In the United States mining law, mineral deposits that are not veins in place are treated as placers so far as locating, holding and patenting arc concerned. But for the purposes of this book, the term "placer" is applied to deposits of sand, gravel, and other detrital or residual material containing a valuable mineral which has accumulated through weathering and mechanical concentration processes. Prerequisites for a placer are:
- A valuable mineral which is relatively heavy and is resistant to weathering and abrasion.
- Release of the valuable mineral from its parent rock.
- Concentration of the valuable mineral into workable deposits. This step usually involves water transport.
The term "placer" as used in this book applies to ancient (Tertiary) gravels as well as to recent deposits, and to underground (drift mines) as well as to surface deposits.
STUDY OF PLACERS - GENERAL
The study of placers is not simple - it involves many of the disciplines of geology, with special emphasis on the theory and habits of streams. Although the location, size, and shape of a placer will reflect the regional forces of erosion, transportation, and deposition which created it, its final form will be controlled or modified by purely local conditions. As a result, each placer deposit can be expected to be unique in one or more ways and the field investigator should approach his work with this in mind.
All placers begin with weathering and disintegration of lodes or rocks containing one or more heavy, resistant minerals such as gold, platinum, magnetite, garnet, zircon, cassiterite, monazite, etc. It should be stressed that the end richness and size of a placer deposit will depend more on there being an abundant supply of source materials, and on conditions favorable for their concentration, than on the actual richness of the primary source. While the following paragraphs refer largely to gold placers, the principles set forth apply to all types. At the risk of oversimplification, a brief discussion of basic placer-forming processes will be taken up under the following headings:
- Sources of valuable mineral.
- Weathering and release processes.
- Stream processes related to placers.
- Concentration of valuable minerals.
- Preservation of the deposit.
SOURCES OF VALUABLE MINERAL
The source of gold or other minerals found in a placer may be one or more of the following:
- a. Lodes or mineralized zones.
- b. Erosion of pre-existing placer deposits.
- c. Low-grade auriferous conglomerates or glacial debris.
- d. Magmatic segregations and associated basic rocks.
- e. Regional rocks containing scattered particles of valuable mineral.
a. Lodes: Although placers are commonly found in lode mining districts, experience has shown that th.ere is no fixed relation between the richness of the parent lode and the richness or size of resultant placers. Some of the most noted gold mining districts such as Goldfield, Nev., contain no significant placers. On the other hand, some highly productive placer areas are not associated with valuable lodes, the Klondike region being an example. In some cases the lode source may have been completely removed by erosion, while in others, it can be demonstrated that the gold or other valuable mineral was not derived from a single source, but instead, from many small mineralized seams or zones scattered through the bedrock. Although individually unimportant, these smaller sources can collectively furnish substantial amounts of gold.
b. Pre-existing placers: In many localities old placers have been destroyed by erosion and their gold content reconcentrated in present day streams. Excellent examples are found in the Sierra Nevada region of California. Here, in Tertiary times, beginning some 60 million years ago, there existed an extensive river system, a prolonged time of rock decay, and a good balance between erosion and deposition. These factors combined to form widespread gold placers and some extremely rich bedrock concentrations in the Tertiary streams. Later, as the present Sierra Nevada range was elevated by mountain-building disturbances, new streams flowing toward the west cut across the ancient channels and swept some of their gold concentrations into today's streams where new, and sometimes richer, concentrations accumulated. Somewhat similar reconcentrations were found in parts of Alaska, the Klondike, and other places.
c. Auriferous conglomerates and glacial debris: In some placer areas the immediate source of gold is found to be a low-grade auriferous conglomerate or a glacial deposit. With few exceptions such material is too poor to be worked but where appreciable quantities have been eroded, superficial placers may form on the erosion surface or within the local drainage. Wind erosion of an auriferous conglomerate occasionally creates an enriched surface veneer amenable to small-scale, selective mining. Under favorable conditions, gold derived from low-value glacial debris is carried into streams and reconcentrated into minable placers and although some of these reconcentrations have an appreciable extent, most are relatively small and spotty.
Examples of placers derived from low-grade auriferous conglomerates are found in the Red Rock, Goler, and Summit mining districts of Kern County, Calif., (Hulin, 1934) II. At Breckenridge and at Fairplay, Colo., gold derived from glacial moraines has been successfully dredged along the Swan, Blue, and South Platte Rivers. According to MacDonald (1910), a gold-bearing glacial till in the Weaverville district of Trinity County, Calif., is an important source of gold for the Trinity River and its tributaries.
d. Magmatic segregations: Disseminated deposits associated with magmatic segregations in basic rocks seldom generate significant placers but a notable exception is found at Goodnews Bay, Alaska, where extensive platinum placers have been derived from a large mass of dunite and related basic rocks. Although minable placers from basic or ultrabasic rocks may be few in number, they can be important, as shown by the Goodnews Bay platinum deposits where dredging has been carried on since 1937.
e. Regional rocks: Monazite placers of the type dredged in central Idaho, provide an excellent example of a placer mineral originating as small, sparsely distributed particles in the country rock. Here, monazite, which is a rare-earth mineral chiefly valuable for its thorium content, originates as an accessory mineral in the regional granitic rocks. Following liberation by the usual weathering processes it finds its way into stream deposits, where because of its moderately high specific gravity, it accumulates in placers along with other heavy minerals. Similarly, most of the magnetite, ilmenite, rutile, garnet and zircon associated with gold placers, could be traced to the regional bedrock where such minerals commonly occur as scattered particles or accessory minerals.