Mineralogy and Stable Isotope Geochemistry of Gold-silver Telluride Mineralization in the Berners Bay District, Alaska
Mesothermal gold mineralization in the Berners Bay district (>2.2 Moz Au) is unique among gold-producing districts along the Juneau gold belt in that tellurides are a significant constituent of the ore. Most of the prospects in the Berners Bay district are hosted by the 105 Ma Jualin Diorite. At the large Kensington deposit (1.9 Moz Au), gold is associated with the third of four hydrothermal vein stages, dated at 56.5 to 53.2 Ma by Miller et al (1995). Gold-bearing minerals at Kensington consist of native gold, calaverite (AuTe2) and petzite (Ag3AuTe2). These minerals occur as inclusions in pyrite, chalcopyrite and quartz. Traces of coloradoite (HgTe), volynskite (AgBiTe2), sylvanite ((Au, Ag)2Te4), tellurobismuthite (Bi2Te3), altaite (PbTe), hessite (Ag2Te), sphalerite, bomite, pyrrhotite, galena, hematite and magnetite are also present in stage 3 veins. Petzite and electrum are the dominant gold-bearing minerals in veins along the Comet shear. These veins also contain greater amounts of hessite and altaite than do those to the east. The presence of pavonite ((Ag, Cu)(Bi, Pb)3S5), bismuthinite (Bi2S3), a member of the friedrichite (Pb5Cu5Bi--S1)-aikinite (PbCuBiS3) solid solution series, and?rucklidgeite ((Bi, Pb)3)Te4) also characterizes the Valentine prospect.
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