The presence and the form of some fissures help to identify the existence of gold and its distribution in the deposit. A line of weakness once established is likely to continue as such through long periods of time, the original gouge seam or zone of attrition material being preserved and offering a plane of easy relief to subsequent stresses; where a fissure has been completely healed by gold mineralization, the resulting vein of brittle quartz or other minerals is likely to be less resistant to fracture than the tougher enclosing rock. It is common, therefore, to find evidence of post-mineral movement along gold veins.
It has been determined that the post-mineral movement is likely to crush the ore and to mix it with waste to such an extent that its value is materially reduced and also to render the country rock loose and likely to cave during mining operations; not infrequently, post-mineral movement so complicates the structure as to render exploration difficult. A favorable effect of post-mineral assuring, either along or across a vein or deposit, is in permitting the access of surface waters, which are thereby given opportunity to form secondary enrichments, is other important consideration. A usual effect of post-mineral movement along a vein is the formation of false walls, or fissures carrying gouge, that cut the vein obliquely; these mask the part of the vein behind them, and also hide the junctions of branch veins.
False walls are frequently the cause of losing a gold vein in development, the tendency being to follow their well-defined fissures and to leave the vein to one side. Where such movements are known to have taken place, frequent cross-cutting may be necessary to make sure that the vein has been exposed. Gouge-filled fissures appear to be unfavorable to ore deposition, probably chiefly because the pasty gouge does not permit the ready passage of mineralizing solutions, and also because the continual movement along such fissures tends to close any minute channels that are permitted to form. Such fissures once formed probably persist as lines of weakness and movement for long periods, and the occurrence of a gouge-filled fissure in connection with an ore-deposit is no proof that it is of later formation than the ore.
It is frequently seen in gold mines that such fissures act as efficient dams, the passage of solutions through them being as difficult as the circulation of solutions along them; they may be considered, therefore, as having frequently determined the limits of gold deposition through the impounding of mineralizing solutions. The fact that an orebody or gold vein is cut off by a gouge-filled fissure is no proof that a fault has displaced the orebody or vein, the continuation of which may never have existed beyond the fissure. It is often difficult to prove whether such a fissure is of later formation, and has faulted a vein, or whether it is older than the vein, which ends upon reaching it. The proof of a fault is, of course, the continuation of the vein beyond it; this, however, is the object of the search. That such a fissure is a post-mineral fault may be indicated by the faulting of associated beds or dikes, or by a drag or trail of ore through the fault filling.
If such a gouge-filled fissure is older than and limits the gold zone, this may be indicated in a filled fissure, by a closing of lines against the fault, equivalent bands being connected, or by a change in the vein filling upon approaching the fault, or by a branching or widening of the vein upon approaching the fault. A gouge-filled fissure older than the mineralization may through recent movement exhibit the characteristics of a fault that has displaced the gold vein, though in reality limiting it.