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



mine occur in the oxidized zone,* and copper glance pseudomorphous 

 after chalcopyrite in the quartz veins near Georgieff, in the Altai 

 mountains, is described as occurring with hematite, chrysocolla, and 

 malachite; also derived from copper pyritcf 



Emmons and Tower have stated I that chalcocite " occurs filling frac- 

 tures in pyrite and chalcop}'rite, and from its relation with these min- 

 erals and bornite it is certain that it is derived from the two latter min- 

 erals by decomposition, and is therefore later than these minerals." 

 Bornite is also mentioned by the same writers as an alteration product 

 of chalcopyrite. 



Kemp has called attention § to the "general experience that the car- 

 bonates form most readily when the original sulphides are in limestone, 

 whereas when granite or some similar crystalline rock constitutes the 

 walls chalcocite or bornite results." 



With these exceptions, I have not found in the abundant literature of 

 mineral alteration and mineral synthesis any reference to the formation 

 of crystalline glance by secondary alteration. I have therefore been 

 somewhat surprised to find abundant evidence of it in copper ores, not 

 only at Butte, but elsewhere in Montana, and also in the ores of North 

 Carolina and Virginia. The glance so found is in all conditions : amor- 

 phous, massive without crystalline texture, massively crystalline, and in 

 crystals. 



Native copper, though commonly considered as a product of oxidizing 

 waters only, also occurs, as already noted, under conditions which show 

 that it has been produced far below the zone of oxidation and where no 

 oxygen has been present. Thus Douglas has described || native copper 

 as occurring in the Copper Queen mine, u not at the surface, where ox- 

 idizing agencies have been most active, but in the deepest layers of the 

 large ore bodies, where apparently some reducing agent has been more 

 actively at work than elsewhere and where the ore is farthest removed 

 from atmospheric disturbance." In my own examinations I find that 

 the native metal frequently occurs under similar conditions at many 

 localities. It is not necessary, however, to suppose any new conditions, 

 and the deposit is easily understood if, as is commonly the case, ordi- 

 nary limestone be present. The carbonate of lime will reduce the copper 

 sulphate to cuprous oxide, and this with either sulphuric acid or ferrous 

 sulphate will form native copper, as shown by the following equations : 



*Silliman and Whitney, Am. Jour. Sci., 2d ser., vol. xx, p. 361. 



fJeremeef: Bull. Academy of Sciences, St. Petersburg, 1897, v. 6, p. 37 ; quoted in Zeitschriffc 

 Kryst. Mineralpgie, 1899, v. 31, p. 508. 

 I Under " Paragenesis," in Butte Special Folio, folio 39, U. S. Geological Survey. 

 I Ore Deposits of the United States, 2d ed., New York, 18915, p. 10-4. 

 || Trans. Am. Inst. .Mining Engineers, February, 1899. 



