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Porgera |
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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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The Porgera gold deposit is located at an altitude of 2500 m in the remote Enga Province of western Papua New Guinea, 130 km west of Mount Hagen and 600 km northwest of Port Moresby, (Location: 5° 28'S, 143° 05'E).
The operation, which was commissioned in 1990, was based on the lower grade, open pit, Waruwari bulk gold orebody (originally calculated at 54 Mt @ 4.3 g/t Au) and the high grade, underground, tabular Zone 7 deposit (5.9 Mt @ 27 g/t Au).
The orebody is associated with the Late Miocene (6.0±0.3Ma), silica-poor, K-rich, shoshonitic Porgera Intrusive Complex (PIC) of micro-gabbro to diorite and feldspar porphyry, which was intruded at an estimated palaeodepth of ~2.0 to 2.5 km, into previously thrusted late Jurassic to Cretaceous pelitic sediments deposited on the shelf of the Australasian plate margin. Intrusion of the PIC magmas occurred slightly prior to, or during, the uplift of the Papuan Fold and Thrust Belt that was formed as a result of the collision of the Indo-Australian plate with the Bismarck Sea and Caroline oceanic plates to the north and east. The mineralising event occurred immediately after the emplacement of the PIC and was locally accompanied by intense phyllic alteration that affected the shallow-level intrusions. A late, steeply dipping normal structure, the Roamane fault, crosscuts all intrusions and hosts the richest gold bearing veins and breccias.
The host shelf sequence includes the Jurassic Om Formation, a thinly bedded, dark grey carbonaceous siltstone with pyritic calcareous nodules, overlain by the Cretaceous Chim Formation which comprises bioturbated and laminated grey calcareous siltstone and mudstone with local sandy horizons. Eocene to Miocene limestone crops out further south and east of the mine.
Regionally, these sediments are broadly folded about east to ESE trending axes, but dip steeply around the intrusive contacts, adjacent to which they are altered. The earliest faults are flat dipping and may be related to early mineralisation. Steeper NE and E-W faults post date the intrusive complex and are also mineralised. The main gold ore is associated with the E-W trending, 60 to 75° dipping Romaine Fault and steeper associated footwall and hangingwall fractures.
The immediate host package comprises Cretaceous sediments including mudstones, shales, siltstones and sandstones, which are often calcareous. Syn-mineral thrusting has placed calcarenite in contact with black shale, the main host rock in the mine area, while post-mineral thrusting on the Boundary Fault has resulted in the emplacement of the calcarenite - black shale package over fissile brown shale.
The gold mineralisation at Porgera is classed as a low-sulphidation, alkalic, epithermal gold deposit (e.g., Hay, Haydon and Robert, 2020). It displays a staged geological development associated with evolution of the PIC as a differentiating alkaline intrusion, intruded as stocks (<500 m diameter) and dykes. Aeromagnetic data indicate that the intrusions exposed at the mine occur as apophyses to a much larger (3.5 km diameter) buried magmatic source, which is inferred to have domed the host sedimentary rocks.
Two main mineralising events have been recognised, namely:
Stage 1 associated with the emplacement of generally porphyritic intrusions of, i). initial augite hornblende diorite, ranging from equigranular to porphyritic, with associated 'andesite' dykes and sills; ii). subsequent hornblende diorite with radiating hornblende rosettes in a fine-grained matrix.
Stage 2 mineralisation was associated with quartz-feldspar porphyry stocks and dykes emplaced as the most differentiated later stage of PIC magmatism and was initiated as localised crosscutting milled matrix fluidised breccia dykes, containing fine silica-pyrite, formed by explosive hydrothermal activity at an elevated crustal setting.
Three types of gold-bearing vein assemblages are recognised:
(i) Pre-Stage I magnetite-sulphide-carbonate veins with economically insignificant gold, localised in the central and deep parts of the mine, some 1000 m below the current workings. Magnetite is dominant, with associated secondary biotite and lenses of pyrite, pyrrhotite, chalcopyrite, and rare inclusions of gold in pyrite;
(ii) Stage I base metal-sulphide±gold-carbonate veins. Sulphide minerals are pyrite, sphalerite and galena, with rare chalcopyrite, arsenopyrite, marcasite, freibergite and proustite/pyrargyrite. The gangue consists of complex Mn-Ca-Mg carbonates and subordinate quartz. The veining carries refractory Au in arsenical pyrite, disseminated in zones of intense phyllic alteration, plus minor Au in the associated base metal sulphide veins localised by NE- to NNE-trending pre-mineral faults and the margins of intrusive bodies. Gold ocurs in pyrite, either in an invisible form, or as microscopic inclusions of native gold. This stage predominates in the Waruwari open pit area; and
(iii) Stage 2 quartz-roscoelite-pyrite-gold and breccia veins, which host the highest gold grades and are the economically most significant vein assemblage. Some pyrite is arsenian, with local arsenopyrite, although chalcopyrite is rare. The Ag-Au tellurides petzite, krennerite and calaverite, and the Ag telluride hessite are present, as well as coloradoite and altaite. Gold dominantly occurs in the native form and as electrum. Veins are strictly fault controlled, in particular within the late normal Romaine Fault.
