200 W. II. WEED — MINERAL VEINS ENRICHED liY SULPHIDES 



The notes by Jenney and lies mentioned show that secondary zinc 

 sulphide is formed at other localities. The writer has also aaan zinc 

 blende as a later formed mineral incrnsting fractured primary ore and 

 resting on secondary quartz and pyrite in specimens from Neihart, 

 Montana. This secondary blende differs greatly in appearance from 

 that of the primary ore, being translucent, with well formed individual 

 crystals, while that of the older generation is dark colored, impure, 

 massive, and generally fractured. 



Conclusions 



From what has been shown it is concluded that later enrichment of 

 mineral veins is as important as the formation of the veins themselves, 

 particularly from the economic standpoint. The enrichment is usually 

 due to downward moving surface waters, leaching the upper part of the 

 vein and precipitating copper, silver, etcetera, by reaction with the un- 

 altered ore below. In many cases the enrichment proceeds along barren 

 fractures and makes bonanzas. In others it forms films, pay streaks, or 

 ore shoots in the body of leaner original ore. In still other cases the 

 leaching, transposition, and redeposition are performed by deep seated 

 uprising waters acting upon the vein. 



As a consequence of this, veins do not increase in richness in depths 

 below the zone of enrichment. 



The practical bearing of the phenomena described and the deduction 

 drawn from them will, I think, be apparent to every mining engineer 

 and geologist. If my views be correct, the future of many ore deposits 

 is to be judged in the light of these facts, and the value of the mine 

 must not be based on the presumption that the ore will continue in 

 unabated richness in depth. 



