Many gold deposits are characterized by fracturing caused by the folding and crumpling upwards of strata into mountains, accompanied by lateral tangential folding and compression of rocks, and takes its maximum intensity when the rock breaks and a fault or slip is the results, with its attendant fault fissure. This relieves the strain for a while, but the shock produces a general breaking up of the adjacent zone into many parallel and smaller faults and cross faults, besides a general shattering of the intermediate zones to the faults. In this way, a zone shattered in that condition forms a mineral belt or mining region when the cracks and scars have been healed and filled by minerals.
When a fault descends to a very great depth, it may tap the molten rock reservoir supposed to lie beneath great mountain ranges and the molten lava rushes upward through the weak line of the fissure, fills it and after cooling becomes a dyke instead of a gold vein. In the case of intrusive rocks may or not reach the surface and overflow it in a lava sheet. If they don’t, they find relief by intruding themselves laterally between the layers of stratified rocks whose leaves or bedding planes may have been partially opened like the leaves of a crumpled book by previous folding. In this circumstance, the dyke or intrusive sheet may, if it be mineralized, will be close to a mineralized gold vein. When the ore is found on one or both sides of such a sheet, in the line of separation and weakness between it and the adjacent strata, it may be mineralized by a substitution process. In this way, a dyke and intrusive sheet itself as well as at its contact with other rocks, gold and silver are located.
If the dyke or sheet is decomposed, clayey and rusty, it may contain free gold disseminated through it, which at a depth which may passes into the auriferous pyrite from which the free gold came. In this case the gold ore is not free-milling. It has been detected that little stringers or veins of quartz if observed in an eruptive rock, it must be carefully examined as most of the richest gold zone. It is worth observing that the dyke may be only valuable as a gold deposit as the decomposition lasts. With depth, the pyrite of the undecomposed lower portion of the dyke may be found too poor in gold. According to some geologist, most dykes and intrusive sheets when mineralized content pyrite and they are usually more gold bearing than silver bearing. The contact deposits adjacent to a volcanic rock may have aided in their deposition by steam from the molten mass or by heated waters or steam ascending with it.