HYDROTHERMAL DEPOSITS The part played by water in the formation of ore deposits is of the first importance, for on account of its mobility and solvent action, which is enormously increased at high temperatures and pressures, it is the universal vehicle for the transference of mineral matter.
In the hydrothermal circulation the mineral-bearing solutions are in part directly connected with igneous magmas, namely in so far as they are formed by the cooling of aqueous vapours given off at a late stage in their consolidation. A considerable proportion, however, are no doubt surface waters (i.e., meteoric in origin), which, descending along fractures, derive their thermal properties from the secular heat of the earth, or by coming in contact with, or into the near neighbourhood of, igneous intrusions.
Of the water that falls as rain one part is the run-off, that is to say, water that reaches the rivers without having penetrated the surface. A second part is returned into the atmosphere by evapo ration; while a third part disappears into the soil and under-lying rocks by percolation. Under the influence of gravity the water of percolation penetrates into the earth's crust, first through the superficial weathered and disintegrated layers and then through the more solid rocks by way of the fractures of jointing and fault ing, the planes of bedding or the pore-spaces between their con stituent minerals.
It is a matter of common observation that at a variable depth below the surface, there is a connected body of water, which per manently fills all openings. The surface of this sea of water is called the level of the ground-water, the permanent water level, or the water-table; it is the upper level of the belt of saturation.
Above the permanent water level is the zone of percolation in which the openings in the rocks are only intermittently filled with water. This water is in active movement and air is present. The thickness of the zone of percolation varies greatly. At sea-level and at, or near, streams or lakes the level of the ground-water reaches the surface; in average areas it is from io to i,000ft.
below the surface; in high-lying districts with small rainfall it may be ioo to 3ooft. below the surface, and in elevated desert regions as much as r ,000 to 2,000ft. below the surface.
The belt of saturation is divisible into two portions. The upper portion, or that part which has a means of horizontal escape and discharge, is the zone of discharge. In general, it lies between the water-table and sea-level. In it the circulation is vigorous, as is evidenced by the vast volume of water discharged by springs.
The bottom part of the belt of saturation is the static zone. In it the waters are practically stagnant, or at best move very slowly. Near the bottom limit of the static zone the water gradually diminishes until the dry zone is reached. The boundary between these two zones is quite irregular, descending to great depth along fractures and rising high in solid ground. The lower levels of most deep mines are in the dry zone, in some places reached at a depth of not more than i,000 to i,sooft. below the surface.
The waters circulating in the zone of percolation are cold, con tain free oxygen and are acid with dissolved carbon dioxide. Such waters have been termed vadose (Lat. vadosus, shallow) by Pogepn. They have a strong oxidizing effect on sulphides, pyrites for instance being decomposed with formation of oxide of iron and sulphuric acid. The net effect of the vadose circulation, there fore, is destruction, and the zone of percolation is practically coin cident with the zone of weathering, although the latter overlaps the upper portion of the zone of discharge.
In the zone of discharge the waters, as they descend, lose their oxygen and carbon dioxide and deposit material brought from the zone of weathering. When they have access to open channels they penetrate deeper and deeper, absorb heat and thus become powerful solvents of the metallic sulphides and tellurides.