Gold prospectors and geologists agree that the right estimation of gold ores reserves play an important role in the development of a gold project. Most engineers will agree fairly well in regard to the quantities of developed gold ore and of probable ore in any mine, but great differences are to be expected in estimates of possible ore, which, of necessity, are forced predictions of the future based upon insufficient data. A reasonable method will help to purchase mines on the basis of the ore reserves plus a royalty for all the ore afterward discovered.
The starting-point in a consideration of the probable depth to which an ore-shoot will persist is the determination whether the gold ore is of primary or of secondary origin; if primary, there is no genetic reason why the shoots should not continue to great depths; if secondary, little or no tonnage may be allowed below the lowest workings, unless the mineralogical character of the ore is such that it seems reasonably certain that the workings are still in the upper part of the zone of enrichment. Secondary ore-shoots, in general, are greater in horizontal than in vertical extent; primary ore-shoots, on the contrary, are commonly greater in vertical than in horizontal extent; the relation between the horizontal and vertical dimensions of exposed ore-shoots should be considered in making estimates of the depth to which they probably continue.
If the ore exposed is the result of geological conditions unlikely to be duplicated in depth, the end of the mine is probably in sight. It is certainly true that the tendency of primary as well as of secondary ore-shoots is to pinch out or to become low in grade with increasing depth; many primary ores, however, probably continue to depths below the limit of mining. A factor of safety for the engineer's personal reputation has no place in a mining report, except in the margin of profit that deems necessary to make the enterprise attractive.
It has been noted that the relation between the width of a gold vein and the stoping width necessary to extract the ore, should receive careful attention in the examination of narrow veins. The minimum stoping width for machine drills is usually from 1.2 to 1.6 m; the new air-hammer drills require less. For hand drilling, 0.80 m., or sometimes even less, is required. The sloping width and the amount of waste broken with the ore varies with the relative hardness of the ore and wall rocks, and also according to the firmness of the hanging wall, which, if loose, will contribute waste by caving. In some gold veins the waste may be shot down first and the ore broken down clean afterward. In the consideration of narrow gold veins the average amount and grade of ore must be determined as it stands, and the values, if any, that are carried by the wall rock; then, upon the assumption of a stoping width, the average value of the broken ore may be calculated.