180 W. IT. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



main mass of the mineral deposit; or sometimes fissures in a low grade 

 ore are filled by a rich ore. Such masses are believed to be generally of 

 later origin than the original vein filling, and as they enrich the vein 

 they are designated as sulphide enrichments. 



Scope of Paper 



This paper is an attempt to explain the genesis of such bodies of 

 sulphide ores as enrichments formed by the redeposition of material 

 leached from the vein, generally by superficial waters, and to show the 

 chemical and mineralogic changes involved in the processes and the 

 physical conditions under which the ores have been deposited. 



Not only is the process one that has played an active part in the 

 later history of ore deposits, but a consideration of its effects leads to a 

 review of the changes in the history of the vein, more especially in re- 

 gions where marked changes of water level have resulted from physio- 

 graphic revolutions. At an early date I hope to present a clear illustra- 

 tion of the importance of this change of level in the ore deposits of 

 Butte, Montana, where for some time past a study of the copper veins has 

 been made by Professor S. F. Emmons and myself for the United States 

 Geological Survey. 



In the present paper I attempt to prove — 



1. That the leaching of a relatively lean primary ore, commonly by 

 surface waters (but it may be by deep seated waters), will supply the 

 material in solution for such enrichment. 



2. That the unaltered sulphides, especially pyrite, will induce precipi- 

 tation ; that the material precipitated is crystalline: and that a number 

 of mineral species are commonly formed, and are now forming, in veins 

 by such reactions. 



3. That such minerals deposited in quantity ma}^ form ore bodies of 

 considerable size (bonanzas) or may be disseminated through the lean 

 primary ore in strings and patches, thus enriching the ore body as a whole 

 and even making a former low grade body of sufficient value to work. 



Definition of Sulphide Enrichment 



By secondary sulphide enrichment is meant the concentration of the 

 metals into high grade sulphide ore bodies. It differs from what is often 

 called secondary enrichment (enrichment in which the character of the 

 ore is improved by the removal of its worthless constituents and the non- 

 removal of the valuable metals), and it should not be confused with that 

 enrichment due to circulating surface waters which results in the forma- 



