Abstract

Silver Hill is a stratiform deposit 15 m wide, 5 m thick, and at least 550 m long in chloritic argillite in a sequence of Cambrian andesitic to rhyolitic submarine pyroclastic-flow tuffs. The orebody has sharp upper and lower contacts and grades laterally into disseminated ore; chert extends for up to 200 m on either side of the orebody. Quartz-sulfide-siderite veinlets cut the argillite above and below the orebody. The ore consists of beds of fine-grained sulfides; chert, calcite, and dolomite (metamorphosed to tremolite) that precipitated with the sulfides; and argillite. Assays average 40 percent Zn, 19 percent Pb, and 0.6 percent Cu. Primary ore minerals are pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, arsenopyrite, silver, freibergite, pyrargyrite, mackinawite, cubanite, bismuthinite, bismuth, magnetite, and pentlandite(?). The effect of greenschist facies metamorphism on sulfide minerals is minor. A lens of banded sphalerite-pyrite ore precipitated from a late-stage ore solution. Arsenic, bismuth, and silver were concentrated in the residual ore solution; antimony was not. An area of intense quartz mineralization marks a conduit of ore solution onto the sea floor. Other massive sulfide and quartz stringer deposits near Silver Hill are genetically related to the Silver Hill deposit.

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