The Geological Society

Burma's rising star

Nick Gardiner* on tin-tungsten mineralisation in Burma, where mining is all set to make a comeback.

lkj Burma – Myanmar – is one of the largest countries in Southeast Asia. Lyng at a historically key geographical juncture between India, China and South East Asia, it is slowly emerging from decades of oppressive military rule. It is extremely attractive to geologists for two principal reasons.

First, as a minerals jurisdiction, it is astonishingly rich in a variety of commodities and ore deposit types: tin-tungsten, copper, nickel, gold, silver, lead and zinc, as well as the famous Myanmar jade and the Mogok rubies and sapphires. It contains at least three world-class mineral deposits (Mawchi (tin-tungsten), Bawdwin (lead-zinc-silver) and Monywa (copper)). Second, it is sited at the eastern end of the Mesozoic-Cenozoic India-Asia collision zone, immediately south of the Namcha Barwa Eastern Syntaxis, providing a pivotal link between Tethys suture zones (to the north in Karakoram-Himalaya-Tibet), and those found further south and east (Thailand, Malaysia and Sumatra).

Picture: Hermyingyi Mine - once operated by the British (before WW2)

However, largely due to the recent political situation (and despite sterling work by indigenous geologists and a few notable UN and other Western-sponsored programmes) it remains geologically enigmatic. Political and economic changes over the last four years have, however, been profound and rapid. While democracy remains a ‘work in progress’, the country’s slow opening up has allowed Oxford scientists to gain unprecedented access to areas that, in some cases, have not been visited by western geologists for 40 years or more.

Oxford Burma Project

The Oxford Burma Project has one simple but broad aim: to unravel the tectonic history of the country, and link it to the metallogenesis of the ore deposits. We would like to answer some fundamental questions regarding the nature and evolution of Burma's geological history and of its mineral deposits, and to address these we use a variety of field, petrological, geochemical and geochronological techniques.

We have particular interest in the petrogenesis of the extensive tin-tungsten deposits. While significant recent research has focused on granitoid-related porphyry systems hosting copper-gold mineralisation (in part due to their immense economic significance) granite-related tin-tungsten and associated mineralisation has received less attention - especially since the tin crash of the mid-1980s. However interest in tin deposits has increased, driven by strong short- to medium-term price forecasts and by associated ‘security of supply’ concerns over co-genetic metals.

‘Critical Metals’ are elements deemed necessary for new and green technologies with a limited supply-base. Such metals include tungsten, the Rare Earth Elements (REEs), lithium, tantalum-niobium and indium - as highlighted on the British Geological Survey’s ‘Risk List’. Most of these are lithophile elements, and are spatially and genetically concentrated by the granite-related processes that give rise to the world’s major tin deposits. Such deposits are markedly associated with crustal-melt granites associated with major subduction zones – of which the principal Mesozoic-Recent examples are the tin-tungsten provinces of the Andean Cordillera and the Southeast Asian tin granite belts. The opening up of a promising new jurisdiction hosting one of these major tin-bearing Southeast Asian granite belts is therefore of great interest both to academia and trade. Accordingly we have been generously funded both by the University and by a minerals investor.

Diminished

The Burmese mining industry has become much diminished in recent decades from peaks before WW2, in 2010 registering barely 0.1% on Burma’s total GDP. But huge potential for careful redevelopment remains, which could be of significant benefit as the country strives to rebuild its economy.

Picture: Tin concentrate being panned at an alluvial deposit.

In the 1930s the Mawchi Mine was perhaps the world’s largest single tungsten producer, accounting for some 60% of Burma's total production - the country then being the world's biggest tungsten supplier. The Bawdwin Mine in the Northern Shan States was, in the early 20^th Century, one of the world's largest lead mines (as well as a major silver, zinc and nickel producer), hosting a major mining and smelting operation (where Herbert Hoover, future US President, made one of his fortunes).

While both Mawchi and Bawdwin retain production potential, present mining operations are now largely artisanal. Despite the temporary lifting of western sanctions in 2011, in my travels around the country it is obvious that a chronic lack of investment, coupled with concerns over tenure, punitive production-sharing licences and a lack of modern mining skills and equipment all contribute to a challenging operating environment.

Within the last year, I have visited Burma five times, usually accompanied by one or other of my Oxford colleagues Mike Searle and Laurence Robb. We have also worked closely with Dr Andrew Mitchell, a geologist who has worked in Burma for over 40 years and who remains one of the few true experts on the region. I have built up a network of local geologists –practising consultants and from academia - and am pursuing joint research opportunities with domestic universities.

While simple to fly to, in many ways Burma still remains a tricky country to travel in - you cannot simply turn up in Yangon, hire a car and drive to an outcrop. Any proposal to visit much of the country beyond the central plains (roughly between Yangon-Mandalay-Bagan) generally requires a government permit issued through the Ministry of Mines. Hiring a guide is a necessity, for both translation and local logistics. Foreign mobile phone roaming does not work, and Wi-Fi is available but often patchy. The local currency is the Kyat, but for many things you need to pay in pristine US$; while ATMs and credit card facilities are only starting to appear in major cities. Regardless, we have forged on and participated in field trips to various localities, including mine visits to tin, gold and lead-zinc deposits. Most recently I have made a couple of trips to southern Burma to study the extensive tin-tungsten deposits and observe present-day mining operations there.

