42 WILLIAM H. EMMONS 



small amount of manganese contributed by the country rock. 

 In view of the relations shown by the chemical experiments it is 

 probable that a very little manganese will accomplish the solution 

 of gold, but that it requires considerably more manganese to 

 form appreciable amounts of the higher manganese compounds 

 which delay the deposition of gold, suppressing its precipitation 

 by ferrous sulphate. In the absence of larger amounts of the 

 higher manganese compounds, the gold would probably be pre- 

 cipitated almost as soon as the solutions encountered the zone 

 where any considerable amount of pyrite was exposed in the 

 partly oxidized ore. From this it follows that deposits showing 

 only traces of manganese, presumably supplied from the country 

 rock, are not enriched far below the zone of oxidation. 



7. Vertical relation of deep-seated enrichment in gold to chalco- 

 citization. — In several of the great copper districts of the West 

 gold is a by-product of considerable value. In another group of 

 deposits, mainly of middle or late Tertiary age and younger than 

 the copper deposits, silver and gold are the principal metals, and 

 copper, when present, is only a by-product. But in some of these 

 precious-metal ores chalcocite is, nevertheless, the most abundant 

 metallic mineral, often constituting 2 or 3 per cent of the vein 

 matter. Frequently it forms a coating over pyrite or other 

 minerals. Some of this ore, appearing in general not far below 

 the water-table, is fractured, spongy quartz, coated with pulver- 

 ulent chalcocite. It frequently contains good values in silver, and 

 more gold than the oxidized ore or the deeper-seated sulphide ore. 

 Clearly, the conditions which favor chalcocitization are favorable 

 also to the precipitation of silver and gold. 



The exact chemical reaction which yields chalcocite is not 

 known. At ioo° C, according to Dr. H. N. Stokes, 1 the reaction 

 with pyrite is probably about as follows : 



5FeS 2 + i 4 CuS0 4 + 1 2 H 2 0= 7Cu 2 S+ 5FeS0 4 + 1 2H 2 S0 4 . 



In the cold, the reaction may differ in details, but- without doubt 

 much ferrous and acid sulphate is set free. Attendant reactions 



1 Unpublished MSS quoted by Lindgren in Professional Paper No. 43, U.S. 

 Geological Survey, 183 (1905), and in Weed's translation of Beck's textbook. 



