] TYPES OF DEPOSITS. 23 



fragments of the shattered rock are usually merely coated with a thin 

 deposit of quartz, fluorite, and other minerals. As the rich tellurides 

 were usually among the minerals last to form, and are particularly 

 abundant on the walls of the vugs, it is probable that had quartz, fluor- 

 ite, or other gangue minerals been more abundantly deposited the 

 ores would have been of much lower grade. 



Sheeted veins. The mineralized sheeted zones constitute the most 

 characteristic deposits of the district and occur in practically all the 

 rocks, although particularly common in breccia. They consist of a 

 varying number of narrow, approximately parallel fissures, together 

 composing a sheeted zone that may range from a fraction of a foot to 

 50 or 60 feet in width. Such uncommonly wide zones of fissuring, 

 however, can usually be resolved into two or more sheeted zones lying 

 so close together that the whole constitutes for practical purposes a 

 single ore body, as in the Captain vein system of the Portland mine. 

 Usually the sheeted zones are from 2 to 10 feet in width. In other 

 cases the fissures may be very numerous, the rock for a foot or more 

 in width being divided into thin parallel slabs, while on each side of 

 this medial portion the fissures become farther and farther apart as 

 the lode grades into the normal country rock of the vicinity. In still 

 other cases there may be two main fissures, 3 or 4 feet apart, accom- 

 panied b}^ more or less irregular fracturing of the intervening and 

 adjacent rock. As a rule the fissures are mere cracks, showing no 

 brecciation, slickensiding, or other evidence of tangential movement 

 of the walls. Usually the tellurides are exclusively confined to the 

 narrow fissures and cracks, and do not, in this type of deposit, in any 

 sense constitute a replacement of the country rock. The rocks in the 

 vicinity of the fissures are partly replaced by dolomite, pyrite, and 

 a little fluorite; the telluride ores, however, do not share this pro- 

 pensity, but coat the open fissures, associated with a little quartz and 

 fluorite. Replacement by tellurides does occur in two other types of 

 deposits, to be described later; but as regards the simple veins and 

 sheeted zones, it will be necessary to modify the results of Penrose by 

 restricting the metasomatic role of the tellurides. In the oxidized 

 parts of the veins, such as were almost exclusively available for obser- 

 vation when Penrose visited the district, these relations can seldom be 

 clearly ascertained, and would easily lead one to overemphasize replace- 

 ment as a feature of vein formation. The fissures are not, in general, 

 planes of faulting. Appreciable movement has undoubtedly occurred 

 in some instances, but the displacement probably rarely exceeded 1 or 

 2 feet. 



Although found most abundantly in the breccia or trachytic phono- 

 lite, sheeted zones and single fissures are often well developed in the 

 granite, as in the El Paso, C. K. & N., and Gold Coin mines. While in 

 some of these lodes the ore minerals are as plainly confined to the fis- 