Tanintharyi

klj Picture:Hermyingyi Mine - veining within the host rock

Burma’s main tin-tungsten district is located along its southern spine, an archipelago stretching down along the Andaman sea, sharing the border with Thailand. This is the Tanintharyi region, difficult of access and my favourite part of the country (albeit a bad malarial zone with a problem of increasing resistance to drugs). Largely jungle, its coastline comprises pristine beaches with a multitude of islands and inlets, while inland are lowland rice-paddies and slow-moving muddy rivers with a backdrop of extensive and vegetated mountain ranges. High precipitation means that during the rainy season from May to September the rivers swell and many dirt roads are rendered impassable - but on the other hand the verdant green of the palm-fringed paddies is stunning.

Dawei (Tavoy) is about an hour’s flight from Yangon and best flown, courtesy of one of the new breed of privately-owned local operators, on modern ATP prop planes. You will probably be the only westerner on board. It is a pleasant, small, fairly undeveloped town, where I was met at the airstrip by my guide. After registering with the local authorities we jumped straight into a 4x4.

The recent geological history of SE Asia is dominated by Mesozoic-Cenozoic subduction, accretion and collision between a series of plates and island-arc terranes that rifted from Gondwana in the south, and sutured onto the South China terrane during the staged closing of Tethys. The history is similar to other terranes along the Tethyan margin (e.g. Tibet), with progressively younger continental collisions and associated suture zones running east to west.

Two principal collision events dominate the geological history of Burma. The Late Triassic Indosinian Orogeny (the closure of Palaeo-Tethys) describes the collision of Sibumasu with the Indochina terrane. The resulting Bentong-Raub Suture is thought to run through eastern Burma, western Thailand and south into peninsular Malaysia. Sibumasu (Siam-Burma-Malaysia-Sumatra) is the basement thought to underlie eastern Burma, and represents a contiguous terrane that rifted from Gondwana in the Early Permian.

The more recent closure of Neo-Tethys and the initiation of the Himalayan Orogeny, has been dated along the Indus-Yarlung-Tsangpo suture further north at c. 50Ma, and its suture is thought to crop out in western Burma, in the Mount Victoria Belt. This suture has, however, been cut by recent Neogene strike-slip faults; most notably the 1200km-long dextral Sagaing Fault, a major active north-south fault dividing eastern and western Burma, and which continues to accommodate most of the northward motion of the Indian Plate to this day.

Mineralisation

Picture: Artisanal miners folowing primary outcrop

Tin-tungsten mineralisation in southern Burma is associated with the intrusion of a suite of largely Cretaceous-Eocene crustal-melt granites, part of the extensive tin granite belts of SE Asia. Collectively, these belts are thought to be the magmatic expression of the staged closing of Tethys. To the east, and running through peninsular Malaysia, through Thailand to the border with both Burma and Laos, two of these major granite belts are found, together representing the Indosinian Orogeny.

These are termed ‘Main Range’ and ‘Eastern Belt’, and are delineated by the Bentong-Raub suture. This suture is accordingly thought to separate dominantly subduction-related ‘I-type’ granites to the east from continental collision ‘S-types’ in the west. Classically, tin mineralisation is linked with the latter; though recent work at Oxford has shown that these granite belts are more chemically and chronologically similar than previously thought, and that tin mineralisation is associated with both.

Together, these Southeast Asian granite belts produced some 54% of all the tin ever won worldwide. Most was found through the destructive dredging of river and coastal deposits in both Malaysia and Thailand. In Burma, however, many primary deposits remain; the Burmese tin granite belt is one of two magmatic belts found within the country. These two are thought to be parallel, probably contemporaneous magmatic-metallogenic provinces related to the closure of Neo-Tethys. However they do exhibit completely distinct mineralisation. The Mogok-Mandalay-Mergui (MMM) belt (central and southern Burma – the site of our study) is rich in extensive tin-tungsten mineralization; while the Wuntho-Popa Arc to the west is thought to be a magmatic arc with Cu-Au porphyry and Au-Ag epithermal mineralisation.

Mineral provinces

Burma can be divided into three major mineral provinces; the aforementioned Wuntho-Popa Arc, the MMM, and the Shan Plateau in the far east of the country.

The Wuntho-Popa Arc is underlain by the Burma Seismic Zone, an eastern-dipping subduction zone with earthquakes recorded down to 230km. Sited above this are several large Pliocene calc-alkaline stratovolcanoes (Popa, Taungthaulon and Loimeye). The Arc hosts the major copper deposit at Monywa.
The Shan Plateau largely comprises a series of Ordovician-Triassic carbonates overlying PreCambrian-Cambrian sediments and volcanics, the latter hosting the major lead-nickel-silver Bawdwin Mine, a VMS-type deposit.

lkj Picture - Roads in Myanmar are increasingly metalled, but many can turn to impassable quagmires in the rainy season.

