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Manganese deposits in the Drum Mountains, Juab and Millard Counties, Utah
The Drum Mountains are in west-central Utah 30 miles northwest of Delta, between the Sevier Desert on the east and Whirlwind Valley on the west. It is a typically barren desert range comprising a westward-tilted structural unit in which is exposed as much as 9,000 feet of quartzite (Cambrian and Precambrian?) and 3,000 feet of carbonate rocks of Cambrian age. These beds, which strike northward and dip west, are cut by myriad east- to northeast-trending faults with displacements of a few feet to a few thousand feet. Quartz monzonite dikes, pebble dikes, and vein deposits are present locally along the faults. The Cambrian rocks are overlain unconformably by volcanic rocks of probable Tertiary age. Bodies of manganese carbonate ore were formed by replacement of two 20-foot beds of impure dolomite at the base of the sequence of carbonate rocks, along their intersection with certain preore faults. The feeding fissures locally contain veins in which rhodochrosite is associated with base metal sulfides. Downward- moving meteoric water has oxidized the ore bodies to a depth of 100 to 200 feet except where they are sealed off by structural or stratigraphic traps. From 1925 to 1953, 72,462 long tons of manganese ore with an average grade of about 25 percent Mn were shipped.
Manganese deposits of the Drum Mountains, Utah
More than 15,000 tons of manganese ore has been produced from small deposits in the Drum Mountains in west-central Utah. Lenses of rhodochrosite, now largely weathered near the surface to manganese oxides, lie parallel to the bedding of Cambrian dolomites and shales near faults that are nearly normal to bedding. Two varieties of rhodochrosite, one fine-grained, dark-gray or black, and massive, the other coarser-grained, pink, and generally occurring in veinlets in the gray variety, are present. The gray contains less MnCO 3 and FeCO 3 , but more CaCO 3 and MgCO 3 than the pink variety. The deposits are interpreted as bodies that replaced favorable dolomite or limestone beds. The gray rhodochrosite first replaced the dolomite or limestone; the pink variety later formed veinlets in the gray rhodochrosite and replaced it to a small extent.