CHAPTER IV.

THE GEOLOGICAL DISTRIBUTION OF GOLD AND SILVER.

Introductory Remarks.

In the preceding pages the geological occurrence and geographical distribution of gold and silver deposits have been given in considerable detail. However, a discussion of the geological distribution of the precious metals with respect to the time of their origin in geological periods involves to a certain extent the consideration of their occurrence and distribution and, therefore, in order to simplify matters, a table has been prepared (see table of Geological Occurrence of Gold and Silver), by means of which the importance of the various periods as precious metal producers and their relation to the different districts can more readily be seen.

After an exhaustive study of the mineral deposits of the West, Clarence King deduced the following interesting generalization:

"The Pacific Coast ranges upon the west carry quicksilver, tin, and chromic iron. The next belt is that of the Sierra Nevada and Oregon Cascades, which, upon their west slope, bear two zones, a foot-hill chain of copper mines, and a middle line of gold deposits. These gold veins, and the resultant placer mines extend far into Alaska, characterized by the occurrence of gold in quartz, by a small amount of that metal which is entangled in iron sulphurets, and by occupying splits in the upturned metamorphic strata of the Jurassic age. Lying to the east of this zone, along the east base of the Sierras, and stretching southward into Mexico, is a chain of silver mines, containing comparatively little base metal, and frequently included in volcanic rocks. Through Middle Mexico, Arizona, Middle Nevada, and Central Idaho is another line of silver mines, mineralized with complicated association of the base metals, and more often occurring in older rocks. Through New Mexico, Utah, and Western Montana lies another zone of argentiferous galena lodes. To the east, again, the New Mexico, Colorado, Wyoming, and Montana gold belt is an extremely well-defined and continuous chain of deposits." 1

1 T. A. I. M. E., vol. 1, p. 33.

That this classification is correct in every detail or complete is open to question, but is valuable as an outline of the general occurrence, of the mineral deposits, and can be further elaborated by adding to it on the east, the slightly argentiferous zone of galena of the Mississippi Valley, and the gold belt of the Southern and Eastern states, besides which are the zones of iron ores, both magnetites and hematites, and the extensive coal measures.

As early as 1859 Sir Roderick Murchison said, in the third edition of "Siluria," "My chief article of belief has now proved to be truethat is, that the rocks which are most auriferous are of Silurian age.' And again, "The Paleozoic accumulations - particularly the Lower Silurian are the chief source whence gold has been or is derived."1 Nor was this wholly without corroborative evidence as gold had been discovered in considerable quantities in both the Ural Mountains and in Australia; however, it was the former which led him to propound the theory in the first place.

In 1864 the work of the Geological Survey of California, conducted by Whitney, disproved once for all the theory that the Silurian was pre-eminently the gold-bearing formation, and since that time similar evidence from other districts has been accumulating. According to Rickard: "The following tabulated statement shows at a glance that the chief gold fields of the world are scattered through the entire sequence of geological strata, from the Archean to the Tertiary:

GEOLOGICAL DISTRIBUTION OF GOLD AS ILLUSTRATED BY THE PRINCIPAL MINING DISTRICTS OF THE WORLD.

"The Lower Silurian of the Urals is now scarcely worth mentioning, the gold production of that region having dwindled to insignificance. Since Murchison's day the geographical center of Russia's gold production has shifted steadily eastward. It was once at Ekaterinburg, in the Urals; it passed to the Yenesei, and then to the Olekma. To-day the chief gold region is that drained by the Amoor and its tributaries. The gold fields of Victoria, in Australia, also refuse, now, to be indentified any longer with Murchison's blunder, several of the best districts in that colony having been developed in the Upper Silurian, as distinguished from the prevailing Lower Silurian of the first discovered gold-veins at Ballarat and Bendigo. Newer mining regions, scattered all over the globe, afford testimony which denies the supposed relation between gold-deposits and the age of the rocks enclosing them. Although numerous rich districts occur in igneous formations of the Tertiary period, no important gold field of the present day is indentified with sedimentary rocks later than the Cretaceous; nevertheless to make the testimony complete, it can be stated that a conglomerate (the San Miguel formation of the Telluride district) of undoubted Tertiary age, covering an extensive area in southwestern Colorado contains goldveins, which have been mined at a profit. If eruptive rocks be included, we have the testimony of J. E. Spurr that the gold veins of Monte Cristo, in the State of Washington, occur in andesite and tonalite of Pleistocene or Quaternary age,1 and at Steamboat Springs, Nevada, gold has been detected in cracks traversing the sinter around a thermal vent; this rock can therefore be labeled Recent. The Laurentian granitoid gneiss of western Ontario is traversed by important gold-bearing lodes. Therefore, the record of the rocks, in regard to their association with the occurrence of gold, is unbroken throughout the main divisions of geological time."

