Bench Placers:
In some regions thick
gold-bearing gravel deposits are left high above the modern flow of water.
This happens by the downward cutting of the streams due to elevation of the
land, resulting in deeply trenched canyons, leaving the uneroded remnants of
the stream gravels as benches along the valley slopes. Cases of this sort
are found on the western slope of the Klamath and Trinity systems in
northern California, the Sierra Nevada in California, on Anvil Creek in the
Nome district of Alaska, and in the Klondike district of the Yukon.
The gold from the destroyed portions of the old channels, together with more set free from the quartz veins during the erosion, accumulated in the modern canons. Along their slopes benches remain in places, indicating transient accumulations of gravel during the process of canon cutting. Somewhat similar conditions exist in some parts of Alaska. Near Nome on the ridges surrounding Anvil Creek are "high gravels" 600 to 700 feet above the present rivers. These gravels, some of which are rich, are the remnants of an old, now almost wholly eroded system of drainage. In the Klondike also high gravels occur a few hundred feet above the present creeks, the most conspicuous instance being gravel deposit termed the "White channel".
Buried Deep Lead Placers:
Not only are placer deposits being formed today, but
they have been formed where conditions were favorable throughout geologic
history since pre-Cambrian time. Becker mentions buried placers in the
southern Appalachian region and in the Bald Mountain region, Wyoming, and
states that the Triassic and Cretaceous in California contain placers. In
the Black Hills of South Dakota, which are famous for their buried placers,
the Cambrian basal conglomerate and sandstone rest unconformably above the
pre-Cambrian schist series that contains the lode of the Homestake gold
mine, and in the conglomerate are found rounded grains of gold, evidently
derived from the pre-Cambrian deposits. Associated beds contain Cambrian
marine fossils. The deposits are believed to have been formed along an
ancient shore. When they were worked out the gold was found to be
concentrated near the bottom of the gravel beds, as in most placers now in
process of formation. Becker mentions many other gold bearing conglomerates
in America, Australia, and South Africa. The presence of heavy residual
minerals, the accumulation of the gold in the lower portions of the gravel
beds, and the rounding of the particles of gold are fairly constant
characteristics of buried placers. In some instances stream placers may have
become buried under other barren gravels, or lava flows. (as in Victoria and
some California deposits.) The gold in such instances has to be recovered by
underground methods.
Where land sediments or lavas cover stream beds, the stream placers will be preserved. The most productive buried placers of North America are the Tertiary gravels of California, which, according to Lindgren have yielded about $300,000,000. This region in early Tertiary time was less rugged than it is today, and on its gentle surface gold accumulated in streams from the weathered lodes near by. The gravel beds were covered with rhyolite tuffs, andesitic breccia, and basalt, in places as much as 1,500 feet deep. Later the country was elevated, and deep canyons were sunk in its surface. The new drainage lines did not follow the former ones, and the ancient gravel beds where crossed by later canyons are now exposed, some of them hundreds of feet above present streams. As the canyons are widened, gold in the ancient gravels is reconcentrated in the present gulches, where it mingles with material that is accumulated from the weathered outcrops of the lodes.
In the two principal regions containing buried placers, viz., California and Victoria, the ordinary covering of alluvium has been capped by thick flows of basaltic lava, and to this capping the ancient gravels of California, at least, largely owe their preservation. Greatly depressed placers are, from their depth, and hence from the great bodies of water contained in them and in the superincumbent strata, generally economically inaccessible; it is when they have, in the course of great earth-movements, been elevated above the permanent water-level of the country, that their gold becomes readily available to mining. The buried placers of California have been elevated to an average height above sea-level of 2,600 feet along the western flanks of the Sierra Nevada, and have shared in the late Tertiary uplift of that great range. In Victoria similar buried gravels are commonly termed "deep leads." Akin to those due to ancient fluviatile action are ancient lacustrine auriferous gravels, as those of the Blue Spur, New Zealand.
The gold of deep leads, as might be expected from analogy with the deposits of modern rivers, is not evenly distributed throughout the lead, either vertically or longitudinally. The deepest part of the lead is termed the "gutter," and normally pursues a sinuous course. The gutter is often the richest part of the lead, but the best runs of gold may nevertheless, as in existing streams, be contained in beaches high above the gutter. Buried " benches" are known along the course of deep leads, which also show all the branching into tributary streams displayed by modern placers. The boulders of the deep lead gravels of Victoria are, on the average, less than 6 inches in diameter. They may, however, range up to 3 feet, and, very rarely, to 12 feet. The general sequence of strata in a deep lead is gravel, sand, and clay, with often a carbonaceous layer overlying the clay; this sequence may be repeated several times. The normal color of the chief Californian leads is blue, from the presence of ferrous compounds. The color changes to rusty brown on exposed surfaces. Cementation by ferruginous oxides is common in deep leads, the resultant indurated mass being termed cement. Notable placer concentrations occur in many deep leads below the junction of two streams and also below the intersection with auriferous zones or reefs.
