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Wafi, Golpu, Nambonga |
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Morobe Province, Papua New Guinea |
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Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Wafi epithermal gold deposit, and the related Golpu porphyry gold-copper and Nambonga porphyry copper-gold deposits around 1 km to the north-east and 1.2 km to the northwest respectively. All three are 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: 51° 46' "S, 146° 27' 9"E).
Gold was originally reported from the Wafi River from the 1930s. In 1977, a geochemical sampling program conducted in the area by CRA Exploration Pty Ltd (but with no gold assays). This was part of a campaign that led to the discovery of the nearby Wamum porphyry copper prospect. It returned low level base metal anomalies which were followed up in in the same year. 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 Resources Ltd 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 NZ Forest Products Ltd, who sold their mineral holdings, which were bought back by CRA. In 2002 the property was taken over by Aurora Gold, which was merged into Abelle Ltd in 2003, who were subsequently bought by Harmony in 2004. In 2008 Harmony formed the 50:50 Wafi-Golpu Joint Venture with Newcrest Mining. Newmont Corporation acquired Newcrest Mining Limited on November 6, 2023, and its share in the joint venture.
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 Late Cretaceous Owen Stanley Metamorphics derived from protoliths 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 are overlain by the Omaura Formation shale and greywacke, with some reef facies limestones that was deposited at ~25 Ma; overlain by the Middle Miocene Langimar Beds, composed of volcaniclastic pebble to cobble conglomerates interbedded with sandstones and reef facies limestones. All of these have been intruded by the middle Miocene (12.9 ±2Ma) Morobe Granodiorite and the Nambonga Intrusion and Golpu diorite porphyries, and are unconformably overlain by the Pliocene Babwaf Conglomerate.
The Golpu Intrusive Complex and Nambonga Intrusion are composed of multiple, hornblende-bearing diorite porphyries intruded into the host sedimentary lithologies. The porphyries are separated based on their spatial position and where not texturally destroyed, into coarse hornblende-rich variants, feldspathic-rich units and porphyries containing quartz-eye inclusions. Cross-cutting intrusive contacts are rare. Enclave inclusions of early intrusions in interpreted later intrusions are also rare. Intrusions range from small dykes to small stocks/bosses and apotheoses. Single intrusions pinch and swell vertically over tens of metres and form dykes, pipes and stocks.
The key mineralisation related intrusives include the 10.33 to 9.95 Ma Nambonga Intrusion, comprising an initial mineralised hornblende- and plagioclase-phyric porphyritic diorite, and late stage, barren, biotite-phyric diorite. These are followed by the 9.01 to 8.59 Ma Golpu Intrusive Complex, composed of the Western or Diorite Porphyry mottled grey, plagioclase and quartz-phyric diorite. This complex also includes the hornblende and plagioclase-phyric diorite that is the Golpu porphyry, in which, quartz phenocrysts are absent.
The 9.05 Ma Wafi Breccia Complex in the centre of the system, is a large multiphase polymictic breccia, which is ~1050 x 480 m at surface, tapering to 500 m at depth. It consists of fine-grained volcaniclastic material, occurring as infill to fragments and lapilli. The metasiltstone and metasandstone clast breccias comprise about 90% of the unit, with fragments occurring in intervals that may be 60 to 200 m thick. These intervals are typically upward-fining, and more fragment rich at the base.
The unmineralised, massive, 70 m wide Hekeng Andesite dyke, consisting of plagioclase crystals in a chlorite groundmass is interpreted to post-date the Golpu intrusion and Wafi breccia.
All of the above are unconformably overlain by the Holocene Wafi Conglomerate, a poorly-consolidated conglomerate composed of Owen Stanley Metamorphic clasts and minor carbonaceous material in a poorly-sorted sandy matrix.
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 Wafi maar-diatreme breccia pipe has steep-dipping margins and comprises a polymictic milled matrix breccia. Wafi is on it's southern margin, whilst Nambonga and Golpu are ~750 m to its NW and 200 m to its NE. 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 2012 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.
Reserves and resources at Golpu at the end of June 2020 were (Newcrest Mining NI 43-101 Report, 30 June, 2020):
Indicated Resource - 690 Mt @ 1.1% Cu, 0.71 g/t Au, 1.3 g/t Ag;
Inferred Resource - 140 Mt @ 0.85% Cu, 0.63 g/t Au, 1.1 g/t Ag;
TOTAL Resource - 727 Mt @ 1.09% Cu, 0.71 g/t Au, 1.29 g/t Ag, containing 516 t of gold.
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 Wafi at the end of August 2012 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.
Reserves and resources at Wafi at the end of June 2020 were (Newcrest Mining NI 43-101 Report, 30 June, 2020):
Indicated Resource - 110 Mt @ 1.7 g/t Au, 4.4 g/t Ag;
Inferred Resource - 37 Mt @ 1.4 g/t Au, 4.2 g/t Ag;
TOTAL Resource - 147 Mt @ 1.62 g/t Au, 4.35 g/t Ag, containing 238 t of gold.
