1170 A TREATISE ON METAMORPHISM. 



oxidized in order that the free gold may be produced. This may take place 

 first by the oxidation of tellurium to an oxide, and then the solution of the 

 oxide by acids. Or the sum total of all the changes may be represented in 

 a given case as follows: 



Au 4 Te 3 +12NaCl+6C0 2 +60=4Au+3TeCl 4 +6Na 2 C0 3 



That this or some similar process of the oxidation of the tellurium, 

 leaving gold behind, has taken place upon an extensive scale at Cripple 

 Creek, is shown by the very frequent pseudomorphs of spongy gold after 

 the various tellurides. 



The free-gold ores thus formed in the belt of weathering' are commonly 

 very much richer than the downward extensions of the deposits, in which 

 the gold is associated with sulphides or tellurides, or occurs as a telluride. 

 This exceptional richness of the upper part of the gold deposits is so well 

 known that it is unnecessary to give many cases illustrative of it. However, 

 one or two of the more important districts may be mentioned. In Australia, 

 Don says, "for ounces per ton above the ground- water level, only penny- 

 weights per ton have been found below it." a In the Sierra Nevada deposits, 

 according to Lindgren, near the surface the values are from $80 to $300 

 per ton, whereas deeper they are somewhat uniform for the given vein, 

 and run from $20 to $30 per ton. 6 In many instances the decrease in values 

 is so great, in passing from the belt of weathering to the deeper workings, 

 that while the belt of weathering and the upper part of the belt of cementa- 

 tion may be very profitable, the deeper portions of the deposits are so 

 lean as not to warrant working. 



These facts seem to me to be conclusive evidence that, in some way, 

 the downward-moving waters have concentrated in a comparatively narrow 

 belt an amount of gold which originally had a much wider vertical extent. 

 That is to say, in some way, as denudation continued downward, the gold 

 deposited in one of the original forms has been taken into solution by the 

 descending waters and has been reprecipitated, thus producing the enriched 

 upper part of the veins. As this process continues the belt of weathering 

 becomes richer and richer, there being segregated within a comparatively 



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

 p. 596. 



b Lindgren, Waldemar: The gold-quartz veins of Nevada City and Grass Valley, California: 

 Seventeenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1896, p. 128. 



