Bawdwin - Shan, China, Meingtha lodes
Zn Pb Cu Ag
Super Porphyry Cu and Au|
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The Bawdwin zinc-lead-copper-silver deposits are located 20 km from the township of Namtu in Northern Shan State, Myanma/Burma, approximately 700 km NNE of the capital Yangoon (#Location: 23° 6' 33"N, 97° 17' 49"E).
Silver mining has occurred at Bawdwin since at least to the 15th century with estimated historical production of ~300 t Ag. The British owned Great Eastern Mining Company started retreating old slag dumps in ~1906, followed by the Burma Corporation which was established by Herbert Hoover in 1914 to access deeper sulphide ore via the Marmion Shaft and the Dead Chinaman and Tiger Tunnels. Peak production was from 1919 to 1940 with total output reported of 12.68 Mt of ore with grades that were typically ~20% Pb, 15% Zn, 0.3% Cu, 500g/t Ag. Significant Ni and Co was also present in parts of the mine although resource and production figures are not available. A run of mine sample reported by Dunn (1973) had grades of 21% Pb, 15% Zn, 0.3% Cu, 0.23% Ni, 0.08% Co, 550 g/t Ag, and 1.2% Sb. The plant and smelter were operated by the Japanese during WW2, but were destroyed during that war. Mining recommenced in 1951 and continued under British ownership until 1965 when the mine was nationalised. Following nationalisation, the mine was run down and starved of capital investment, with the resultant refined lead metal production declining from 77 700 t in 1938 to 16 518 tonnes in 1960 and 4843 t in 1974/75.
The Bawdwin deposits lie within the Palaeozoic to Mesozoic main Asia Plate of eastern Myanma/Burma, which is separated from the Tertiary Burma Volcanic Arc in the western half of the country by the major, dextral, Sagaing (or Shan) Fault system, which includes basement metamorphic nappes along its eastern margin. Bawdwin is ~100 km to the east of this suture. The Australian-Indian Plate has been moving obliquely to the NNE relative to the Eurasian Plate since the early Cretaceous, accommodated by a generally north-south structural zone that includes the 1500 km long Sagaing Fault system. This motion comprises two components i). east-directed plate convergence resultant in the uplift of the Indo-Myanmar mountain ranges, and ii). dextral strike-slip motion (Nielsen et al., 2004).
The main Asian plate is composed of Cambrian quartzites and shales which have associated acid volcanics to the north including the area around Bawdwin. 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.
The mineralisation at Bawdwin is hosted by, and has a close association with, a 2000 m thick Lower Palaeozoic (Early Ordovician) sequence of volcaniclastic sedimentary rocks and shallow rhyolite intrusives that are intercalated with shallow-water sedimentary rocks, together known as the Bawdwin Volcanic Centre. The sequence comprises coarse volcaniclastic tuffs of the Bawdwin Volcanic Formation that interfinger with calcareous sedimentary rocks of the Pangyun Formation, both of which are intruded by co-magmatic rhyolite porphyry bodies. The main controlling structure is the Bawdwin Fault zone which comprises a complex NW-trending, SW-dipping (~75 to 80°), array of faults, splays and relays.
The late phases of volcanism was characterised by pneumatolytic and/or hydrothermal solutions which produced three, almost upright, pipe like high-grade massive sulphide orebodies (from NW to SE, the Shan, Chinaman and Meingtha lodes) that were mined historically underground. These lodes lie along 4 km of strike of the NW-SE oriented Bawdwin Fault zone, which is offset by later faults. The lodes were not apparently uniform, but were composed of massive mineralisation localised in dilational structural zones as veins and breccias, with semi-massive to disseminated sulphide mineralisation and stockwork sulphide veining occurring in the intervening silicified lithic breccia of the Bawdwin Tuff. The high grade lodes are associated with dome-like rhyolitic intrusives, and are surrounded by lower-grade disseminated and stockwork mineralisation hosted in volcaniclastics sediments that has been exploited in the open pit on the China Lode. The most extensive 'halo' mineralisation is up to 150 m wide in the footwall of the main China Lode. This zone encompasses mined stopes in the footwall of the China Lode and is characterised by high-grade veins, breccias, stockworks and shear zones and extensive intervals of disseminated mineralisation associated with silicification of the Bawdwin Tuff lithic breccia. This zone is interpreted to reflect a relay zone extending north towards the Shan Lode (Myanmar Metals Ltd., 2017).
The loade have the following dimensions in longitudinal section - China Lode, strike length ~600 m, vertical extent >400 m and thickness of 100 to 250 m; Shan Lode, strike length ~400 m, vertical extent ~400 m and similar thickness to the China Lode; Meingtha Lode, strike length ~700 m, vertical extent ~300 m and thickness of generally half that of the other two lode (sections in Myanmar Metals Ltd., ASX releases, 2017).
