au and 1.05% Cu from 1062 m (including: 156 m @ 1.09g/t Au and 1.48% Cu from 1149 m), and 528 m @ 0.70g/t Au and 1.41% Cu from 551 m (including: 187 m @ 1.57g/t Au and 3.16% Cu from 892 m). These and other intersections suggest an exploration target in the range of 500 to 800 Mt at grades of between 0.7% and 1.1% Cu and 0.5 to 0.7g/t Au, including the current resource (Harmony Gold press release, April 2010).
Reserves and resources at the end of August were (Newcrest Mining ASX Release, 29 August, 2012):
Indicated Resource - 810 Mt @ 0.92% Cu, 0.64 g/t Au, 1.1 g/t Ag;
Inferred Resource - 190 Mt @ 0.80% Cu, 0.61 g/t Au, 1.0 g/t Ag;
TOTAL Resource - 1000 Mt @ 0.90% Cu, 0.63 g/t Au, 1.1 g/t Ag;
Probable Reserve - 450 Mt @ 1.2% Cu, 0.86 g/t Au, 1.4 g/t Ag.
The Wafi Gold deposit comprises four key zones, (Zone A, Zone B, the Link Zone and the Western Zone) distributed around a core dacitic vent. Two breccias are the centre of interest for the gold mineralisation. The first is limited in extent and is found in the east and south-east of the complex and is fragment rich. The second in the west is a diatreme complex with surface dimensions of around 500 x 1000 m, narrowing drastically with depth to a wine glass shape. It is matrix supported and has milled shale, siltstone and feldspar porphyritic diorite clasts, and a matrix of rock flour, lithic fragments, feldspar and rare quartz crystals. Intense alteration makes the recognition of primary textures difficult, although accretionary lapilli have been recognised. The unmineralised Pliocene Babwaf Conglomerate overlies both the altered and mineralised Owen Stanley Metamorphics, intrusive feldspar porphyritic diorite of the Golpu porphyry and the diatreme. The diatreme breccias which host the Wafi gold orebodies are younger than the main feldspar porphyritic diorite and the potassic and phyllic alteration and associated Golpu porphyry copper, gold and molybdenum mineralisation. The main gold ore occurs with disseminated sulphides within the breccias or as veins in the porphyry and metamorphics. A 2.3 sq km surface zone of pyrite alteration centred on the main diatreme, and extending out into the surrounding Owen Stanley Metamorphics, is overlain by a soil gold anomaly of >0.2 g/t Au and encloses the gold rich bodies that comprise the gold resource. Alteration is concentric from a central advanced argillic core surrounded by an acid-argillic zone of quartz-kaolinite (with up to >75% silica) and then to irregular zones with phyllic and argillic assemblages and finally to propylitic alteration of the surrounding country rock. Gold, with rare exceptions, is submicroscopic and occurs in advanced argillic and phyllic alteration zones.
The high sulphidation gold event alteration overprints the Golpu porphyry style alteration and mineralisation, with the diatreme carrying fragments of the earlier porphyry alteration. The high sulphidation event has been interpreted to have remobilised pre-existing porphyry-related copper from the phyllic-argillic altered upper porphyry and deposited this as zoned enargite-tennantite-covellite-chalcopyrite mineralisation, while most gold was introduced in association with pyrite of the high sulphidation event.
The low sulphidation Link Zone, which occurs on the diatreme margin, between and below the Zones A and B high sulphidation gold mineralisation, is characterised by veins of mainly of pyrite with lesser quartz (quartz-sulphide-gold style), which are overprinted by more than one generation of pyrite-sphalerite-galena-carbonate veins (carbonate-base metal-gold style). Selective sampling sows the gold in this zone is related to the arsenian pyrite of the quartz-sulphide style veins, while the multi-stage low-arsenic carbonate-base metal veins contribution to the gold grade is insignificant.
In mid 2004 the identified mineral resource at Wafi Gold was quoted at 109.6 Mt @ 1.85 g/t Au for 202.5 tonnes (6.51 Moz) of gold (Harmony Gold, website).
Reserves and resources at the end of August were (Newcrest Mining ASX Release, 29 August, 2012):
Indicated Resource - 110 Mt @ 1.7 g/t Au, 3.6 g/t Ag;
Inferred Resource - 23 Mt @ 1.3 g/t Au, 2.5 g/t Ag;
TOTAL Resource - 133 Mt @ 1.56 g/t Au, 3.4 g/t Ag, containing 207 t of gold.
The project is controlled by the Morobe Mining Joint Venture (Harmony 50%, Newcrest 50%).
The most recent source geological information used to prepare this summary was dated: 2004.
Record last updated: 20/9/2012
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:
Andrew R L 1995 - Porphyry copper-gold deposits of the south-west Pacific: in Mining Engineering, Jan 1995 pp 33-38|
Funnell F R 1990 - Wafi River 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 1731-1733|
Ryan S J and Vigar A, 1999 - Discovery of the High-Grade Gold Link Zone at Wafi, PNG: in Proceedings of Pacrim 99 Conference, Bali, Indonesia, AusIMM, Melbourne pp. 65-72|
Tau-Loi D, Andrew R L 1998 - Wafi copper-gold deposit: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 827-832|
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