6 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
CHAPTER VII.-STRUCTURE OF THE GOLD DEPOSITS. 
Principal types of deposit .—The ore bodies are in almost all instances causally 
related to fissures. They comprise (1) lodes or veins and (2) irregular replacement 
bodies, usually in granite. These two types are not sharply distinct. All of the 
ore deposits are characterized by the narrowness of the fissures which gave passage 
to the ore-bearing solutions and by the comparatively small volume of material 
deposited in these fissures. 
Lode fissures .—These occur mainly within and near the volcanic neck, and 
have a rough radial plan. They are usually nearly vertical, although there are 
some notable examples of productive “flat veins.” Individual fissures rarely 
exceed half a mile in length, but some well-defined zones of Assuring are traceable 
for twice that distance. Some of the most productive fissures are less than 300 
feet in length. The persistence of a fissure down the dip is roughly proportional 
to the length of the same fissure. In general the fissures seem to be smaller and 
less abundant in depth than near the surface. 
Productive lodes occur in all the rocks of the district, with the possible excep¬ 
tion of the schist. They are most abundant in breccia and in granite. Many lodes 
follow phonolitic or basic dikes. 
The structure most characteristic of the Cripple Creek lodes is the sheeted 
zone, which occurs in various degrees of regularity and in widths ranging from a 
few inches to over 100 feet. The fissures of the zone are usually very narrow, 
although there are a few notable exceptions to this rule. Evidence of tangential 
movement or faulting along the fissures is rare. 
Intersections of one fissure zone by another are common and usually show no 
visible displacement. In general the fissures are not fault planes, although there 
are some exceptions to this statement. 
The fissures were probably formed about the same time as the intrusion of the 
basic dikes and represent a late phase of volcanic activity. They were probably 
opened under relatively light load by local compressive stresses due to a slight 
subsidence of the solidified breccia, and associated intrusive rocks forming the 
volcanic neck. 
CHAPTER VIII.-THE ORES. 
The characteristic feature is the occurrence of the gold in combination with 
tellurium and chiefly as calaverite. Native gold is present in the unoxidized ores 
only as a rarity. Pyrite is widely distributed in the country rock and also occurs 
in the veins, with tellurides. Galena, sphalerite, tetrahedrite, stibnite, and molyb¬ 
denite are sparingly present . Among the gangue minerals quartz, fluorite, and dolo¬ 
mite prevail; celestite is also often present, but is frequently changed into quartz. 
The ore occurs chiefly as filling of narrow fissures, and consequently the ores as 
mined have the approximate composition of the country rock. The average 
tenor of gold in the ores is about $30, or 1.5 ounces per ton, and at various mines 
ranges from 1 ounce up to 3 or 4 ounces. Ore with less than $12 per ton is rarely 
mined. Small amounts of ore with up to 2,500 ounces of gold per ton have been 
mined. The pyrite is rarely auriferous except when admixed with tellurides. 
