Placers are deposited by the action of water or wind. Aeolian or wind-deposited placers are generally not of commercial tenor, at least on any large scale, and will not receive further attention in this section.
Water-deposited placers include most placers mined commercially in the world today. Engineering data relates that the carrying power of water varies as the sixth power of its velocity. Whether carrying capacity varies as the fifth or sixth power, it will be appreciated that minor changes in velocity change the carrying capacity of a stream dramatically.
In planning land developments today, consideration is given to the effects of 25-year storms and 100-year storms, i.e., the severest storm to occur in those periods. When one thinks in terms of geological time, there are also 1,000-year storms and 10,000-year storms. Some of us have been privileged to see the results of very severe storms in our lifetimes, when brooks that one stepped over one day became raging torrents transporting 27-t (30-st) boulders the next. Severe storms result in major earth movements. Subsequent stream flows cause reconcentration of storm-deposited materials.
It is assumed that readers are familiar with the criteria that streams above a certain velocity will erode stream banks and bottoms, that streams at a certain velocity will transport suspended material, and that streams below a certain velocity will deposit transported material. Fine materials are, of course, deposited at lower velocities than coarse materials.
In accordance with the laws of hindered settling, coarse particles (gravel and coarse sand) will settle in general relationship to their specific gravities. Particle size and shape have effects but are subordinate to specific gravity. Moderately sized particles (sand) will settle similarly. Fine particles (slimes) have greater surface area per unit of mass than coarse particles, and the gravity component is limited by surface effects.
Heavy mineral concentrations may be expected to occur where streams lose velocity. Streams lose velocity when their gradients become flatter, where they change course, or deepen. More specifically, where a stream emerges from a canyon, on the inside of bends in a watercourse, or in deeper pools or “pot holes.” These are the traditional places that placer miners look for concentrations.
Over the tens or hundreds of thousands or more years during which a placer is in formation, stream courses change and sites of heavy mineral deposition change. Glass models of placer deposits, based on extensive drilling results, have shown concentrations in similar patterns but at different sites. Certain levels of deposition are richer than others, and reflect the higher heavy mineral content transported during those periods of deposition.
In shallower placers the highest concentrations of heavy minerals are often on bedrock, which may be defined as the firm base on which alluvial material is deposited.
The division between ancient and less ancient and recent placers has never been established precisely to the author’s knowledge. In some parts of the world ancient placers are called paleo-channels (Brazil); in others they may be called tertiary channels (California) reflecting the period of depositions. Many placers fall in the time frame between carbon-14 dating and potassium-argon dating.
It is believed that the tertiary placers of California were deposited 8 to 65 million years ago (Lindgren, The Tertiary Gravels of the Sierra Nevada of California, USGS Professionals Paper 73-1911) before and during the uplift of the current Sierra Nevada range. During this period, stream systems eroded auriferous sections of the western slope. The tertiary rivers were subsequently covered with volcanic ash, volcanic flows and later, alluvials. Today, the tertiary channels may be covered with a hundred to many hundreds of feet of later material. In general, channels in the headwater areas are higher in heavier mineral content. Channels are often narrower in headwater areas and coarse gravel and boulders prevail. Subsequent faulting and displacement may change original gradients, and add to the miner’s and explorationist’s problems.
Sites where recent drainage cut tertiary channels in California were first located and mined during the 1850–1884 period. The scale of the workings of that period is little appreciated today. Over 8046 km (5000 miles) of ditches carried water to hydraulic monitors that excavated 0.038 km3 (50 million cu yd) of material annually, and discharged tailings into the river systems. A court order suspended this practice in 1884. The Ballarat/Bendigo area in Victoria, Australia, is another site of ancient placers which is well described in literature.
When areas of the world are first explored and developed by ambitious men, placer deposits, especially gold placer deposits, are among the first to receive attention. The gold fields of California in 1849, the Klondike of the Yukon in 1898, Nome, Alaska beaches in the early 1900s the South Island of New Zealand in the later 1800s, and New South Wales in the late 1800s are a few examples. The placer tin fields of Southeast Asia followed in the 1900s. Lucrative gold fields in Colombia, South America were developed in the 1908–1920 period, expanded in the 1930s and late 1940s, and continue to produce today. The placer tin fields of the Brazilian Amazon were first recognized in the 1960s and production continues to expand annually.
What are known as heavy mineral placers, generally beach sand minerals not of the precious metals (Au, Pt) or high value specific gravities (SnO2) types, have been
developed in this century. They include rutile, ilmenite, and related minerals, zircon and monazite.
Large and rich placers may occur in areas where lode gold mines are few or nonexistent. Placer areas west of the Sierra Nevada range of California lie below serpentine belts where lode mines have been of the small vein type with rich localized ore shoots, often called pocket mines. In placer areas of Alaska, there have been few lode mines. In British Columbia, gold mines of size have been infrequent. In Colombia, substantial lode gold mines have been very few. In New Zealand, the best placer areas of the South Island have supported few lode mines. In Bolivia, the richest gold placers have been found in ancient conglomerates, or in more recent placers resulting from their erosion and redeposition. Some placers have resulted from the erosion and direct deposition of
vein materials, but they have been small. Most of the great placer gold fields of the world are second or third generation placers, i.e., the auriferous material has been eroded and redeposited two or three times.
Ancient placers may lie on varying grades due to subsequent tectonic movements. More recent placers may lie on grades of 0.5 to 3% or more. Smaller deposits with coarse gravel may be expected at the steeper gradients, and deposits with more sand and silt at the flatter gradients.
In areas of moderate to heavy rainfall, 76 to 760 cm (30 to 300 in.) per year, placers may be expected to contain rounded well-washed gravels and sands. Gravel with a shingled appearance on river bars provides an excellent environment for the collection of placer gold. In areas of lower rainfall, material may be subangular and contain more clay resulting from major movements during infrequent floods. In semiarid areas, alluvial fans, formed where intermittent streams emerge from hills and discharge onto flood plains, may contain heavy minerals. Fan deposits are found in Nevada, Arizona, and around lake basins in other states of the western United States. The monazite/zircon/ilmenite deposits of the Cascade basin of Idaho, which were mined with AEC/GSA support in the 1950s, were of this type.
In addition to placers deposited by the action of water, there are in-situ deposits which have been enriched by the erosion of lighter material accompanying the heavy minerals. When tin-bearing granites decompose and are subjected to erosion, the feldspars become clays and are washed away by rains and stream action. Quartz fragments remain behind along with heavier cassiterite. Unless quartz fragments are considerably coarser than the cassiterite, they too will be washed away, leaving rich concentrations of cassiterite behind.
Outcroppings of gold-bearing gossans subjected to weathering are other examples of in- situ enrichment. The enriched upper sectors are often mined profitably, but profitability generally declines rapidly as material becomes harder, and in-situ enrichment decreases. These surface enrichments have been called saprolitic in areas of the southeastern U.S. Enrichments may also occur on the slopes between in-situ or eluvial deposits and stream bed alluvial deposits. These slope deposits are referred to as colluvial deposits. Minerals found in placers in economic concentrations include gold, platinum, cassiterite, rutile, ilmenite, zircon, monazite, tantalite, columbite, diamonds (gem and industrial), gemstones (other than diamonds), abrasives (garnet and carborundum), chromite, and others in minor amounts. Silica and other light residuals may be salable.