Gold ore shoots have some characteristics that define their structure and mineralization. A prerequisite of gold mineralization is the existence of a fissure or fissures to give access to the mineralizing solutions; the mineralizing effect of these solutions is probably controlled to a large degree by the character and changes of these circulation channels. One extreme of fracturing may be considered the solid, unbroken rock, through which solutions work their way very slowly. The other extreme is a crushed or ground-up condition where the fineness of comminution is sufficient to produce a gouge or clay-like mass, which is also relatively impervious to the passage of solutions. Between these two extremes lie the favorable conditions for gold deposition.
It has been established that gold is deposited from solutions either in open spaces or by replacement of the fissured rock. In replacement deposits the degree of mineralization is frequently proportional to the degree of brecciation, as replacement proceeds most rapidly where it has the greatest area of rock surfaces on which to work, until the point is reached where the fineness of comminution commences to retard the passage of the solutions. There is, however, a point where brecciation ceases to be an advantage where it is so extensive that the solutions are dissipated through a large mass, and the resulting mineralization is too scattered to be an economic gold deposit.
It sometimes happens that two or more systems of fracturing or brecciation have effected the rocks; here the time of the fissuring becomes important. Fissures heal and become closed in time, and so cease to afford circulation channels; the most favorable time of fissuring or brecciation may be considered that just before the introduction of the solutions, perhaps due to the same igneous disturbances to which the solutions owe their origin. Post-mineral fracturing is, of course, of no primary effect, but may be of the greatest importance in working secondary changes. In the prospection of a mineralized gold area, therefore, the distribution and extent of the brecciated masses, and the degree and relative time of brecciation should be studied; mineralization is frequently co-extensive with or confined to the brecciated areas. The brecciated structure of an non-mineralized structure is sometimes not visible on fresh fractures, but becomes plain on weathering, or upon being wetted.
The degree of brecciation varies greatly in different rocks; massive or rigid rocks yield breccias where soft or plastic rocks yield to strain without brecciation. The continuity of a brecciated zone from one rock into another is, therefore, uncertain. The degree and character of brecciation is of the greatest importance in the study of the surface exposures where the existence of disseminated gold is suspected. In the gold deposits formed by the filling of open spaces the distribution of these open spaces is usually the controlling factor in the gold deposition, and the segregation of metals into gold ore shoots bears the closest relation to the gold vein structure. The pinching and swelling of veins in fissures of small displacement; where this condition can be shown to exist, the pinching out of a gold ore shoot may be reasonably expected to be followed by at least a widening of the vein, which may or may not contain ore. A gold vein that is the result of both replacement and of the filling of open spaces is likely to vary in mineral content according to which of the two processes took place during the gold deposition, the open spaces perhaps contributing the gold ore shoots while the replacement of the walls where the fissure was tight is represented by intervals of barren or low gold grade filling. In a crustified vein, if it is determined that the valuable mineral was among the first formed, and relatively near the walls, the gold ore is likely to be persistent as compared with a mineral that was among the last to form and so confined to such spaces as were kept open the longest.