24 BESUKVEY OF CRIPPLE CREEK DISTRICT. [BULL. 254. 



sures as in the breccia, in other cases the ore to some extent permeates 

 the granite alongside the fissure, this constituting a deposit interme- 

 diate in nature between types 1 and 2. They also frequently follow 

 phonolite dikes, the general tendency of these dikes to develop a platy 

 parting parallel to their walls being particularly favorable to the produc- 

 tion of a well-defined sheeted zone when the direction of fissuring hap- 

 pens to coincide with that of the dike. 



The metasomatic alteration accompanying these sheeted zones is sur- 

 prisingly slight, and consists of a partial replacement of the breccia, 

 phonolite, trachytic phonolite, or "basalt" by dolomite and pyrite 

 accompanied by a small amount of sericite and a little secondary potash 

 feldspar. But even in the most altered rock the newly formed min- 

 erals rarely form more than a small percentage of the rock mass. The 

 alteration in granite exhibits a somewhat different phase, described in 

 a subsequent paragraph. 



Not all the sheeted zones carry ore, nor is the ore of a productive 

 sheeted zone necessarily coextensive with the fissuring. The ore occurs 

 in pay shoots up to 2,000 feet in length and 1,000 feet in depth, but 

 usually very much smaller than is indicated by these limits. The 

 boundary between the ore and the barren portions of the lode can be 

 determined, as a rule, only by assays. No single factor that can 

 account for the localization of the ore in these pay shoots has been 

 discovered. In some mines the pay shoots occur where the lode is 

 intersected by cross fissures; in other mines no such relation exists. 

 In some mines ore occurs where the fissures pass through phonolite 

 dikes; in other mines the lode, elsewhere productive, becomes barren 

 when it enters phonolite; while in still others the presence of the 

 phonolite has had no apparent influence upon ore deposition. It 

 thus appears that the occurrence of two or more favorable factors is 

 necessary to determine the position of a pay shoot in a lode. The 

 discovery of these factors is one of the unsolved problems connected 

 with the Cripple Creek district. 



Replacement deposits in granite. The replacement deposits in 

 granite all occur in close proximity to the contact with the breccia, 

 and are well developed in the Elkton (Thompson), Ajax, Independence, 

 and Portland mines. Although these bodies of ore are related to fis- 

 sures and occur particularly where several fissures intersect, or where 

 they meet a dike, the ore is not confined to the actual fractures. The 

 rock in the vicinity of these fissures is often extensively altered. The 

 change from altered to unaltered rock, while never perfectly sharp, is 

 often fairly abrupt and may take place within a distance of a few feet. 

 The most obvious characteristic of the metamorphosed rock is a porous 

 texture and a change of the reddish color of the normal granite to 

 grayish or greenish tints. Closer examination shows that, while the 

 porphyritic aggregates of pink microcline, so prominent in the Pikes 



