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The vein liquid. Following injection, with sudden relief to pressure, the more volatile constituents would tend to escape and react with adjacent minerals of the wall-rock or find their way towards the surface and would thus be entirely lost to the remaining ore liquid. It is noticeable how completely the fluorite and much of the tourmaline was deposited at the beginning of the sequence. It is possible that a considerable part of the H20 content of the ore liquid also was dispersed after injection. The remaining liquid still contained some B,0; and was obviously high in silica, tin, wolfram, iron, manganese, sulphides, calcium, with also a little beryllium, alumina, magnesia, lithia and alkalies,-a liquid which must obviously be alkaline in reaction. The mode of combination of the Sn was certainly not as a fluoride, for all F had now disappeared; it was presumably in the form of SnO2. The stability of such a liquid cannot be reconciled with a normal solution in which H20 is the dominant constituent. It may be suggested that, following on the sudden change in composition of this ore liquid after injection and loss of volatiles, the constituents separated out as a dispersed colloid. The prolonged and gradual crystallisation from this colloid eventually gave rise to very coarsely crystalline vein minerals; the early sequence of tourmaline, cassiterite, wolfram, beryl and quartz being · followed by sulphides and carbonates. There is no evidence and no justification for the assumption that in these veins, the ore liquid was in constant upward movement, and, apart from loss of water in an upward direction along the lode channel itself as the colloid crystallised, there could have been no great redistribution of material. Shrinkage accompanying crystallisation of the colloid, with adjustments along the lode under pressure, would give rise to sufficient mineral fracturing to permit movement of this final water. The last minerals to form, carbonates and scheelite, were probably the results of the reactions of this residual water.
In the above picture it has been suggested that the ore liquid was injected as a liquid. There is the possibility that, with relief to pressure, the residual magmatic liquid as a whole was injected as a vapour phase which condensed to a liquid in the higher and cooler parts of the granite before deposition, or deposition may even have taken place direct from the vapour phase. Such a process would give rise to a more marked differentiation than is noticeable in these lode minerals, in which volatile and relatively non-volatile
constituents are closely associated. Apart from the earliest minerals in the ore sequence, fluorite and tourmaline, which may have been formed as the result of the reaction by F and B,0, vapour given off directly from the injected ore liquid, the rest of the lode materials examined appear to have been deposited from an ore liquid which had not passed through a vapour phase.
One point which I have endeavoured to bring out is that in these magmas, which contain an abundance of highly volatile constituents, there is not, during crystallisation, one single stage which can be referred to as pneumatolytic. The action of these volatile constituents both as vapours and in the liquid phase is continuous right from the moment of intrusion to the final stage of ore deposition. Loss of volatiles, whether it be by mere vaporisation or consequent upon phases of ebullition, is a continuous process.
The activity of a vapour phase is, however, relatively unimo portant. The main function of the volatiles is the prolonged retention of a liquid phase down to a very low temperature, permitting a long-continued reaction of this liquid with the minerals throughout the body of the rock, and thus giving rise to widespread deuteric changes. These changes take place right from the commencement of crystallisation of any part of the magma. Kaolinisation is a continuous process from early crystallisation of felspars in such magma. In the deep-seated residual magma the volatiles were responsible for the extremely high vapour pressure which ultimately caused injection of the liquid into the overlying granite.
It is doubtful whether the ore liquids, as a whole, were the product of condensation from a vapour phase. They always were a liquid, although volatiles passed off as vapour at any stage that the rock pressure permitted, and reacted either with the wall rock or continued to the surface. With loss of volatiles the “ solid” constituents of the ore liquid possibly separated into a dispersed colloid from which crystallisation finally took place.
The earlier cassiterite in the granite may have been precipitated from SnF, but in the ore liquid at least it was in the form of Snog, presumably as a dispersed colloid.
