Volume 16, pages 93-96, 1931

     One general mineral association which may be referred to the deeper environment just described¹ is the granite pegmatite. Intrusive bodies of this type have compositions and structures which have led to the conception that they have been derived from a residuum, or from successive residua, of cooling granitic magma. These residua become increasingly hydrous, and enriched in a variety of compounds which, because of high solubility in the liquid, or slight original concentration, have remained in solution until cooled by intrusion as detailed above. Reopening of the fissures after periods of cooling allows successive invasions of liquids, each in general presumably cooler than the preceding, and thus in a state of saturation ready to deposit new compounds, or to dissolve or react with and replace existing minerals. Both Schaller² and Hess³ have presented much evidence for a common replacement sequence-potash, soda, lithia.

     Minerals rich in the elements barium, strontium, nickel, cobalt and chromium are almost absent from acid pegmatites, but these elements are known to be present in solid solution in common rock-forming minerals. Therefore their concentration in a residual liquid would be partly prevented.

     Copper, zinc and lead are not absent from pegmatites, but pegmatites are not the home of their ores. They are not definitely known as solid solution constituents of common rock-forming minerals. When we have learned, for the oxygen, sulfur and arsenic compounds of the several elements enumerated, something about the relative solubilities in dry and in highly hydrous rock liquids, then we shall be able to picture more definitely the relative importance of early and late segregation of the elements in a cooling deep-seated magma.

     Now we may consider that the temperature gradient from the surface is sufficiently steep to produce in the liquid of the rock a vapor pressure higher than the minimum environmental pressure. This is the condition required for the presence of a gas phase. If the temperature gradient is only slightly higher than in the condition previously described the gas phase will develop at intermediate depths, while if the gradient is steep⁴ hot gas will fill the pores of the rocks near or at. the surface.

     Consider a finely brecciated vertical zone of rock of intermediate composition with pores at first filled with water. At a depth of a few thousand feet the mass has been subjected from below to a rapidly increasing temperature, so that the joint blocks are being dissolved to form a hydrous liquid highly concentrated in the constituents of the rock. The vapor pressure of the solution is decreased by concentration and increased by temperature. At some intermediate depth the vapor pressure of the solution exceeds the environmental pressure, and vesicles form in the liquid of the narrow joints. But because of the postulated rising temperature, the rock in the vicinity of the vesicles is getting hotter, therefore the vesicles become extended over a vertical range⁵. At the ends of the range, liquid, gas, and crystals exist in contact, but at intermediate levels, liquid tends to disappear and leave the rock pores filled only with gas. The tendency, however, is only partly realized, for in a mass of rock in which several crystal phases are present in irregular distribution, and in contact with films of liquid, there is a range of temperature at a given pressure at which gas and liquid phases may persist.

     When vesicles first form, the liquid just above them has the same temperature and composition as that just below them, but as vesicles elongate vertically the liquid above them has both lower temperature, and lower concentration of rock constituents. As the thickness of the vesicular zone increases, the difference between the upper and the lower liquids increases. Pressure is no longer maintained at substantial equality at the top and the bottom of this gas cushion. The higher temperature at the bottom causes streaming of the gas through pores toward the top, where absorption of the gas by the upper liquid takes place.

     Transfer of heat by the streaming gas is much faster than through the intervening blocks of rock. The shapes and sizes of the blocks and pores will thus affect the distribution of gas and liquid in pores and crevices. The interiors of blocks will be enough cooler at first to maintain liquid films into which volatiles from the gas in the crevices will be absorbed. But such films will gradually be displaced by gas as the whole environment gets hotter, and as the volatiles of the liquid evaporate. Thus as the gas cushion grows thicker the rock into which the gas is penetrating is subject to reactions with gas and liquid, some volatiles are absorbed and held in new crystals, others already present in the rock may be given up to the liquid or gas.

     Increased volume due to thickening of the gas cushion forces the upper liquid toward the surface through the more open rock crevices. The flow includes liquid which is derived from near the cushion, and which contains volatiles from the lower liquid, or released from the rocks as indicated above. Such a process of leaching, recrystallization and introduction of volatiles may well be considered in attempts to account for the loss of lime, magnesia and soda from, and the introduction of sulfur and copper into, the porphyries of the disseminated copper deposits.

 NOTES