PRECIPITATION OF SULPHIDES. 1111 



of the Vermont copper mine described by Cazin." The direct effect 

 of the carbonaceous material in the production of the sulphides is also 

 illustrated by the indicator veins of the Bendigo reef of Australia 6 and 

 by the pyrite in the argillite described by Lindgren. Another clear 

 case of the direct reduction of oxidized salts by organic matter is that of 

 the lead and zinc deposits of the Mississippi Valley. In Wisconsin, as 

 shown by Blake/ lead and zinc are precipitated by the organic matter of 

 the oil rock, and in Missouri, as shown by Bain," by the organic matter of 

 the Devonian shales. More frequently, however, the direct action of 

 organic material upon the sulphates and sulphites is to form sparsely 

 disseminated sulphides, which must be further worked over in order to 

 produce ore deposits. 



Recently Jenney has brought together a large number of instances 

 in which there is a close association of ores with organic compounds, and 

 these suggest that the organic material has directly or indirectly reduced 

 sulphates to sulphides, and thus resulted in their precipitation/ 



Probably sulphites and sulphates also may be reduced to sulphide by 

 ferrous iron in the rocks. In the case of silver sulphate the reaction 

 may be supposed to be as follows: 



Ag 2 S0 4 +12FeS0 4 +12H 2 = Ag 2 S+4F&A+ 12H 2 S0 4 



The reaction for the production of chalcocite from cuprous sulphate is : 



Cu 2 S0 4 +12FeSO i +12H 2 0=Cu 2 S+4Fe 3 4 +12H 2 S0 4 



and for cupric sulphate is : 



2CuS0 4 +15FeS0 4 +16H 2 0=Cu 2 S+5Fe a 4 +16H 2 S0 4 



It is to be noted that in these reactions magnetite is formed simultane- 

 ously with the production of the sulphides, and in order that the reactions 



«Cazin, F. M. F., Discussion of genesis of ores: Trans. Am. Inst. Min. Eng., vol. 24, 1894, pp 

 604-608. 



&Rickard, T. A., The origin of the gold-bearing quartz of Bendigo reefs, Australia: Trans. Am. 

 Inst. Min. Eng., vol. 22, 1894, p. 314. 



« Lindgren, Waldemar, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Ann. 

 Eept. U. S. Geol. Survey, pt. 2, 1896, PI. VIII, C, pp. 140, 156. 



d Blake, Wm. P., Lead and zinc deposits of the Mississippi Vallev : Trans. Am. Inst. Min. En°-., 

 vol. 22, 1894, pp. 630-631. 



«Bain, H. F., Van Hise, C. E., and Adams, Geo. I., Preliminary report on the lead and zinc 

 deposits of the Ozark region : Twenty-second Ann. Eept. U. S. Geol. Survey, pt. 2, 1901, pp. 128-129. 



/Jenney, W. P., The chemistry of ore deposition: Trans. Am. Inst. Min. Eng., vol. 33, 1903, 

 pp. 445^48. 