The Nambonga porphyry porphyry copper-gold deposit is hosted within the Nambonga diorite porphyry stock, that is typically medium-grained, containing plagioclase and hornblende phenocrysts set in a feldspathic matrix. It is cut by a late barren diorite phase at depth. The host diorite has intruded meta-sandstone and minor meta-conglomerate lithologies of the Owen Stanley Metamorphic Complex.
The deposit appears to be localised along major northwest arc-parallel and northeast trending transfer faults.
What is described as a typical porphyry alteration zonation has been recognised within the porphyry system, grading outwards from an inner potassic core dominated by potassic feldspar, into peripheral propylitic alteration, and upwards into a sericite-dominant phyllic zone. In the upper levels, within the carapace of the porphyry and the adjacent metasedimentary contact, strong silicification and brecciation are evident, with pronounced quartz stockwork veining. Phyllic alteration, characterised by a sericite-K feldspar-chlorite-pyrite assemblage, is associated with the brecciation in the upper levels, particularly in the hanging wall of the porphyry. This alteration envelope is mushroom-shaped, and tends to decrease in width with increasing depth. The phyllic alteration overprints an early potassic event, and may have remobilised primary copper mineralisation, while enhancing the overall copper/gold grades.
Potassic alteration commonly occurs as selvedges to veins as well as pervasive zones with increased vein density. Potassic alteration partially overprints and grades into a peripheral propylitic zone, which is characterised by a chlorite-carbonate-epidote-pyrite assemblage.
The Nambonga stock hosts a low-grade copper and gold mineralised system that extends over an area of ~200 x 200 m and to a vertical extent of at least 800 m. Much of the mineralisation is associated with silicification occurring that is either pervasive or occurs as veins. Quartz veins that may be as thick as 10 mm, form a stockwork that overprints the porphyry, especially in the shallower levels. Mineralisation comprises disseminated and vein-style copper–gold, as well as structurally-controlled base metals in steeply-dipping lodes. Chalcopyrite is the dominant copper sulphide. Chalcopyrite and pyrite occur as anhedral grains that can be as much as 0.2 mm across, and tend to occur as centre-lines to quartz veins. Magnetite occurs as anhedral grains that may be as much 0.4 mm in diameter, and is generally found on the margins of the quartz veins or in the wall rock adjacent to quartz veins. Minor magnetite, pyrite and chalcopyrite are disseminated through the host rock.
Gold has not been seen in thin section, and is likely intergrown in the lattice of chalcopyrite or pyrite. Arsenic is negligible to low throughout the porphyry and surrounding metasediment carapace, with the exception of the massive sulphide shoots. Structurally-controlled base metal mineralisation forms steeply-dipping lodes of variable thickness. These lodes are usually at the margins of the diorite porphyry, where a competency contrast may have formed a dilational zone between the porphyry and wall-rock meta-sediments. Paragenetically, the massive sulphide bodies have formed much later than the porphyry intrusion and associated mineralisation.
Reserves and resources at Nambonga at the end of June 2020 were (Newcrest Mining NI 43-101 Report, 30 June, 2020):
Inferred Resource - 48 Mt @ o.20% Cu, 0.69 g/t Au, containing 33 t of gold.
The information in this Nambonga description has been largely drawn from: Gleeson, K., Manco, P., Curry, D. and Jones, C., 2020 - Wafi-Golpu Project, Morobe Province, Papua New Guinea, an NI 43-101 Technical Report prepared by Newcrest Mining Limited, 345p.
The project is controlled (in 2024) by the Morobe Mining Joint Venture (Harmony 50%, Newmont 50%).
The most recent source geological information used to prepare this decription was dated: 2020.
Record last updated: 26/11/2024
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.
Wafi-Golpu
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Andrew R L 1995 - Porphyry copper-gold deposits of the south-west Pacific: in Mining Engineering, Jan 1995 pp 33-38
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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
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Lunge, M. and Espi, J.O., 2021 - Trace Element Geochemistry of Chalcopyrites and Pyrites from Golpu and Nambonga North Porphyry Cu-Au Deposits, Wafi-Golpu Mineral District, Papua New Guinea: in Geosciences (MDPI) v.11, 39p. https://doi.org/ 10.3390/geosciences11080335.
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Rinne, M.L., Cooke, D.R., Harris, A.C., Finn, D.J., Allen, C.M., Heizler, M.T. and Creaser, R.A., 2018 - Geology and Geochronology of the Golpu Porphyry and Wafi Epithermal Deposit, Morobe Province, Papua New Guinea: in Econ. Geol. v.113, pp. 271-294.
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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
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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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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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