Smeltery of the Balaklala Consolidated Copper Co.,
Coram, California.
[XII
THE QUESTIONS OF DEPTH AND
GRADES OF ORE.]
The prevailing belief of a few years ago that ore bodies always improve with depth has been discredited. Not a single mining geologist will longer maintain such a notion. The evidence of many thousands of mines has refuted this older belief and it has been proven that quite the opposite view is the correct one concerning changes of value with depth. Values, instead of getting better, do actually, in the majority of cases, grow poorer as depth is gained.
President C. R. Van Hise, of the University of Wisconsin, was among the early expounders of the newer theories to account for this fact. The writer heard him state, years ago, before a scientific gathering (which, at that time, was not quite ready to agree with him), that if he were given his choice, he would much prefer to own the upper thousand feet of the earth's crust than all the rest of the globe. In this remark, he was referring only to mineral values, of course.
This belief that the best values are to be found not far from the surface has since become popular, for it is based upon proven facts. It is not claimed that values are never mined below an elevation that is a thousand feet from the surface. There are many mines, and great ones, too, that are operating at depths greatly exceeding this distance; but in these same mines there will be found valid reasons for not applying the general statement to their particular cases. For instance, the great copper mines of the Keweenaw Peninsula are productive at depths of a mile or more from the surface; but we believe that here the ore must have been originally deposited at, or near, the surface, that it was then overlain with rock strata; and subsequently steeply tilted by earth movements which carried some of the ore bodies down to the depths where they are now found.
The "reefs" or bankets of the Rand are so termed because these ore bodies were undoubtedly ancient coast beaches or sea placers. The gravel, sand, and gold particles were cemented together into a conglomerate, then covered with many later sedimentaries, and finally the continent of Africa was so raised or altered in some manner as to bring these gold deposits into their present inland and tilted positions.
In veins or lodes, it is not supposed that ore-making minerals could have been precipitated from solutions travelling either upward or downward and obeying chemical laws if the depth were sufficient to furnish great temperature or high rock and hydrostatic pressures. Therefore minerals which were deposited from aqueous solutions rising from depths, for example, must have retained their dissolved condition until they ascended to horizons in which both pressure and temperature were low enough to permit the precipitation and crystallization that create ores. Contrarily, descending solutions must have given off their contents before reaching the deep zones of heat and pressure, or not at all.
It is a quite common phenomenon to observe that the richest gold ore in a mine is found close to the surface, if not actually at "grass roots." The explanation is simple. The gold, being the most stable of the aggregate of minerals composing the original ore, has the better resisted the corrosive attacks of atmospheric agencies and has remained nearly intact, while its associated minerals have been dissolved or altered and carried away. The same amount of gold remaining with a diminished quantity of the worthless, non-metallic minerals—the "gangue"—inevitably renders the ore richer per unit of weight (such as a ton), although per unit of volume the value remains constant, or nearly so, so far as the gold is concerned.