270 H. V. WINCHELL — SYNTHESIS OF CHALCOCITE AND ITS GENESIS 



veins, sometimes many feet in thickness and of most remarkable vertical 

 and lateral extent. 



Paragenesis of Chalcocite in Butte 



A study of the veins and ores and the paragenesis of the various min- 

 erals furnishes convincinj^ proof that the copper glance is one of the 

 latest minerals to be formed in the veins. It occurs crystallized or 

 massive on pyrite, quartz, zinc blende, altered granite, and enargite. 

 It fills fault crevices and fractures either in the veins or the country 

 rock. It is also found in extremely thin films on pyrite, chalcopyrite, 

 covellite, and blende, as well as upon the non-metallic rock and vein 

 minerals. 



The lodes themselves are quartzose veins and represent more or less 

 completely replaced country granite. The nature of this granite has 

 been fully described by Mr W. H. Weed.* 



There have been two or more separate periods of fracturing and min- 

 eralization in the copper area ; but chalcocite is found in the fractures 

 of all ages. It is frequently found cementing vein minerals like pyrite 

 which ma}^ have been broken apart by earth movements or internal 

 strains. It is also found in great purity and abundance associated with 

 the clay " gouges " of strike or cross-faults, where there is the best oppor- 

 tunity for the circulation of waters from the surface. As depth increases 

 the percentage of pyrite and enargite increases in comparison with that 

 of ohalcocite. While the ores of the camp, as mined, may have averaged 

 8 per cent or 10 per cent copper for the first thousand feet from the sur- 

 face, they will not exceed an average of 6 per cent copper for the second 

 thousand feet of vertical extent. It is impossible to arrive at exact 

 figures as to the ratio of chalcocite to total copper ; but it is a fact that 

 the percentage of chalcocite is decreasing, and that of enargite increas- 

 ing, with the depth of the mines. 



Chemistry of Copper Sulphide 



Coming now to the question of the genesis of this chalcocite, it should 

 be borne in mind that copper glance or chalcocite is cuprous sulphide 

 (CUjS), and that covellite is cupric sulphide (CuS). 



It should also be remembered that hydrogen sulphide and soluble 

 sulphides normally precipitate CuS (not Cu.^S) from copper solutions, 

 whether acid or alkaline.f Cupric salts, with an excess of hydrogen 

 sulphide, always yield an appreciable amount of cuprous sulphide ; X 



♦Journal of Geology, vol. vii, no. 8, pp. 737-750. 



t Douglas and Preseott's Qualitative Analysi.«, 7th edition, p. 89. 



:Brauner: C. N., 1896, 74, 99; Compt. rend., 1884, 98, 1492. 



