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PLATE 1, Fig. 1.- Aravalli schist country, south-west of Kundanpur, Jhabua
Fig. 2.-General view of Aravalli quartzite ridges, Umria, Bariya State. PLATE 3, Fig. 1.-Aravalli quartzite (dip-slope), Hadaph river-section, Umria,
FIG. 2.-Weathering of granite, Kothamba, Lunavada State.
PLATE 5. Geological map of Gujarat and Southern Rajputana.
Tin-TUNGSTEN MINERALISATION AT MAwcHI, KARENNI STATES,
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 6 to 12.)
PROCESS OF MINERALISATION
Crystallisation and mineralisation within the magma
230 233 234
INTRODUCTION. Until recently, very little known about the complex tin ores of Karenni and there is no published detailed geological literature the subject. Mr.
Mr. J. J. A. Page and Mr. H. Walker, both of the Geological Survey of India, visited Mawchi in 1909 and 1915-16 respectively. Walker's report, which was published by the Burma Government, concerned
1 H. Walker, Report on the Mawchi Mine, Bawlake State, Karenni, Burma. Government Press, Rangoon.
with ore reserves and assay values than with detailed geological and mineralogical descriptions. Although we have a broad idea of the geology of the Mawchi area, no detailed maps have yet been published.
Early in 1935, Dr. J. C. Brown suggested to me that a mineragraphic study of the complex tin-wolfram ores of the Mawchi mine would supply interesting information. Fortunately, towards the end of the same year, Mr. G. V. Hobson, until lately of the Geological Survey of India, joined the staff of Mawchi Mines Ltd., and, by courtesy of the General Manager he was able to send me specimens from time to time.
It was hoped that Mr. Hobson and I would be able to write a joint account on the Mawchi mineralisation, but stress of work has prevented Mr. Hobson from carrying out our original object. The mineragraphic work finished early in 1936, and as I am about to proceed on leave, and other work will undoubtedly require attention on my return, it has been thought advisable to publish the results of my petrological studies immediately. However, although this account will lose by its lack of detailed field descriptions, its prior submission to Mr. Hobson for comment will, I hope, preclude the intrusion of serious errors.
Country rock. Mr. G. V. Hobson describes the Mawchi area as consisting of argillites and fine sandstones with some limestones, intruded by a large granitic mass. The argillites show no cleavage. The tinwolfram deposits occur in each of these rock types, but with one exception the veins very rapidly die out on meeting the limestone. In the upper horizons of the mine, slates are the principal country rock, whilst in the lower horizons the lodes are in granite.
The limestones are fine-grained and have been somewhat recrystallised. Some appear to be slightly carbonaceous. Three specimens were analysed as in Table 1.
Specimen A is a dark grey limestone containing tremolite and much fine quartz; secondary sulphides are scattered throughout. Specimen B is quite a fine-grained rock, whilst C is a pure white saccharoidal limestone. In consequence of their metamorphic character they might aptly be described as marbles.
Mineralisation at Mawchi is obviously associated with the granite, which frequently contains a noticeable amount of cassiterite. Veins of aplite and pegmatite traverse the granite, as also do tourmalinequartz veins, in which there may be a little cassiterite. Finegrained “segregations" containing tourmaline, cassiterite, wolfram and scheelite (the latter replacing wolfram) occur quite commonly. The granite is rather variable both in texture and mineral assemblage. Occasionally it is banded or gneissic, especially the tourmaline variety. Kaolinisation has been widespread, and this is by no means an effect of weathering, but appears to be largely a
late magmatic change due to the removal of alkalies by either volatiles or hydrothermal solutions. It was necessary to impregnate almost every specimen with bakelite before thin sections could be made, using kerosene as a lubricant.
Oligoclase and orthoclase are present in variable amounts, but the former is typically the more abundant. Orthoclase is usually more severely ka olinised than is oligoclase. Oligoclase is often idiomorphic, and is earlier than orthoclase. Evidence of resorption of oligoclase was noticed, quartz replacing oligoclase crystals. Quartz is usually only in subordinate amounts, but is often quite abundant in the more sodic varieties. Most of the quartz shows a curious persistent relation to the felspars ; it followed oligoclase but preceded orthoclase in the crystallisation sequence. A later fine-grained quartz does occur occasionally, interstitial to orthoclase, but the normal sequence was undoubtedly oligoclase, quartz, orthoclase. Very rarely the orthoclase shows a graphic texture with quartz.
Sometimes microcline is observed in fresh specimens --it is possibly abundant, but obscured by kaolinisation. Porphyritic varieties usually consist of oligoclase phenocrysts in an oligoclase-quartz-microcline groundmass.
The rock is a tourmaline-granite throughout. Sometimes tourmaline is present to the exclusion of other ferromagnesians. Biotite occurs in only a few specimens, and, with the exception of a little green hornblende in one specimen, appears to be the only primary ferromagnesian mineral present in these rocks. Usually the brown biotite is somewhat altered to chlorite. The tourmaline is frequently seen to replace other minerals; in those specimens in which it is the only ferromagnesian present, tourmaline possibly originally replaced first biotite and then continued growth by replacing felspar. It is both blue and brown in colour, and is occasionally zoned. Sometimes the centre of the tourmaline may be a well defined crystal, which has continued growth as an outer zone of blue tourmaline replacing felspar and quartz. A common position of replacement by tourmaline is at the interface between two minerals such as felspar and quartz.
Muscovite is entirely secondary, usually replacing felspars ; no primary muscovite was observed. It frequently occurs as radiating rosettes, especially when it shows the pink-green pleochroism of lepidolite. It is noticeable that the muscovite tends to replace oligoclase rather than kaolinised orthoclase. Muscovite and