On reading this manuscript Mr. G. V. Hobson made two apposite observations. Concerning the relationship between cassiterite and wolfram he remarks:
“From fleld evidence one is bound to accept the very close relationship between cassiterite and wolfram as regards their period of deposition. If, however, wolfram preceded cassiterite one might expect that as one progressed outwards from the granite, or from zones of higher temperature to zones of lower temperature there should be a fall in the ratio of wolfram to tin. Actually the reverse appears to be the case. I have made no calculations on this point, but taking the mine as a whole I think I am right in saying that, with increasing depth, the tin has risen very considerably in relation to wolfram.”
He makes a similar observation concerning arsenopyrite and pyrite :
Here again the lower temperature character of the arsenopyrite and pyrite would lead one to expect them in greater profusion in the outer zones of the deposit. The reverse is the case ; as development progressed inwards towards the more central part of the granite, both vertically and laterally, sulphides became more prevalent. In the lower barren sections of veins sulphides (all of them) occur and oxides do not."
However, Mr. Hobson supplies the partial answer himself by suggesting that
“All these anomalies may be due to extreme rapidity of formation, so that relative concentration rather than relative temperature of formation, was the controlling factor.”
In addition, I would emphasise again what has been said in discussing the vein liquid. The injection of these liquids was sudden and deposition was not from a moving liquid as is pictured in most mineral veins which show zoning. Deposition was first determined by relief to pressure and secondly to temperature. Under such conditions the lower temperature minerals may be expected to occur rather more abundantly in the lower parts of the veins where deposition was prolonged a little. Only considerable re-opening of the veins during deposition could have permitted movement of the ore-liquid and zoning of the lower temperature minerals in the upper levels—and such movement was absent or negligible in these veins. The relationship which Mr. Hobson points out is, therefore, not unexpected.
EXPLANATION OF PLATES.
PLATE 6, Fig. 1.---Cassiterite (C) interstitial to wolfram (W). Quartz (Q).
P. S. 252.
quartz (Q). P. S. 243. X 54.
cy). P. S. 231. X 54.
PLATE 7, F10. 1.-Wolfram (W) crystal replaced by scheelite (S). Cassiterite
(C) and quartz (Q). Bakelite (B). P. S. 251. X 28. Fig. 2.—Quartz (Q) replacing wolfram (W). A little pyrite (P). P. S.
x 54. Fig. 4.–Pyrite (P) veining tourmaline (T), and both replaced and veined by chlorite (C). P. S. 239 B.
PLATE 8, Fig. 1.--Pyrite (P) and chalcopyrite (C), veining arsenopyrite (A).
Quartz (Q). P. S. 164. X 28.
bonate (black). P. S. 164. X 180.
places quartz (lighter grey) and galena (white). Pyrite
(P). P. S. 236. X 54. Fig. 4.-Sphalerite (S) veining and replacing arsenopyrite (A). Also
chalcopyrite (C) and carbonate (Ca). P. S. 242. X 81. PLATE 9, Fig. 1.--Quartz veins in sphalerite. P. S. 173.
x 40. Fig. 2. Stannite (S) and galena (G) which veined stannite but is al
tered in part to cerussite (Ce). Wolfram (W). P. S. 166.
the bismuthinite replacing the quartz along the molybdenite.
areas of bismuthinite. P. S. 175. X 54. FIG. 4.-Intergrowth of bis athinite in galena. Crossed nicols. P. S.
235. X 54.
PLATE 10, FIG. 1.--Galena vein (white) in cassiterite (C). Quartz (Q). P. S. 235,
and stannite (S) vein along cleavage in galena. P. S. 166.
X 235. FIG. 3.--Zoning in cassiterite (thin section). Mic. slide 24690. X 24. Fig. 4.-Tourmaline (T) replaced by muscovite (M) and chlorite (Ch)
in cassiterite (C) and olfram (W). Mic, slide 24681. X 24.
PLATE 11, Fio. 1.-Cassiterite replaced by interstitial felspar. Mic. slide 24680.
x 24. Fig. 2.-Cassiterite replaced and veined by beryl. Mic. slide 24687.
X 24. PLATE 12, Fig. 1. -Mica veins (M) in cassiterite (C). Note how one of them
stops at the border of scheelite (S) after wolfram, and its
x 24. FIG. 4.–Platy carbonate. Mic. slide 24696.