TUNGSTEN 



677 



small amounts of scheelite and estimate grade quite 

 accurately. Most discoveries of workable tungsten- 

 bearing tactite deposits in the United States were 

 made by use of this method through systematic 

 scanning of recognized tactite zones. 



Geophysical methods are particularly applicable 

 to the selection of geologic environments in which 

 tungsten deposits might logically exist, but no direct 

 measurements of the tungsten content can be made 

 by these techniques. Broad regional studies by means 

 of airborne magnetic and radiometric surveys and 

 regional gravity measurements are useful in identi- 

 fying variously anomalous rock bodies that are 

 known to relate to possible environments of ore 

 deposition. Such measurements have located igneous 

 contacts, have provided data on location and depth 

 of buried plutons and cupolas, have delineated 

 tungsten-associated granites (as in the Kazakhstan 

 region of the U.S.S.R.) that have relatively low 

 magnetic field intensity and high radioactivity, and 

 have helped define such regional structures as the 

 Colorado Mineral Belt, whose intrusive igneous 

 masses are reflected in low gravity anomalies even 

 though the associated molybdenum-tungsten min- 

 eralization is not so reflected. 



Detailed geophysics may apply directly to the de- 

 tection of ore zones, individual ore bodies, or prom- 

 ising environments. Many tactites that contain 

 magnetite and (or) pyrrhotite are responsive to 

 magnetic methods of detection ; those with high sul- 

 fide contents respond to various electrical tech- 

 niques ; and detailed gravimetry may usefully refine 

 the location, size, and depth of small plutons or the 

 configuration of significant contacts. 



Geochemical surveys, though used sparingly in 

 the United States to date, can be useful for future 

 tungsten exploration. The simplest and perhaps most 

 useful technique is the panning and analysis of 

 stream sediments. Because of their high specific 

 gravity and low solubility, tungsten minerals tend 

 to concentrate as resistate fractions in streambeds, 

 and the systematic sampling of stream sediments 

 will often identify anomalous drainage basins and 

 pinpoint source areas. The sampling and analysis 

 of individual plutons of an intrusive complex has 

 shown that, in some instances, plutons associated 

 with tungsten mineralization contain anomalous 

 amounts of tungsten and associated elements. This 

 technique, even though not specific for individual 

 deposits, does narrow the target for the application 

 of other methods. 



Tungsten, because of its low chemical mobility 

 under weathering conditions, generally produces 



geochemical anomalies of only limited range sur- 

 rounding the source material. Analysis for indicator 

 elements such as fluorine, arsenic, and (or) phos- 

 phorus that have greater mobility and are commonly 

 associated with tungsten deposits may, in some cir- 

 cumstances, be more useful because of the wider 

 halos they produce in rocks, soils, and (or) plants. 

 In some special circumstances, other metallic ele- 

 ments commonly associated with tungsten such as 

 molybdenum, copper, bismuth, antimony, and tin, 

 may be used as indicators. 



PROBLEMS FOR RESEARCH 



Much has been learned about tungsten deposits 

 from long experience with them. The application of 

 empirical data has identified the common modes of 

 occurrence, the effects of local geologic controls, the 

 general geologic environment, associated minerals, 

 general distribution, and other characteristics, and 

 it has been useful in the discovery of new deposits 

 and even new districts. There are limits, however, 

 to this approach and a full evaluation of the re- 

 source potential of tungsten must await the answers 

 to many basic questions : Why is tungsten found in 

 some geologic provinces and not in others? Why is 

 it associated with igneous rocks of certain composi- 

 tion or of certain ages? Why is it frequently 

 pockety and erratic in distribution ? Why has it been 

 deposited over such a wide range of temperature- 

 pressure conditions? Why is it deposited as scheelite 

 in one place and wolframite in another? These are 

 but a few questions, most of which relate to tung- 

 sten's geochemistry — its distribution in the earth's 

 crust and the processes by which it is concentrated 

 into ore deposits. 



Answers to these and other pertinent questions 

 can come only from basic research that should, 

 ideally, combine geologic mapping and sampling of 

 carefully selected field areas with detailed laboratory 

 work on the mineralogy and chemistry of all re- 

 lated geologic components. One of the key restraints 

 in this approach has been the inability to analyze 

 with accuracy for small amounts of tungsten (less 

 than 10 ppm) in rocks and minerals. It is hoped 

 that the refinements in techniques already made or 

 being developed will lower the level of accurate de- 

 tection into the parts-per-billion range. Such refined 

 measurements may lead, for example, to the identi- 

 fication of source rocks, the delineation of channels 

 of migration of ore-bearing solutions, or the recogni- 

 tion of dispersion halos of buried ore bodies. 



Many prospecting techniques have been used in the 

 search for tungsten, but new approaches are needed 

 and undoubtedly wifl be developed. Immediate help 



