242 R. H. Rastall — The Genesis of Tungsten Ores. 



veins, some 1-5 metres thick, and mainly consisting of quartz, cany 

 wolframite, pyrite, and some gold. The wolframite occurs in 

 thickenings of the veins, something like a chain of beads. Here 

 there is little evidence of any kind of pneumatolytic action. 



In the United States tungsten ores are very abundant in some 

 localities, and the total output is now the largest of any country in 

 the world. The tungsten boom of 1916 in the Western States has 

 already been referred to. The chief producers are the states of 

 Colorado, Arizona, and Nevada, while some important deposits of 

 scheelite are now being largely worked in California. The most 

 important area of all is undoubtedly Boulder County, Colorado, 

 north-west of the city of Denver. The occurrences of wolfram ores 

 here have been exhaustively described by Messrs. Hess and 

 Schaller.^ The ferberite area of Boulder County, of which the town 

 of Nederland is the centre, lies on an elevated plateau some 8,000 feet 

 above the sea, forming the eastern margin of the Rocky Mountain 

 system. The country rock consists of biotite-hornblende granite, 

 gneiss, and quartz-mica schist, all of pre-Cambrian age. The ferberite 

 occurs in a group of veins striking south-west to north-east and 

 accompanied by gold and silver veins of the same general trend. 

 The gold veins are of two types, characterized by sulphides and 

 tellurides respectively, and the ferberite veins are more closely 

 connected with the telluride type of gold vein. The only gangue 

 mineral of any importance is quartz ; occasionally a little felspar is 

 found, together with chalcedony and calcite. The sulphides 

 actually found in the ferberite veins include only chalcopyrite, 

 galena, and blende, and these only in small quantity. Some 

 molybdenite has been recorded from one locality only. These 

 veins are extraordinarily rich in ferberite, which sometimes 

 comprises the greater part of the veins, being accompanied only by 

 a little quartz. It is believed by Lindgren that these deposits are 

 a product of comparatively recent thermal activity, and the associa- 

 tion with tellurides is noteworthy. 



At Leadville, Colorado, wolframite and scheelite are associated 

 with quartz in pyrite-gold veins; the scheelite seems, as usual, to be 

 somewhat later than the wolframite. These veins appear to be 

 connected with a monzonite porphyry.^ 



In the Snake Range, White Pine County, Nevada, wolframite is 

 found with scheelite, pyrite, fluorite, and a little gold and silver in 

 veins connected with a granite-porphyry and cutting quartzites and 

 slates. Here the presence of fluorite indicates pneumatolytic 

 tendencies.^ 



In the Black Hills of Dakota tungsten ores occur in two very 

 different forms: the first, seen at Etta Knob and Nigger Hill, has 

 already been mentioned ; the second type is quite unlike anything 

 hitherto described. According to Irving,* the ore shoots of this area 



1 " Colorado Ferberite and the Wolframite Series " : Bull. 583, U.S. Geol. 

 Snrv., 1914. 



^ Fitch and Loughlin, Economic Geology, vol. xi, p. 30, 1916. 



2 Weeks, Bull. 840, U.S. Geol. Surv., 1908, p. 263. 



■* Irving, Trans. Amer. Inst. Min. Eng., 1901, and Professional Paper 

 No. 36, U.S. Geol. Surv., 1904, p. 363. 



