By hydrothermal metamorphism is meant the alteration of rocks by hot ascending solutions; it is distinctly a metasomatic process and is a frequent accompaniment of ore deposition.
The walls of veins are commonly fractured or strained by the stresses that produced their fissures, especially the hanging walls, and in this way ready access is afforded to the solutions that accomplish their alteration. The alteration is usually intense immediately along the veins, but is likely to diminish rapidly in degree with distance from them. It appears that the walls of a vein tend to keep within them the heavy bases, and gangue minerals, but permit the passage of the depleted solutions that effect the rock alteration. Not infrequently, disseminated sulphides are found in the altered wall rocks, although these minerals commonly carry low values in gold and silver as compared with the sulphides deposited within the veins. Pyrite is the most widely disseminated sulphide in wall rocks, and is frequently the result there of the alteration of the rock minerals themselves, in which case it does not carry values in the precious metals. Zincblende and galena are more rarely found as disseminations.
The results of hydrothermal metamorphism are frequently valuable guides in the search for ore-deposits, especially where they are confined to the vicinity of the veins or stocks. In many cases, however, and especially where mineralization and attendant primary rock alteration have taken place at slight depth below the surface, the porosity and shattered condition of the wall rocks has permitted a wide distribution of the altering waters, and the altered areas are likely to be large, and so to lose their value as aids in the search for ore-bodies.
In many parts of the arid regions of the western United States and Mexico there are large areas of brilliant red hills, the rocks of which have suffered hydrothermal metamorphism; the iron mineralization of these red hills is likely to be more apparent than real, the surface being stained a deep red, yellow or brown, but upon fresh fractures the rock is seen to carry little iron, present in most cases as pyrite, resulting from the alteration of the rock minerals.
The alteration of wall rocks is likely to be less along deepseated veins than in deposits of shallower origin; it is also likely to reach a less development where it accompanies deposits resulting from the filling of open spaces than in connection with replacement deposits.
The most common effect of intense alteration immediately along veins is sericitization and the introduction of pyrite, which alteration is likely to grade into propylitization with greater distance from the veins, the latter process being the less intense and more widespread; in basic rocks, propylitization is likely to persist close up to the veins, there to give way perhaps to sericitization. While the degree and extent of alteration by one set of solutions is likely to vary markedly in different rocks, it not infrequently happens that the final products of alteration and recrystallization are similar, and it becomes a matter of difficulty to distinguish one rock from another. The chief chemical effects of hydrothermal metamorphism are an increase in potash with attendant loss in soda, silicification, and the introduction of sulphides, chiefly pyrite.
The effects of hydrothermal metamorphism may usually be distinguished readily from the effects of hydro-metamorphism, or the alteration accomplished by surface waters: the latter alteration is rather more likely to be uniform, and is essentially a process of oxidation, hydration, and solution, while the former is characterized by sericitization and silicification. Kaolin is the chief guide in making this distinction, being a typically surface product, often formed from sericite as well as directly from feldspars through the action of sulphuric acid solutions set free by the oxidation of pyritic sulphides.