BASTIN AND HILL: ORE DEPOSITS OF GILPIN COUNTY 161 



authors. The tungsten district of Boulder County is the prin- 

 cipal producing district in the United States. 



(4) Copper ores. Ores poor in silver and worked primarily 

 for copper occur in one mine, the Evergreen, near Apex. The 

 copper sulphides here are primary constituents of monzonite dikes. 



(5) Titaniferous iron ores. These are products of magmatic 

 differentiation within the monzonite stock at Caribou. They 

 have no present or prospective commercial value. 



Veins are the predominant structural type among the ore 

 deposits. These commonly follow fracture zones rather than 

 single fractures. The longest is the Mammoth vein near 

 Central City which has been traced continuously for 6000 feet. 

 The greatest depth to which a vein has been followed has been 

 2250 feet. The vein mineralization has been accomplished in 

 part by fissure-filling and in part by replacement, the latter 

 process being in general predominant. In addition to the veins 

 there are in the region a few stockworks, the best known being 

 the so-called "Patch" near Central City, a pipe-shaped mass of 

 brecciated rock formed at a place where a number of strong 

 vein fractures approach unusually close together. The miner- 

 alization of the ''Patch" is continuous with that of the veins 

 which enter it and is of the same mineral character. Chamber 

 stopes large enough to accommodate a three-story house have 

 been excavated in the ''Patch." 



Gold-silver ores. These occur mainly as veins with a few 

 stockworks. The main values are in gold and silver; the sub- 

 sidiary values in copper, lead, or zinc. They may be classed, 

 on the basis of mineral composition, into the pyritic type, galena- 

 sphalerite type, composite type, and the telluride type. The 

 predominant minerals of the pyritic ores are pj^ite and quartz; 

 chalcopyrite and tennantite are usually present. They are 

 essentially gold ores. Their usual composition is as follows: 

 Au 1 to 3 ounces; Ag 4 to 8 ounces; Cu commonlj^ less than 

 1| per cent, but in some 15 to 16 per cent. Within a small 

 area near Russell Gulch a number of the pyritic veins carry 

 enargite in place of tennantite, and some of the veins in this 

 region also carry fluorite. 



