1184 A TREATISE ON METAMORPHISM. 



Penrose cites the Arizona copper deposits as instances of secondary 

 concentration. These deposits he regards as produced by leaching of the 

 copper from a lean copper-bearing pyrite, and its segregation at the places 

 where the rich ores occur. In this process Penrose, however, says that the 

 volume of the deposit must be decreased; but he makes the point that 

 the smaller amount of the rich product is more valuable than a larger 

 lean deposit, because more easily mined and more readily reduced." This 

 process of concentration is further described by Douglas, who notes, also, 

 that the changes have resulted in the production of enriched sulphides from 

 very lean sulphides in the Copper Queen mine. Here, according to Doug- 

 las, a large very low-grade copper-bearing pyrite deposit running from the 

 60- to the 120- meter level contains rich oxysulphides and black sulphides 

 on the outside and mainly lean pyrite in the interior. 6 The original mate- 

 rial in the Arizona locality is as plainly a lean cupriferous pyrite as in 

 Tennessee. Here, however, on account of the peculiar climatic conditions, 

 the alterations have been of a different kind and have not extended to a 

 uniform depth. Instead of the rich belt being a horizontal sheet, it occurs 

 in a zone about a large cupriferous pyrite mass; but the principles of 

 concentration are identical, and the rich products are unquestionably in 

 part due to reactions between the oxidized salts and the lean sulphides. 

 The rich oxidized products of the Southwest doubtless were produced 

 directly from the enriched sulphurets. Therefore, in the formation of the 

 rich oxidized products there were two stages of alteration; first, the pro- 

 duction of rich sulphurets by the reaction of oxidized products upon the 

 lean pyritiferous material, and after that oxidation of the rich sulphurets. 

 These oxidized ores formed partly in situ, but there has been also more or 

 less of transfer of material from one place to another. 



An excellent illustration of an enriched upper belt in the case of gold 

 is furnished by the gold-quartz veins of Grass Valley, California, where, 

 according to Lindgren, the decomposed belt of weathering, about 50 meters 

 in depth, contains "from $80 to $300 per ton, while the average tenor in 

 depth is from $20 to $30."° Furthermore, the rich 50 meters, which con- 



" Penrose, E. A. F., jr., The superficial alteration of ore deposits: Jour. GeoL, vol. 2, 1894, pp. 

 306-308. ' , 



''Douglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29, 1900, 

 pp. 532-533. 



» Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven- 

 teenth Ann. Rept. V. S. Geol. Survey, pt. 2, 1896, p. 128. 