Sulphide mineralisation is characterised by argentiferous galena, sphalerite and pyrite, together with smaller amounts of chalcopyrite, covellite, tetrahedite, gersdorffite and cobaltite. Copper mineralisation accompanies lead and zinc but also separately where it can be associated with nickel and cobalt. Sulphides are generally coarse grained in massive lodes and when disseminated in altered tuff (Myanmar Metals Ltd., 2017). Co-Ni±Cu is found in the lower levels, followed by Pb-Zn-Ag, with Ba at the top of the ore lenses.
Weathering and oxidation is from 20 to 50 m deep on the ridges, but much thinner in the valleys. The existing open pit (2017) at Bawdwin has largely removed the weathered zone and fresh sulphides occur in the pit floor (Myanmar Metals Ltd., 2017).
The known resource in 1938 comprised: 10.8 Mt @ 13.9% Zn, 22.8% Pb, 1.05% Cu, 670 g/t Ag.
In 1976, following additional exploration and drilling, a German Geological aid mission reported resources of:
Underground - 6.69 Mt @ 7.59% Pb, 3.54% Zn, 151 g/t Ag, 0.15% Cu, and
Open pit - 9.69 Mt @ 5.37% Pb, 2.54% Zn, 0.25% Cu.
In 1997, RSG (for Mandalay Mining) reported JORC compliant resources, that included the lower grade halo mineralisation outside the main lodes:
Total Resource - 104.3 Mt @ 5.61% Pb, 2.34% Zn, 0.21% Cu, 71 g/t Ag (@ 2% Pb cut-off), including
Indicated Resource - 23.81 Mt @ 6.49% Pb, 2.84 % Zn, 0.26% Cu, 177 g/t Ag (@ 4% Pb cut-off), and
Recoverable resource - 94.5 Mt @ 6.49% Pb, 2.84% Zn, 0.26% Cu, 118 g/t Ag.
Hannington et al., 1999 quoted: 34.1 Mt @ 13% Zn, 9.09% Pb, 0.48% Cu, 232 g/t Ag, 0.06 g/t Au.
JORC compliant Inferred Mineral Resources for the individual lodes (Myanmar Metals Ltd., 2017) were:
Shan Lode - 27.9 Mt @ 4.9% Pb, 2.3% Zn, 0.29% Cu, 116 g/t Ag; comprising
fresh sulphides - 26.4 Mt @ 5.1% Pb, 2.4% Zn, 0.29% Cu, 120 g/t Ag;
oxide-fresh transition - 1.5 Mt @ 2.0% Pb, 0.2% Zn, 0.45% Cu, 47 g/t Ag;
China Lode - 32.3 Mt @ 5.1% Pb, 2.8% Zn, 0.20% Cu, 130 g/t Ag; comprising
fresh sulphides - 30.3 Mt @ 5.1% Pb, 2.9% Zn, 0.18% Cu, 130 g/t Ag;
oxides - 0.1 Mt @ 9.0% Pb, 1.1% Zn, 0.27% Cu, 140 g/t Ag;
oxide-fresh transition - 1.9 Mt @ 1.7% Pb, 1.3% Zn, 0.50% Cu, 135 g/t Ag;
Meingtha Lode - 15.7 Mt @ 3.2% Pb, 1.4% Zn, 0.26% Cu, 101 g/t Ag; comprising
fresh sulphides - 13.1 Mt @ 3.6% Pb, 1.6% Zn, 0.30% Cu, 101 g/t Ag;
oxides - 0.7 Mt @ 1.0% Pb, 0.04% Zn, 0.07% Cu, 88 g/t Ag;
oxide-fresh transition - 1.9 Mt @ 1.7% Pb, 0.3% Zn, 0.07% Cu, 102 g/t Ag;
TOTAL - 75.9 Mt @ 4.6% Pb, 2.3% Zn, 0.25% Cu, 119 g/t Ag;
After further drilling, JORC compliant Indicated and Inferred Mineral Resources for all lodes (Myanmar Metals Ltd., 2019) were:
Indicated Resource - 24.790 Mt @ 5.1% Pb, 2.8% Zn, 0.22% Cu, 134 g/t Ag;
Inferred Resource - 57.008 Mt @ 4.6% Pb, 2.2% Zn, 0.24% Cu, 112 g/t Ag;
TOTAL Resource - 81.797 Mt @ 4.8% Pb, 2.4% Zn, 0.24% Cu, 119 g/t Ag.
Made up of:
Oxide Resource - 1.551 Mt @ 4.9% Pb, 2.5% Zn, 0.26% Cu, 165 g/t Ag;
Transition - 20.716 Mt @ 4.8% Pb, 2.3% Zn, 0.19% Cu, 130 g/t Ag;
Sulphide Resource - 59.531 Mt @ 4.7% Pb, 2.4% Zn, 0.25% Cu, 114 g/t Ag.
The most recent source geological information used to prepare this summary was dated: 2017.
Record last updated: 23/3/2018
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.
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.|
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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