Page images

Kaolinite. From one of the fault planes which intersect the granite in this area, a peculiar white, soft talcose material was analysed and proved to be of the kaolin group :

[ocr errors]
[merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]


Mineral sequence.

The accompanying sequence diagram (Fig. 1) has been constructed from a study of both polished and thin sections. The relationship of the minerals may now be discussed in the order of their deposition.

Mr. G. V. Hobson has found that garnet occurs quite commonly in the lower horizons of the mine, and, as a rule, in veins or parts of veins that are barren. Hence, in this collection of specimens. selected to study the major ore minerals, garnet is rare and there is little on which to judge its position in the sequence, but it is clearly earlier than beryl and quartz. It occurs in a vein in No. 2 horizon in granite country with beryl and is replaced by calcite.

Zoisite, found in vein specimens from No. 1 horizon in marble country rock, is possibly a high temperature contact mineral and clearly preceded associated fluorite,

[merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Fluorite was at first assumed to be late in the sequence, but, surprisingly, it was found to be one of the early minerals. It may have a much wider range than that indicated in the sequence diagram, but it is more closely associated with the earlier minerals than with the later, and its period of crystallisation was presumably brief; the mineral is not particularly abundant. It replaces orthoclase and is, in one place, veined by blue tourmaline, and in another by pyrite. Occasionally it is replaced by calcite.

The orthoclase of the veins in No. 1 horizon is clear and unaltered in contrast to the cloudy kaolinised felspars of the granite. It is replaced by cassiterite and calcite. It is sometimes apparently interstitial to cassiterite (Pl. 11, fig. 1) and the inference is that the two minerals crystallised more or less together.

Blue tourmaline is abundant; brown tourmaline is not seen so frequently. In every case tourmaline is closely related to cassiterite and wolfram. In some sections blue tourmaline was seen to have grown outwards from euhedral wolfram, and in one case it replaces the latter. It commonly shows euhedral outlines against wolfram (Pl. 6, fig. 3), and occasionally prisms of the mineral are included in the latter. Crystallisation of these two minerals obviously overlapped. Cassiterite sometimes shows euhedral outlines against tourmaline, but more often the tourmaline has well developed prismatic faces against cassiterite. It is sometimes replaced by quartz and muscovite (Pl. 10, fig. 4). Blue tourmaline has apparently been replaced by scheelite, but this may, at times, be a relict relation and the scheelite may have actually replaced wolfram which had previously replaced tourmaline.

Although wolfram and cassiterite are closely associated, in no case was either mineral found to replace the other. Wolfram shows a consistent euhedral form against cassiterite, the latter being usually interstitial to wolfram prisms, as if moulded upon them (Pl. 6, figs. 1 and 2). For the most part, then, it would appear that wolfram commenced crystallisation prior to cassiterite, so far as the formation of the veins is concerned. However, in one section a grain of cassiterite included in wolfram was definitely veined by the latter, the two sides of the vein dovetailing into each other. The evidence points to the two minerals crystallising closely in time, the wolfram preceding the cassiterite but deposition of the two overlapping.

Cassiterite in these ores displays its characteristic stability and is not usually replaced by other minerals. In only one or two cases was the mineral seen to be replaced by pyrite and by quartz. Very rare veins of galena (Pl. 10, fig. 1), scheelite and calcite have been seen in it. In contrast, wolfram has been widely replaced and veined by scheelite, and in places is veined and replaced by sulphides and such gangue minerals as quartz, mica and calcite.

Beryl occurs in lodes in granite country and is closely associated with cassiterite and early quartz. It is interstitial to cassiterite and even veins the latter (Pl. 11, fig. 2). Quartz appears to have followed beryl immediately, as it is slightly interstitial to it and occasionally replaces it. Beryl is replaced and veined by sphalerite, galena and muscovite; in one case, where scheelite had replaced laths of wolfram, the scheelite at the border had replaced and veined beryl along the latter's cleavage (Pl. 11, fig. 3).

Phenacite replaces tourmaline, and in one case minute prisms of the mineral radiate from a tourmaline crystal. It is veined and replaced by sulphides, carbonate and scheelite. It occurs in lodes. in both slate and granite country.

Molybdenite appears to be the earliest of the sulphides. It commonly occurs as small thin flakes isolated in quartz or at the border of wolfram and quartz. It is replaced by calcite in one section, where it occurs as flakes in tourmaline, but the latter mineral has been largely replaced by calcite and micaceous material. In quartz and bismuthinite (Pl. 9, fig. 3) it often appears to form wavy veins, but these are really thin elongated, distorted flakes of the mineral; between crossed nicols they show the characteristic wavy extinction along the length of the flakes. Molybdenite has the appearance at times of having formed at the interface between quartz and bismuthinite, but in such cases the later sulphide has usually replaced the quartz up to the molybdenite flake. Molybdenite flakes were seen in scheelite which had, however, replaced wolfram. The evidence, on the whole, indicates that it is earlier than quartz and other sulphides and that it is closely associated with cassiterite and wolfram, apparently immediately succeeding the latter.

The early phase of mineralisation closed with the deposition of a large amount of white quartz. This replaced some of the earlier minerals such as wolfram (Pl. 7, fig. 2)-in one specimen this replacement was so complete that only the outline of the original mineral

was left. The relation of quartz to cassiterite is well illustrated around a vugh in a narrow vein. Fine cassiterite occurs along the wall of the vein, then quartz, then coarse crystals of cassiterite have grown out into the vugh, whilst cutting across such individual crystals minute quartz veinlets may be seen.

On the whole it might be said of this first phase of mineralisation that, apart from fluorite and zoisite, the minerals show little tendency to replace one another, but mainly separated out as a mineral aggregate in the form of veins which exerted comparatively little replacing effect on the country rock. It should not be understood that this tin-tungsten phase was sharply demarcated from the later mineralisation. Quartz continued to be deposited and veinlets are found to penetrate later minerals in succession.

Further mineralisation gave rise to the deposition of arsenopyrite and pyrite. Well defined veinlets of pyrite occasionally cut across arsenopyrite (Pl. 8, fig. 1). In one specimen quartz veins cut across arsenopyrite, whilst pyrite veins intersect the quartz (Pl. 7, fig. 3), clearly indicating an interval between the two sulphides. Although neither mineral was. seen to show a euhedral boundary against the other, pyrite is frequently euhedral against quartz and other gangue minerals. Veins of pyrite occur in tourmaline (Pl. 7, fig. 4) and orthoclase, and are especially common in wolfram; such veins often also contain chalcopyrite. Pyrite frequently accompanies quartz veinlets in wolfram. Both arsenopyrite and pyrite may be found along the interfaces between wolfram, cassiterite and quartz. Both sulphides are sometimes veined and replaced by chalcopyrite, quartz, calcite and other gangue minerals.

Sphalerite is usually closely associated with wolfram. The posi tion of sphalerite with respect to pyrite and arsenopyrite cannot be directly observed, but that it is later is suggested by its relation to chalcopyrite and stannite. Sphalerite contains, in these specimens, innumerable minute almost sub-microscopic ex-solution droplets and veinlets of chalcopyrite and occasionally droplets of stannite; these minute inclusions are, of course, characteristic of sphalerite in many ore deposits. The droplets are sometimes discernible only with the highest magnification under oil immersion, and they have separated out from solid solution in sphalerite with lowering of the temperature. It is obvious that at high temperatures the sphalerite is capable of containing a number of other


« PreviousContinue »