ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1187 



lodes ceased comparatively early in the history of the district. Regarding 

 these ores Ransome says: 



In spite of the diversity shown by the different ore bodies, there is after all 

 remarkable uniformity to be found in the change at very moderate depths — usually 

 less than 300 feet [91 meters] — from an ore consisting chiefly of argentiferous galena 

 to highly argentiferous silver-copper ores, and then a gradual diminution of value 

 downward through the increasing proportion of low-grade pyrite in the ore bodies. 

 These changes are best recorded in the Yankee Girl, Guston, and Silver Bell mines." 



In some of the deeper lodes of the district the values have become so 

 low that they do not warrant working. While in some lodes there has also 

 been a decrease in the amount of gold with depth, the gold values in gen- 

 eral have held up very much better than the silver, and in some of the 

 mines at a depth of 500 or more meters below the surface, as at the Camp 

 Bird, the values are still very high in gold. Possibly the explanation of 

 the difference between the two metals is that from the descending solutions 

 the silver was thrown down more rapidly than the gold. Since silver is 

 precipitated as a sulphide, it is very quickly thrown down from its soluble 

 salts in consequence of the reaction of the sulphides of the base metals, 

 such as galena, sphalerite, and pyrite; whereas by these compounds the 

 gold is thrown down from its solutions in metallic forms, and is therefore 

 probably more slowly precipitated, and consequently secondary enrichment 

 extends much deeper. 



The Monte Cristo district of Washington, described by Spurr, gives a 

 most excellent illustration of the principle of decreasing richness with 

 depth. In that district there is a definite arrangement of the ores verti- 

 cally, which arrangement has a very marked relation to the topography. 

 The order downward is, (1) oxidized ores; (2) upper sulphide belt, char- 

 acterized by galena, blende, and chalcopyrite, carrying the highest values 

 in gold and silver and extending not more than 100 meters below the 

 surface; (3) intermediate sulphide zone, having reduced amounts of galena, 

 blende, and chalcopyrite, and with these realgar, arsenopyrite, pyrite, 

 and pyrrhotite; and (4) lower sulphide zone, low in gold and silver, and 

 consisting of pyrite, pyrrhotite, and arsenopyrite, and in which galena and 

 blende are subordinate or absent.'' In the upper sulphide belt the "galena 



« Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull. 

 U. S. Geol. Survey No. 182, 1901, pp. 111-112. 



b Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S. 

 Geol. Survey, pt. 2, 1901, pp. 841-857. 



