proceedings: geological society 253 



magmas. They comprise three main classes: contact-metamorphic, 

 fissure-vein, and replacement deposits. 



The oldest are the contact-metamorphic deposits. Their formation 

 was accompanied by mineralization and the development of the usual 

 garnet zone containing a dozen or more contact-metamorphic minerals. 

 The ferruginous or metallic constituents of these minerals, as andradite, 

 gedrite, and others, were derived chiefly from the magmatic or ensuing 

 solutions of the intrusive rock and not from the host limestone. The 

 deposits are mainly copper-bearing; the chief primary ore minerals 

 are chalcopyrite and cupriferous pyrite. From these two minerals, 

 copper carbonates, oxide, and secondary sulphides, constituting the 

 present workable ore bodies, were formed by oxidation and secondary 

 enrichment. 



The fissure veins are numerous and widely distributed. They are 

 about 6 feet in average width. Some are a mile long, and have a known 

 vertical range of nearly a thousand feet. The filling is chiefly quartz, 

 fluorite, and barite. The ore minerals are chiefly argentite, cerargyrite, 

 bromyrite, native silver, gold, pyromorphite, pyrargyrite, various 

 lead, copper, and zinc minerals, molbydenite, wolframite, and scheelite. 



The replacement deposits are associated with the contact-meta- 

 morphic deposits and some of the vein deposits described above. They 

 occur chiefly in limestone in association with the intrusive rocks. Their 

 principal metallic constituents are lead and silver. To this class appar- 

 ently belong in large part the deposits at the Mowry mine and at the 

 Total Wreck mine. The Mowry deposits occur on a steeply north- 

 ward-dipping fault-contact between quartz monzonite on the south 

 and Paleozoic limestone on the north, with altered gabbro underlying 

 the limestone and forming the hanging wall in the deep part of the 

 mine. They are opened to the depth of 500 feet, and for 600 feet 

 longitudinally along the fault, and they extend laterally 100 feet or 

 moi'e back from the fault into the limestone. They consist mainly 

 of lenses or chimney-shaped, nearly vertical bodies, standing or lying 

 parallel with the fault plane, and are composed chiefly of argentiferous 

 ore minerals, cerusite, coarse galena, anglesite, and bindheimite, all 

 contained in a manganiferous and ferruginous gangue consisting prin- 

 cipally of psilomelane and massive pyrolusite and hematite. The ore 

 is mostly oxidized to the depth of 300 feet. The deposits are thought to 

 be genetically connected with the gabbro, which seems best able to have 

 supplied the iron and manganese constituents found in the gangue. 

 They were probably deposited in the sulphide form as metasomatic 

 replacements; from the sulphide minerals the present ore minerals and 

 gangue were derived by processes of oxidation. They seem to be sim- 

 ilar in character and origin to the Leadville, Colorado, deposits recently 

 described by Philip Argall. (A fuller account is to appear in the forth- 

 coming U. S. Geol. Survey Bulletin 582). 



Discussion: N. L. Bo wen referred to the analysis of the gabbro 

 as exhibited on the blackboard, and pointed out its remarkable com- 

 position, referring to the 7.25 per cent of alkalies and the 1.29 per cent of 



