676 



UNITED STATES MINERAL RESOURCES 



Table 146. — Geology and reserves of principal tungsten deposits of the world 



^ Identified deposits from which minerals 



be extracted profitably with existing technology and under present 



the Yukon within the last 20 years is a specific 

 example. In other parts of the world, tungsten, pre- 

 sumed to be of Precambrian age, is widely scat- 

 tered in parts of the Brazilian, African, and Cana- 

 dian Shield areas, as well as in the Precambrian 

 crystalline cores of the Rocky Mountains of Colo- 

 rado and Wyoming. Although mostly small as now 

 known, these deposits are indicative of a time-and- 

 place relation that once thoroughly understood 

 might lead to the discovery and exploitation of sig- 

 nificant amounts of tungsten ore. The association 

 of tungsten with porphyry-molybdenum deposits in 

 Colorado is well known and such association is 

 suggested in other areas as well. In contrast, the 

 association of tungsten with porphyry copper de- 

 posits is thought to be rare or is undetermined, in 

 spite of the fact that these deposits are strikingly 

 similar in mode of occurrence and origin to the 

 molybdenum-bearing porphyries. Recovery of tung- 

 sten as a byproduct from some of the porphyry 



copper operations is a speculative possibility. 



PROSPECTING TECHNIQUES 



Many early discoveries of tungsten deposits were 

 made from the recognition of scheelite and wolf- 

 ramite, accompanying other heavy minerals, in 

 panned concentrates collected in the search for gold 

 and other precious metals. In outcrop, wolframite, 

 as black bladed crystals in quartz veins, was easily 

 identified, but scheelite, because of its similarity in 

 color and luster to quartz and feldspar, was often 

 overlooked. Panning is still a very useful technique 

 for tracing tungsten to its source. Development of a 

 portable shortwave ultraviolet light during the 

 1930's greatly stimulated the search for scheelite, 

 which fluoresces a bright blue-white to creamy- 

 yellow at this wavelength. The degree of yellow 

 fluorescence is controlled by the amount of molyb- 

 denum in the mineral structure. A person experi- 

 enced in the use of this technique can detect very 