The principal deep leads of California lie on the western Sierra Nevada ranges, in Yuba, Sierra, Placer, and Nevada counties, in the region drained by the Feather, Yuba, and Bear Rivers. The general age of the Californian tertiary placers is Neocene (Miocene and Pliocene); that of the Victorian deep gravels is late Pliocene. It will be evident that during regional depression and subsequent elevation, the drainage system of a country may be materially modified. The modification has been notable in California, where the existing streams have intersected the ancient buried channels almost at right angles, and have cut great gorges in them, the bed of the present stream being occasionally 1,500 feet below that of the ancient channel. In this way also great lengths of the course of the ancient rivers have been obliterated, and the adjacent country so eroded that the course of the ancient channel may now be traced only at intervals high up on the flanks of a mountain range. It may even follow a ridge, a feature of not uncommon occurrence when a basaltic lava flow has filled an ancient valley. In this case the lava has resisted denudation while the softer bed-rock of the valley sides has been worn away, leaving a lava-capped ridge, as shown in the accompanying section. In the same way hill-tops of cemented or lava-topped gravel may be formed.
In Victoria, the general direction of river drainage has not changed since the deposition of the deep leads, and the channels of modern streams are therefore either superimposed on the deep leads or are parallel to them. No great amount of elevation has taken place in Victoria, and the country has rather been subjected to a general north and south tilting, depressing the lower (northern) portions of the channels and raising the upper (southern) portions. The tilting has occasionally proceeded so far as locally to change the direction of fall of the bed-rock. On account of the absence of marked regional elevation, the ancient placers of Victoria fail to show the high-level "benches" (river terraces) so characteristic of the Californian deposits. The driving of tunnels or drifts, often called "bed-rock" tunnels, to reach and develop auriferous deposits occurring in channels having rocky sides, is one of the initial operations in drift-mining. The channel sides called "rim-rock" consist of the same material as the bed-rock or floor of the channel, and it is through the rims and often even below the level of the channels, that the tunnels must be driven in order to properly develop the contained deposits of gravel. However, before driving the tunnel it is considered advisable to sink one or more shafts to bed-rock in order that the level or altitude of its entrance may be determined, otherwise there is danger of its being too high or too low. If too low a connecting passage must be made with its inner extremity and the channel above.
Marine placers. Gold is very durable and not easily dissolved or weathered. Where fine gold is transported downstream to the ocean, marine or beach placers may form. These are formed by the sorting action of the waves along coasts where auriferous gravels or sands are exposed. They are known in along the coast of California, Washington and Oregon, as well as Canada, but the best examples are those of Cape Nome, Alaska. At these locations, the fine gold is concentrated by the surf and the oblique shore currents into thin, temporary pay streaks on the sandy beach.
The best-known examples of marine placers are confined almost entirely to the shores of the Pacific Ocean, but they also occur on the coasts of Nova Scotia, Ireland, and Portugal, which shows that the foregoing restriction to the Pacific is purely coincidental. They perhaps attain their greatest development on the west coast of the South Island of New Zealand, on the beaches of Oregon, and below Port Arthur on the Liau-Tung Peninsula in China. The richest beach sands known appear to have been those of the Gold Bluff, Klamath County, Oregon, where narrow beaches at the foot of overhanging bluffs serve as concentrating floors for the auriferous sands thrown within reach of the ocean waves by cliff falls. Concentration on these beaches takes place only when the surf strikes the shore line at an acute angle; when the surf beats at right angles gravel -and sand are cast up. The process of concentration is simple: the strong surf casts up gold, gravel, heavy minerals, and sand, while the weaker and less rapidly flowing undertow removes only the sand and lighter stones. With the gold is generally associated much black sand. On the auriferous marine beaches of New Zealand pebbles or stones are absent, and the gold is contained entirely in "black sand." The distribution of beach sands, both in time and place, is erratic, the requisite degree of concentration being generally attained only after heavy storms. Once formed, beach deposits, when being worked for their gold content, are immediately removed beyond the reach of subsequent storms, since these, if coming from a slightly different direction, destroy the previously formed deposit. South-west gales are in New Zealand considered the most favourable for the production of rich layers of black sand.
The gold of these deposits is invariably flaky; that of Oregon is often bi-concave with well-defined rims, due possibly to rolling edgewise when moving up and down the beach. The deposits of Oregon are continued north to Washington and south to California. Other notable beach placers are those of Carelmapu and Punta Arenas, Chili; the shores of the Sea of Okhotsk; at Unga Island, Nome, and Cape Yagtag, Alaska; and on the northern coast of New South Wales. Those of Cape Yagtag are remarkable in that they carry garnet without the ordinary associates of gold in beach sands, viz., magnetite and ilmenite. In Oregon and New South Wales and at Nome, ancient auriferous beach sands are worked above present high water level. At Nome elevated beaches have been mined, where there are two old beach lines 37 and 70 feet above the present level of the ocean. In Santa Cruz County, California, a similar elevated beach was mined for some time. Gold-bearing beach sand occurs) all along the Pacific coast from San Diego to Alaska, and in many other parts of the world. It is not always easy to classify every placer deposit into one of the types noted above, There are intermediate types may be found which belong to both of two groups. Hillside placers occur on hill slopes and do not occupy any well-defined channels. They grade on the one hand into placers of residual origin and on the other into placers of the stream or gulch type. Creek and gulch placers occur both in material that has been assorted once and in that which has passed through several cycles of erosion.
Continue on to:
Types of Placers – Part
I: Residual Placers
Types of Placers – Part
II: Stream Placers
Return To:
All About Placer Gold
Deposits
