1 6 WILLIAM H. EMMONS 



waters, but it also inhibits the precipitating action of ferrous salts, 

 and thus permits the gold to travel farther before final deposition. 



These statements apply to the action of surface waters descend- 

 ing through the upper parts of an auriferous ore deposit, since such 

 waters are cold, dilute, acid (i.e., oxidizing) solutions. In deeper 

 zones, where they attack other minerals, they lose acidity, until 

 the manganese compounds, stable under oxidizing conditions, are 

 precipitated together with the gold. Thus, manganite, as well as 

 limonite and kaolin, is frequently found in secondary (i.e., dis- 

 solved and reprecipitated) gold ores. Moreover, in the precipi- 

 tation of secondary copper and silver sulphides, ferrous sulphate 

 is generally formed; and, consequently, the secondary silver or 

 copper sulphides frequently contain gold. 



Those deposits in the United States in which a secondary 

 enrichment in gold is believed to have taken place are, almost 

 without exception, manganiferous. Since secondary enrichment 

 is produced by the downward migration, instead of the superficial 

 removal and accumulation, of the gold, it should follow that both 

 gold placers and outcrops rich in gold would be found more exten- 

 sively in connection with non-manganiferous deposits; and this 

 inference is believed to be confirmed by field-observations. 



Among the papers which treat the superficial alteration and 

 secondary enrichment of copper, gold, and silver deposits are 

 those of S. F. Emmons, 1 Weed, 2 Penrose, 3 Winchell, 4 Van Hise, 5 

 Kemp, 6 and Rickard. 7 The processes upon which the changes 

 depend are clearly outlined in these, and subsequent work has, 

 in a large measure, confirmed the premises stated. The chemical 



1 "The Secondary Enrichment of Ore-Deposits," Trans., XXX, 177-217 (1900). 



2 "The Enrichment of Gold and Silver Veins," Trans., XXX, 424-48 (1900). 



3 "The Superficial Alteration of Ore-Deposits," Journal of Geology, II, No. 3, 

 288-317 (Apr.-May, 1904). 



4 Bulletin oj the Geological Society of America, XIV, 269-76 (1902). 



5 "Some Principles Controlling the Deposition of Ores," Trans., XXX, 27-177 

 (1900). 



6 "Secondary Enrichment in Ore-Deposits of Copper," Economic Geology, I, No. 1, 

 1 1-25 (Oct. -Nov., 1905). 



7 "The Formation of Bonanzas in the Upper Portions of Gold- Veins," Trans., 

 XXXI, 198-220 (1901). 



