L92 W. II. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



occurs secondary after cupriferous pyrite, as shown by several speci- 

 mens. The ores of the copper-bearing diabase dikes of the " Blackfoot 

 ceded strip "' in northern Montana also show this same alteration, with 

 specular iron ore accompanying the glance. 



Bornite altering to glance is a common feature of the ores of the Vir- 

 ginia field in Virginia and North Carolina. Specimens from the Blue 

 Wing mine show bornite cracked by minute fissures, along which alter- 

 ation has taken place and left rounded cores of bornite surrounded by a 

 shell of dull, conchoidally fracturing glance with iron oxide outside of 

 this. The specimens taken below the level of permanent water show 

 the bornite altered to chalcocite and the iron concentrated in nests of 

 laminated specular iron ore. 



The evidence showing the secondaiy nature of these copper minerals 

 has been summarized from a large mass of material, for while many 

 writers have assumed the secondary character of ore deposits formed of 

 these minerals, no evidence has been given on which to base a judgment, 

 and the facts have lately assumed a great economic importance in the 

 legal fight now being made for the possession of several of the great 

 copper veins at Butte, Montana. 



The evidence of a secondary origin of other metallic sulphides is less 

 abundant, but quite sufficient to be convincing. The first writer to 

 describe the general occurrence of such minerals appears to be Walter 

 P. Jenney, who notes their appearance in the lead and zinc deposits of 

 the Mississippi valley.* He ascribes the formation of secondary sul- 

 phides of lead and zinc to the descending oxidized solutions which leach 

 the upper parts of the original ore body. He says : 



" The minerals of secondary deposition are sulphides — pyrite and marcasite de- 

 rived from the iron of the wall rocks; blende, galena, ehalcopyrite, and greenock- 

 ite produced by alteration from the primary ores in the zone of oxidation in the 

 upper portions of the ore bodies and reformed as sulphides by the reducing action 

 of organic matter in the deeper levels. In addition to these sulphides, sulphates, 

 carbonates, silicates, and phosphates also occur, the most abundant being now 

 anglesite from the alteration of galena. Barite occurs locally, probably derived 

 from a gangue of the ore. Soluble sulphates of zinc, iron, and other metals are 

 also found in the drip of the mines." 



In discussing the order of deposition of the minerals he states that 

 minerals of secondary deposition vary in their paragenesis, and the same 

 mineral may occur more than once in the series. Though conforming 

 to no absolute order, the prevailing sequence is as follows : 



1. Crystallized white and rose-colored dolomite lining cavities in the 

 ore body and filling interstices in the breccia. 



* Trans. Am. Inst. Mining Engineers, August, 1893, p. 29. 



