More than 45 tonnes of sapphires are estimated to have been mined from Montana. Most production has been from three deposits: the Yogo dyke famous for blue sapphires, alluvial deposits along the Missouri River and alluvial deposits in the Rock Creek district. Sapphires were first discovered in these deposits by gold miners in the 1890s. Major production of Montana sapphires was from 1900 into the late 1920s, largely for watch and instrument bearings. Montana was the largest USA source of sapphires for these uses. World War II brought renewed interest in mining sapphires for industrial uses until 1943, when synthetic corundum replaced sapphires in these applications. Montana sapphires have been intermittently mined for gemstones during the last 50 years. Since the 1970s, many of the pale-hued sapphires from the Missouri River and Rock Creek deposits have been heated to enhance colour and make them more attractive gemstones. Recent detailed study of the Yogo dyke and a sill in the vicinity of the Missouri River deposits has shown that corundum in these intrusive bodies was derived from the partial assimilation of corundum-bearing xenoliths. Xenoliths of lithologies known to be abundant in the Precambrian basement of southwestern Montana have been identified in both of these Cretaceous or Tertiary intrusives. Although the corundum-bearing sill in the vicinity of the Missouri River deposits is clearly too small to be a significant source of alluvial sapphires, it provides a clue as to their possible source. The occurrence of corundum gneiss in the Precambrian basement and the occurrence of basement xenoliths in these igneous bodies raises the possibility that Precambrian corundum-bearing gneiss may have been a source of sapphires in alluvial deposits in Montana. We speculate that magma generated in the upper mantle or lower crust incorporated corundum gneiss that was partly assimilated, leaving corundum xenocrysts that were subsequently released during weathering of these igneous rocks.
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