The formation of secondary alterations in gold deposits is controlled by the channels that permit surface waters to circulate through the rocks and ores. In massive gold deposits where heavy rainfall and steep slopes give rise to erosion more rapid than percolation and alteration, these processes are of no effect, and primary ores and minerals appear at the surface. Where post-mineral fracturing permits surface waters to pass through the gold ore, however, a thorough rearrangement of the gold bearing minerals and values may be accomplished, even in districts of heavy rainfall, steep slopes, and rapid erosion. Solution is a characteristic feature of the action of surface waters upon the upper parts of ore-bodies, which commonly become more and more porous under their attack and through seepage a supply of water is thus maintained under hydrostatic pressure for the continuance of the process, even in districts of scanty rainfall.
It has been noted that conditions of heavy or slight rainfall, with attendant rapid or slow erosion, may be balanced or counteracted by the presence of good or poor circulation channels, and gold enrichment may form under either set of conditions. Under conditions of heavy rainfall, sulphide enrichments are likely to form without important zones of oxidation; under desert conditions, the oxidized zone is likely to be deep and important though characteristically irregular, often merging irregularly into the sulphide enrichment. A close relation exists between the depth of secondary alteration and the ground water level. Oxidation commonly reaches to or somewhat below the water level, and enriched sulphides commonly occur at or just below this horizon.
While not a uniform relation, the water level commonly follows the topography. The ground water may be considered to fill a complicated but connected system of porous rock masses, fissures, joints, and other open spaces. In most instances the water level reflects the surface in a modified form; being deepest below the middle slopes, and becoming shallow again in the foothills, where the water reaches the surface in springs. In the Southwestern States, the valleys are commonly filled with debris; the apparent foothills, therefore, are often the middle slopes as referred to the outlet of ground water, and oxidation may proceed to important depths beneath them. The ground-water level depends upon the permeability of the rocks traversed as well as upon the height of outlet. The water level should be horizontal beneath a hill of loose conglomerate, for example, but might easily reach the surface in a tight rock that does not permit seepage.
In investigating the probable depth of the water level in a gold zone, it should be borne in mind that springs may exist at levels considerably above the general ground-water level, being the mouths of accidental drainage channels that permit a more easy escape of waters than does continued seepage. A number of springs at approximately the same elevation are commonly a sign of the true water level. Where there has been much faulting, a gold zone may be divided into fault blocks, each of which constitutes a separate hydrostatic basin, the water being impounded to a certain extent by the impervious gouge along the fault planes. The groundwater level is likely to be deep in arid regions, and shallow in regions of heavy rainfall, and the same relation holds for the depths to which oxidation may be expected to penetrate. Where the water level is permanent, well-defined, and not too deep, oxidation and enrichment tend to form relatively regular zones; where the water level is irregular or practically lacking, oxidation and enrichment of gold are likely to be quite irregular. The water level in any gold zone is likely to change with time, and the level at some past epoch may have been the determining factor in the location of enrichments as now found.