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Monywa - Letpadaung, Kyisintaung

Myanmar/Burma

Main commodities: Cu Au Ag
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The Monywa copper deposits of Letpadaung and Kyisintaung, are located on the western side of the Chindwin River in central Myanmar/Burma, 110 km west of the city of Mandalay, and 600 km NNW of the capital, Yangoon, #Location: 22°15' N, 95°05' E.

The Monywa project comprises two main zones of chalcocite blanket ore 6 km apart, the Letpadaung and Kyisintaung deposits, which together constitute a global resource of around 1 Gt @ 0.41% Cu (Win and Kirwin, 1998), most of which is amenable to cheap SX/EW treatment. These two, with the associated Sabetaung and South Sabetaung deposits are distributed within an area of 20 sq. km. USGS reports a resource of 1.7 Gt @ 0.37% Cu.

This mineralisation falls within the younger western half of Myanmar/Burma, which is occupied by the Burma Volcanic Arc, the northward continuation from Indonesia, through the Andaman Islands, of the Sunda-Andaman magmatic arc. Within Myanmar, this arc has been divided into distinct structural elements from west to east that comprise:
i). the Indo-Burmese Ranges, that mark the Outer Volcanic Arc, which to the east comprise the uplifted Mt Victoria-Kawlum belt of mica and graphite schists, overthrust by tectonised Upper Triassic turbidites and mudstones, and by ophiolite and amphibolite. The schists form the core of a prominent antiform. To the west the ranges consist of late Upper Cretaceous (Campanian) pelagic limestones and mudstones and Tertiary clastic sedimentary rocks in thrust contact with the Mt Victoria belt.
ii). the Inter-Arc Trough, with >10 km of Late Cretaceous to Quaternary sediments;
iii). the Inner Volcanic Arc, which consists of a terrestrial acid to intermediate volcanic suite, with the Monywa deposits being situated within a local sedimentary basin that was the site of Oligocene volcanic and intrusive activity; and
iv). the Back Basin filled by a thinner Tertiary sedimentary sequence.
All of these elements are related to the eastward subduction of the Australian-Indian oceanic plate beneath the main Asian Plate to the east, with collision between the continetal crust of the Indian and Asian plates to the north.

The Monywa deposits fall within the Inner Volcanic Arc, and are exposed within an uplifted physiographic section of the Burma Volcanic Arc, the Monywa-Banmauk segment, a geanticlinal structural zone occurring as a chain of inliers overlain by late Cenozoic sediments of the Inner Burmese Tertiary Basin. The geanticline is composed of a basement of amphibolite, chlorite and pelitic schists and rare gneisses, overlain by Jurassic basaltic pillow lavas and volcaniclastics, intruded by andesitic sills, with unconformable late Lower Cretaceous (Albian) limestones in synclinal cores. The pre-Albian rocks are intruded by 106 to 85 Ma granodioritic to dioritic plutons and batholiths of a west-facing arc system. Younger rocks include rhyolitic sills in Palaeogene marine mudstones, Eocene non-marine clastics, Miocene to early Pliocene quartz andesite porphyries, and three extinct late Quaternary stratovolcanoes. To the south, the Pegu Yoma geanticlinal fold-belt of Upper Oligocene and Miocene sediments lies along the projected axis of the magmatic arc.

The Burma Volcanic Arc is separated from the main Asian Shan-Thai plate by the major, 700 km long, north-south oriented Sagaing (or Shan) Fault system. The Sagaing fault accommodated dextral transverse displacement as the Indian plate moved northward, subsequent to the initial collision between the Indian and Shan-Thai continental plates in the north between 54 and 49 Ma, although east dipping subduction of the Indian oceanic plate continued into the Tertiary to the present in the south. Displacement on this structure is at least 300 km and possibly >450 km.

