194 W. H. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



In a memoir on the silver deposits of Neihart, Montana, I have 

 described* the paragenesis of the ore and gangue minerals. From 

 a careful study of the order of superposition of the minerals in distinct 

 crusts, and in crystals upon earlier minerals, supplemented hy that of 

 thin-sections of the ores, it was found that galena, pyrite, and blende, 

 which form the primary ore minerals, are decomposed, and that the silver 

 is concentrated, together with arsenic and antimony, as polybasite and 

 dark, ruby silver, and more rarely pure, transparent blende, pure galena, 

 and pyrite are deposited as secondary minerals. The thin-sections show 

 galena altering to spongy polybasite. Both polybasite and pyrargyrite 

 occur on all other minerals as crystalline aggregates and crusts and are 

 not in any case coated or dotted by other minerals. Some of the ma- 

 terial shows that the deposition of these silver minerals is still going on 

 in water holes below the normal water level at the Florence mine at 

 Neihart. As described later, the occurrence of the minerals also affords 

 good proof of their secondary character. 



SUMMARY OF CHEMICAL AND MINERALOGIC EVIDENCE 



The evidence afforded by a study of the original unaltered ore and of 

 ore in various stages of alteration, together with that of the super- 

 ficial waters draining masses of such ore, show that the original ore is 

 leached by surface waters which take into solution the various metals, 

 and, trickling downward, meet with and are decomposed by the sulphides 

 of iron present in the unaltered ore below and deposit new sulphides of 

 the metals. The copper, silver, and gold contents of the original ore are 

 thus concentrated, usually with antimony and arsenic. 



The mineralogic evidence shows that the chemical precipitates formed 

 by the foregoing reactions are true minerals, and the study of hand speci- 

 mens and of microscopic slides confirm the chemical changes outlined. 



Mode op Occurrence of Deposits of Secondary Sulphide Ores 

 method adopted of presenting the subject 



In the following pages the occurrence of masses of exceptionally rich 

 sulphide ores, such as copper glance, polybasite, ruby silver, etcetera, 

 will be noted, and it will be shown that these facts prove the secondary 

 nature of the ore. The genetic association of ores of this character with 

 direct superficial alteration or secondary post-vein fractures will be in- 



*<Jeology of the Little Belt mountains, Montana, with notes on the Neihart, Barker, Yogo, ana 

 other mining districts of the region, Twentieth Ann. Kept. U. S. Geological Survey, 1899-1900, pp. 

 267-572. 



