SECONDARY DEPOSITS OF SILVER MINERALS 203 



lode carrying- the ore faults earlier veins, and that the lode worked is rich 

 only where such intersections occur. He says : "All the evidence yet 

 produced by the mine's development points to an essential connection 

 between the ore deposits and cross-veins.' 1 His explanation of electric 

 currents seems to be unnecessary, but may well supplement a leaching 

 of the earlier vein and the concentration of material at the point shown. 

 It is significant that the ore occurs under the vertical vein, as shown in 

 his figure, and not above its faulted portion. 



The only examples of secondary sulphide enrichment of silver veins 

 which I have carefully studied are those of Neihart, Meagher county, 

 Montana. The Neihart ore deposits occur in metamorphic gneisses of 

 supposed igneous origin and Archean age, and extend upward into the 

 basal beds of the Belt series of Algonkian age. They are sheeted fissures 

 that cut both ancient and recent igneous rocks, and are believed to be 

 of post-Cretaceous age. 



The veins contain silver-lead ores ; more rarely rich silver sulphides. 

 The ratio of gold and silver is one dollar in gold to 5 ounces of silver. 

 The common ores consist of galena, blende, and pyrite in a gangue con- 

 sisting of lime-magnesia-iron and manganese carbonate. The rich silver 

 ores consist of polybasite, with a lesser amount of pyrargyrite, and in 

 the oxidation zone of native silver chalcop}^rite also occurs. Barite is a 

 common gangue mineral, but occurs in much smaller quantity than the 

 carbonate -' spar." The primary ore minerals are those mentioned above, 

 excepting perhaps pyrargyrite. Polybasite more commonly occurs, 

 however, as a secondary mineral. 



The silver-lead ores vary from $20 to $60 per ton ; the richer ores $100 

 to $200 or more per ton. The vein fissures are part of a general fissure 

 system, running about north and south magnetic, and dipping west at 60 

 to 80 degrees. The width of the fissure varies in the different rocks. It 

 is widest in the softer schistose rocks, narrow but sharp cut in the mass- 

 ive diorite, is irregular and narrow in tough and knotty amphibolites, 

 and becomes lost in a multitude of little fissures in rhyolite porphyry. 



The veins are commonly zones of closely sheeted rock. This rock is 

 intensely altered and decomposed, and the vein walls practically limit 

 this alteration. The ores occur in more or less persistent streaks of car- 

 bonates or " spar," and rarely show quartz in this altered rock or vein 

 matter. The payable ore bodies occur in shoots. Ore deposition was 

 by ascending carbonated waters, producing metasomatic replacement 

 along fissure lines. The veins have suffered later fracturing and secon- 

 dary enrichment of the zone has occurred at or below the water level, 

 while quartz and barite have been deposited in the open spaces of some 



XXIX— Bull. Geol. Soc. Am., Vol. 11, 1899 