"Deposits of gold ore occur in rocks of every age, and in rock of every kind. The metal was deposited later than the encasing rock and it is likely that since it was so deposited it has been subject to constant solution and precipitation, by which it has been redistributed and concentrated. The first deposition, the time when it was brought from below the zone of rock fracture to the place of precipitation, was associated with a thermal activity following upon regional movements and volcanic eruptions; that time of first for

'The Ore Deposits of Monte Cristo, Washington, 1902, U. S. G. S. 22 Ann. Rept., Pt. 2, p. 864.

2 Min. and Sci. Press, Vol. 93, pp. 477-478.

mation may have been late or early, in the Cambrian or the Cretaceous of geological history. But once so deposited, it became at once the sport of the chemical waters that find a passage both from the surface and from the deep. These may have effected no noteworthy redistribution of the gold along the rock-fractures where the ore lies.1

"Gold bearing veins cluster in certain localities. A critical examination will reveal the fact that many vein-systems are massed about the contacts of intrusive masses, which consolidated far below the original surface of the earth at the time of the igneous activity, and which have been exposed by subsequent erosion. Most commonly, perhaps, these intrusive rocks are diorite, monzonite, quartz-monzonite, granodiorite, or their porphyries, more rarely typical granites. Under favorable conditions it can often be proved, and in other cases established with probability, that the upper part of the vein has been removed by the same erosion which laid bare the intruded rock masses. In other words the top of the vein has been removed, the root remains. . . . The age of these veins must, in general, be considerable, for the great erosion involved has usually required a long time-interval.

"Another large class of vein-systems cut the recent or comparatively recent lavas, which cover the surface of the older, eroded rocks in the form of successive volcanic flows. Frequently the age of these lavas may be established with accuracy.' When it is possible to determine approximately the position of the earth's surface at the time of an eruption or flow of lava, any vein cutting the formations may be said to have a certain age, assuming that the vein-deposition was coincident with the lava flow, and that the portion of the vein near the original surface may be considered as the true apex of the vein.

By far the larger number of gold-bearing districts of the United States occur in or contiguous to igneous rock either in the form of dikes or intruded masses. Further, probably the majority of the intrusions did not reach the surface at the time of the volcanic flow, but formed bodies of consolidated, granular or coarsely porphyritic rocks, which were laid bare by subsequent erosion.

There is no doubt but that gold-bearing fissure-veins have been formed throughout the geological history of the continent. 'Cambrian conglomerates bear witness to pre-Cambrian gold-veins, and 1 Min. and Sci. Press, Vol. 93, p. 480.

2 T. A. I. M. E., Vol. 33, p. 794, 1903.

very recent thermal deposits at Steamboat Springs, Nevada (according to Becker), and at Boulder, Montana (according to Weed), prove that gold is deposited by thermal waters to-day. But the process has evidently not been a continuous one. Cambrian, Silurian, Devonian and Carboniferous gold-deposits are not definitely known to exist in North America. Continuous sedimentation, absence of dynamic movements and relatively slight igneous activity characterized these periods." 1

"The great eruptions of the Cordilleran belt of North America began during the Triassic period of the Mesozoic age, and igneous activity has continued almost without interruption from that date to recent time. Each eruption has probably been accompanied by more or less extensive deposition of gold in fractures near the igneous focus. On the Pacific coast the eruptions began at an earlier date than in the region of the Rocky Mountains; and, likewise, many of the gold-deposits of the Pacific coast antedate those of the Rocky Mountains. In the latter province the igneous rocks began to break out at the close of the Cretaceous period, and have continued at least up to the beginning of the Pleistocene. Certain periods of deposition, however, stand out prominently, and we may, with good reason separate the distinctly Cretaceous or late Mesozoic gold-belt of the Sierra Nevada and the Pacific coast in general from the Tertiary, mostly post-Miocene, veins so extensively developed in Mexico, Nevada and Colorado. The former are genetically connected with great intrusions of granitic and dioritic rocks, the latter with big flows of surface-lavas which erosion has not, as yet, removed. But both in the Great Basin and in the Rocky Mountains there are also many deposits of late Cretaceous or early Tertiary age genetically connected with intrusions of granitic rocks and very commonly, porphyries. In very many cases the age of these deposits is doubtful. If erosion has been exceptionally active in the particular district in which they occur, they may well, though occurring in connection with deep-seated intrusions, be of Tertiary age. To this class of doubtful age belong, for instance, many of the gold-veins of Montana. Miocene and later igneous rocks are often lacking in this region, so that an accurate determination of age becomes very difficult.

"Still another complication to be borne in mind consists in possible, though probably rarely occurring, reopening of veins and superimposition of deposits of two or several epochs. All this being

1 T. A. I. M. E. Vol. 33, p. 795, 1903.