Placer Deposits

Fortunately for humanity, gold, platinum and a limited quantity of silver have been being separated for millions of years from country rock and deposited into commercially useful concentrations. Such deposits are called placers. Their accidental discovery probably started people digging in the earth searching for them many thousands of years ago. Jeffrey St. John of Time-Life books gives a synopsis of the process, and several of his comments are printed below. Much of this will probably be considered old-hat to sourdoughs.

Placers are created by wind, running water and gravity. The sequence begins when wind, running water and temperature changes gradually break down the original ores. Weathering crumbles and dissolves the rock into a collection of loose grains that may range in size from boulders to microscopic particles.

When a slope erodes, heavier minerals, including the noble metals such as gold, platinum, and infrequently silver, move downhill more slowly than lighter materials which are washed or blown away. The heavier minerals left behind are concentrated into deposits. Because of their chemical inertness, the noble metals cannot be decomposed by oxidation or by other chemicals present.

The primary agent in the formation of placers is water, and the resulting deposits are called stream placers. When eroded sediment reaches a vigorously flowing stream, the water sweeps away lighter materials, such as quartz, while the heavier placer minerals sink to the bottom.

On the trip downstream, the minerals are pounded, scraped and tumbled until the brittle ones among them are reduced to a powder. Even the most durable elements are rounded and compacted. A prospector who comes upon bits of sharp, angular gold knows that the source area cannot be far upstream.

Alluvial placers most often occur at points where the water velocity drops. Thus, deposits are likely to be found immediately upstream from any obstruction that blocks the water's natural progress. They are also found in the potholes and pools that form at the bases of waterfalls and rapids, and just downstream from the confluence of a swiftly flowing tributary into a larger, slower river. Deposits also form in the slack waters along the inner shores of a curve in the stream's course.

While gold is sometimes found in nuggets as big as a man's fist, most placer gold appears in fine specks called dust.

Despite their high density, some gold and platinum eventually reaches the sea, and the pounding waves and shoreline currents concentrate portions along beaches. This was the case at Nome in 1899, when frenzied prospectors found that the beaches were thick with black sand containing gold. But that bonanza was brief. Only the more persistent of the prospectors found that the bulk of Nome's gold lay in placers deposited as far as three miles inland, and buried under as-much as 50 feet of frozen tundra.

Alaska's major dredging operation for platinum -- and the largest in the United States -- lies beneath the Salmon River, near Goodnews Bay. The dredge working that deposit excavates more than a million cubic yards of gravel during the short 160 day dredging season.