Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
8. MISCELLANEOUS TYPES
There are other less publicized types of gold placers that are not economically important today, but which may achieve importance at some future date. For this reason a brief discussion and some selected references are offered here.
a. Beach placers: Beach placers may form where gold-bearing material is carried into the ocean by streams, or along the wave-cut base of a gold-bearing coastal plain. With exception of the highly productive beach placers discovered at the turn of the century at Nome, Alaska, none have been of great importance in the Western States. Typical beach placers along the Pacific coast are found as erratically distributed, somewhat lenticular concentrations or streaks of black sand minerals with varying amounts of finely-divided gold and in some places, with platinum-group minerals. Beach-placer black sands can be expected to consist largely of magnetite and ilmenite but significant amounts of chromite are found in some Oregon beach sands. In the case of gold-bearing beach placers, the individual black sand concentrations are seldom over 100 feet long or more than a few feet thick. Those found on the active beaches are the result of storm and tidal action, and they come and go with changing conditions of the beach. Some of the most productive placers have been found in ancient, elevated beaches that are now several miles inland.
Beach mining reached its height following discovery of the rich gold placers at Nome, Alaska, in 1898. Here, over two million dollars were produced from a 20-mile section of modern beach about 200 feet wide, and another 15 million from a series of inland elevated beaches. Subsequent discoveries on other Alaskan beaches, and elsewhere along the Pacific coast, resulted in other gold rushes but little production. Some beach placers along the California-Oregon coast have been worked in the past by simple hand mining methods and although a number of large-scale operations have been attempted, none, to this writer's knowledge, have been successful.
Over the years a number of attempts have been made to mine magnetite-rich beach sands for their iron content and during World War II some chromite was recovered from ancient beach deposits in Oregon. (Kauffman and Baber, 1956, pp. 12, 13).
When investigating a beach placer, all reports and sampling data relating to the property should be examined critically. First it should be ascertained if the samples represent a minable volume, of if they actually represent selected or scattered streaks of enriched material. Second, it should be ascertained whether or not the reported gold values were determined by fire assay. In the absence of information to the contrary, it must be assumed that the reported values were obtained by fire assay. Little or no credence can be given sample results, or the reliability of a report, based on fire assay returns. There are persistent stories that beach deposits contain gold and platinum in forms not amenable to recovery by conventional processes, in fact, many of the larger-scale attempts to work beach deposits have been based on some kind of "revolutionary" or secret process intended to recover this elusive gold. The fact that few such schemes have gone into commercial production and none have become sustained operations is significant.
A number of excellent technical articles relating to beach deposits in Oregon, Alaska and California are available, among which are those by Pardee (1934); Dasher, Fraas and Gabriel (1942); Twenhofel (1943); Thompson (1915); Thomas and Berryhill (1962); and by Jenkins (1946). Recent work by the United States Geological Survey (Clifton, 1967) points out the problems normally encountered when sampling and analyzing beach or marine deposits containing detrital gold.
b. Glacial deposits: The mineral examiner working in the Western States may seldom encounter a placer directly associated with glacial deposits but, on the other hand, it is not unusual for a miner 'to assert that a particular deposit, particularly if its origin is obscure, is a "glacier" placer. For this reason the engineer should know about glacial deposits as they relate to placers.
The fundamentals have been well set out by Blackwelder (1932) as follows:
"Since it is the habit of a glacier to scrape off loose debris and soil but not to sort it at all, ice is wholly ineffective as an agency of concentration for metals. Gold derived from the outcrops of small veins is thus mixed with large masses of barren earth. Attempts to mine gold in glacial moraines, where bits of rich but widely scattered float have been found, are for that reason foredoomed to failure.
"On the other hand, the streams born of glaciers or slowly consuming their moraines have the power to winnow the particles of rock and mineral matter according to size and heaviness. Such streams may form gold placer deposits in the well-known way by churning the load they carry and allowing the heavy minerals to sink to the bedrock. Placers may therefore be found in the deposits of glacial rivers if there are gold veins exposed in the glaciated area upstream. Nearly all the gravel which has been dredged for gold along the foothills of the Sierra Nevada was deposited by rivers derived in part from glaciers along the crest of the range, but most of the gold was probably picked up in the lower courses of such rivers. Since glacial rivers choke themselves and build up their channels progressively, their deposits are likely to be thicker and not so well concentrated as those of the more normal graded rivers which are not associated with glaciers."
Where a glacier-related placer is encountered, the field engineer should, as an early step in his investigation, search out and study all available technical literature relating to the glacial history of the region. In particular, he should seek any reliable information on past mining of the deposit or similar deposits in the district, the object being to determine if significant gold concentrations are to be expected and, if so, under what conditions they are likely to be found.
Two districts having glacier-related placers that are well described in the technical literature are those near Breckenridge and Fairplay, in Colorado. At Fairplay (Singewald, 1950), the actual moraines were mined locally but the most extensive and productive placers were found in outwash aprons extending away from the true moraines. At Breckenridge (Ransome, 1911, pp. 175-181), bench gravels associated with Pleistocene glacial deposits were mined by ground sluicing and hydraulicking while younger gravels derived from glacial moraines have been extensively dredged along the Swan and the Blue Rivers. Both districts are well described in the literature referred to.
In general, glacial debris that has been scoured from highly mineralized bedrock areas may be expected to contain gold but it will probably have little or no economic value unless resorted by post-glacial streams.
c. Eolian placers: In desert regions the wind may act as an agent of concentration by blowing sand and the lighter rock particles away from a body of low-value material and leaving an enriched surface veneer containing gold or other heavy minerals in a somewhat concentrated state. There have been many cases where wind-caused surface enrichments supported the activities of itinerant miners using hand tools and simple dry washers.
Although commercial-grade eolian placers are not likely to be encountered by today's mineral examiner, he should be aware of their existence and should be alert to their misleading appearance. In other words, when taking near-surface samples from desert placers, he should guard against unintentional salting which could result from the inclusion of non-representative, wind-caused surface enrichments.