Continuing a boyhood passion for rockhounding, our speaker became a mineralogist of global experience. In 1971, returning from a four-year work stint in Africa, he became affiliated with the Colorado School of Mines Research Institute (CSMRI). Shortly after, he was approached by the inheritors of 35 acres of gold placer ground along Clear Creek below Black Hawk. This is not strictly a mineralogy problem and he endeavored to refer them to an expert in gold placering. To his surprise he discovered that there were no experts, owing to the decades-long stagnant gold price which drove all knowledgable persons from the field. Always one to try new things, he embarked on a gold placer research program of his own. In the end, he pleased the Clear Creek heirs by recovering 200 ounces with the then value of $7,000. He did not sell his own share of this, which increased in value by many times. He concedes that with the knowledge he subsequently gained, he might have recovered more.
His timing was good. The price of gold climbed steadily through the 1970's and the value of his research and expertise climbed with it. His employment arrangement with CSMRI allowed him access to state-of-the-art, and very expensive, research equipment not available to potential competitors. He seldom published and he said that much of what he revealed to us is being made public for the first time. His research mostly applies to commercial-size operations and especially to by-product gold recovered from stream-borne sand and gravel operations, but his talk was nevertheless both interesting and informative.
His experiments corroborate published data that in a sluice box the ideal ratio of the height of the riffles to the riffle spacing should be 1:3.5. If, for example, the riffle height is one inch, then the riffles should be three and one-half inches apart. Anyone with a home-made or store-bought sluice box can check theirs and adjust it to that spec.
He mentioned that he had greatest success with "Australian riffles." This is the style which is vertical for half its height and then sharply bends backward and upward at 45 degrees. Also, in his work in Clear Creek, where many of us prospect, he recovered almost no gold larger than 10-mesh. (Mesh sizes are measured in holes per linear inch, such that, for example, a 10-mesh screen has ten holes per inch.) Because there is seldom any gold particle in Clear Creek larger than 10-mesh, it is most productive to classify your pan or sluice to include only material that can pass through a 10-mesh screen.
For very fine gold, smaller than 50-mesh, the losses were 50% using even the most sophisticated equipment because in that size range the gold "floats like a leaf off the [concentrating] table." He mentioned too, that the typical gold yield from local sand and gravel operations is five to ten cents per ton. May times never get so bad that we are compelled to duplicate this with shovel and wheelbarrow! He graciously gave thanks that his childhood hopes and dreams became the reality of a fulfilling, fascinating, and remunerative life's work, the enjoyment of which he expects to continue. May we all have such good prospecting!