Gold: Where is it in the River?
Excerpted from "Let's get Physical", by James C. McNeill (copyright © 1995)
Gold is found in lode deposits, residual deposits, alluvial deposits, bench deposits, streambed deposits, ancient rivers, and flood layers. A lode deposit is a crack or fissure in hardrock that's full of gold. This is the original source of placer deposits.
Residual deposits are pieces of ore that have eroded away from a lode. They are usually directly under the lode that they broke away from.
Alluvial deposits are pieces of ore that have eroded away from a lode, but haven't been deposited in a stream. The hill that they came from may no longer exist, or may even be further downhill.
Bench deposits are found on the banks of a stream, and streambed deposits are found under the water. You can start your exploration in the streambed. If you don't see any signs there, chances are that the entire basin is bare.
Look for cracks or crevices in the rock at the bottom of the stream. Gold will settle into them. Any rough or irregular bedrock surface will act as a gold trap. Potholes in the bedrock will trap gold, so dig until you find the hard edges of the hole. Smooth and polished surfaces don't trap gold well.
Dikes in the bedrock will trap gold in different ways. If it angles downstream, gold will to collect on the downstream side. If it angles upstream, it will tend to collect on the upstream side. Rock outcroppings from the stream sides work about this same way.
Any sudden drop-off into a deeper and larger volume of water is a good place to look. Boulders at the base of a waterfall will protect gold deposits from being boiled away by the falling water. Sometimes the gold will settle out just beyond the boilout point. If the slope of the streambed lessens and smoothes out, there may be a good sized deposit there. Look on topographical maps for places where the grade levels off and check it out.
Gold tends to follow the shortest route between bends.
Boulders in the stream may trap gold on the downstream side. Of course, if they are in the shortest path, they are even more likely to do so.
During the Tertiary period, about 2 million years ago, the mountains underwent a lot of twisting and faulting. Many streams were formed, most of which ran in a South-East direction. The benches of these ancient rivers and streams are well known for the rich deposits they contain. These deposits often have a deep blue color, and are called 'Blue lead', which turns a rusty reddish brown after being dug up and exposed to the air. They are often very hard and compacted.
Flood gold can be found at the bottom of flood layers where heavy storms with enough force to move large amounts of gold will produce concentrations. Watch for layers of differing color, hardness and consistency. Some hard layers may masquerade as bedrock, so don't give up if the going gets a little hard. The shortest route idea applies here, also. Sharp bends may show good return in the inside edges quite far from the normal water line.
Excerpted from "Let's get Physical", by James C. McNeill (copyright © 1995)
