| Description |
Gilsonite, a soft asphaltite found in near-vertical veins over a large area around Bonanza, Utah (Fig. 1), is mined by American Gilsonite Co. (AGC) using air-powered chipping hammers instead of explosives and mechanical equipment. A vacuum air lift moves 100,000 st/yr of ore to the surface through bored shafts at several mines. The system virtually eliminates dust explosion and fire hazards inherent in mining this hydrocarbon, which, depending on grade and quality, sells to specialized markets that include the oil well drilling industry, automotive sealer manufacturers, ink and paint producers, foundries, and the nuclear industry. AGC has recently completed a $6 million processing plant that doubled its capacity from 5,000 st/m to 10,000 st/m while improving product quality. "Processing stages that were performed at four widely separated locations are now done at one complex," notes Robert Haffner, president. "This includes cleaning, drying, screening, pulverizing, storing, and packaging. The new plant replaced a 30-year-old mill and complies with all state and federal environmental and safety regulations at substantially reduced operating costs." The gilsonite occurs in parallel, near-vertical fractures in the Uinta Formation of the Uinta Basin of Utah and Colorado. The Green River Formation also contains rich oil shales that may have been the source of the hydrocarbons that formed the gilsonite. The gilsonite-bearing factures (Fig. 2) often outcrop and may be as much as 2,000 feet deep. They vary in width from a few inches to 22 feet and may be as long as 25 miles. Average vein thickness is 6 feet, but veins as thin as 18 inches have been mined. It is believed that these fractures were once filled with a heavy, viscuous crude oil that lost most of its volatile constituents and then solidified. Gilsonite resembles obsidian and has a specific gravity of 1.04. Its softening temperature varies from 250° to 500 °F, depending on the vein and depth at which it is mined. |
