1182 A TREATISE ON METAMORPHISM. 



ILLUSTRATIONS OF SECONDARY ENRICHMENT AND DIMINUTION OF RICHNESS WITH DEPTH. 



The processes have now been explained by means of which a rich upper 

 belt may be produced. If the argument be correct, it is an inference from 

 this that ore deposits which have undergone a second concentration are 

 likely to diminish in richness with depth, provided a considerable belt be 

 considered. It remains to give the facts which confirm the above hypoth- 

 esis, and illustrate diminution of richness with depth. 



At Ducktown, Tenn., at the level of ground water, appeared a belt 

 of rich black copper ore (copper glance), which varied from less than 1 to 

 about 2J meters in thickness. Above this belt was gossan, very poor in 

 copper; below it a very low grade cupriferous pyrrhotite." In this instance 

 it can hardly be doubted that originally the lean cupriferous pyrrhotite 

 extended not only to the present surface but probably much higher. The 

 downward-moving waters transported copper to its locus near the level of 

 ground water. Here the copper salts reacted on the iron sulphide and 

 produced rich sulphurets. 



A case which has been, perhaps, more closely studied than any other 

 in the United States is that of the deposits of Butte, Mont. Douglas 

 states that here rich oxysulphurets are found near the surface. On the 

 summit of the hill "it seems as if the copper, leached out of the 400 feet 

 (120 meters) of depleted vein, had been concentrated in the underlying ore, 

 and had thus produced a zone of secondary ore about 200 feet (60 meters) 

 deep, which contains, as might be expected, about thrice its normal copper 

 content." 6 Of the Butte deposits Emmons says: 



Secondary deposition, or transposition of already deposited minerals, has played 

 an unusually important r61e. In the case of the copper veins it has not heen confined 

 to the oxidizing action of surface waters, which has resulted in an impoverishment of 

 the ore bodies, but below the zone of oxidation it has resulted in the formation of the 

 richer copper minerals bornite, chalcocite, and covellite, in part, at least, by the 

 breaking up of original chalcopyrite. Unusual enrichment of the middle depths 

 of the lodes has thus been caused. Whether the two processes of impoverishment 

 and enrichment have been differing phases of the action of descending waters, or 

 whether the latter may have been a later result of the rhyolite intrusion, has not yet 



« Blake, W. P., The persistence of ores in lodes in depth: Eng. and Min. Jour., vol. 55, 1893, 

 p. 3. Henrich, C, The Ducktown ore deposits and the treatment of the Ducktown copper ores: Trans. 

 Am. Inst. Min. Eng., vol. 25, 1896, pp. 206-209. 



6 Douglas, James, The copper resources of the United States: Trans. Am. Inst. Min. Eng., vol. 19, 

 1891, p. 693. 



