Abstract

The Ima mine, which has been one of the leading producers of tungsten in the United States, is of exceptional interest because of (1) the many different minerals contained in its ore, (2) the zonal arrangement of the minerals about a mass of granite exposed in underground workings, and (3) the close relationship between the mineralization and the formation of granite.The mineralization is confined to a complex group of fractures along the crest of an anticlinal fold of quartzitic strata belonging to the Belt series (pre-Cambrian) near a small irregular mass of early Tertiary (?) granite. The fractures hold quartz veins, some near the granite being relatively large. Although composed mainly of quartz, the veins also contain microcline, fluorite, muscovite, sericite, rhodochrosite, siderite, and calcite, the microcline, muscovite, and fluorite occurring near the granite, the sericite and rhodochrosite, generally more distant therefrom. The metal-bearing minerals are molybdenite, pyrite, sphalerite, tetrahedrite, chalcopyrite, galena, gratonite, scheelite, and huebnerite, the huebnerite, tetrahedrite, and pyrite being most abundant. The scheelite and molybdenite occur in and near the granite. The others have a less-restricted distribution, the huebnerite least of all. The huebnerite appears to be one of the youngest minerals. Microscopic features of the granite suggest that it formed at least in part by "granitization" of quartzite and that its formation may represent the initial stage of the mineralizing process.

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