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A Guide to the Mother Lode Country (1948) by C. Frank Brockman


THE ORIGIN OF THE GOLD

What was the origin of the riches that characterized the region known as the Mother Lode Country? How and why was it concentrated in this relatively limited area? By what processes was it distributed?

The answers to these and similar questions lie rooted in an understanding of the geological story of this region; a story that goes back through time to a period over 200,000,000 years ago when this entire region lay beneath the sea. Through long periods various types of sediments—mud, silt, and sand— were gradually deposited in layer-fashion upon the ocean floor from nearby land masses. These layers were interspersed with beds of limestone. Eventually these sedimentary rocks accumulated to a thickness of thousands of feet.

Upon at least two occasions this long process of deposition was interrupted by the folding of these stratified layers. By such means the original submarine rocks were slowly folded, twisted, and uplifted above the sea to form a series of roughly parallel, northwestward-trending mountain ranges, occupying the approximate position of the present Sierra Nevada. The second uplift took place some 125,000,000 to 150,000,000 years ago and was associated with the presence of hot magma or molten rock which welled up from deep within the earth. This magma or molten rock was prevented from breaking through the surface by the great thickness of the older, overlying surface materials which they invaded and penetrated. Thus imprisoned by the “roof” of older rocks the magma cooled very slowly and eventually formed a granite “core” beneath the surface of these mountains. The tremenduous pressures and heat generated by the process of forming these mountains, as well as the presence of great volumes of hot, slowly cooling magma, modified the character of the original overlying rocks. For instance shales, originally mud on the ocean floor, became slates and limestone was transformed into marble.

With the passage of time these pre-Sierra mountains were largely destroyed by various forces of erosion. Eventually the area was characterized by ridges of only moderate height and the granite core lay exposed over wide areas. During this period the principal drainage pattern in the area was more or less at right angles to what it is today; the principal streams flowed from north to south instead of from east to west as is the case at present. Finally, about 60,000,000 years ago, a great block of the earth’s surface was slowly tilted to the west. The culmination of this uplift brought the Sierra Nevada into being. It is a great block range composed largely of granite, from 60 to 80 miles wide, about 420 miles long, and with a crest averaging about 13,000 ft. above the sea. Its gradual westward slope is sharply contrasted with the abrupt, precipitous eastern escarpment. As the tilting of this granite block progressed the drainage pattern of the area was transformed from a general north-south pattern to the east-west character of the present time. All of these events were to have a profound effect, not only on the formation of gold in this region but on its eventual distribution and, thus, on the manner by which it was reclaimed.

The formation of metals, and gold is no exception, is intimately associated with rocks that are derived from the cooling of molten magma deep within the earth. Such magma is the source of metals. But the gold in the Mother Lode Country was not found in the granites—which were originally molten magmas, and which were eventually exposed to view following the destruction of the older, overlying strata. It was in the foothills, bordering the exposed granite core, where the roots of these older mountains still remained, that the precious metal was recovered. What then is the connection between these geological processes?

While some metals form directly by concentration as the magma cools, others, particularly gold, are carried off from the molten rock by highly heated gases and vapors. These gases penetrate cavities, fissures, and pores of adjoining and overlying rocks, cooling gradually as they extend themselves into the surrounding mass. As temperatures are lowered the mineral substances contained are deposited by various means along the sides of cracks, fissures, and cavities. One might roughly compare this process to the deposition of salt as sea water evaporates. Most of the world’s gold, and this is particularly true of that in the Mother Lode Country, was formed by low temperature deposits at relatively shallow depths. Thus, roughly as outlined, the gold of the Mother Lode was deposited in older, overlying rocks adjoining the granite core as veins, or pockets, in cracks, fissures, pores, and cavities, or through the replacement of certain material such as limestone, which was readily soluble in the mineralized solutions. Such deposits were reclaimed by lode mining, by following a vein of gold-bearing quartz by means of shafts or tunnels which were sunk into the earth. It was a process that required technical knowledge, great skill, ample equipment, and sizable capital. Consequently it did not come into prominence here until placer mining was well developed. But unlike the placer deposits which were eventually worked out, lode mining continues to the present day in many sections of the Mother Lode Country. The free gold is reclaimed from the ores by pulverizing the gold-bearing rocks and separating the metal from the associated valueless materials by various technical processes.

But much of the gold obtained from the Mother Lode Country in the early days came from placer deposits. It was panned from the streams or washed from the surface gravels. Where gold-bearing gravels were not adjacent to water flumes were constructed or the gravels were carried laboriously to the nearest stream for washing. The history of this region is replete with interesting and often amusing incidents resulting from the destruction of buildings, and even entire sections of towns, as the miners undermined them in their search for gold-bearing gravels.

Placer deposits were responsible for the quick development of numerous boom camps. As the deposit was worked out they quickly declined. In consequence many famous old-time camps, dependent wholly upon rich placer deposits, are now no longer in existence. The origin of such placer deposits resulted from the gradual disintegration—by natural means—of the original gold-bearing rocks which once overlaid the captured magma. As the original surface layers which formed the pre-Sierra terrain were slowly destroyed the more valuable minerals were separated by nature and transported by gravity or water to other nearby sections. The disintegrating materials moved slowly down the slopes to the nearest stream. Here the water carried the lighter, valueless materials away at a rapid rate, but the heavier free gold readily sank to the stream bed and was carried for only short distances. This process went on for long periods of time, eventually resulting in the concentration of free gold at places where the currents were not sufficiently strong to transport it—such as in eddies of other quiet position of the streams. It was in such places—the placer deposits—where free gold derived in that fashion was washed from the gravels by sluicing, by the use of the pan, the rocker, or the “long tom.” Unlike lode mining it required little previous mining experience, only relatively crude equipment, and little if any capital.

CAPTION
[click to enlarge]

Old metamorphic rocks in
foothill region. It was in
these strata that the gold
was found.

Anderson Photo



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