A Medial Metamorphic Belt, which extends from Yangoon to northern Myanma, is found immediately to the east of the Sagaing Fault System on the western margin of the Shan-Thai plate. It consists of two metamorphic units, both of probable Proterozoic protoliths: (i) the Mogok belt in the north, which is largely composed of sillmanite gneiss, diopside-phlogopite marble and schist, and (ii) the Kyaukse-Mopalin belt in the south, which comprises layered migmatites, schist and minor marble. Both of these units are intruded by discordant plutons of biotite granite and K felspar megacrystic granite batholiths.

The Shan-Thai plate in eastern Myanmar/Burma is composed of Cambrian quartzites and shales which have associated acid volcanics to the north. The Cambrian is succeeded in the same areas by Ordovician limestones and shales. These carbonates are overlain in the north by Siluro-Devonian shales and sandstones which are now largely represented by quartzites and phyllites. The overlying Carboniferous sequence comprises greywackes mudstones, pebbly mudstones and siltstones with minor limestones, overlain by a Permian sequence of predominantly carbonates with varying amounts of acid to intermediate volcanics, argillites and arenites, and a thick Triassic sequence of greywackes. The Jurassic and Cretaceous is basically a red-bed sequence, marking a major break at the end of the Triassic. These rocks are variously intruded by Permo-Triassic high level granites to the east, a belt of Cretaceous granite further to the west and Palaeozoic granitic complexes to the north. Much of the western part of the Shan-Thai plate has undergone early Jurassic and Cretaceous thrusting resulting in complex nappe stacks.

Mitchell et al., 1999, regard the Medial Metamorphic Belt as representing a continental foreland of the Shan-Thai plate, onto which other units were thrust from the west in the Mesozoic. Mid Jurassic to Early Cretaceous eastward and westward subduction were followed by emplacement of late Lower Cretaceous volcanics onto the Shan-Thai margin. The Mt Victoria-Kawlum and equivalent metasedimentary rocks to the west may represent a separate block which collided with Myanmar prior to the Albian. In the late Cretaceous, following deposition of Albian limestone, eastward subduction generated the calc-alkaline batholiths of the Inner Arc.

Copper mineralisation at Letpadaung in the Monywa district is associated with high level 19 Ma (K-Ar on sericite and alunite) dacitic intrusive rocks, while the intrusives at Kyisintaung are dated at 13 Ma. Post-mineralisation hornblende-biotite porphyry dykes have yielded a single age of 5.8 Ma. The volcanic rocks are overlain by Quaternary clastic sediments and younger olivine basalt flows.

Pyrite and primary/secondary copper sulphide mineralisation principally chalcocite, is of high sulphidation style, occurs as breccia bodies which are best developed at the Letpadaung deposit. At this deposit, these breccia bodies occur as northeast-trending, vertical to sub-vertical, dyke-like breccia bodies outcropping over an area of 5 sq. km. Individual breccia dykes vary from centimetres to >70 m in thickness, and may be traced for strike lengths of more than a kilometre. The clasts are of variable angularity, composed mainly of dacite porphyry and wall-rock pyroclastics in varying proportions, set in a matrix of silicified rock flour and chalcocite. Although there is evidence of multi-phase brecciation, the bulk of activity appears to have been related to a single event. The breccia bodies appear to have been initiated in the uppermost portions of sub-volcanic dacite intrusions that were highly enriched in volatiles, copper and sulphur.

While the copper mineralisation at Letpadaung is mainly found within the breccia matrix, at Kyisintaung, Sabetaung and South Sabetaung occurs within sheeted fractures and subordinate, narrow, sub-parallel, linear breccia zones. A high-grade pipe at Sabetaung grades up to 30% Cu, and much of the ore at Sabetaung South is in a NE-trending zone of mega-breccia and stockworked sandstone. Chalcocite occurs as fracture coatings, disseminations and breccia matrix. The gold content of the copper ore is negligible. Low sulphidation epithermal quartz veins to the north and east of the breccia bodies contain sub-economic levels of silver and gold.

