162 BASTIN AND HILL! ORE DEPOSITS OF GILPIN COUNTY 



The galena-sphalerite type is less widespread than the pyritic. 

 The predominant minerals are galena, sphalerite and a carbon- 

 ate (usually calcite or siderite) ; pyrite, chalcopyrite, and quartz 

 are present in lesser amounts. In the region northeast of 

 Idaho Springs a number of the veins of this type carry rhodo- 

 chrosite. The metal content is more variable than in the pyritic 

 ores, the usual content being as follows: Au 0.1 to 5.5 ounces; 

 Ag 2 to 25 ounces; Cu commonly less than 1| per cent, rarely 

 exceeds 10 per cent; Pb to 55 per cent; Zn to 25 per cent. 

 The varieties poor in gold have, in many cases, undergone down- 

 ward enrichment in silver and constitute the typical silver ores 

 of the region. In contrast to these, certain veins, such as the 

 Klondike in the Topeka mine, are very rich in gold. The 

 Klondike locally carried free primary gold in large plates; an 88- 

 pound piece from this mine yielded upon smelting S5,500. 



The composite ores are the result of dual mineralization, first 

 with ore of the pyritic type and then, after brecciation, with 

 ore of the galena-sphalerite type. The reverse relation of 

 pyritic ore filling brecciated galena-sphalerite ore was not ob- 

 served. Certain veins are pyritic at one end, composite in the 

 middle, and of the galena-sphalerite type at the other end. 

 Composite veins are in general most abundant between areas 

 characterized by pyritic ores and areas in which galena- sphalerite 

 ores predominate. 



It is beheved that the ores of this region are genetically related 

 to the Tertiary igneous rocks. The copper and iron ores, being 

 products of magmatic differentiation, are obviously so related. 

 In the case of the veins, the agent of ore deposition is believed 

 to have been ascending thermal solutions of an alkaUne or neu- 

 tral character. That the composition of these solutions varied 

 in different parts of the region even at the same period is shown 

 by the presence of enargite in a locahzed group of the pyritic 

 veins, and the presence of rhodochrosite in a localized group 

 of the galena-sphalerite veins. The composition* of the solu- 

 tions also changed progressively during the mineralizing period, 

 the pyritic ores being deposited in the early part of this period 

 and the galena-sphalerite ores at a later time. From the extent 



