Page:The Journal of geology (IA journalofgeology21894univers).pdf/312

Another physical effect of surface influences on ore deposits is seen in certain forms of brecciation due to physical or chemical causes, such as expansion by hydration, etc. Such brecciation, however, has usually occurred in the country rock before the concentration of the ore deposit; in fact its existence, by offering favorable conditions for deposition, has often been the cause of the formation of the ore deposit in a given place. Though brecciation, therefore, is very important as a factor in the concentration of ore deposits, it does not belong, to any large extent, in a discussion of the surface alteration of ore deposits after their formation, and, therefore, it will not be treated further in this paper.

Depth of alteration.—Having thus discussed briefly the means by which superficial alteration in ore deposits is produced, and the results of this alteration, the next feature to be taken up is the depth to which it extends. As already shown, the alteration is primarily one of oxidation and generally of hydration; and, though either may occur without the other, they both very often occur together. When surface waters percolate into the rock, their influence is more active near the surface, because they carry large quantities of oxygen, and because the oxygen of the air itself also has some influence. As they sink deeper, the effect of the oxygen of the air becomes less active, and the oxygen dissolved in the water is consumed in oxidizing various materials which it meets on the way, until finally most of the oxygen is lost and active oxidation ceases. Theoretically, this oxidizing action may extend down as far as, and sometimes below, the level of the drainage of the surrounding country, which is called also the zone of permanent saturation. Above that level, there is a constant circulation of water from the surface downwards, thus affording means of active oxidation; but when the water reaches that level, not only has most of the oxygen contained in solution generally been used up, but also the circulation of the water is much more sluggish, so that oxidation is less active.^[1]