530 



GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



sists of iron-stained clay, with layers of light-colored banded chert, from a 

 few inches to two or three feet in thickness, which pass into coarse silicious 

 sandy material, resembling quartzite ; this latter constitutes the richest gold 

 ore. Impure kaolin and Chinese talc are also found in the vein material, 

 and are generally gold-bearing. The deposit is thus in almost every respect 

 a counterpart of those of Leadville, except that gold replaces silver and that 

 lead is wanting. From this vicinity was obtained direct evidence that tho 

 iron oxide in these vein materials results from the decomposition of pyrites, 

 in the kernel of undecomposed pyrite that sometimes is found in the center 

 of a nodule of vein material. 



Figure 6. El Capitau Mine. 



\Vrhrr f Me.. 



firnj Porphyry. 



The following table shows the result of a chemical examination of one 

 of these nodules from No Name gulch, to the south of the El Capitan mine. 

 Analysis No. 1 is tho pyrite kernel, No. 2 the dark zone next to it, and No. 

 3 the lighter-colored outer zone. From these it would seem that the proc- 

 ess of change is not only an oxidation of the sulphide, but a gradual 

 removal, first of the sulphuric acid and later of part of the hydrated oxide 

 of iron, and a replacement of this by insoluble material, mainly silica. 

 From the two analyses of rich ore (Nos. 4 and f>) in the same table, the 

 richer of the two appears to be that in which this change has proceeded 

 furthest, and which is called "silicious ore," although its contents in alu- 

 mina would seem to indicate a mechanical as well as a chemical interchange. 



