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TIN-TUNGSTEN MINERALISATION AT HERMYINGYI, TAVOY DISTRICT, BURMA. BY J. A. DUNN, D.Sc. (MELB.), D.I.C. (LONDON), F.N.I., F.G.S., Petrologist, Geological Survey of India. (With Plates 13 and 14.)
At the instigation of Mr. E. L. G. Clegg, Superintending Geologist of the Burma Geological Department, Geological Survey of India, Mr. Jackson, Manager of the Hermyingyi mine, Tavoy, Burma, sent to me selected specimens of the mine ore for examination under the ore-microscope. The Hermyingyi specimens were of interest from the point of view of comparison with the Mawchi ores in Karenni which had been previously studied.
Only eighteen specimens were sent, and all of lode material, so that the study cannot be regarded in any sense as exhaustive. In comparing them with the Mawchi ores, the greater coarseness and simplicity of the mineral development of the Hermyingyi lodes becomes at once apparent. This could, of course, be merely an incidental feature of the specimens submitted, but almost any specimen from Mawchi would show a diversity in mineral assemblage greater than the whole of this set of Hermyingyi ores, and no Mawchi specimen examined by me has such coarse crystals as those from Hermyingyi.
The geology of Tavoy has been described in detail in a memoir by Drs. J. C. Brown and A. M. Heron.1 In this memoir a full bibliography is provided of the geological work which had been undertaken in this part of Burma prior to 1923.
Briefly, the rocks consist of a presumably pre-Cambrian group of sediments, the Mergui series, intruded by granite. The Mergui series comprises argillites, volcanic agglomerates, quartzites, occasional limestones and conglomerates, which strike N.N.W.-S.S.E. and dip at a high angle from 60 degrees to vertical.
At Hermyingyi about sixty different veins had been worked. In general they strike N.N.E.-S.S.W. and have an easterly dip. They have been followed for a distance of 500 to 1,100 feet and have a width of 10 inches to 5 feet. The veins continue upwards from the granite into the sedimentary rocks above. The proportion of WO, to SnO2 in the concentrates from the veins was approximately 2 to 1.
The veins usually have well definedwalls with often a thin selvage of mica, and, almost throughout, narrow bands of the wall-rock in granite country have been altered to a quartz-mica greisen which sometimes contains sufficient cassiterite and wolfram to be profitably mined.
The ore minerals in the specimens examined consist of wolfram, cassiterite, pyrite, sphalerite, chalcopyrite, galena, bismuthinite, molybdenite, and native bismuth in that order of abundance. The gangue minerals include quartz, muscovite and fluorite. In addition, Brown and Heron record the presence of topaz along the wall of one vein. Supergene minerals include WO, ochre, covellite and a little limonite.
Four minerals, present in the Mawchi ores, are absent from the Hermyingyi specimens: scheelite, arsenopyrite, tourmaline and calcite. This cannot be entirely incidental to the specimens examined because there is scarcely a specimen from Mawchi in which at least scheelite and tourmaline are not abundant. Brown and Heron have remarked on the absence of tourmaline from the wolfram veins in Tavoy. They do, however, record the occasional presence
1 J. C. Brown and A. M. Heron, The Geology and Ore Deposits of the Tavoy District, Mem. Geol. Surv. Ind., XLIV, Pt. 2, 1923.
of scheelite, and even more rarely of arsenopyrite and calcite. The abundance of these minerals in the Mawchi ores provides a strong contrast. Fluorite is rather more abundant in the Hermyingyi ores than at Mawchi.
The microscopic characters of these minerals have already been described, so that there is no necessity to repeat them here.
Wolfram. This is almost invariably in coarse and thin bladed crystals, up to 6 inches in length, with excellently developed cleavage, and often very friable. Against quartz it commonly shows crystal faces. A film of WO, ochre, of a white or pale yellow colour, is very often found along the cleavage.
An analysis of carefully picked material gave the following results :-
This, then, is a wolfram in which the Mn Fe ratio is 2: 1. It may be compared with the wolfram from Mawchi in which the Mn: Fe ratio is approximately 3: 2.
Cassiterite. This is often in coarse crystals over one inch diameter. A characteristic of the Hermyingyi cassiterite is its rather pale brownish colour as compared with the more usual deep brown to black colour elsewhere. The cleavage is well developed and the mineral is often extremely friable, especially where there has been slight crushing. Zoning is not particularly common in the Hermyingyi mineral.
