204 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
the free gold of the ordinary oxidized ore is very often entirely free from silver. 
Possibly, as suggested by Pearce, the silver has been dissolved by ferric sulphate. 
No evidence has been found to show that any part of the gold set free by 
oxidation has been dissolved and reprecipitated. The silver dissolved from the 
oxidized ores should be deposited as chloride and native metal just above or as 
sulphides below the water level. No evidence has been found that this process 
has taken place. The quantity of silver in the veins is, however, usually very 
small. 
The analysis quoted in the paper by Pearce contains in the oxidized portion 
far more tellurium in oxidized form than would be required to form tellurides with 
the gold and silver. If this were a general fact it would indicate that a part of the 
gold had been dissolved. The two analyses by Hillebrand of samples from Stratton’s 
Independence mine (p. 202) show, on the contrary, a very considerable deficiency 
of tellurium, and it is in fact easily to be comprehended that the relatively soluble 
oxidized tellurium compounds could have been carried away or locally concentrated. 
SECONDARY SULPHIDE ENRICHMENT. 
If oxidation of a deposit is accompanied by leaching of one or more metals, like 
copper, silver, or lead, b} r surface waters, it sometimes happens that the materials 
dissolved from the oxidized part of the veins will be precipitated as sulphides at a 
lower elevation, and generally just below the water level. 
In districts where this so-called sulphide enrichment is known to have taken 
place the ore minerals exhibit in general an orderly sequence, both in relative 
abundance and in kind, from those characteristic of the most highly enriched ore 
to those constituting the original lean and unaltered ore. The secondary minerals 
produced are such as can result from rearrangement 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 recog¬ 
nizable 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 sequence, nor 
does the present investigation find anything to support the view that the rich 
telluride ores, as a rule, pass with increasing depth into low-grade pyritic ores. 
Frequently 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. Tetrahedrite, 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. 
No particularly rich ore occurs immediately below the oxidized zone. Briefly, 
no evidence has been found indicating that sulphides or tellurides have been formed 
in or below the oxidized zone by deposition of material dissolved by surface waters. 
