Mineral resource studies of national forest roadless areas in Idaho (original) (raw)

Within the group 1 study area, there are tracts permissive for the occurrence of undiscovered deposits of five types: Tertiary low-fluorine porphyry molybdenum, Cretaceous low-fluorine porphyry molybdenum, Comstock-type epithermal veins, polymetallic quartz veins and lodes, and radioactive black sand placers. One tract permissive for Tertiary low-fluorine porphyry molybdenum (fig. 3) has a 10 percent chance of containing at least one undiscovered deposit of a type that has a median size of 94 million tonnes of ore and a median grade of 0.085 percent molybdenum. Estimated mean metal endowment for this tract is 50,000 tonnes of contained molybdenum. One tract permissive for Cretaceous low-fluorine porphyry molybdenum (fig. 4) has a 50 percent chance of containing at least one and a 10 percent chance of containing at least two undiscovered deposits of a type that has a median size of 94 million tonnes of ore and a median grade of 0.085 percent molybdenum. Estimated mean metal endowment for this tract is 200,000 tonnes of contained molybdenum. One tract permissive for Comstock-type epithermal veins (fig. 5) has a 50 percent chance of containing at least one and a 10 percent chance of containing at least two undiscovered deposits of a type that has a median size of 770,000 tonnes of ore and a median grade of 7.5 grams/tonne gold and 110 grams/tonne silver. Estimated mean metal endowment for this tract is 25 tonnes of contained gold and 1,700 tonnes of contained silver. One tract permissive for polymetallic quartz veins and lodes (fig. 6) has a 10 percent chance of containing at least one undiscovered deposit of a type that has a median size of 12,000 tonnes of ore and a median grade of 14 grams/tonne gold. Estimated mean metal endowment for this tract is 0.2 tonnes of contained gold. Tracts permissive for radioactive black sand placers (fig. 7) were defined, but no estimate was made of the number of undiscovered deposits present. There is not adequate information on this deposit type to develop descriptive or grade/tonnage models. Group 1 Group 1 8 Polymetallic quartz veins and lodes Deposits are in veins of vuggy brecciated quartz within and next to massive composite quartz veins that commonly form prominent topographic features. Some of the massive quartz veins are exposed for distances exceeding 2 mi. This is a common deposit type in this region, occurring throughout the Idaho batholith. Examples within the map area include the Atlanta and Rocky Bar deposits. The ore zones constitute only a portion of the extensive quartz veins and lodes and are often separated by several tens to hundreds of feet of barren quartz. Most of the production was for gold and silver, although some veins contain significant amounts of lead, zinc, copper, and antimony. Individual ore bodies range from a few tonnes in isolated pods to pipe-like lenses as much as 450 ft long, 8 ft wide, and more than 1,000 ft deep. Ore grades vary considerably but across the larger ore bodies average 11-16 grams/tonne gold and 34-58 grams/tonne silver. Radioactive black sand placers Several alluvial deposits in central Idaho contain black sand minerals containing thorium dioxide, rare-earth oxides, niobium and tantalum pentoxides, and U2O8. The known deposits within the study area include the Big Meadow and Payette placer deposits. These deposits are generally large alluvial sand blankets, containing more than a million cubic yards of material. Gold placers There has been major gold production from placer deposits in the region, especially in the Placerville and Idaho City areas, just to the west of the study area. Placer deposits exploited prior to 1950 ranged from a few acres to several thousand acres in area and a few feet to more than 100 ft in thickness. Small placer operations continue along many of the major streams in the study area and small pockets of placer ore will continue to be found for many years. MINERAL RESOURCE ASSESSMENT Group 2 14 One tract permissive for polymetallic veins (fig. 14) has a percent chance of having at least two, a 50 percent chance of at least four, and a 10 percent chance of at least seven undiscovered deposits of a type that has a median size of 7,600 tonnes of ore with a median grade of 820 grams/tonne silver, 9 percent lead, and 2.1 percent zinc. Estimated metal endowment for this tract is 400 tonnes contained silver, 30,000 tonnes contained lead, and 20,000 tonnes contained zinc.