The gold in auriferous placers may come from one or more of the following sources:
It should be noted that the geological history of productive placers is frequently complex, much more so than the sequence:
Often an intermediate collector of gold is involved, mainly auriferous conglomerates, quartzites, etc. A number of variants are recognized in the lode-placer sequence as follows:
The primary agency that produces gold placer is weathering; a process that involves numerous complex chemical reactions. Three things may happen to gold in primary deposits:
The formation of placers is, therefore, a combination of both mechanical and chemical processes working in consort over long periods of time.
The mechanical agencies that assist in the transport and winnowing of gold into placers are gravity, the running water of streams and rivers, the agitation of waves along the shores of lakes, seas and oceans, the wind, and glaciers. Gravity is a force that operates in the formation of all types of placers; it is the principal agent that concentrates gold in eluvial placers.
The other agents give rise respectively to the following types of placers:
There are few authenticated aeolian gold placers of any size, and we shall not consider them further except to note that near the outcrops of some of the primary deposits in Australia the wind had blown away the finer detritus exposing coarser material in which gold was enriched in places. Auriferous sand dunes have been recorded in the Silver Peak quadrangle, Nevada. These apparently contain only traces of gold and silver. Placers due entirely to glaciation are uncommon.
The minerals concentrated in placers include two types:
All of these minerals have three features in common:
The last is of importance since flaky minerals such as molybdenite, scaly gold and specularite are difficult to concentrate in spite of their high specific gravity. Another factor is the degree to which a mineral can exist in a subdivided form and yet remain relatively chemically stable. This is a factor with respect to gold since the metal can exist in almost infinitely subdivided form giving the so-called 'flour', 'float', 'flood' or 'skim' gold. Such gold does not sink readily in sand and gravel and hence is not concentrated to any extent in placers; on the contrary it may be transported hundreds of miles in running water to be deposited over broad deltas or on the floors of lakes and oceans where it is commonly disseminated throughout the sediment. This particular feature is of importance in discussing the origin of the very finely divided gold of deposits such as the Witwatersrand. Commonly flour gold accumulates temporarily in streams and rivers on bars on the inside of curves or meanders or in other areas of slack water ('skim bars'). Unless these are recognized they may give an inflated impression of the potential of a placer stream or river. Skim bars are also misleading during geochemical prospecting within a restricted area since they give false anomalies.
'Moss gold' is flour gold trapped by mosses and roots of other plants along river and stream beds during high water. 'Moss miners have collected this gold for centuries along the auriferous rivers throughout the world. It is easily recovered by burning the moss.
With skim and moss gold a new 'gold crop' appears each year after high water in auriferous watercourses. This is partly the reason for the adage that 'gold grows in placers'. It is, however, probably not the whole story as will be seen later.
The most common minerals with low to medium specific gravity in placers are quartz (S.G. = 2.65), muscovite, amphibole, pyroxenes, tourmaline, garnet, diamond, chromite, rutile, barite, corundum, wad, limonite and zircon (S.G. = 4.5); those with medium to high specific gravities in placers include monazite (S.O. =5), magnetite, ilmenite, cassiterite, woiframite, scheelite, cinnabar, gold and platinum (S.G. = 22 when pure). A great variety of sulphides and sulphosalts may accompany gold in placers. Pyrite (often abundant), galena, sphalerite, arsenopyrite, boulangerite and jamesonite are found in placers in Yukon Territory and elsewhere. Native bismuth is present in some placers; native mercury in others; and more rarely native arsenic, native silver, arquerite,(Ag-Hg amalgam), native copper, native lead (not shot), native zinc, cinnabar, realgar, sperrylite, molybdenite, chalcopyrite, hematite, carbonates, feldspars, kyanite, topaz, spinel, allanite, epidote, sphene, tantalite-columbite and apatite. Some gold placers contain diamonds and other gems such as rubies, sapphires, emeralds, topaz, garnets, etc. Commonly the gems are secondary in value to gold, and in some placers they are so sporadic as to be a curiosity. Certain placer sands and gravels are cemented or coated by limonite, wad or mixtures of these mineral aggregates. These aggregates commonly contain much silica, alumina, titania and other hydrolyzed oxides and frequently large amounts of humic material. Placer gravels or sands strongly cemented by limonite and other iron oxides are often referred to as 'cement gravels' in English speaking countries or 'cangalli' in South American countries. A few placer sands and gravels have been cemented by carbonates, silica or clay, and most placers in the permafrost of Canada, Alaska and Russia are frozen solid, requiring a procedure to thaw them out in places.
The gold of placers has varied characteristics. The most common habit is as dust that comprises particles ranging from specks the size of the tip of a needle or less to those the size of a flaxseed (in quantitative terms, <0.1-2 mm). Also common are small scales and spangles; less common are nuggets, crystals, wires, leaves, tufts and hairs and arborescent, reticulated, dendritic, filiform, mossy and spongy forms. Extremely finely divided gold (flour, float or skim gold), the bane of the placer miner and geochemist, is common in some placer districts.
Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting
Rafal Swiecki, geological engineer email contact
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