The June 2006 meeting featured Mr Gold, widely experienced professional gold placer operator. He is the employer of Allen Mershon, our Program Chairman, who finally coaxed him to come in and share some of his vast expertise with us. Mr Gold gained his early experience in Alaska and the Yukon, the "big leagues" of gold placering. He currently operates thirty-two gold placers in Colorado. All are adjuncts to sand and gravel operations. His talk took the form of a press conference as he fielded questions from the audience.
Among the information revealed, is that sluice boxes are more effective if they are varied rather than of a single design. Placerers constantly argue among themselves about the effectiveness of this or that sluice box design, i.e., riffle height and shape, spacing, floor material, etc. Mr Gold's solution is both simple and elegant: put in some of everything that you can. In even a small sluice box you can put in several riffle sizes and shapes, space the riffles irregularly, use different types of matting, e.g., ribbed rubber, Astroturf, and "miners moss." Mr Gold's argument is that the mechanics of gold-capture in a sluice are so complex that the more you vary the conditions, the better chance you have of catching more gold. For that reason he believes that the "store bought" type of hobby sluice is not very good for catching fine gold.
Regarding sluice box angle, Mr Gold points out that too much water is better than not enough. If the flow is so low that the riffles quickly silt up, then very little gold will be captured. Also, it helps to feed a sluice evenly. Sudden surges of material into the sluice will upset the balance of the system. Steady, even hand shoveling is okay. The best way to test sluice box efficiency is to catch and pan the reject material. What can't be panned probably can't be caught by any gravity method. Mr Gold pointed out that most of the gold he has caught is in the size range between minus 50 and minus 425 mesh. That is mighty fine! (White flour is about minus 200 mesh.)
Equipment, such as jigs or shaking tables, are usually suitable only for commercial applications. They undoubtedly capture more gold, but they yield so much "heavies" that clean-up becomes a big problem. The heavies in many operations are typically 3%, but of course can be more or less.
We wish Mr Gold good digging and hope that he can return and enlighten us some more!