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Sepon - Padamand Thengkham, Khanong, Vang Ngang, Discovery, Nalou / Namkok |
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Laos |
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Au Cu
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Super Porphyry Cu and Au
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The intrusive-centred, sediment-hosted Au-Cu Sepon mineral field lies approximately 40 km north of the town of Sepon, in Savannakhet province of south-central Laos. The Sepon mineral field includes seven gold and two copper deposit (Khanong and Thengkham) which occur as separate adjacent deposits over a 5 km interval containing over 120 tonnes (4 Moz) of gold and 1.2 million tonnes of copper.
(#Location: 16° 57' 26"N, 106° 0' 17"E).
Gold and copper deposits were discovered in the Sepon district by CRA Exploration and then Rio Tinto between 1993 and 1999 before being on-sold to Oxiana Limited who continued exploration between 2000 and 2003. In 2008, the Australian based Oxiana and Zinifex merged to form OZ Minerals who assumed control of Sepon. In 2009, most of the assets of OZ Minerals, including Sepon, were acquired by China Non Ferrous Metals Co Ltd (MinMetals) to become the company subsequently operating as MMG Limited. In 2018, MMG agreed to the sale of its 90% interest in Lane Xang Minerals, which owns the Sepon mine in Lao PDR, to Chifeng Jilong Gold Mining Co Ltd.
The 1.25 Mt of ore per annum Sepon gold mine poured its first gold in December 2002 and ceased in 2013 due to price/costs and other factors. The copper operation commenced in 2005 and until 2020 had produced >1 Mt of copper cathode using a whole-of-ore leach, solvent extraction and electrowinning (SX-EW) process. In quarter 3, 2020, Sepon transitioned from copper to a gold focused operation with gold mining re-starting. Copper production continues at Sepon through the heap leach pilot program, and is expected to produce around 45 000 tonnes of copper cathode in 2020 (Lane Xang Minerals Limited 2021).
At Sepon, gold occurs as fine disseminations hosted by strongly altered calcareous sedimentary rocks with similarities to the Carlin style gold deposits of Nevada in the USA.
The separate Khanong supergene-enriched, carbonate-replacement and sediment-hosted magmatic-hydrothermal Cu-Au resource is located immediately adjacent to and within the main 6 x 4 km cluster of gold deposits.
Sepon lies within the Truongson Fold Belt (or Annamite Cordillera) and the Kontum Massif. The Truongson Fold Belt is a NW elongated belt of Early to Mid Palaeozoic sediments and lesser volcanic rocks, interspersed with lesser tectonic slices of metamorphic terranes, bound to the north by the Song Ma-Song Da suture zone and to the south by the NW trending Truongson Fault. The Kontum Massif comprises an Upper Proterozoic basement of ortho- and para-gneiss, crystalline schist and migmatite which abuts the Truongson Fold Belt to the south.
In the Sepon district, the Truongson Fold Belt is represented by a series of E-W trending basins which are truncated by the NW-striking Truongson Fault. Granitic intrusions have been mapped in the district as well as rhyodacite and andesite dykes and stocks, interpreted to be part of the Late Palaeozoic Variscan Orogeny. At least three phases of around 300 Ma rhyodacite porphyry intrusion are recognised, including a late, quartz-stockwork veined phase, which is interpreted to be genetically associated with all known Cu and Au mineralisation in the field. Minor remnants of Jurassic to Cretaceous Khorat Basin continental sedimentary rocks unconformably overlie the Palaeozoic marine sedimentary sequences to the east and north, while west of the Kontum Massif and south of the Truongson Fault, Khorat sedimentary rocks are dominant. Cenozoic volcanic rocks unconformably overlie the Palaeozoic rocks to the north-east.
The stratigraphy within the Sepon district comprises Devonian to Carboniferous continental fluvial and shallow to deep marine sediments deposited in a half graben basin. Pre- to syn-rift, dominantly siliciclastic sediments, grade upwards to transgressive and regressive, carbonate-dominant sequences. This sedimentary sequence has been divided into nine lithology based formations. The lower and upper contacts of formation 6, a calcareous, carbonaceous mudstone, host most of the known gold mineralisation.
The sediments generally dip moderately north and NW, while the dominant fault directions are NW, parallel to the Truongson fault system, and east-west, parallel to the basin bounding faults. Intersections of east-west and NE trending faults are important in that they localise mineralisation and rhyodacite porphyry intrusive stocks and associated dyke and sill complexes of similar composition.
Four broad alteration/mineralisation styles are recognised namely: i). quartz stockwork porphyry Mo, ii). Cu-Au skarn, iii). Cu-Au carbonate replacement, and iv). sediment-hosted Au.
Individual deposits include the Padamand Thengkham porphyry Cu-Mo mineralisation within a rhyodacite porphyry intrusive plug, the adjacent Thengkham skarn Cu occurrences, the Khanong Cu carbonate replacement ore, the Vang Ngang silica-pyrite replacement Au deposit, and the Discovery and Nalou/Namkok jasperoid associated gold ores (adjacent to dykes and sills of rhyodacite porphyry), and finally Pb/Zn/Ag carbonate replacement mineralisation.
