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Placer Ore Defined
Gold-encrusted quartz and other ores erode as they become exposed to the elements. Water is the primary mover and concentrator of these deposits, called "placers." Placer ore is gathered by two primary methods: "panning" or digging just below the surface of dry stream beds or along a shoreline, and sifting wet gravel gathered from the bottom of a flowing stream. This latter method can be seen in any number of western movies and television shows. The California Gold Rush of 1849 began after concentrated placer gold deposits were discovered in stream beds. The gold had accumulated after gradually washing down from the Sierra Nevada mountains over thousands of years.
Ore in Streams
Gold, silver and other metallic ores sink to a creek bottom because they are heavier than other minerals. The speed of the moving water determines whether the ore settles out quickly or remains suspended in the flow with sand and other debris for a longer period. A great deal of ore movement takes place during floods when the flow rate is highest. Following a flood, placer deposits are often found further downstream compared to their usual locations. Ore tends to settle on the inside of stream bends because water velocity decreases at these points. Depressions in the stream floor are another location where ores collect. Underwater sand bars cause a flow reduction and allow placer ore to settle next to the downstream side of the bar.
Ore Found on Coasts
Placer ores are also found on coasts, often at or near the location where a river meets the ocean. Gold and other ores are washed into the sea during seasonal high water periods, such as yearly snow-melts or during floods. These ores are captured by wave action and some of it is washed up onto the beach. The ore settles down into the sand over time, so several feet of digging may be required to reach beach placers.
Placer Prospecting Sites
Alaska contains the highest concentration of gold and silver placer ore in the United States. Other locations in the United States that offer opportunities for gold and silver placer ore discovery are generally in mountainous regions ranging from California and Nevada to Idaho and Colorado. Areas that produced ores with a low metal concentration in the past, and abandoned, may be worth exploring now. In 2011, gold was selling at $1,500 per ounce -- more than 60 times its worth of a hundred years ago. In addition to streams and beaches near river outflows, placers are also known to exist near extinct volcanoes, even those worn down over time to little more than small hills.