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.

Paper No. 114-0
*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*
SLACK, John F., USGS, MS 954, Reston, VA 20192, jfslack@usgs.gov, KELLEY, Karen D., USGS, MS 964, Denver, CO 80225, ANDERSON, V. Michelle, U.S. Geol Survey, MS964, Denver, CO 80225, CLARK, Jeffrey L., Cominco American Inc, 15918 East Euclid Ave, Spokane, WA 99216, and AYUSO, Robert A., U.S. Geol Survey, MS954, Reston, VA 20192 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.
GSA Annual Meeting, November 5-8, 2001 General Information for this Meeting
Session No. 114 Sediment-Hosted Lead-Zinc Deposits: Roles of Basin Evolution, Tectonics, and Geochemistry in Ore Genesis I Hynes Convention Center: 306 8:00 AM-12:00 PM, Wednesday, November 7, 2001

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