The following is from a presentation given at the Gold Prospectors of the Rockies general meeting by long-time member, Allen Mershon. ~ed
The specific gravity (SG) of a substance is the ratio of its weight to the weight of an equal volume of water. Average specific gravities of some of the materials a prospector might encounter in Colorado's Rockies are
2.7 - rock
2.7 - quartz
4.2 - copper
4.3 - garnet
5.1 - pyrite
5.1 - magnetite
5.3 - hematite
7.3 - tin
7.5 - galena
7.9 - iron
10.5 - silver
11.3 - lead
13.6 - mercury
19.2 - gold
21.5 - platinum
As you can see, most rock (and sand and dirt) is one-half to one-seventh lighter than other materials a prospector may find and will float over the edge of a gold pan leaving the heaver materials in the bottom. The old miner knows that garnet is found in the Rockies and is often found associated with placer gold. So, when you are gold panning in the mountains, remember that "garnet is your friend!"
The secret of gold deposition is in the "venture effect" or, more properly, the reverse of this effect. The speed of a fluid increases when forced through a more restricted area--this configuration forms a venturi. In a suction nozzle, the venturi effect, produced by the lessening of the diameter of the water intake to the system, creates a powerful fluid jet. In a flowing creek, after water enters the wide end of the top a "V," such as in the wide end of a V-shaped canyon, the speed of the water at the narrow end of the convergence is much faster. And, as we know, fast water can carry and move more and heavier elements. The reverse is true, however, when the enclosing walls of a creek widen; the speed of the flow of the water slows and any large and heavy materials carried by the stream are more likely to settle to the bottom because the water flow no longer has the force to carry them.
Moving water stratifies material. Thus, dropped into the ocean's surf, a gold ring can sink more than two feet in a matter of minutes. The same is true of the materials carried along by the moving water of a mountain stream. The flare on a sluice creates a venturi effect as it narrows and a reverse venturi effect when the water flow encounters the wide spacing of the sluice's walls, dropping heavier materials between the sluice's riffles. Thus, when using a sluice, you need to be aware of the optimum flow of the speed of water over your riffles. The "magic number" is 8 feet per second. This can be approximated over time by the experienced miner or can be determined accurately with a flow meter attached to a sluice. Water flowing faster than 8 feet per second continues to carry gold. Slower than 8 feet per second, gold can no longer remain suspended and is dropped out of the water flow.
Knowledge of the venturi effect can help you find gold in nature. Just as in a V-shaped canyon, anything that creates a venturi in a gold-laden stream will deposit gold when the venturi is no longer in effect (so to speak). It is why gold collects where the stream flows more slowly through a curve and deposits a sand bar. It is why gold collects behind a boulder. It is why gold is deposited into the bottom of v-shaped cracks in bed rock. It is why gold can be found where a stream widens into a pond. It is why, in slow water, you have to dig deeper to find gold. And, it is why you can find values in the thousands of v-shaped, vortex-producing pockets of the blue Nomad Matting (Miners Moss) that you have used to replace that outdated rippled carpeting in your sluice! (You haven't done that yet? Then you're losing precious gold!)
Virtually every commercially-produced sluice can be modified in some manner to make it more useful to the weekend miner. Whereas the ancients used sheepskin and the not-so-ancients used burlap bags, the materials produced today to capture gold in a sluice are far superior. Under the riffles in your sluice, consider laying a three-layer medium constructed of (from top to bottom) Miners Moss, astro turf, and ribbed rubber matting. With this configuration, you can capture even the gold fines that you may have been missing.
Gold is most likely to be found under a stream bed at the level of bed rock. When viewing and "reading" a stream, however, remember that below the surface rocks, gravel, and sand of the bottom, carbonates can form a more or less cemented layer. This layer is called "caliche" and it forms a "false bottom" that also can catch and retain particles of heavy materials, such as gold. So, look around streams for places where prehistoric channels left their benches. Although Colorado doesn't have a lot of productive benches, if you can find a caliche layer, prospecting on top of that layer in a known gold-producing area might prove rewarding!
Everyone, well every prospector, has heard the saying, "Gold is where you find it." Sometimes, you just have to get away from the obvious places where you expect to find gold and take the "dumb man's path." That path is the trail away from your diggin's and along the banks and benches of the stream where only an animal would trek. Way back when, some of those dumb miners tripped over "lodes" of gold that way.
Besides your shovel, pan, and sluice, what else should you take with you when prospecting? Waders and skivvies, of course! Why? To keep out the cold! When the large muscles of your body are immersed in cold water, you lose body heat rapidly. When you get very cold, your autonomic nervous system takes over those muscles and you start shivering uncontrollably. If this happens, you are suffering from hypothermia--get out of the water and begin walking around, especially uphill, because seventy-five percent of large muscle activity produces heat!
The expected survival time of an adult submersed in water of 40 degree Fahrenheit (4.44 degrees Celsius) can be as little as 1/2 to 1 hour. In air of the same temperature, you can live as long as 8 hours or so. On the other hand, were you to lie down unprotected on 40-degree ground to go to sleep, you could wake up dead just 8 hours later!