There is popular idea known as Igneous Theory that explains the filling of gold veins. According to this theory all gold veins have been filled from below by the material being forced into them while in a molten condition, this would involve intense heat. While there can be no question that the dikes have been filled in this manner, a multitude of facts compel to look for some other explanation for the majority of metallic veins. That the intrusion of dikes has been accompanied with heat is abundantly proved by the changes which have been effected in the rocks through which they pass, or into and between which they have been squeezed as sheets.
In some locations, the country rock is close to the dike and has been altered, being spotted with white feldspar crystals, which gradually become less conspicuous as the vicinity of the dike is left, until they fade out entirely in the plain-colored, unaltered general country rock. The same lava flow contains large fragments of the country rock which have been torn from the sides of the fissure during its formation, and altered in exactly the same way, just as when clay is converted into brick by the action of the heat. Form these considerations, it is important to look for similar changes in the walls of gold veins. Basically, it is possible to find changes in the walls produced by the action of the heat, but they are such as result from the action of hot water and steam, and partially for molten matter. The later when once injected will gradually cool off and when once cold will produce no further change, while the hot water may continue to circulate for ages, so long as the source of heat remains unchanged, and produce changes much more extensive and more widely disseminated than the action of injected lavas.
For example, at the old Head Center Mine, Tombstone, Arizona, crystals of gold were found planted on the surface of horn silver (silver chloride); yet when melted together gold and silver form an alloy with the greatest facility, and the affinity of the metals for each other is so great that native gold always contains more or less silver. Other example is found in the old mines at Batopilas, Mexico, where native silver occurred in veins of calcspar, which had an extraordinary persistence, frequently dwindling down to a mere seam, often less than 1 in. wide for long distances, and then opening out to a width of several feet. In places probably 90% of the ore is crystallized native silver, a large proportion of the remainder being crystallized ruby silver and crystallized black silver sulphide, the latter often in branching flakes like moss formed in the joints of the rock alongside the veins. Near Yankee Hill, Butte County, California, beautiful specimens of crystallized gold were found in the joints of the porphyries, usually in thin flakes, taking the shape of combs or fern fronds. In this case there was no indication of vein matter. At West Point, Calaveras County, California, gold occurred in the solids granite. At Drytown, Amador County, California, gold occurred in limestone. Other important example is Fiddletown, in Eldorado County, gold was found in the pyrite, which was in the slates; the crystals were often of considerable size, chiefly cubes, and the gold was seen on the smooth faces of the crystals.