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tourmaline are frequently closely associated and muscovite veins the tourmaline.
Fluorite, usually purple in colour, is seen in several specimens, interstitial to felspars and quartz, and usually associated with fine muscovite. It commonly replaces oligoclase.
Calcite is present in the more altered types, as are also minute prisms of topaz. Such more altered types contain a large amount of muscovite and secondary quartz as well as pyrite. Pyrite and magnetite replace each other, but it is not clear in which exact order. Epidote is occasionally present.
Grains of cassiterite are found in many of these granite specimens. It is often zoned and may show a red to colourless or pale green pleochroism. It is usually interstitial to the quartz and felspar, and is more closely associated with tourmaline, muscovite and fluorite. Occasionally it appears to replace felspar and quartz. In some cases, where the granite has been crushed and the cassiterite appears to have replaced crushed quartz, it is obviously later than the crushing. Analyses of three specimens of this granite are given in Table 2.
No. 8.-A fresh fine medium-grained tourmaline granite. Notwithstanding that fluorine is not recorded in the analysis, a few minute grains of fluorite can be seen in thin section. Cassiterite can also be determined in the thin section.
No. 19.-A fresh fine medium-grained tourmaline granite with a little biotite. Zoned cassiterite is present in the thin section, although not determined in the analysis.
No. 20.-A slightly kaolinised coarse-grained porphyritic granite with much biotite and epidote, and a little hornblende and apatite. Cassiterite is also present.
A number of lodes have been worked at Mawchi. As long ago as 1916, Walker recorded seventeen veins varying up to five feet in thickness, and ten were of a payable nature. Workable veins now number twenty-seven, some of which, in places, are double or treble veins, thus increasing the actual number considerably. They strike N. N. E. over a length of a few hundred feet and are usually almost vertical. Cropping out on the surface of the hill, they have been readily worked from adits,
H2O at 110°C
H2O above 110°C
Analyst R. Dutta Roy.
The lodes are mainly of cassiterite, wolfram and quartz, with calcite as quite a common gangue mineral. The grade varies very considerably from place to place in the veins, but mill heads average
just below 3 per cent. A coarse banding is occasionally noticeable. as a result of an arrangement of the minerals parallel to the vein walls. The origin of a curious vein variety described by Mr. Hobson as a sand lode" is difficult to interpret; the lode channel here is occupied by fine loose grains of quartz with cassiterite of a sandy nature. It calls to mind the unusual occurrence at Mount Bischoff in Tasmania, where in places porphyry dykes were attacked during mineralisation and the felspars removed as well as much of the quartz, leaving merely a loose angular cassiterite-quartz-sand which filled the dyke space completely.1
The lode minerals show little disturbance under the microscope, apart from slight movement which permitted later minerals to form veins along cracks in earlier minerals. The platy calcite often shows considerable signs of crushing. In the hand specimens slickensided faces are sometimes seen.
This work was completed before the installation of the lead lap polishing machine in the Geological Survey Laboratory. Most of the microphotos accompanying this account were taken on surfaces polished on cloth laps. The ores and gangue minerals are described separately, and in the order of decreasing importance.
Only the mineragraphic descriptions of cassiterite, wolfram, scheclite and molybdenite are given here; those for the sulphides are well enough known to need no repetition.
Cassiterite.-Usually the cassiterite in these ores is quite coarsegrained but fine grains may be seen in polished sections. and calcite it often shows well developed crystal faces, but where associated with wolfram it is commonly interstitial to the latter and shows no crystal boundary (Pl. 6, fig. 1). The mineral is readily detected by its dark yellowish brown colour and great hardness. A large grain gave a specific gravity of 6.92. As a result of crushing and the brittle character of the mineral it often crumbles very easily. In thin sections it is beautifully zoned (Pl. 10, fig. 3) and exhibits a red to pale green pleochroism.
1J. A. Dunn, The economic geology of the Mount Bischoff tin deposits, Tasmania, Econ. Geol., XVII, pp. 162, 163, 166, 167, 174, 176, (1922).
It is more difficult to polish this than perhaps any other mineral. With cloth polishing it is almost impossible to obtain any considerable area of the mineral free from deep pits and grooves, and, with its high relief, flat, smooth surfaces are rare. However, better results are obtained by polishing on lead laps, but even then the mineral's brittleness gives rise to difficulties. Hardness: G+. Reflectivity: about 11. Colour: grey, sometimes zoned, and multiple twinning shows up at times as a result of slight pleochroism. On smooth surfaces polished on lead laps, a prismatic cleavage is sometimes seen. A yellow inner reflection is occasionally noticeable under polarised light. Anisotropism: moderate, colours usually obscured by the strong variegated (mainly amber yellow) inner reflection. (Negative.-HCl, HNO3, aqua regia, H2O2, H2O, +H,SO4, HgC1,, FeCg, KOH, KCN, Etch tests Positive.-A drop of HCl and a fragment of metallic zinc gives a film of metallic tin over the surface.
Wolfram. This mineral is very abundant throughout all of these ores. It is usually coarse-grained, often forming laths several inches in length with well defined crystal faces in quartz (Pl. 6, fig. 3) and in many cases grew inwards from a thin selvage along the vein walls. Without exception it retains its prismatic form against cassiterite (Pl. 6, figs. 1 and 2). The prismatic cleavage is so well developed that the mineral crumbles readily on breaking. An analysis on carefully hand-picked material, which previous microscopical examination had shown to be free from replacing scheelite, gave the following results :
Analyst R. Dutta Roy.
Assuming the correctness of this analysis the high percentage of MgO is surprising in view of the care with which the fine grains of wolfram were hand-picked. It can only be assumed that some tourmaline escaped detection in picking. But even if this were so the amount of SiO2 and Al2O3 should be greater than that determined.
The mineral polishes rather well, with few pits; the prism directions are smoother than the basal. Hardness: About D, scratched by a needle, shows a decidedly lower relief against cassiterite and against quartz. Reflectivity: about 18, with slight pleochroism noticeable only between adjacent grains. The reflectivity and colour are exactly the same as those of sphalerite, which is sometimes associated with the wolfram, and the two are distinguishable only between crossed nicols. The sphalerite in these ores always contains minute ex-solution droplets of chalcopyrite and sometimes of stannite. Colour grey-white, with a hint of brown. Prismatic cleavage is often noticeable. Sometimes the mineral shows zoning with scheelite, apparently as a result of selective replacement by the latter. Anisotropism: moderate, colours are yellow and dark grey, sometimes with a hint of violet or green. A red inner reflection is noticeable between crossed nicols, particularly where the surface is not well polished.
Etch tests. Negative.-HCl, HNO3, aqua regia, H2SO4, H2O2,
Scheelite. Although scheelite is so widely distributed throughout these ores, it is not usually detected in the hand specimen. However, occasionally a occasionally a white material, interstitial to wolfram, can be diagnosed as scheelite. Its principle mode of occurrence is as a replacement product of wolfram, when it has almost the same colour as the latter. Where wolfram loses its easily separated cleavage and takes on a dull colour it is invariably found to be largely replaced by scheelite.
An unusual specimen was obtained from one of the veins. This was a brownish, rather, porous, and almost incoherent material, which had obviously been subjected to considerable leaching.