30 RESURVEY OF CRIPPLE CREEK DISTRICT. [BULL. 264. 



The dependence of the ore zone on the surface would thrn merely 

 express the depth to which fissuring extended in a conical volcanic 

 mountain. 



We have thus two factors of importance to account for the scarcity 

 of ore shoots below the 1,000-foot level first, difficulties of develop- 

 ment and exploration, and second, the disappearance of fissures in 

 depth. They do not seem to be sufficient, however, and it is believed 

 that a third factor, as yet undiscovered, exists, and that it is related to 

 the chemistry of the actual ore deposition. 



In those districts where so-called secondary sulphide enrichment is 

 known to have taken place the ore minerals exhibit irr general an 

 orderly sequence, both in relative abundance and in kind, from those 

 characteristic of the most highly enriched ore near the zone of oxida- 

 tion to those constituting the original, lean, and unaltered ore. The 

 secondary minerals produced are such as can result from rearrange- 

 ment and concentration of elements present in different combinations 

 in the primary ores. At certain points within this range of alteration 

 it is possible to detect direct mineralogical evidence of the change of 

 one mineral to another, effected by solutions moving downward from 

 the zone of oxidation. In most cases the secondarily enriched ores 

 bear a recognizable relation to the lower limit of oxidation. 



Careful study of the Cripple Creek ore deposits has failed to discover 

 that the hypothesis of secondary enrichment is supported by crucial 

 evidence of the kind just indicated. The minerals are not arranged 

 in any discoverable definite sequence, nor does the present investiga- 

 tion find much to support the view that the rich telluride ores, as a 

 rule, pass with increasing depth into low-grade pyritic ores. Fre- 

 quently such ore as occurs below a depth of 1,000 feet is precisely the 

 same in character as ore found within 100 feet of the surface. Tetra- 

 hedrite, which has been regarded by some, without definite proof, as a 

 secondary mineral, occurs sporadically throughout the district and at 

 all depths reached by present workings. The richest ore does not 

 uniformly occur immediately below the oxidized ore. There is, in 

 fact, little indication of enrichment in the oxidized zone such as is so 

 often found in gold-quartz veins of the normal type. Frequently the 

 fresh telluride ore is extremely j*ich, and high-grade pockets occur 

 impartially in oxidized and fresh portions of the veins. Neither 

 would it be correct to say that there is a gradual decrease in the value 

 of ore in depth. It is quantity, not value, which decreases. 



While it is certain that pyrite, and possibly other minerals, has 

 formed at more than one period during the mineralization of the dis- 

 trict, and while it is equally clear that in general the rich tellurides 

 were the last of the ore minerals to be deposited, there is apparently 

 no evidence that any one of these minerals has been formed by enrich- 

 ing solutions at the expense of primary minerals. So far as definite 



