All veins, whether bearing gold or not, have a common the origin, the result of movements in the upper portions of the earth’s crust, producing cracks or crevices, which have been subsequently enlarged and filled with some material different in its physical and chemical properties and appearance from the rocks in which the fracture has taken place. These movements, which may be the result of earthquakes or the readjustment of the pressure caused by the thinning of one portion of the crust by denudation and thickening of another portion by the deposition upon it of all the material brought down from the mountains by the action of rivers, are intimately connected with the process of mountain building, and consequently shoe themselves most strikingly in mountain regions, in which also volcanic agencies have played a most important part.
In this way, it is important to make reference to fissures, faults, contact veins and contact veins as important aspects of the origin and consequence of gold veins. Veins filled with porphyry or similar rocks or other kind are usually dikes. The term gold vein is more properly restricted to those which are more or less filled with free gold and/or gold bearing minerals, whether in workable quantities or otherwise. Owing to their different origin, the strike of true veins is more likely to have a uniform direction than in beds, as the inclosing rocks have not usually been subjected to so much movement since the formation of the fissure as has been suffered by the strata containing the beds; or if movement has taken place, the weakness of the fissure has directed the motion into that plane and simply caused a reopening of the fissure, and not infrequently a refilling of it with a different class of gold-bearing minerals.
Red arrows indicate the direction of the gold vein