TUNGSTEN 



671 



Table 145. — Minerals of tungsten 



Relative 

 Name Composition abundance 



Wolframite group: 



Huebnerite MnWOi Common. 



Wolframite -— (Fe,Mn)WO. Major. 



Ferberite FeWO, Common. 



Scheelite CaWO, Major. 



Powellite Ca(Mo,W)04(W03 Common. 



to about 10 percent) 



Stolzite PbWO, Uncommon. 



Tungstite HoWO, Common. 



Cuprotungstite Cu^WOjCOH): Uncommon. 



principal tungsten mine in the United States, the 

 tungsten is a coproduct with molybdenum and cop- 

 per; but in the second largest producer, tungsten is 

 a byproduct of molybdenum mining. Ore minerals of 

 other commodities that commonly occur in the tung- 

 sten deposits of the world are molybdenite (MoSo), 

 cassiterite (SnOs), chalcopyrite (CuFeSs), bismuth- 

 inite (BioS,), native bismuth (Bi), and fluorite 

 (CaFa). In a few places in the United States, tetra- 

 hedrite (SCuaS-SbaSs) and sphalerite (ZnS) are co- 

 products. 



TYPES OF DEPOSITS 



Concentrations of tungsten occur in a wide variety 

 of geologic environments that range from high- 

 temperature, deep-seated deposits to those deposited 

 at the surface from relatively low-temperature hot 

 springs. In addition, tungsten has been recovered 

 from placers and is known to exist in significant 

 though as yet-uneconomic amounts in the brines of 

 Searles Lake, Calif. The principal types of occur- 

 rence from which most production has come are 

 contact-metamorphic deposits (tactites), hydrother- 

 mal veins, and stockworks and related porphyry- 

 molybdenum deposits. Other deposits that have been 

 productive or represent significant resources either 

 are variants of these major types or result from 

 special geologic situations. These include pegmatites, 

 iron-manganese-bearing hot-spring deposits, replace- 

 ments of various host rocks under special conditions, 

 placer deposits, and the Searles Lake brines. 



CONTACT-METAMORPHIC DEPOSITS 



Tactite, one of the products of contact metamorph- 

 ism, is the principal commercial source of tungsten 

 in the United States and is second only to quartz 

 veins as a commercial source on a worldwide basis. 

 Tactites are the products of high-temperature re- 

 placement and recrystallization of pure or impure 

 limestone or dolomite at or near the contact of in- 

 trusive igneous rocks. The resulting dark-colored 

 deposits, comprising various combinations of such 



dark calc-silicate minerals as garnet (usually alman- 

 dine or andradite), epidote, hedenbergite, and horn- 

 blende, together with magnetite and some quartz and 

 calcite, are usually conspicuous in contrast to the 

 lighter-colored granitic rocks or the sedimentary se- 

 quence into which the granite was intruded. The 

 name "tactite" was applied to deposits of this type 

 by Hess (1919) and is analogous in a broad sense to 

 the Swedish term "skam." 



Contact-metamorphic deposits have been produced 

 by two basic processes — simple thermal metamorph- 

 ism of impure carbonate strata, and additive meta- 

 morphism of either pure or impure carbonate layers 

 wherein at least part of the material essential to the 

 deposit is added from the magma. Presumably, both 

 processes are active in the formation of tactites, and 

 essential materials added are principally iron and 

 silica with lesser amounts of aluminum manganese, 

 and possibly some magnesium. Locally, significant 

 amounts of tungsten, molybdenum, copper, zinc, sul- 

 fur, and fluorine are added — usually at a late stage. 

 Determination of which major elements have been 

 added and in what amounts is often difficult unless 

 the tactite zone can be traced along strike into the 

 unaltered strata where the original composition can 

 be determined. However, the presence of one or 

 more of the metallic elements, and of sulfur, fluorine, 

 or boron, is considered to be indicative of additive 

 processes. 



Tungsten, where introduced into tactites, occurs 

 only as scheelite or molybdenum-bearing scheelite. 

 Magnetite is usually present and may be abundant as 

 at the Long mine, Pershing County, Nev., and the 

 Old Hickory mine, Milford County, Utah. Pyrite, 

 pyrrhotite, molybdenite, sphalerite, chalcopyrite, 

 tetrahedrite, stibnite, and bomite are formed at a 

 late stage in some deposits, and fluorite, usually pres- 

 ent, is locally abundant as at Darwin, Calif., and 

 Tem Piute, Nev. 



In most places, tactites form distinctive, sharply 

 bounded bodies whose shape and distribution are 

 controlled by many factors but mainly by the contact 

 of an intrusive body which usually forms one wall, by 

 favorable composition of the invaded host rock, and 

 by the structural relations of the strata to the con- 

 tact. Tactite developed along conformable contacts 

 tends to form tabular bodies parallel to the contact 

 that seldom maintain a uniform width and vary from 

 a few inches to many feet. Where a contact crosses 

 bedding, beds more favorable lithologically may be 

 replaced outward from the contact for greater dis- 

 tances than less favorable ones — in some instances 

 for more than a thousand feet. Steep beds may be 

 replaced for considerable vertical extent, the tactite 



