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The Mawchi tin-tungsten mine is located in Kayah State of eastern Burma/Myanmar and lies within the northern section of the 3500 km long Thai-Malay tin-tungsten belt. It is located within 40 km to the west of the Thai-Burma border, 250 km NNE of Yangoon (#Location: 18°50'N, 97°10'E).

The Thai-Malay tin-tungsten belt extends for some 3500 km from the Shan States of Burma in the north, along the Burma-Thailand border into Thailand and Malaysia and along a chain of islands in Indonesia to Belitung Island in the south. The oldest rocks in the belt outcrop in north eastern Burma and north western Thailand, and on the southern extremity of peninsular Thailand. These are 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 largely present as quartzites and phyllites in Thailand and Burma. In Malaysia the Silurian is represented by carbonates and argillaceous sediments with minor sandstone and volcanics. The Devonian in Malaysia is also composed of limestones and argillaceous sediments. The Carboniferous in Thailand and Burma comprises greywackes mudstones, pebbly mudstones and siltstones with minor limestones. In Malaysia, the Carboniferous is largely sandstones and shales with minor limestones. On the east coast of Malaysia, a higher proportion of quartzites is found within the sequence, particularly in the Late Carboniferous. The Permian sequences of Burma, Thailand and Malaysia are predominantly carbonates with varying amounts of acid to intermediate volcanics, argillites and arenites. In Burma, the Triassic is represented by a thick sequence of greywackes. The Jurassic and Cretaceous on mainland Southeast Asia is basically a red-bed sequence, marking a major break at the end of the Triassic which is not obvious on Sumatra. Tertiary volcanics are extensively developed in Sumatra and Burma.

The Thai-Malay tin-tungsten belt is intruded by three belts of granitic batholiths younging to the west:
i). Permo-Triassic high level granites outcrop along the east coast of Malaysia.
ii). Late Triassic batholiths extend from Belitung through Bangka and Singkep and along the west coast or Main Range of Malaysia to south eastern peninsular Thailand. Other Triassic granites are found to the south east of Bangkok and along the Thai-Burma border.
iii). Further to the west, a belt of Cretaceous granite extends from the Shan States along the Thai-Burma border through Phuket to Sumatra where inferred Cretaceous granites outcrop along the main central range.

Palaeozoic granitic complexes are also found in the Shan States and north western Thailand.

The Mawchi deposit is a cassiterite wolframite-scheelite bearing quartz lode system largely within a granitic host. In addition to hard-rock mining, tin and tungsten dredgeing was undertaken around Mawchi and in the region. The Mawchi tin-tungsten mine began operation in 1909. By 1939, 9 dredges were operating on the alluvial deposits and Mawchi had reached its peak annual production of 3143 tonnes of tin and 2771 tonnes of tungsten concentrate. During, and just prior to the Japanese occupation, most of the dredges were destroyed and, following the war, only three restarted. Just one was still working by 1970.

The geology of the Mawchi hard rock mine area is characterised by a series of sedimentary rocks known locally as the Mawchi Series, consisting of sandstones, grits, calcareous mudstones, shales and limestones, which strike northwest-southeast and the dip is steeply to the west. They may correlate with the mainly Carboniferous Mergui Series of the Tavoy district of Tennanserim. A small granitoid pluton intruded these sedimentary rocks of the Mawchi mine area, producing a low grade thermal metamorphic aureole of marbles, quartzites, spotted grits, and indurated slates. This granitoid pluton is one of the smallest bodies in the Sn-W-related late Mesozoic to early Tertiary Burmese granitoid belt.

In the mine area, the NW-SE elongated, oval-shaped, granitoid pluton is bounded by limestones to the northwest and on part of its eastern flanks, and by sandstones and shales on the southeastern and western margins. Although it is elongated broadly parallel to the strike of the enclosing sedimentary rocks, the contact with these rocks is mostly irregular and discordant, with many apophyses along the granitoid roof. Thin dykes or veins of aplite and pegmatite, which are thought to be related to the late stage of granitoid emplacement, are seen penetrating the sedimentary rocks. Field evidence suggests these dykes are older than the mineralised quartz veins. The granitoid rock is biotitic and medium grained, and locally has a porphyritic texture with large K feldspar phenocrysts, and discrete microscopic grains of cassiterite. In many places, these intrusive rocks have been strongly tourmalinised and kaolinised. Both pre- and post-ore faults are found in the mine area. Post-ore faults displace the mineralised quartz veins from several cms, up to some metres.

Tin and tungsten mineralisation is found in more than 60 major quartz veins developed in the apical parts of the granitoid intrusive and for short distances into the enveloping sediments.   The majority of the veins strike northeast at 6 to 60°, are inclined at 75 to 80°W, and have maximum lengths of 570 m and thicknesses of up to 2.5 m, averaging 1 m. They have been mined for up to 300 m below the surface.

The veins of the system have a simple and regular attitude within the granitoid, but change into a stockwork with numerous branches and fingers in the metasedimentary rocks. Individual veins are drusy and contain cassiterite and wolframite intimately intergrown with sulphide minerals which include pyrite, chalcopyrite, arsenopyrite, molybdenite, bismuthinite, sphalerite and galena.

Sulphides have been reported to be relatively and quantitatively more common at lower levels within the mine, while accompanying wolframite in the majority of ore veins decreases with depth. Some of the ore veins contain scheelite, which is commonly interstitial to wolframite and sometimes replaces it.

Gangue minerals are quartz, feldspars, mica, calcite, magnetite, fluorite, tourmaline, minor topaz and beryl. Fluorite commonly replaces feldspars and is usually associated with fine-grained muscovite, sometimes forming massive veinlets cutting the mineralised quartz veins. Tourmaline is very widespread in the Mawchi orebody and is found as vein selvages, as disseminations in the granitoids, and as quartz-tourmaline rocks. Greisens is rarely found bordering the Mawchi quartz veins.

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1983.    
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.
Li, H., Myint, A.Z., Yonezu, K., Watanabe, K., Algeo, T.J. and Wu, J.-H.,  2018 - Geochemistry and U-Pb geochronology of the Wagone and Hermyingyi A-type granites, southern Myanmar: Implications for tectonic setting, magma evolution and Sn-W mineralization: in    Ore Geology Reviews   v.95, pp. 575-592.
Zaw A K, Thet D K M  1983 - A note on a fluid inclusion study of Tin-Tungsten mineralization at Mawchi Mine, Kayah State, Burma: in    Econ. Geol.   v78 pp 530-534

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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