Eruptive rocks are formed from volcanic activity are frequent accompaniments of gold bearing veins, but they are not absolutely essential in the siliceous rocks or sandstone reefs. There are cases in which the precious metals in the deposits undoubtedly appear to have been derived from them. But outside of these instances there are others where the outburst of eruptive rocks have simply produced the necessary conditions of heat and fractured rocks, furnishing waters of the requisite temperature and providing ample facilities for their percolation through materials so crushed as to be easily soluble.
Without the aid of eruptive rocks, by the enormous pressure involved in the complicated folding or folded strata, so it would appear that any cause which will produce heated fluids and fractured rocks is sufficient to furnish the conditions for gold vein filling, the material for forming the deposit being drawn from any one or all of the rocks indiscriminately through which the water has been circulating, whether above, below or alongside, near to or at considerable distances from the point to the final deposit; and that this place of final deposit is largely determined by the character of the rock through which the water may be circulating at the time it is compelled to part with some portion of its mineral burden, and not by the character of the cause which produced the fissures, or the nature of the associated eruptive rocks, unless the ore is deposited in that rock itself.
For example, in the Potsdam series of rocks in South Dakota, these ores were in some sections almost exclusively gold-bearing; in others, they carried partly gold and partly silver, and again on other zones the silver predominated. Also, it is possible that the porphyry at bald Mountain brought mainly gold; at Ruby basin only a few miles distant, gold and silver in nearly equal quantities. Basically, gold predominates in the quartzites, but gives place to silver as the calcareous portions forming the upper parts.
It is important to indicate that all waters will not be charged with the same mineral or the same water always with the same mineral, as their contents vary with the nature of the rocks from which they draw their supply, and consequently they can only deposit what they have in solution for the time being, and may carry that for long distances for want of a suitable precipitating agent. They may have only one or many minerals in solution, but at any rate it is possible to detect the occurrence of gold in the joints of porphyry, of silver in the joints of quartzite and the variability of the ore when vein passes from one series of rocks to another (e.g. gold-bearing arsenical pyrite in the underlying granite).