The goal of the prospector is to find gold, and one of the keys to that is finding the minerals that normally accompany gold ores. Gold occurs very widely diffused in nature, chiefly in the free state, but invariably alloyed with the minerals silver or copper, and occasionally with bismuth, mercury, and other metals. By far the greater part of gold occurs as the native metal. It enters into only one series of compounds, the tellurides, and these minerals, while at times forming rich ore deposits, as at Cripple Creek, Colorado, are found in only a few districts. The structure of the rocky envelope of the earth has been in many localities been so carefully deciphered that the mining engineer can unravel the geology of an ore deposit with a success to which the mines of Leadville, Bendigo and Gympie bear ungrudging testimony. When, however, he endeavors to ascertain the causes which have determined the presence of gold at one spot and its absence at another he hesitates, and, saturated with the brutal experience of widely separated regions, he confesses that it is a phenomenon yet to be fully explained.
The working of gold mines in different countries has yielded an accumulation of scattered evidence which needs only scant examination to emphasize how incomplete and contradictory it is. The absence of accurate knowledge on this subject has encouraged the growth of attractive fallacies, in the combating of which the whole matter often comes up for informal discussion wherever mining men congregate. During recent journeying over the goldfields of West Australia, I found that many an old fallacy had sprung again into vigorous life amid the congenial atmosphere of a community over which the windy breath of a gold rush boom had but lately passed.
It may seem a thankless task to oppose those plausible theories, which become rampant only when facts are scarce, yet I am convinced that in the search after truth the first step must be to sift the little that I really do know from the much that we think we know; the first is science, the second is popular knowledge. The old industry of mining was formerly guided by the venerable rule of thumb, and would forever have remained but a blind sort of groping in the dark had it not obtained the willing aid of the younger science of geology. The immediate problem to which my most recent experience has called attention may be summarized thus: In examining a lode only incompletely developed and in a new country, what is the evidence on which a correct estimate of the prospective value of the mine may best be based? The two most common answers would be a plain denial of any man's ability to see further into the ground than the point of his pick, and against this obvious surrender would come the reply that the best indication would be found in the presence of particular minerals in association with the gold. It is this question of indicative minerals which I purpose discussing. The experience of certain mining districts has gone to prove that gold is notably accompanied by particular minerals, and this to such an extent that they are considered to assure the richness of an ore in which the gold itself cannot visibly be discerned. These "indicative minerals," as they may be termed, are not the same in every locality. A few examples, such as have come within my own experience, may be quoted. Every mining engineer can add to the list.
In Boulder County, Colorado, roscoelite (a vanadium mica) is closely associated with the tellurides of gold, calaverite and sylvanite. This fact is rendered of additional interest because the same rare mineral has been recognized by me in the telluride ores of Kalgoorlie, in West Australia. At San Andreas, in California, uranium ochre (the yellow oxide of uranium) is found to distinguish the pockets of specimen gold ore to such an extent as to serve as a guide in prospecting. In several parts of Arizona, in Yuma, Yavapai and Final counties, especially the last, vanadinite and descloizite (both vanadates of lead) characterize ores rich in gold and silver. Wulfenite (the molybdate of lead) often accompanies the vanadium minerals, and has been noticed in the ores of two celebrated lodes, the Comstock in Nevada and the Vulture in Arizona. The association of gold with these lead ores is notable, because the particular minerals mentioned are all of great beauty and delicacy. The same may be said of crocoite (the chromate of lead), which characterizes several gold veins in the North Coolgardie goldfield, especially at Menzies. The more common sulphide, galena, is also an accessory mineral in the richest mines at Menzies, and accompanies coarse native gold. At Niagara, Pinyalline, and Wagiemoola, three widely separated localities in West Australia, I found gold closely associated with tourmaline in the form of acicular crystals in contact with coarse gold, and also in a condition of minute diffusion forming dark blotches in the white quartz. The gold ores of the Mysore District of India carry tourmaline.
Instances might be multiplied, but it would be to no purpose. Those already cited will serve as a sufficient text for the discussion of the subject. Such occurrences as these would seem at first sight to afford a much needed aid to the explorer. It is unfortunately easy to prove that as evidence they, and any number more of them, are of very uncertain value in a new mining territory, because they are contradicted by similar testimony of a negative kind, which compels us to regard them as mere coincidences. Take the case of sphalerite. One locality was quoted where this mineral is a sure sign of a generous amount of gold and silver in the ore. The Morgan Mine, in Wales, affords, I understand, corroborative testimony. At the East Murchison Mine, in West Australia, this mineral has been found to be an almost unerring guide in separating rich from poor ore. But in Arizona it is a common experience that the impoverishment of lodes in depth, when it does occur, is concomitant with an increasing percentage of zinc. Other regions echo this unpleasant fact. Broken Hill knows it.
Similarly, there is the beautiful mineral fluorite or fluorspar. The association of fluorite with the tellurides of gold was early recognized in both the Boulder and Cripple Creek districts of Colorado, and the purple tint imparted by this mineral was speedily bailed as a distinction peculiar to rich veins. Later experience, notably in Park County, has proved that poor ores are favored with fluorite no less than the bonanzas. I have mentioned a locality where rhodochrosite is esteemed a favorable mineral. But while it is thus characteristic of rich ore at Rico, in Colorado it is a negligible factor in certain lodes at Butte City, Montana. Again, consider calcite. When it is seen amid the gold bearing quartz of California it is recognized with regret, because it so often means a falling off in values, while at Kalgoorlie the same mineral characterizes ores rich in calaverite (the telluride of gold), and at Rhuda in Romania, a very valuable gold vein has been worked whose matrix was essentially calcite.
We are all familiar with mines in which iron pyrites is so intimately associated with the gold that a lessening of the one means a diminution in the other, but there are also cases where an excessive percentage of pyrites coincides with impoverishment. Moreover, there are lodes in which the coarse cubes of pyrites are less favorable to the presence of gold than the finely crystalline variety, but there are also those in which the reverse is true. Much in the same way, there used to be an idea that coarse cubical galena was less silver-bearing than the fine grained kind, but this as a generalization has long since been exploded. Thus, therefore, it requires but little sifting of this sort of evidence to emphasize its contradictory character. The rich lodes of the same district frequently differ widely in their mineralization. Poor veins often carry the ores considered characteristic of the rich ones. The neglect of the former causes this fact to be overlooked. "When the field of comparison is enlarged from mines to whole districts the divergence of evidence becomes tenfold emphasized.
Continue on To:
Minerals Which
Accompany Gold Ores, Part II