Pyrite. Where present, this mineral usually shows a euhedral outline against quartz, and commonly occurs in perfectly developed little cubes, but against other minerals it is much more irregular.
Sphalerite. The characteristic ex-solution droplets of chalcopyrite are more beautifully developed in the Hermyingyi sphalerite than perhaps in any other specimens which I have previously seen. Sphalerite is not particularly abundant and occurs only in occasional specimens.
Chalcopyrite. Wherever sphalerite occurs, there, also, chalcopyrite is invariably found. Besides forming ex-solution droplets in sphalerite, small patches replace and vein other minerals.
Galena. Small patches and veins of galena are occasionally seen but are not particularly abundant, and are usually minute.
Bismuthinite.-Fine-grained granular bismuthinite is usually associated with galena and is found as rare veinlets in other minerals. The two are not readily distinguishable under reflected light although the reflection pleochroism of bismuthinite can be observed; however, with crossed nicols the pronounced anisotropic colours of bismuthinite are characteristic.
Bismuth.-Ex-solution droplets of native bismuth were seen in some of the bismuthinite (Plate 14, fig. 4). The mineral is very rare and no large patches were seen. It is readily diagnosed because of its extreme softness, high reflectivity and characteristic colour. It is so soft that it flows during polishing.
Molybdenite.-A few flakes of molybdenite may be seen in quartz, but its more usual mode of occurrence is as aggregates of fine-grained flakes either in quartz or interstitial to wolfram.
Quartz.-Coarse white quartz is the main gangue mineral. Occasionally the quartz shows crystal faces in the centre of the veins, where the interfaces may be filled in with muscovite.
Muscovite.-Usually the muscovite occurs in small silvery flakes or pale greenish aggregates. It occurs on the vein walls, often as a thin selvage, or as thin lines in quartz parallel to the vein walls and giving to the veins a somewhat banded appearance. Commonly, also, it is interstitial to wolfram and to quartz, in which case the flakes are often distorted. Sometimes a very fine, almost earthy, white sericitic mica veins the wolfram.
Fluorite. Although by no means abundant, a little fluorite is seen in the majority of specimens. It is pale yellowish, or pale greenish in colour. It is usually interstitial to wolfram and muscovite and is commonly associated with the latter.
Tungstite.-WO, ochre occurs as a fine-grained powder which veins wolfram and other minerals and occasionally replaces patches. of wolfram. Under the reflecting microscope it is readily distinguished by its softness, a reflectivity which is just lower than wolfram, and its pale yellow to white inner reflection.
Covellite. This occurs as thin veinlets in any of the hypogene minerals, and it occasionally directly replaces chalcopyrite.
"Limonite ".-A little limonite is developed where the veins have been leached and are rather cellular, particularly on the vein walls. Wolfram has occasionally altered partly to limonite.
The veins show a consistent structural relation between the minerals wolfram and cassiterite form more particularly along the walls and quartz predominates in the centre of the veins. The coarse bladed wolfram grew outwards from the vein walls, extending into the quartz towards the centre of the veins. Wolfram, in these specimens, is much more abundant than cassiterite. Green and silvery muscovite is commonly found as a thin selvage along the vein walls, but in addition it occurs interstitially to the wolfram and as thin lines in the quartz, sometimes giving to the veins a banded appearance.
The position of muscovite in the sequence is by no means definite. In only one case was it found to be veined by wolfram (Pl. 13, fig. 1) and no other mineral replaces it. Yet it appears to be interstitial to wolfram, although on close examination such muscovite is usually bent, as if pushed aside by the wolfram. It is often closely associated with fluorite. Its consistent position on the vein walls suggests that muscovite was the earliest mineral, but some of it does undoubtedly vein quartz. The assumption follows that there were two periods of deposition of muscovite. A very fine white sericitic mica sometimes veins wolfram.
On the whole, cassiterite appears to be interstitial to wolfram and tends to crystallise after the latter, but an unusual example of cassiterite grains replaced by wolfram was observed in one specimen (Pl. 13, fig. 3). As in the Mawchi ores, crystallisation of these two minerals obviously overlapped.