204 \V. II. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



of the veins, being usually accompanied by rich silver sulphide ores 

 showing marked crustification.* 



The secondary deposition of rich silver sulphides has played an un- 

 usually important part in the ore deposits of Neihart, Montana. In 

 these veins it consists of a breaking up or decomposition of the primary 

 ore minerals, chaleopyrite, pyrite, and an impure argentiferous galena, 

 of the lode, and the formation of the rich silver minerals, polybasite, 

 pyrargyrite, with secondary pyrite and blende. These secondary min- 

 erals are always found in connection with open water pipes or with post- 

 vein fractures, are often well crystallized, and occur commonly with 

 recent quartz in vugs and along open fractures by which descending 

 waters could trickle downward. 



The products of superficial alteration are largely removed, and the zone 

 of gossan or the barren leached lode is less than 50 feet deep, or may not 

 exist. Beneath this leached and impoverished part of the lode there is 

 generally an irregular accumulation (sometimes regular enough to be 

 called a layer) of a sooty, black ore consisting of manganese and silver 

 sulphide, the material often being quite rich in silver. In the rhyolite 

 porphyry, where the veins are not well defined but are a mass of shat- 

 tered rock, the crevices for 40 to 100 feet or more downward are filled 

 with this material. Where the lode is well defined the secondary anti- 

 monial sulphides occur below at first in considerable abundance, but 

 deeper down only in crevices and fissures, partly or wholly lining filled 

 fractures, so that they become less and less abundant ingoing downward 

 on the vein. There is therefore an unusual enrichment of the upper 

 part of these veins — that is, of what is now the upper part. 



The transposition and redeposition of ores with enrichment in silver 

 can be easily conceived to take place by means of water-courses convey- 

 ve} r ing the deoxidized surface waters to considerable depths, a common 

 feature in many of our western mines, t The ruby silver of the Trout and 

 Hope mines and the rich sulphides of the Granite Mountain mine, near 

 Phillipsburg, Montana, 2,000 feet below the surface, associated with 

 clear evidence of partial oxidation, are examples seen by me. The very 

 rich ores of the Ruby mine on Lowland creek, near Butte, Montana, and 

 the high grade ore shoot of the Hope mine of Basin, Montana, show geo- 

 logical conditions suggesting a similar origin. Microscopic examinations 



* The foregoing description is an abstract of a chapter on the Neihart ore deposits forming part 

 of a report on the geology of the mining districts of the Little Belt mountains, Montana. Twen- 

 tieth Annual Report, V . S. Geological Survey for 1898-'99, part iii, p. 403. 



-(-These water-courses occur at Elkhorn, Montana, 2,000 feet below the surface. That they are 

 the channels of descending waters is clearly proved by the main workings, although the perma- 

 nent water level of the country is only a short distance below the ground, and the oxidized ores 

 extend down for 600-700 feet only. The rocks are limestone. 



