A Story About Assaying Oil Shales
Fire Assay of Fly Eyes
By Dr. Ralph E. PrayColorado-Utah-Wyoming oil shale was first reliably discovered to contain gold and silver by the U.S. Bureau of Mines in the 1920's. Although most of the shale, up to 300 meters (1,000 or so feet) in thickness, contains nil or barely detectable values, many brown shale samples fire assayed here over the past three decades have yielded up to 0.02 opt Au and 2.0 opt Ag. The shale contains almost one percent pyrite in its thousands of sediment layers, but the values are neither in the pyrite nor in any any other identifiable host mineral.
A thin section of the Mahogany Layer, carrying 190 liters (50 gallons) of oil per ton, shows insect legs, wings, eyes and other parts as black against a tobacco-brown matrix. From this viewpoint the material does not appear to be a willing candidate for the fire, particularly if the values occur as metal chelates such as the porphyrin complexes present in crude oil. These organometallics may have a thermal stability up to only 120-150 deg C (250-300 deg F) and would normally disappear like water in a hot furnace.
The trick then is to bring the temperature up gradually until the metal atom vibrates out of its capture ring to re-attach onto something more friendly in the crucible cauldron. This has to occur long before' initial carbon dioxide emission from the fifty percent carbonate shale sample, and has been found to take place in excess PbO powder.
Following 15 years of research and careful cupellations, patents were applied for and granted showing how oil shale retort vapor (kerogen) could be scrubbed of entrained Au-Ag in a molten Pb spray. But the trillions of tons of oil shale carry elusive stringers of precious metal in their entombed fish, algae, mosses, ferns, spider legs and fly eyes. The next sample, and the following five or fifty, were usually barren.
The many years of assaying mountainsides inch-by-inch, where insect parts took the place of minerals and rock samples burned like candles, and where mostly there wasn't anything to weigh at the end of the day, now seem like a long trip to an unseen place.
Published in Mining Engineering Magazine
Reprinted with permission of the author.
Dr. Ralph Pray, Mining & Metallurgy
Telephone: 626-357-6511 Fax: 626-358-8386