In matters like these the experience of each mining engineer is a personal equation by which every theory must be eventually tested. Out of the whole sad wreck of glittering generalizations on this particular subject, only one or two have survived my own particular trial of them, and even they, I fear, await the destructive testimony which may at any moment be found in the development of new mining regions. When gold occurs in pyrrhotite ores it has been as yet invariably proved to be in immediate association with a small, often overlooked, percentage of chalcopyrite. The testimony of Montana, Colorado, and British Columbia is at one in this deduction. Again, while many veins carrying coarse gold encased in white quartz persist to great depths, and in this respect Bendigo does especially set at naught the dictum of American experience, nevertheless I have not known gold quartz to be persistent when wholly barren of the sulphides of the baser metals, while, on the other hand, I do remember innumerable examples of ore quite destitute of pyrites, galena, and sphalerite which proved particularly short lived. I might venture one other.
There is no better indirect evidence of the size of a body of gold ore than uniformity in the distribution of the gold. A patchy occurrence is less likely than homogeneity to indicate continuity or size; samples which vary between narrow limits are more encouraging than those which swing between wide extremes of richness and poverty.
It is with a desire to avoid a merely destructive attitude that I have dared to offer these three observations. While therefore the evidence in support of the value of the supposed indicative minerals is, as we have seen, of a very contradictory nature, it also has another feature which must not be overlooked. At its best, the aid of these minerals would be delusive, because, even if it proved the invariable association of gold with particular minerals, it could not predicate the actual amount of that gold. That a lode should carry gold is quite insufficient to the mining engineer, whose operations for its profitable extraction require that it should be there in paying quantity. This is the difference between science and business. The union of the two creates an industry. One might discuss in a learned and entertaining manner, the suggestiveness of the association of gold ore with such a compound as the silicate of boron and aluminum (tourmaline), because it indicates a deep seated origin. The presence in notable quantity of the fluoride of calcium (fluorite) might prompt speculations of a vapourous and corroding kind. But the proof of gold having either a profound or a gaseous origin, were it attained by the geologist or chemist, would not permit the mining engineer to infer that the gold persists in paying quantity to any particular depth. An ore which carries 2 dwt. per ton in a region where the conditions are such as to require the equivalent of 15 dwt. to be expended in the extraction and reduction of a ton, is to all practical intent as valueless as one which is wholly barren.
The question of indicative minerals bears many points of resemblance to that of the plants which have been observed to distinguish the soil enriched by particular ores. The Viola Intea was supposed to be peculiar to the soil covering the zinc deposits of Westphalia, and was subsequently recognized growing on the outcrop of the zinc ores of the Horn Silver Mine in Utah. It became known as the "zinc plant." Similarly there is a so-called "lead plant," the Amorpha canescens, which characterizes the lead deposits of Michigan, Wisconsin, and Illinois. These plants are local varieties rather than a distinct species, their color being affected by their absorption of the particular metallic ingredient in the soil. Their occurrence has long ceased to have anything more than academic interest. If then we cannot accept the belief that certain minerals are indicative of the plentiful occurrence of gold, what shall be said for the idea that they give an assurance of persistence in depth?
Every one has read serious statements to the effect that this or that mine was of undoubted value because its ores contained particular minerals, the presence of which promised that the vein would increase in richness in depth. In young mining regions such ideas find a fertile soil. In West Australia the finding of tellurides such as calaverite and sylvanite in an ore is now generally considered to permit of the inference that the lode will go down to a great depth. The early discoveries of extraordinary pockets of native gold in veins of white quartz, such as made the Londonderry famous, proved sporadic and bunchy to a distressing degree. When, therefore, the telluride ores of Kalgoorlie became recognized, and were found to occur in bodies of magnificent size and very satisfactory persistence, the conclusion was jumped at that if tellurides were only present in an ore, continuity in depth became thereby guaranteed.
In these matters a very small portion of ascertained truth is swamped amid a mass of supposition quite unworthy of the name of theory. A theory embodies an underlying principle of universal application. The idea which to-day dominates the mining of Western Australia is a vain imagining, delusive as a promoter's dream. Those who are acquainted with the history of the mining of tellurides need not be told how ill founded is the statement that these particular minerals characterize lodes of peculiar permanence. The combinations of tellurium with gold and silver have proved of notable commercial importance in three mining regions, namely, Romania, Colorado, and West Australia. There are other districts where the mining of them is an incident in the working of ordinary gold ores. Such is the case in certain localities in California, South Dakota, and the North Island of New Zealand. In none of these, however, have they been indicative of any special persistence in the richness of the veins, and their occurrence has been merely an added obstacle to the successful extraction of the gold.
It was in 1802 that Klaproth discovered the presence of a new element, tellurium, in the ores of Zalathna, and so led to the recognition of a large number of the compounds of that metal with gold and silver. Zalathua, Nagyag, and Verospatak were gold mining centers when Romania was the Roman province of Dacia and was ruled by Trajan. The very complex ores of these districts have been a puzzle to the metallurgists of many generations. The veins penetrate young volcanic rocks (andesites) and Tertiary limestones, sandstones, and conglomerates. This very ancient mining country exhibits to-day a very forceful example of impoverishment in depth, while the refractory character of the ores has tried the resources of the smelting establishments of Schemnitz, Zalathna, and Freiberg as well as numerous districts in Nevada.
In Nevada, veins in young volcanic rocks generally do not persist deeply, but do not generally contain tellurides. Nevada ores contain gangue minerals such as quartz, calcite, some rhodochrosite, fluorite, alunite and barite. Associated gangue minerals in hot springs related deposits can include cinnabar, sulfur, stibnite and pyrite. Nevada ore minerals include acanthite (argentite), electrum, gold, stephanite and polybasite. Though the Comstock ores do not contain tellurides, the silver ores of the Comstock Lode were mined as deep as 5000 feet down dip on the lode. The lode went deep, even though there was a lack of tellurides in this ore hosted in young volcanic rocks.In Colorado the tellurides were recognized as early as 1874 in Boulder County, more especially at the mines of Magnolia, Salina, and Sunshine. An experience of nearly twenty-five years has proved to the miners of that county that these ores occur there in comparatively small bodies of remarkable richness but of very irregular and uncertain behavior. In two other districts of the same State, namely, in Hinsdale County and amid the La Plata mountains, valuable mines, carrying ores rich in the tellurides of gold and silver, have been worked during the past ten years, but their record corroborates that of Boulder. There remains the more important goldfield of Cripple Creek, where veins penetrating a remarkable complex of volcanic rocks have proved so persistently rich to a depth now approaching one thousand feet that they have obliterated the reputation which this class of ore won in the three older parts of the same State.
Continue on To:
Minerals Which
Accompany Gold Ores, Part III
