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 (Pb5Cu5Bi7S18)-aikinite (PbCuBiS3) solid solution series, and ?rucklidgeite ((Bi,Pb)3)Te4) also characterizes the Valentine prospect. The distribution of tellurides in the district reflects variations of physicochemical conditions of the ore-forming fluids, different levels of uplift along the shear zones, and fluid compositions. Values of [Delta]34S of vein sulfides in the Berners Bay district range between -10.7 and 3.20/00 (n = 84), with the majority ranging from -3.0 to 1.O0/00. These data are consistent with an external, reduced source of sulfur. Values of [Delta]13C and [Delta]18O of calcite and ankerite from vein stages 2-4 in the Kensington deposit range from -7.57 to -4.70/00 and 11.07 to 16.640/00 (n=15), respectively, whereas values of [Delta]13C and [Delta]18O of calcite in unknown vein stages from elsewhere in the district range from -7.70 to -1.330/00 and 11.41 to 16.580/00, respectively. Some of the isotopically light values of [Delta]13C of vein carbonates reflect local interaction of the hydrothermal fluids with the carbonaceous metasedimentary rocks of the Treadwell Formation. Gold-forming fluids resulted from metamorphic dehydration reactions produced by movement along the Coast Shear Zone.