The gold mineralisation is localised by structural and stratigraphic fluid traps which are said to have close affinities with the sediment-hosted, 'invisible' gold deposits of the Carlin Trend, Nevada, USA. The mineralisation is finely disseminated and is closely related to decalcification and variable silica (jasperoid) replacement of carbonatic rocks following structures and lithological contacts. The geometry of the gold ore is controlled by a combination of anticlinal structures, shallow dipping stratigraphy, structure and porphyry sills.
The copper mineralisation includes both primary and secondary styles. Cu-Au skarns are developed along the contacts of the intrusive stocks. Spatially, the main hypogene mineralisation occurs as a lower temperature, silica-sulphide replacement style, semi massive sulphide body, hosted by shallow dipping, highly sheared carbonate rocks (calcareous cataclastitic, fault rocks, in the flat-lying, extensional, Khanong Fault), which occur immediately out-board of the skarn-hornfels Cu-Bi front and in-board of sediment hosted Au-(As-Sb) mineralisation in a zoned intrusive-centred hydrothermal system. Weathering and in situ supergene enrichment of the hypogene sulphide protore produced a high-grade chalcocite-clay zone (approx. 10.3 Mt @ 4.6% Cu) overlain by leached limonite-clay, and underlain by a very high grade copper oxide zone, comprising malachite ±azurite ±cuprite ±minor native copper developed above an impermeable dolomite. Exotic Cu-Mn-wad mineralisation is developed for up to 1 km down the hydraulic gradient from the chalcocite-clay zone.
Ore Reserves and Mineral Resources in December 2005 (Oxiana Ltd, 2006) were:
Hypogene Gold, Proved + probable reserves - 6.30 Mt @ 1.99 g/t Au, 9.71 g/t Ag;
Copper, Proved + probable reserves -15.90 Mt @ 4.97% Cu;
Oxide Gold, Measured + indicated + inferred resources - 38.1 Mt @ 1.27 g/t Au, 7.27 g/t Ag;
Hypogene Gold, Measured + indicated + inferred resources - 72.8 Mt @ 1.55 g/t Au, 7.82 g/t Ag;
Copper, Measured + indicated + inferred resources - 96.5 Mt @ 0.064 g/t Au, 5.52 g/t Ag, 2.03% Cu at Khanong and Thengkham.
Ore Reserves and Mineral Resources at 30 June 2017 (MMG Mineral Resources and Ore Reserve Statement, 2017) were:
Mineral Resources
Oxide Gold - Indicated + Inferred Resources - 1.7 Mt @ 3.0 g/t Au;
Partial Oxide Gold - Indicated + Inferred Resources - 1.1 Mt @ 4.3 g/t Au;
Hypogene Gold - Indicated + Inferred Resources - 7.2 Mt @ 3.9 g/t Au;
Stockpiled Gold - Indicated Resources - 0.9 Mt @ 1.7 g/t Au;
TOTAL Gold Mineral Resources - 10.9 Mt @ 3.6 g/t Au for 39 t of contained gold.
Supergene Copper - Indicated + Inferred Resources - 7.0 Mt @ 4.4% Cu;
Hypogene Copper - Indicated + Inferred Resources - 12.2 Mt @ 1.1% Cu;
Stockpiled Copper - Indicated Resources - 6.1 Mt @ 1.4% Cu;
TOTAL Copper Mineral Resources Reserves - 25.3 Mt @ 2.1% Cu.
Ore Reserves
Supergene Copper - Probable Reserves - 3.5 Mt @ 4.7% Cu;
Hypogene Copper - Probable Reserves - 0.35 Mt @ 1.1% Cu;
Stockpiled Copper - Probable Reserves - 5.6 Mt @ 1.4% Cu;
TOTAL Copper Ore Reserves - 9.4 Mt @ 2.6% Cu.
NOTE: Mineral Resources are inclusive of Ore Reserves.
The current owner (in 2021) Chifeng Jilong Gold Mining Co Ltd., do not appear to publicly release resource or reserve estimates.
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.
Sepon
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Selected References:
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Cromie P W, Manini A J and Zaw K 2004 - Geological setting and characteristics of the Au and Cu deposits in the Sepon Mineral District, Lao PDR: in Muhling J, Goldfarb R, Vielreicher N, Bierlein F, Stumpfl E, Groves DI and Kenworthy S 2004 - SEG 2004, Predictive Mineral Discovery Under Cover, Extended Abstracts, Centre for Global Metallogeny, The University of Western Australia, Publication No. 33, p 411
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Cromie, P., Makoundi, B., Zaw, Khin, Cooke, D.R., White, N. and Ryan, C., 2018 - Geochemistry of Au-bearing pyrite from the Sepon Mineral District, Laos DPR, Southeast Asia: Implications for ore genesis: in J. of Asian Earth Sciences v.164, pp. 194-218. doi.org/10.1016/j.jseaes.2018.06.014.
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Loader S E 1999 - Supergene enrichment of the Khanong copper resource, Sepon Project, Lao PDR: in Pacrim 99, International Congress on Earth Science, Exploration and Mining Around the Pacific Rim, Proc. Pacrim 99 Conference, Bali, Indonesia, AusIMM, Melbourne, pp 263-270
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Manini A, Aquino J, Gregory C and Aneka S, 2001 - Discovery of the Sepon gold and copper deposits, Laos: in NewGenGold 2001 Conference, Perth, Western Australia, 26-27 November 2001, AMF, Adelaide, Proceedings, pp. 93-107.
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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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