ILLUSTRATIONS OF SECONDARY ENRICHMENT. 1185 



tain from four to ten times as much gold as the sulphurets below the level of 

 ground water, are depleted in silver. In some veins the sulphurets extend 

 almost to the surface. Lindgren further states that the sulphurets below 

 the level of ground water continue with undiminished richness to a depth 

 of 500 or more meters." He adds that the California region is one in which 

 denudation has extended to a dejitth of 500 to 1,500 or more meters.' From 

 these facts it is highly probable, as suggested by Lindgren, that sulphurets 

 similar to those below the level of ground water were deposited above the 

 present surface. If this were the case the only possible explanation of the 

 belt of weathering rich in gold and depleted in silver is that descending 

 waters have abstracted a large part of the gold from the material removed 

 by erosion, and have deposited it in the belt of weathering. Its precipita- 

 tion there was, doubtless, mainly due to reduction by the sulphides, 

 producing sulphurets richer in gold. Later, the sulphides have been 

 oxidized, leaving the enriched belt of free gold. The silver apparently has 

 been transported downward to a greater extent. One would expect that 

 correlative with the belt above the level of ground water poor in silver, 

 there would be a belt at and below the level of ground water richer in 

 silver. Upon this point Lindgren does not give us information. 



Another very interesting case of richness of the belt of weathering 

 in gold, as compared with the unaltered sulphides below, is furnished by 

 the Australian gold fields, where the belt above the level of ground water 

 is several times as rich as the unaltered tellurides and sulphides below; 

 some mining men say ounces above to pennyweights below. c This rich belt 

 is from 15 to 120 meters wide. In a portion of the mines of some districts — ■ 

 for example, the Kalgoorlie district — when the bottom of the oxidized zone 

 is reached, the ores are so lean as to be valueless, so that mines which were 

 profitable in the weathered zone did not pay below it/ Many of the mines 

 of that district, however, are profitable below the weathered horizon. If it 

 had not been for the secondary enrichment by denudation and downward 

 transportation of material, many of the mines would not have been exploited, 



« Lindgren, cit, pp. 161-163. 



b Lindgren, cit., pp. 182-183. 



"Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898, 

 p. 596. 



<* Hoover, H. C, The superficial alteration of western Australian ore deposits: Trans. Am. Inst. 

 Min. Eng., vol. 28, 1899, pp. 762-764. 



HON XLVII — 04. 75 



