A layer of fine material in a fault is known as gouge and usually lines one or both walls of a gold vein between the rock and the gangue or vein, like a sandwich. It is derived from the elements of the close rock altered and decomposed by water and in some circumstances by the friction of the walls of the fissure against another, or against the gold vein during the process of faulting, which is most the time noted by a smoothed, slickensided, polished or grooved surface. Some gold prospectors have noted that that gouge contains some rich decomposed mineral such silver sulphosalts and sometimes occur in the heart of a vein, especially if the vein was reopened by movements of the strata.
A gouge may be formed by the decomposition of the feldspar in an adjacent porphyry zone. In some deposits, gouge is derived from the feldspar of the granite. Considering the mineralogy of the gold deposit, a gouge is useful in defining the limit of the gold vein between walls, thus preventing unprofitable exploration into other parts. It is also a guide for following down a vein when mineralization and gangue may be wanting or obscure. Also, both walls are not always clearly defined by slickensided surfaces, by gouge or other mark, and so at times the vein is lost.
False walls caused by movements in the adjacent strata by joints tend to mislead. It is not unusual for a fissure vein to have one clearly defines wall, the other, if it exists, being obscured or changed by mineral solutions. It has been noted that two cracks or fissures occur parallel to each other and the intervening rock was altered and mineralized into a vein. It is believed that many gold veins were formed in this way. In cases where percolating waters were charged with the proper metals and veinstone compounds and the required chemical and physical conditions, the rocks lying between cracks were altered into ore. It has been noted that a gold faulted region is often marked by a step like out-line, each step representing the fallen or risen side of a fault zone. Also, these fault lines must be carefully observed for gold presence, especially if the fault line is occupied by a dyke or quartz vein.