GEOCHEMISTRY OF SILICIFIED ROCKS IN THE RED DOG ZN-PB-AG DISTRICT, WESTERN BROOKS RANGE, ALASKA: EVIDENCE FOR MULTISTAGE DIAGENETIC AND HYDROTHERMAL FLUID FLOW AND FLUID-ROCK REACTIONS

The giant Red Dog (RD) and recently discovered Anarraaq (AN) Zn-Pb-Ag deposits occur in black shales of the Carboniferous Kuna Formation.  Four types of silica-rich rocks are recognized at and near these deposits: (1) silicified shale in wall rocks to massive sulfide and Zn-Pb-Ag veins, at both RD and AN, and in zoned alteration fronts adjacent to small mafic intrusive bodies at RD; (2) silicified limestone at AN; (3) chalcedony (rare) in Zn-Pb-Ag veins at RD; and (4) late quartz in silica rock within sulfide ore and wall rocks at RD, and in veins at RD, AN, and throughout the western Brooks Range.  On a regional scale, sedimentary rocks of the Kuna Formation are dominantly organic-rich, variably siliceous mudstone and shale (40.0-97.6 wt % SiO₂; n=70) having locally high Zn (to 1300 ppm) and other metal concentrations typical of black shale sequences, but consistently low As (<46 ppm), Sb (<20 ppm), Tl (<1.2 ppm), and Ge (<2.0 ppm), and uniformly negative Eu anomalies (Eu/Eu*=  0.37-0.80; chondrite-normalized basis).  Silicified limestones at AN (>90.7 wt % SiO₂; n=4) have similar low values and negative Eu anomalies.  However, silicified shales at RD (>75 wt % SiO₂, n=  27) differ in containing relatively high As (to 145 ppm), Sb (to 58.6 ppm), Tl (to 30.8 ppm), and Ge (to 7.8 ppm), and positive Eu anomalies (Eu/Eu*=1.0-8.9).  High Mn (to 8.7 wt % MnO) occurs in silicified shales locally above the AN deposit.  Rare chalcedony in Zn-Pb-Ag veins at RD has relatively high Sb (23.8 ppm), Tl (37.7 ppm), and Ge (13.8 ppm), but low As (13 ppm) and Eu/Eu* (0.61).  Silica rock at RD displays large positive Eu anomalies (Eu/Eu*=2.2-19.9; n=3) that probably reflect inherited values from precursor barite (Eu/Eu*=1.5-16.7; n=  3).   The late quartz veins (n=4) lack elevated As, Sb, Tl, or Ge, or clearly positive Eu anomalies, and formed from younger fluids that were genetically unrelated to Zn-Pb-Ag mineralization.

Textural and geochemical data suggest that regional-scale silicification of the mudstones and local silicification of the limestones in the Kuna Formation reflect one or more Carboniferous diagenetic events unrelated to Zn-Pb-Ag ore formation at RD and AN.  Silicified shales at the deposits record hydrothermal fluid flow that was broadly coeval with, and probably related to, Zn-Pb-Ag mineralization.