The mineralogical evaluation of gold deposits is a dynamic process, which starts during the prospecting activities and continue during the exploration and development of the mine. In the opening of any property two distinct objects should be kept in mind, the development of known ore-bodies, and the exploration for further deposits near the project. Where high gold grades has been found, it should be followed with a view to the development of ore and also to gain knowledge in regard to its occurrence, each ore-shoot being opened individually without the testing of theories. In general, the time to go slowest in following an ore-body is when it shows signs of giving out; every detail should then be studied and recorded before the rock has been dirtied by further blasting, as frequently small stringers or gouge-filled fissures will be found to lead from one gold deposit to another.
In the exploration for further ore-shoots, however, the major features of the distribution of ore should be borne in mind, and the general trend of the deposit should be cross-cut .thoroughly. Exploration should be confined at first to cross-cutting the known gold horizon or zone of mineralization, perhaps a sedimentary bed favorable to the deposition of ore, a zone of crushing or brecciation, or a certain horizon known to be the most favorable for secondary enrichments. Within these broad zones or horizons considered favorable to the existence of ore, the minor features, such as stringers of ore, low-grade gold, sheeted zones and so forth, may be considered important. Upon cross-cutting the apparent trend of the deposit at the horizon considered most favorable to the existence of ore, the most promising stringer or seam should be followed along its strike for an appropriate distance, where another cross-cut is in order.
It has been noted that the plan to be followed demands detailed study in each individual case, and a state of mind is necessary that is receptive of new impressions as the work progresses. Preliminary exploration is frequently entrusted to a practical miner, when as a matter of fact, such work constitutes perhaps the most important field of the trained geologist. 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.
Where a tabular orebody or gold vein is faulted, the problem is likely to be simpler, as the recovery of any portion of the vein beyond the fault will lead to the discovery of the continuation of the ore shoot. Where the enclosing rock is composed of a series of strata a knowledge of the succession of the strata is of the greatest assistance in the calculation. The cases are rare where complete data for the calculation of the total displacement are available, as, for instance, where a fault cuts a dike that itself cuts a series of known strata, or an older fault, or any combination of intersections that permits the recognition of a point rather than of a plane on both sides of the fault.