Gold grades and grain size increase from Stages 1 to 2. In addition, Stage 2 mineralisation is characterised by high silica and low sulphur, and consequently by higher Au:S (10:1) ratios compared to Stage 1 (2:1) which has low silica and high sulphur.
The majority of gold is present as extremely fine grains contained within pyrite. The Romaine Fault is around 4 km long, with the main mineralised zone being over a strike length of 1 km, with a width of up to 20 m. The highest grade ore has a vertical extent of 240 m, and is found below a depth of 400 m from the surface. At depth the roscoelite mineralisation gives way to a gypsum-rich zone with less sulphide. Mineralisation is commonly associated with the altered contact zones of the intrusive complex.
At December 2007, reserves and resources were:
Proven + probable reserves of 97.96 Mt @ 3.98 g/t Au
Measured resources of 34.8 Mt @ 2.55 g/t Au
Indicated resource of 24.0 Mt @ 1.96 g/t Au
Inferred resource of 11.1 Mt @ 2.89 g/t Au
In 2010, proven + probabe reserves contained 230 t (7.4 Moz) of gold (Barrick website, 2012), presumably at similar grades to the 2007 reserve.
Barrick is the owner of a 95% joint venture interest and is the operator of the Porgera gold mine.
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Porgera
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Selected References:
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Corbett G, Leach T, Stewart R, Fulton B 1995 - The Porgera gold deposit: structure, alteration and mineralisation: in Mauk J L, St George J D (Eds.), PACRIM 95 congress, Exploring the Rim, Auckland, New Zealand, 19-22 November 1995, The AusIMM, Melbourne, Proceedings volume, pp 151-156
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Cotton R E 1975 - Porgera gold deposits: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 872-874
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Fleming A W, Handley K L, Hills A L, Corbett G J 1986 - The Porgera Gold deposit, Papua New Guinea: in Econ. Geol. v81 pp 660-680
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Gunson M, Hall G, Johnston M 2000 - Foraminiferal coloration index as a guide to hydrothermal gradients around the Porgera intrusive complex, Papua New Guinea: in Econ. Geol. v95 pp 271-282
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Hall G C 1996 - The Porgera exploration model: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996 AMF, Adelaide pp 12.1-12.8
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Handley G A, Henry D D 1990 - Porgera gold deposit: in Hughes FE (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1717-1724
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Hay, J.P., Haydon, M.M. and Robert, F., 2020 - Geology of the Porgera Gold Deposit, Papua New Guinea: in Sillitoe, R.H., Goldfarb, R.J., Robert, F. and Simmons, S.F., (Eds.), 2020 Geology of the Worlds Major Gold Deposits and Provinces, Commemorating the 100th Anniversary of the Society of Economic Geologists, Inc., SEG Special Publication 23, Chapter 27, doi.org/10.5382/SP.23.27
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Munroe S M, 1995 - The Porgera gold deposit, Papua New Guinea: The influence of structure and tectonic setting on hydrothermal fluid flow and mineralisation at a convergent plate margin: 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. 413-416
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Orogen Minerals Ltd 1996 - Porgera gold mine (Extracts): in Extracts from Orogen Minerals Ltd Prospectus, 1996 pp 51-59 and 141-158
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Richards J P 1997 - Controls on scale of Porgera-type porphyry/epithermal gold deposits associated with mafic, alkalic magmatism: in Trans. IMM, Section B v106 B1-8
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Richards J P, Bray C J, Channer D M DeR, Spooner E T C, 1997 - Fluid chemistry and processes at the Porgera gold deposit, Papua New Guinea : in Mineralium Deposita v32 pp 119-132
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Richards J P, Kerrich R 1993 - The Porgera gold mine, Papua New Guinea: Magmatic hydrothermal to epithermal evolution of an alkalic type precious metal deposit (Full paper): in Econ. Geol. v88 pp 1017-1052
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Richards, J.P., 2009 - Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere: in Geology v.37, pp. 247-250.
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Ronacher E, Richards J P and Johnston M D 1999 - New mineralisation and alteration styles at the Porgera gold deposit, Papua New Guinea: in Pacrim 99, International Congress on Earth Science, Exploration and Mining Around the Pacific Rim, Proc. Pacrim 99 Conference, Bali, Indonesia AusIMM, Melbourne pp 91-94
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Ronacher E, Richards J P and Johnston M D 2000 - Evidence for fluid phase separation in high-grade ore zones at the Porgera gold deposit, Papua New Guinea: in Mineralium Deposita v35 pp 683-688
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Ronacher E, Richards J P, Reed M H, Bray C J, Spooner E T C, and Adams P D 2004 - Characteristics and Evolution of the Hydrothermal Fluid in the North Zone High-Grade Area, Porgera Gold Deposit, Papua New Guinea: in Econ. Geol. v99 pp 843-867
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Ronacher E, Richards J P, Villeneuve M E and Johnston M D 2002 - Short life-span of the ore-forming system at the Porgera gold deposit, Papua New Guinea: laser 40Ar/39Ar dates for roscoelite, biotite, and hornblende: in Mineralium Deposita v37 pp 75-86
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White, N.C., Leake, M.J., McCaughey, S.N. andd Parris, B.W., 1995 - Epithermal gold deposits of the southwest Pacific: in J. of Geochemical Exploration v.54, pp. 87-136.
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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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