prospecting for placer gold deposits
Are you looking to strike gold? Then you need to know about placer gold. This type of gold is found in a secondary deposit, usually near the surface along river beds or creeks. It's the simplest form of prospecting and can yield significant results.
The term placer gold refers to gold that is found in a secondary deposit. Normally, gold is found in lode deposits. However, due to weathering of hard rock and the action of rivers and glaciers in certain places, fine gold nuggets and particles have been carried away from the original lode deposit and deposited elsewhere, usually near the surface and along river beds or creeks. This secondary deposit gold is known as placer gold, and prospecting for it is generally the simplest form of prospecting, except for buried placers. Gold, platinum, and tin are the principal metallic minerals that are won from placers, but gold alloyed with varying percentages of silver is the only metal that has been recovered in commercially important quantities from placers in the United States.
The following brief discussion applies to gold placers. The most significant type of placer deposit results from the weathering and disintegration of lodes and rock formations containing gold disseminated through them. The erosion and transportation of these rocks by running water cause further disintegration and liberation of the gold from the enclosing rock minerals. The concentration of gold occurs in places where the velocity of the transporting medium (flowing water) is such that the gold and other heavy minerals come to rest, while the lighter and finer material is carried away. Another type of placer is the residual placer, consisting of disintegrated gold-bearing lode material in or near its original position, with only a relatively small amount of barren material removed by erosion. In some of these, a considerable part of the soluble constituents may have been removed by chemical action, resulting in enrichment by gold.
Transported placers have been classified by Brooks, based on their present position relative to that of streams and other waters, as creek placers, beach placers, hillside placers, river-bar placers, gravel-plain placers, sea-beach placers, ancient beach placers, and lake-bed placers. The most productive types are shown in figure 1. Placers consist mostly of unconsolidated alluvium, rounded pebbles, and boulders. From the above brief discussion of their origin, it is apparent that prospecting for them should begin along stream beds (existing or ancient) and bars, from which they may extend to benches and hillsides. Ancient placers sometimes have been buried under considerable thicknesses of later glacial or stream deposits or lava flows, making their discovery more difficult.
Prospecting for placer gold begins along streams because placers are the result of transportation and concentration of gold particles by running waters. The most favorable streams are those originating in and flowing through areas of igneous rocks, which are the chief source of lode gold. If these rocks are known to contain gold, the chances of forming placers along streams flowing through them are obviously enhanced. In actual prospecting, it more often happens that gold is found first in placers, and the lode source of the gold is then sought, rather than the reverse. It should be noted, however, that in many instances, valuable placers have been found and worked without finding the source of their gold, and many rich gold lodes have been worked in areas where no placers result from their erosion.
Placer gold is usually first and most easily found along streams and their bars, even though the richest deposits may lie in benches or on hillsides above. Since the heaviest particles of gold are deposited nearest their source in the host rock, prospectors should work upstream from the original discovery, especially if the gold is fine. At some point upstream, the gold may disappear, indicating that the source of the gold has been passed.
When prospecting along a stream, the prospector pans the gravel at various points, selecting particularly places that show concentrations of heavy minerals ("black sands") with which placer gold is commonly associated. These usually will be found on and just above bedrock and in cracks and crevices therein. During the 1840s through 1860s, gold mining in the United States focused on rivers where miners recovered loose gold nuggets from river beds using simple mining tools such as sluice boxes. Placers were primarily worked for the first few years until the richest deposits were depleted from the riverbeds. When the richer placers began to play out, miners would venture up into the hills and look for the hard rock sources.