Regional uplift, resistance to erosion and leaching of the altered and mineralised rocks have resulted in porous limonite-stained leached caps over 200 m thick which form the Letpadaung and Kyisintaung hills. These barren caps pass abruptly downwards at the water table into the highest grade ore at the top of the supergene enrichment zone, within which copper grade, supergene kaolinite and cubic alunite decrease, and pyrite increases with depth. In contrast, marcasite is mostly shallow. Much of the copper to depths in excess of 200 m below the water table is present as supergene digenite-chalcocite and minor covellite. Disseminated chalcocite is mostly near-surface.

The intensely acid-leached capping at these deposits occurs within altered volcaniclastic rocks and dacite porphyry to a depth of 80 to 200 m, characterised by purple to black indigenous and exotic hematite (indicating multi-cycle leaching and oxidation of chalcocite), with a lack of secondary copper minerals indicating the high pyrite:chalcopyrite ratio of the primary mineralisation. This cap is essentially barren of Cu and Au, although it may carry anomalous As and Pb, but no above background Mo or Zn.

The underlying supergene mineralisation, which varies from 20 to 50 m in thickness, is characterised by sooty chalcocite and digenite usually replacing pyrite, and as coatings on crystalline chalcocite and covellite. The development of this zone is controlled by the intensity of fracturing and proximity to sheeted veins and breccia bodies, and does not form a continuous sheet, as most of the inter-breccia zones within the hypogene system are barren.

The mixed supergene-hypogene mineralisation that occurs below, is structurally controlled, and is tens of metres in thickness in places, characterised by pyrite coated with chalcocite within breccia matrix and fractures. The hypogene zone contains abundant pyrite, coarse-grained chalcocite, digenite, covellite and enargite. Pyrite is partially replaced by copper minerals, with rare chalcopyrite and bornite also occurring as inclusions within the pyrite.

At Letpadaung the alteration zone is developed over an area of 5 sq. km and persists to at least 500 m below the surface. Alteration comprises a high sulphidation central zone of intense leaching which produces a residual vuggy quartz, with the original volcanic fabric being obliterated. This grades outwards into quartz+pink crystalline alunite+diaspore, which continues outwards into quartz+alunite+diaspore+pyrophyllite accompanied by sericite and illite. Pyrophyllite alteration predominates on the peripheries. A late stage supergene assemblage of kaolinite ± cubic alunite ± rare dickite and/or carbonate is also evident.

Two low-sulphidation vein systems have been outlined at Kyuakmyet (5 km ENE of Kyisintaung) and at Nachetaung-Shwebontha east (2 km north of Letpadaung).

The minable ore reserves in 1998 at a cut-off of 0.15% Cu, were (Indochina Goldfields Ltd., 1998):
    Sabetaung and Kyisintaung deposits - 155 Mt @ 0.47% copper
    Letpadaung, Sabetaung and Sabetaung South Mines - 560 Mt @ 0.32% Cu

For more detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1998.    
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 & Additional Information
   Selected References:
Gardiner, N.J., Searle, M.P., Robb, L.J. and Morley, C.K.,  2015 - Neo-Tethyan magmatism and metallogeny in Myanmar - An Andean analogue?: in    J. of Asian Earth Sciences   v.106, pp. 197-215.
Kyaw Win U and Kirwin D J  1998 - Exploration, Geology and Mineralisation of the Monywa Copper Deposits, Central Myanmar: in Porter T M (Ed.), 1998 Porphyry and Hydrothermal Copper and Gold Deposits - A Global Perspective PGC Publishing, Adelaide    pp. 61-73
Mitchell A H G, Myint W, Lynn K, Htay MT, Oo M and Zaw T,  2011 - Geology of the High Sulfidation Copper Deposits, Monywa Mine, Myanmar: in    Resource Geology   v.61 pp. 1-29

   References in PGC Publishing Books: Want any of our books ? Pricelist
Kyaw Win, Kirwin D J, 1998 - Exploration, Geology and Mineralisation of the Monywa Copper District, Central Myanmar,   in  Porter T M, (Ed.),  Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective,    pp 61-73
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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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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