22 RESURVEY OF CRIPPLE CREEK DISTRICT. [BULL. 254. 



STRUCTURAL CHARACTER OF DEPOSITS. 



With few exceptions the ore bodies, of whatever shape, are causally 

 connected with fissures, and most of them constitute fissure veins of 

 various types. The fissure system of the district appears to radiate 

 from a point near the northern limit of the volcanic area. In the 

 eastern part the prevailing directions are northwest or north-northwest, 

 gradually changing to a northerly strike in the southern portion and 

 to predominant north-northeast or northeast courses in the western 

 side of the district. 



Individual veins are rarely over half a mile in length, but linked- 

 vein systems often extend for a mile in the same direction. The dip 

 is generally veiy steep. The movement along these fissure planes 

 appears in all cases to have been very slight. The fissures charged 

 with ore are sometimes simple veins with one fracture plane; much 

 more commonly, however, they are composite veins or lodes which 

 consist of several closely spaced and frequently linked fissures, all 

 more or less ore bearing. A better expression for this structural type 

 as it appears in Cripple Creek is the term " sheeted zone." 



TYPES OF DEPOSITS. 



The most important types of auriferous ore bodies occurring in the 

 district are: 



1. Tabular in form and strictly following simple fissures or sheeted 

 zones. A subtype comprises lodes in which the sheeted zone follows 

 "basalt" or phonolite dikes. 



2. Irregular bodies adjacent to fissures and formed by replacement 

 and recrystallization of the country rock usually granite. 



These types are not always sharply distinct, but may be connected 

 by deposits of intermediate character. 



All the ore bodies, of whatever type, exhibit certain common features 

 which serve to distinguish the deposits of Cripple Creek from those 

 of most other mining districts. In the first place, the actual openings 

 in the rocks available for the deposition of ore are, as a rule, remark- 

 ably narrow. In the second place, the amount of material carried in 

 the mineralizing solutions and deposited as gangue and ore minerals 

 was comparatively small. In consequence of these two conditions, the 

 district contains no such massive veins, solidly filled with quartz or 

 other vein minerals, as are characteristic of the San Juan region in 

 Colorado or the Mother Lode region in California. Even the small 

 fissures of the Cripple Creek district are rarely completely filled, but 

 exhibit a characteristic open or vuggy structure. Where the fractures 

 are of unusual width, or where the i jcks are extensively shattered, as 

 in the Midget and Moose mines, the small volume of available vein 

 matter is particularly noticeable. The walls of such fractures and the 



