200 \\\ II. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



This changes in depth to a rusty, earthy, silicious mass, holding carbon- 

 ates, oxides, and native copper, passing downward into black sulphuret, 

 and this to the unaltered sulphide. These changes are generalized, since 

 there is no definite level of any one kind of ore. Owing to vertical frac- 

 turing, the upper and lower limits of the brown stained oxidized ore are 

 very irregular. This is especially true of the lower limit, channels and 

 pipes of oxidizing waters extending down 180 feet or more below the sur- 

 face and half that distance into solid, unaltered quartz ore. The zone 

 of enrichment marked by chalcocite, in part altered to cuprite, is there- 

 fore not well defined, the secondary sulphides being scattered about in 

 accordance with the irregularities of the water line. 



At the very interesting copper veins of the Virgilina district of Virginia 

 and North Carolina no evidence on secondary enrichment was obtained, 

 as the quartz veins carry chalcocite and bornite and near the surface 

 the superficial alteration products of these minerals. The deepest shaft 

 is now but 300 feet down, and it will be interesting to see if the glance 

 changes to chalcopyrite in depth. The bornite is seen altered to glance 

 and hematite below the zone of oxidation by the surface waters. This 

 and the general absence of either pyrite or chalcopyrite from the glance 

 are quite unlike other deposits of the east, and, in fact, of the west. 



Specimens of glance from the Gila river, Arizona, kindly given me by 

 Mr S. B. Ladd, are similarly free from pyrite or chalcopyrite, but both 

 this material and that of Virgilina are distinctly crystalline and lack the 

 dull fracture and structure of Butte ores. In the " Blackfoot ceded 

 strip," on the east side of the fruit range in northern Montana, dikes of 

 diabase traversing Algonkian slates carry copper ores. The original 

 chalcopyrite is seen cracked and fissured with alteration chalcocite. 



SIL VER 



The u bonanzas " of rich silver ores like those of the Smuggler and 

 Mollie Gibson mines of Colorado, and of silver-lead or p3>ritic ores car- 

 rying ruby silver, polybasite, or silver glance, are believed to be, in many 

 if not most cases, examples of secondary deposition. 



The first writer to call attention to secondary enrichment of silver 

 veins is De Launay. In discussing the superficial alteration of silver 

 deposits he gives a summary of his views,* from which the following 

 extract is taken : 



Near the surface the silver in the veins is in the native state, with chlorides, 

 bromides, iodide, etcetera, associated with oxide of iron, manganese, and often of 

 copper. If the gangue is silicious, it shows a porous honeycomb aspect, resulting 



* Ann. des Mines, vol. xii, 1S!)7, p. 221. See also description given in L'argent, p. 9G. 



