OCCURRENCE, VEIN SYSTEMS, AND RELATIONS. 9 



Biotito-hornblende granite, in some places gneissoid, granitic 

 gneiss, and quart /-mi<-a schist form the count rv rocks, The gneiss 



*""*"' ^LM^MMMMMMHMfH^ . ^^^ AMMMMN*- 



and schist are older than the granite, and all three are of pre-Cam- 

 man age and are cuTbTlater dikes that range in composition from 

 imburgite to granite pegmatite. 



Dynamic metamorphism has in places made the granite, gneiss, 

 and schist difficult to differentiate. Some of the dikes are also more 

 or less squeezed. The gneiss is believed by R. D. George l and Edson 

 S. Bastin 2 to be of sedimentary origin. The pegmatites show only 

 a little crushing and carry few metallic minerals. 



OCCURRENCE, VEIN SYSTEMS, AND RELATIONS. 



The ferbcrite occurs in a group of veins which in general extend 

 from southwest to northeast, though individual veins strike toward 

 nearly every point of the compass. 



( )n the northwest and southwest sides of the tungsten-bearing area 

 are gold and silver bearing veins having the same general trend as the 

 tungsten veins. The veins carrying the precious metals are acontin- 

 uation of the gold-bearing belt of Clear Creek and Gilpin counties. 3 



Two types of veins carry the gold and silver those in which the 

 minerals are mostly sulphides and those in which the gold occurs as a 

 telJTirijo. 4 The sulphidic veins are in general quartz veins carrying 

 gold x and silver bearing sulphides, such as pyrite, galena, chalcopyrite, 

 and zinc blende. -Silver predominates. The telluridic veins 5 occur 

 in sheeted zones and contain only a little quartz. They carry gold 

 mostly in a telluride, which is probably sylvanite, and the\gold pre- 

 dominates over the silver. Pyrite, molybdenite, a vanadium mineral 

 allied to roscoelite, barite, lidularia, and chalcedony accompany the 

 telluride. The veins are thin and commonly frozen to the walls. 



The ferberite veins are more or less connected with the gold veins, 

 as might be expected from the general grouping, and their connec^ori 

 with the telluridic veins seems closer than with the sulphidic veins. 

 Specimens from Magnolia, at the southeastern edge of the field, 

 showed both gold telluride and ferberite, but it could not be deter- 

 mined which was the older. George reports a like specimen in which 

 the telluride was evidently the older mineral. 8 He also reports the 

 occurrence of the two minerals in the Wheelmen Tunnel, in Boulder 

 Canyon, 8 and at Sunshine, on the northern a dge of the field. A speci- 



1 George, R. D., The main tungsten area of Boulder County, Colo.: Colorado Geol. Survey First Kept., 

 pp. 19-20, 1909. 

 1 Oral statement. 



* Lindgren, Waldemar, Some gold and tungsten deposits of Boulder County, Colo.: Econ. Geology, voL 

 2, p. 453, 1907. 



4 Idem, pp. 456-457. 

 & Idem, pp. 457-459. 



George, R. D., op. cit., p. 76. 



