224 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
ORE DEPOSITION. 
NATURE OF ORES. 
Near the surface the ores of the Aspen district occur as oxides, sul- 
phates, and carbonates, mixed with sulphides, from which they are evidently 
derived. With increase in distance from the surface the oxides, sulphates, 
and carbonates disappear and give place to pure sulphides, showing that the 
latter was probably the universal condition previous to the action of surface 
agencies. ‘The most important and common of these sulphides is argentif- 
erous galena. Blende is also very abundant, especially in certain locali- 
ties, and other sulphides are of less frequent occurrence. Pyrite and 
chalcopyrite, with occasional bornite, are also found. The sulphide ore in 
Dubuque tunnel in Queens Gulch contains a considerable amount of these. 
In the Mary B. tunnel on West Aspen Mountain the ore contains large, per- 
fect crystals of iron pyrite, which carries small amounts of arsenic, lead, 
copper, zine, cadmium, cobalt, and nickel. Tetrahedrite and tennantite are 
also very common. These minerals are called ‘‘gray copper” by the 
miners, and always form valuable ore, since they contain a large propor- 
tion of silver sulphide. An analysis of tennantite from the Mollie Gibson 
mine, by 8. L. Penfield, is as follows:* 
Analysis of tennantite from Mollie Gibson mine. 
Per cent. 
Sulphare2 5355252 e ae oe eee eee re St 25.04 
HAT SOTIIG ia eee re Oey ra ea ence Seem hee 17.18 
ONG alieih coYoy aly gaeeanipe ea Ne eA an Sear oe .13 
Copper ie sere ee ee ees eren Ee Be een 35. 72 
Silver 202528 ols WS eee I Dee Pelee ee ee a Ee wa 13.65 
TANG EOE Re ah aie ae ee ee Sis eee aay 6.90 
Tromso tier Sout ea pa aetna ce teeta i ere perenare at 42 
Dee die oh Se A ee eet erect ce ar ae 86 
Total 2h eee eee ten ee eer 99. 90 
In the Mollie Gibson and Smuggler mines there is much polybasite, 
which generally occurs in flesh-colored barite. Two analyses of this poly- 
1 Am. Jour. Sci., July, 1892, 3d series, Vol. XLIV, p. 18. 
