TUNGSTEN 



673 



tricts quartz veins that are mined primarily for 

 other metals — especially gold — contain scheelite as 

 a sporadically distributed secondary component. The 

 Mother Lode of California, the Murray district, 

 Idaho, and the Jardine district, Montana, are ex- 

 amples. 



Tungsten-quartz veins are similar in most respects 

 to typical hydrothermal vein deposits. They are 

 tabular bodies that vary greatly in length, vi^idth, 

 and depth. Although some vein zones extend for 

 thousands of feet, the individual component veins 

 tend to pinch and svi^ell or to be arranged as a 

 series of echelon lenticular segments both along 

 strike and downdip. The attitude of the veins de- 

 pends largely on the local structural environment. 

 Most known occurrences tend to dip steeply, but a 

 tew are controlled by nearly flat joint systems in 

 igneous terrane. 



Tungsten minerals in veins are usually localized 

 in ore shoots. These shoots, as vi^ell as the veins 

 themselves, are controlled by general factors of 

 temperature, pressure, chemical composition of 

 mineralizing solutions, available channelways for 

 solutions, the sequence of mineral formation, and 

 the local effect of structure and type of country 

 rock. Quartz-tungsten veins are, as a rule, spatially 

 very closely related to the contacts of igneous intru- 

 sive rocks and notably to the roof zones of such 

 intrusions. This relation is illustrated by the not- 

 uncommon association of major districts with small 

 plutons as at Boriana, Ariz., and the Ima mine, 

 Idaho, which are probably in cupolas of larger sub- 

 jacent masses, and the alinement of major vein 

 zones with granite-cored anticlinal ridges as in the 

 Chinese deposits. Tungsten ore shoots in quartz 

 veins range from small isolated pockets of scheelite 

 or wolframite to nearly continuously mineralized 

 vein material that may measure a thousand feet or 

 more on strike and downdip. Such extensive ore 

 shoots are the exception in the United States, how- 

 ever, and most individual shoots have a vertical 

 range of less than 500 feet. Cumulatively, the eche- 

 lon ore bodies both at the Ima mine, Idaho, and in 

 the Hamme district. North Carolina, extend well 

 beyond a depth of 1,000 feet. In most occurrences, 

 however, the ore shoots are small and even though 

 the vein itself may be extensive, the concentrations 

 of tungsten minerals within it are widely and 

 erratically dispersed. Such "pockety" habit is com- 

 monplace and makes exploitation difficult. The 

 thickness of tungsten veins varies as widely as do 

 length and depth, but the economically minable size 

 depends as much on grade, attitude, and other physi- 

 cal factors as on actual extent. For example, a 



1-inch-thick vein of pure ferberite (60 percent WO3) 

 in the Boulder district, Colorado, was mined; else- 

 where, shoots averaging less than 1 percent across 

 15 feet or more of vein material have been worked. 

 The average grade for all productive tungsten vein 

 deposits in the United States is probably close to 1 

 percent. Major productive vein deposits in the 

 United States are at Boriana, Ariz., Atolia, Calif., 

 Boulder district, Colorado, Ima, Idaho, and Hamme, 

 North Carolina. Numerous smaller deposits are 

 widely scattered in Arizona, Nevada, Colorado, 

 Washington, Idaho, and Montana. 



STOCKWORK AND RELATED DEPOSITS 



Deposits of tungsten minerals as fracture fillings 

 and replacements in stockworks and breccia zones 

 are known at several places in the United States. 

 These occurrences, though comprising varied host 

 rocks, mineral associations, shape, size, and grade, 

 have at least two common prerequisites of origin — 

 a source of hydrothermal solutions with suitable 

 access channelways, and suitable sites of deposition 

 prepared through intimate fracturing, brecciation, 

 or shearing of the host terrane. In most deposits of 

 this kind (except the closely related porphyry- 

 molybdenum occurrences), scheelite is the only 

 tungsten mineral. Where breccias and stockworks 

 were formed in carbonate rocks, the composition of 

 the host undoubtedly was a major factor in the 

 deposition of the scheelite. Similar chemical controls 

 may have been an important, even if subordinate, 

 factor in other environments. 



At Henderson Gulch, Mont., parts of a highly 

 fractured granitic body are mineralized along frac- 

 ture planes by iron sulfides with some gold and 

 scheelite. A small amount of gold and scheelite has 

 been recovered from placer deposits downstream 

 from this occurrence although the source rock it- 

 self has so far proved to be too low-grade for min- 

 ing. At the Goat Creek property near Thompson 

 Falls, Mont., an elliptical area of moderately frac- 

 tured and altered nearly horizontal sedimentary 

 rocks of the Wallace Formation of the Precambrian 

 Belt Supergroup carries thin seams and veinlets of 

 iron sulfides, molybdenite, and some scheelite. This 

 occurrence, located on the crest of a broad anticline, 

 coincides with a strong, nearly circular aeromag- 

 netic anomaly that has been interpreted to reflect 

 a buried stock at some unknown depth. Although 

 this deposit has not yet been proved economically 

 feasible, it illustrates a mode of occurrence that 

 may have future potential either here or elsewhere. 



Small calcite-scheelite replacement pipes and 

 stockworks have been mined in Nevada and Call- 



