ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1183 



been definitely decided. It is, however, fairly well determined that the enrichment 

 of the copper deposits is so closely associated with the secondary faulting that it may 

 be considered to be a genetic result of it." 



While the larger and richer sulphide bodies were found at from 60 to 

 125 meters below the water level, rich sulphide bodies of smaller size are 

 here and there found at a depth of 450 meters below the water level, 

 "though in apparently decreasing amount as compared with the immense 

 thickness of pyritous ore.'"' Brown states that in the same area oxidized 

 products extend to the level of ground water. These oxidized products, 

 according to Brown, promptly change at water level into normal sulphurets. 

 " There follows below a region of varying height, of valuable rock,, which 

 again slowly deteriorates in depth, this deterioration, however, being so 

 retarded finally as to be scarcely appreciable." He further says that 

 above the level of ground water is gossan " carrying high values in silver, 

 and particularly in gold." d Thus at Butte we have enrichment in silver and 

 gold and depletion in copper in the belt above the level of ground water as 

 compared with the material below it ; and at and below the level of ground 

 water we have rich sulphides of copper which grade into leaner sulphurets. 

 In the case of the Butte deposits it can hardly be doubted that the compar- 

 atively lean sulphides in the deeper workings represent the product of a 

 first concentration, and that the modifications of this material found above 

 and below the level of ground water represent the work of downward- 

 moving waters. To account for the high values of gold and silver above 

 the level of ground water, one must suppose that this belt has received con- 

 tributions of these metals from the upward extension of the veins which has 

 now been removed by erosion. The great richness of the copper below 

 the level of ground water Douglas clearly attributes to the downward trans- 

 portation of the material from the depleted copper veins. However, a part 

 of this material was doubtless derived from an upward extension of these 

 veins, precisely as in the case of the gold and silver. For my own part, I 

 have little doubt that the precipitation of the rich sulphides was produced 

 by reactions upon the lean sulphurets, as given in the equations on 

 pages 1161-1164. 



"Emmons, S. F., Economic geology of the Butte special district, Montana: Geologic Atlas U. S., 

 folio 38, 1897. 



& Emmons, S. F., The secondary enrichment of ore deposits: Genesis of ore deposits, Am. Inst. 

 Min. Eng., 2d ed., 1902, p. 444. 



c Brown, R. C, The ore deposits of Butte City: Trans. Am. Inst. Min. Eng., vol. 24, 1895, p. 556. 



<* Brown, cit., p. 555. 



