vin, a, t Edding field: Calcite-quartz-manganese Gold 133 



in the oxidized ore, * * *. This deposit yielded important placers. 

 Good ore was found at or very near the surface; and, according to the 

 best obtainable data, the values increased somewhat for a short distance 

 below the surface. Some concentration has taken place by the removal 

 of calcite and other valueless material more rapidly than gold; but there 

 is no evidence of secondary enrichment in gold below the water-table. The 

 indications are that the gold has not been extensively transported since 

 the deposit was formed. 



XIII. Georgetown, Colo., silver-lead deposits. — The principal metallic 

 minerals are argentiferous galena and blende, with pyrite and chalcopyrite; 

 the ores usually carry about $2 gold per ton. * * * The gangue is 

 quartz, chalcedony, barite, with carbonates of lime, iron, manganese, and 

 magnesia. * * * The zone of complete oxidation extends from 5 to 

 40 ft. below the surface. The oxidized ore often contains several hundred 

 ounces of silver per ton. Below this ore are friable black sulphides and 

 secondary galena. This secondary ore, according to Spurr and Garrey, is 

 rich in silver and lead, and carries more gold than occurs at greater 

 depth. — Quoting from Spurr and Garrey: 



* * * These richer ores diminish in quantity as depth increases, 

 though gradually and irregularly, so that the lower portion of the veins 

 contains relatively less silver and lead. The best ore in most veins has 

 been found in the uppermost 500 feet, * * *. 



XIV. Auriferous deposits of the Georgetown Quadrangle, Colorado. — 

 They carry pyrite, chalcopyrite, * * * quartz, adularia, and gold, with 

 minor amounts of barite, fiuorite, telluride, etc. Carbonates of iron, mag- 

 nesium, lime, and manganese occur, but are relatively rare. * * * They 

 have yielded some moderately-productive placers. In several mines, the 

 oxidized is much richer than the average ore. 



XVI. Cripple Creek, Colo. — * * * Calaverite is the chief primary 

 constituent; native gold is rarely present in the unoxidized ores. Pyrite 

 is widely distributed; tetrahedrite, * * * and molybdenite are spar- 

 ingly present. The gangue is quartz, fiuorite, adularia, carbonates (in- 

 cluding rhodochrosite) , some sulphates, etc. Some of the deposits were 

 workable at the surface, but the placers which have formed are relatively 

 unimportant. * * * Manganese oxides are often present in the oxidized 

 zone, and, according to Penrose, form nodules in the Pharmacist and 

 Summit mines. They result from the alteration of rhodochrosite, manga- 

 niferous calcite, or other minerals, and are generally distributed in the 

 oxidized zone as stains filling cracks and fissures. * * * Whether a 

 slight enrichment of gold has taken place in the oxidized zone it is not 

 easy to decide. Lindgren and Ransome are inclined to the belief that the 

 oxidized zone as a whole is somewhat richer than the corresponding tellu- 

 ride zone. If this is true, no extensive downward migration of gold can 

 have taken place. The trivial enrichment in the oxidized zone may have 

 resulted from the removal of some constituents of the primary ore. 



XX. Tonopah, Nev. — The deposits at Tonopah, Nev., are silver-gold 

 replacement-veins in andesite. * * * Placers are not developed. The 

 primary ore, according to J. E. Spurr, is composed of quartz, adularia, 

 sericite, carbonates of lime, magnesia, iron, and manganese, with argen- 

 tite, stephanite, polybasite, chalcopyrite, pyrite, galena, blende, silver 

 selenide, and gold in an undetermined form. * * * The waters which 

 descend through the oxidized zone carry sulphates and chlorides, and "wad" 



