The tungsten occurrences in the United States are discussed from the standpoint of origin. Broad zones in which tungsten deposits are more abundant than elsewhere are indicated. These encircle the Colorado plateau, an eastern arc embracing the Colorado-Wyoming area, a central arc extending from Arizona to Montana through Utah and eastern Nevada, and a western arc trending northward from southern California. Aside from an occurrence in North Carolina the localities east of the Cordillera are of little consequence.
The tungsten deposits are related to igneous intrusives which are prevailingly Laramide in the central and eastern arcs and Nevadian in the western arc. Related intrusives vary widely in size, and the deposits are found in several relationships with respect to the igneous masses. The intrusives themselves are largely acidic, while tungsten deposition is related to the aftermath rather than earlier stages of invasion.
Deposits of direct magmatic formation are doubtful, the nearest approach being irregular disseminations in end-stage phases. Pegmatitic origin exists in a number of places, while both pegmatites and aplites have probably frequently contributed to tungsten accumulation.
Most abundant are the deposits of the contact-metamorphic type. These occur either at the contact of limestone with an intrusive or some distance within the calcareous rock. Tactite is a common accompaniment of contact action but does not always carry scheelite. The tactite itself shows stages of development from the pale moderately mineralized phase to a coarsely crystalline quartz-garnet-epidote rock frequently associated with scheelite. In a few places scheelite is associated with magnetite-garnet zones in limestone which are called skarn.
The great igneous areas of the Sierras contain large blocks of metamorphosed sediments, frequently schistose. Larger masses separating granite massifs are described as septa. Within these septa the calcareous strata may be replaced in part by tactite and accompanying scheelite. The emplacement of dikes in the adjacent massif may participate in the process of tungsten migration and accumulation.
Tungsten-bearing quartz veins are widely distributed. These occur both within areas of intrusives and cutting bordering rocks of several types. Common minerals of both the scheelite and wolframite groups are found. A frequent accompaniment is wall-rock alteration to sericite or muscovite. Metallic sulphides may be introduced in the veins in a later epoch of mineralization.
A later stage of deposition is represented by hot-spring deposits. Limonitic ores and manganese ores high in tungsten may be found beneath outflow layers of calcareous tufa.
A complete outline is given of localities on record prior to January 1942 with notes on more important discoveries reported since. Brief statements are furnished descriptive of many occurrences. At the risk of confusing the reader the bibliography includes obscure references and frequently casual accounts, since even such meager data have occasionally contributed to discoveries. More substantial articles will be recognized in most instances through their source.