The chemical geology of gold deposits play an important role to understand and detect the characteristics of some gold zones. Quartz veins have generally been produced by the slow deposition from aqueous solutions of silica on the surfaces of the inclosing fissures. From the general parallelism with its walls of the planes of any fragments of the inclosing rock which may have become imbedded in a vein, it is to be inferred that they were mechanically removed by the growth of the several layers to which they adhered, and that a subsequent deposition of quartz took place between them and the rock from which they became detached. In this way were introduced the masses of rock.
It is important to indicate that the formation of quartz veins was due to hydro-thermal agencies, of which evidences are still to be found in the hot springs and some metallic veins met in some parts. From the variable temperatures at which the vacuities in their fluid cavities become filled, it may be concluded that they are the result of an intermittent action, and that the fissures were sometimes traversed by currents of hot water, whilst at others, they gave off aqueous vapor or gaseous exhalations. This is precisely found at Steamboat Springs in Nevada where the formation of a vein has not finished yet and from which currents of boiling water are often poured forth, whilst at other times the fissures gave off currents of steam and heated gases only. That gold may be deposited from the same solutions which rise to the formation of the inclosing quartz, appears evident from the presence of that metal in pyrite enclosed in siliceous incrustations, as well as from the fact of large quantities of gold having been formed in the interior of the stems of trees, which in deep diggings were often converted in pyrite.
It is possible that the constant presence of pyrite in auriferous veins, and when so occurring its invariably containing a certain amount of gold, indicates the possibility of this sulphide being in some way necessarily connected with the solvent by which the precious metals was held in solutions. It has been shown that finely divided gold is soluble in iron chloride and more sparingly in the sulphate of this metal. It is also known that pyrite sometimes result from the action of reducing agents. If, therefore, iron sulphate in solution containing gold must be transformed by the action of a reducing agent into pyrite and the gold at the same time being reduced to the metallic state. This would be found enclosed in the resulting crystals of that mineral. The silica and other substances forming the cementing material of the ancient auriferous river-beds have been slowly deposited at low temperatures.