BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 11, pp. 179-206 April 4, 1900 



ENRICHMENT OP MINERAL VEINS BY LATER METALLIC 



SULPHIDES* 



BY WALTER HARVEY WEED 



(Presented before the Society December 30, 1899) 



CONTENTS 



Page 



Introduction 179 



Scope of paper 180 



Definition of sulphide enrichment 180 



Zone of oxidation. 181 



Zone of enrichment, 181 



Zone of primary sulphides 181 



Enrichments described by previous writers 182 



Chemical changes 1 83 



Leaching of gossan zone 1 83 



Deposition of material from solution in the enrichment zone 186 



Mineral alteration 189 



Summary of chemical and mineralogic evidence 194 



Mode of occurrence of deposits of secondary sulphide ores 194 



Method adopted of presenting the subject 194 



Copper 195 



Silver 200 



Zinc 205 



Conclusions ■ 206 



Introduction 



In the early history of the development of many ore deposits, espe- 

 cially those of silver and of copper, masses of rich sulphide ore are 

 encountered which lie below the limit of surface decomposition, and 

 which are very often soon passed through in mining and found to give 

 place more or less abruptly to comparatively poor and base sulphide 

 ores. In other cases " bonanzas " are encountered, which in many in- 

 stances consist of rich ores of a mineralogic character wholly unlike the 



* Published by permission of the Director of the U. S. Geological Survey. Abstract printed in 

 program of meeting December, 1899. 



XXVI— Bum,. Geot,. Soc. Am., Vol. 11, 1899 (179) 



