70 ECONOMIC GEOLOGY 



Fissure veins are the most pronounced and persistent type of ore 

 deposits in the district. Some of these may be traced for more 

 than a mile along the line of their outcrop. Sometimes they reach 

 a width of 75 to 100 ft. and are well mineralized throughout. 

 Much free gold occurs in the Camp Bird, Revenue Tunnel, Atlas 

 and Torpedo-Eclipse mines. Samples from the latter mine have 

 assayed over $50,000 per ton. The ore is often associated with 

 tellurium also encased in chalcopyrite with a gangue of country 

 rock and clay. 



Quartz, calcite, fluorite and bariteare common gangue minerals. 

 The silver occurs in part native, in part with galenite and tetra- 

 hedrite and in part as stephanite. Less important replacement 

 deposits occur in the quartzites, sandstones and limestones. 

 These deposits in the sandstones are more or less irregular but in 

 the limestones they occur as broad flat ore bodies associated with 

 the fissure veins that penetrate the limestones. 



The fissuring appears to be late Tertiary and the mineralization 

 in some period later than the introduction of the volcanics. 



Georgetown District: This district is in the Continental 

 Range in Clear Creek County, about 50 miles west of Denver. 

 The base of the Geological series consists of pre-Cambrian schists 

 of sedimentary origin which are overlain by highly metamor- 

 phosed schistose rocks of igneous origin. This series of terranes 

 was later penetrated by both acid and basic intrusives. The latest 

 irruptives of probably late Cretaceous or Tertiary age, consist of 

 porphyry dikes. These porphyries are of special interest because 

 they stretch in a northeasterly and southwesterly direction nearly 

 the entire length of the state. The lodes occur in fissure veins 

 that cut the pre-Cambrian schistose rocks. 



Auriferous pyrite with a quartz gangue predominates in the 

 neighborhood of Georgetown. These veins may or may not bear 

 silver. At the Silver Plume mine much fine-grained argentiferous 

 galenite is encountered. At Idaho Springs the prevailing ore is 

 an argentiferous galenite-sphalerite which contains but little gold. 

 According to J. E. Spurr, descending meteoric waters have 

 effected from the wall rock a mixture of quartz, calcite, kaolin 

 and sericite. The walls of the fissure appear to have been the 

 source of the gangue minerals, while the gold and silver were 

 contributed to the veins by magmatic waters. 



The gold-bearing veins appear at the lower level and the silver 

 at the higher altitudes in this deeply incised region. The former 



