MANGANESE IN GOLD DEPOSITS 45 



Cretaceous, and some of them are probably early Tertiary. 

 They are extensively developed in Montana, Nevada, Utah, and 

 Colorado. Mr. Lindgren calls this group the Central Belt. Many 

 of its deposits have yielded considerable gold, and in certain other 

 districts very closely related genetically (Butte, Georgetown 

 silver-gold lodes, Cortez Nevada, Tintic, etc.) much gold has 

 been obtained as a by-product to copper or silver mining. Some 

 of these deposits have yielded placers and some have not. At 

 Philipsburg and Neihart, Mont., Georgetown, Colo., and else- 

 where, the deposits show a secondary enrichment of silver below 

 the water-table. At Philipsburg, and probably at some other 

 places, an enrichment in gold accompanies this concentration of 

 silver. Some of the lodes of group 3 carry much manganese, and 

 some carry none. Present data are meager for most of these dis- 

 tricts. The determination of gold from the surface down in a 

 large number of deposits would serve as a useful check to the con- 

 clusions based upon the chemistry of the processes involved in its 

 solution and precipitation. 



4. Group 4 includes the most recent ore deposits in the United 

 States. All of them are Tertiary, and most of them are Miocene 

 or Pliocene. In general, they were formed relatively near the 

 surface, and in some places it is highly probable that not more 

 than a thousand feet of vein material has been removed by erosion 

 since the ores were deposited. The majority of these deposits 

 carry silver, and in many of them its value is greater than that of 

 the gold; but they have supplied, notwithstanding, about 25 per 

 cent of the gold production of North America. They are typi- 

 cally developed in Nevada (Comstock, Tonopah, Goldneld, Tus- 

 carora, Gold Circle); California (Bodie); Idaho (De Lamar); 

 South Dakota (later than Homestake type); Colorado (Cripple 

 Creek, Idaho Springs, Rosita Hills, San Juan, etc.) ; Montana 

 (Little Rockies, Kendall, etc.). Many occurrences in Mexico 

 should probably be placed here, also. The deposits of this group 

 have not supplied much placer gold. Many of these deposits are 

 in arid countries, where conditions for working placers are not 

 favorable; but even those in well-watered districts supply rela- 

 tively little placer gold. Manganese is abundant in some of 



