ALTERATION OF COPPER MINERALS 189 



tion of ferrous silicates.* Spurr found native silver occurring in similar 

 situations at Aspen, Colorado. 



In the lead mines of Missouri, Jenney says that the lead and zinc 

 brought into solution by oxidizing waters are reduced and precipitated 

 as sulphides by organic matter in the presence of alkaline sulphates, 

 PbO CO, + CaO SO s + H 2 S + 2C = PbS + CaO S0 3 -f 2C0 2 t + H 2 0. 



MINERAL ALTERATION 



A study has been made both in the field and office of the changes 

 which take place in the alteration of the primary sulphides and of the 

 paragenesis and the association of the redeposited (secondary) sulphides. 

 The data are more abundant for copper than for the other metals be- 

 cause the facilities have been more abundant for the observations of such 

 deposits, and many notes have been gathered in brief trips to Southern 

 copper mines during the past four months. Dana sa}^s chalcopyrite 

 alters to bornite, and bornite to chalcocite. I have observed the fol- 

 lowing alterations : Chalcopyrite to chalcocite, this to cuprite, this to 

 malachite, and the latter to chrysocolla; also malachite to tenorite, 

 and cuprite to native copper. Bornite is seen altering to chalcocite and 

 hematite. The oxide and carbonate are, of course, formed only by super- 

 ficial alteration, but native copper, like native silver, is found in places 

 where its formation must be ascribed to the deep-seated reactions, not 

 to oxidizing waters. 



The black copper of Ducktown and Ore Knob obtained from the Na- 

 tional Museum collection, as well as that collected by the writer at the 

 mines, is proven by chemical tests made in the United States Geological 

 Survey laboratory to be entirely chalcocite. 



The fact that copper glance may be of secondary origin is not gener- 

 ally recognized, and has been denied by some geologists. Of this an ex- 

 amination of the specimens leaves no doubt whatever. Not only is the 

 cupric sulphide formed by the , alteration of chalcopyrite and bornite 

 through the agency of descending surface waters, but the material is 

 actuall}'- taken into solution and transported and redeposited as crystal- 

 line glance. In literature the first mention of crystalline material thus 

 formed is in an account of the Ducktown material by Sterry Hunt, who 

 states that crj'stals of glance were seen by August Rant in druses in the 

 ore. In this ore, however, I myself have been unable to find any crys- 

 tallized material, even after a very careful search on the ground. 



The un equaled chalcocite crystals of the Bristol, Connecticut, copper 



* Jour. Praktische Geologie, April, 1899, p. 250. 



|W. P. Jenney, in Trans. Am. Inst. Mining Engineers, 1893, p. 202. 