The MMM can be subdivided into the Mogok Metamorphic Belt and the Slate Belt. The former is a high-temperature kyanite-sillimanite grade metamorphic terrane dominated by ruby-hosting phlogopite and diopside-bearing marbles, with occasional pelites.

The Slate Belt runs broadly north-south from Mandalay to Mergui and Phuket, and is a Late Palaeozoic succession of meta-mudstones and sandstones. It is within this we find our tin granites. Both hornblende and biotite ‘I-type’, and two-mica occasional toumaline-bearing ‘S-type’ crustal-melt granites punctuate the Slate Belt. Tin-tungsten mineralisation is strongly associated with the intrusion of the crustal-melt granites, and is found both as Cornish-type greisen-bordered veins within the granite cupolas themselves, and as quartz-hosted veins in the country rock.

Unlike the Variscan tin deposits of SW England, no significant copper mineralisation is associated with the tin granites - they are distinctly tin-tungsten producers. In a typical deposit, tin and tungsten are found as varying proportions of cassiterite and wolframite-rich pegmatites, and greisen-bordered quartz veins. There is marked regional zonation; tungsten becomes progressively dominant over tin towards the north. Indeed, from its general predominance, some workers have asserted that collectively the mineralised granites of Burma should be called a “tungsten province” with subsidiary tin. The granites extend as far south as Phuket in Thailand, where mica-tourmaline pegmatites have recorded significant amounts of monazite and other REE- and yttrium-bearing minerals.

The Dawei region is home to over 50 historical tin and tungsten mines - although tin production stretches both further north and south, following the granite belt. Many worked deposits are a mixture of primary with elluvial and alluvial association, but quite a few are solely alluvial, often following river and stream gravels, or as palao-terraces. Working alluvium is favoured by locals who run small-scale artisanal operations, usually with rudimentary processing plants (predominantly sluices) to produce tin concentrate. As cassiterite is largely resistant to the intense lateritic-type weathering, much tin is found as SnO₂ dispersed in the alluvium. Thus, a simple gravity separation process is all that is required to produce a concentrate grade suitable for sale to local wholesalers. There is no local smelter, so most tin concentrate is exported either to Thailand or Malaysia.

Historic mining

klj Picture: Backroads in the Dawei area

Many of the major mines in the area have a long history. We visited the Hermyingyi mine, a primary deposit which was, pre WW2, a significant British-operated tin mine, and one whose redevelopment is already afoot. Here, greisen-hosted quartz veins of cassiterite and wolframite are found within the host granite, and underground mining has been operating since the 1930s. The mine is accessed via a small rail bridge, and although the railway engines have long rusted solid, wagons still run on man-power, transporting hand-excavated ore to a basic processing plant a few hundred metres along the valley.

The Bogyoke Mine is another colonial-era working. Its main underground shaft is now flooded, but a Chinese-sourced processing plant has recently been installed, aiming to produce tin concentrate from the dredgings of nearby rivers. The Mawchi Mine, once the world’s pre-eminent tungsten producer, exemplifies a typical primary deposit. The Mawchi granite is a relatively small intrusion of porphyritic biotite granite, intruding metasediments of the Slate Belt. Mineralised veins up to 2.5m wide are present mainly within the upper part of the granite, and economic tungsten-tin grades are focused within this cupola, decreasing rapidly with depth. These veins show an earlier generation of wolframite and cassiterite, with paragenetically later molybdenite, chalcopyrite and magnetite, gangue minerals such as fluorite.

Up-sides

lkj Picture: It is estimated that 80-89% of Burmese are Buddhists.

There are huge up-sides to travelling and working in Burma. Despite having been ruled by one of the world’s more unpleasant regimes, the Burmese people remain among the kindest and friendliest on the planet. While travelling around to more rural areas, children and adults alike will come up to say hello (“Mingalaba”), give you a toothy grin (the degree of toothiness contingent on whether they chew betel nut or not) and shake your hand. This is a profoundly Buddhist country, and wherever you travel you are greeted by the sight of a dozen golden pagodas rising above the trees, or monks in red robes zooming about on the back of motorbikes. You also enjoy the sight of bullock carts travelling the wrong way down the only dual-carriageway (one which uses the furlong as a measure of distance).

The country itself contains a variety of landscapes: hot central plains, a patchwork of rice paddies, is usually baked to dust by late May waiting for the monsoon; those idyllic, pristine beaches and a thousand waving palm trees greet you in the deep south. In the north, the Himalayan foothills grade from teak forests to rise to Hikabo Razi; and you can get seriously lost in the intractable malarial jungle of the east, host to tiger, elephant - and several armed separatist groups.

We have more trips to Burma planned in the next year - hopefully going further north. It will be a pleasure to return.

Geoscientist Online