Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska

Abstract

The Prince William Sound region is located approximately 100 km east-southeast of Anchorage, Alaska, and contains the geologically diverse Late Cretaceous to Eocene Valdez and Orca Groups, which comprise a portion of the accretionary prism complex of south-central Alaska. The rocks host more than 600 known massive sulfide deposits and prospects. Historically, the deposits have been described as epigenetic vein, replacement, and shear zone-hosted ores. We define the Prince William Sound massive sulfide deposits in terms of two genetically related, yet geochemically distinct, types of deposits, both related to submarine volcanism. The first type is hosted in submarine pillow basalt, massive basalt flows, and mafic tuffs and is analogous to Cyprus-type deposits. The second type is hosted in intercalated slate and graywacke and is analogous to Besshi-type deposits. Both types are similar morphologically and are characteristically stratiform to strata bound. The deposits vary in size from less than 1 million tons to greater than 5 million tons, with the majority of the largest deposits forming in sediment-dominated Besshi-type environments.The massive sulfide deposits are Fe-Cu-Zn systems dominated mineralogically by pyrite, pyrrhotite, chalcopyrite, and sphalerite. Minor phases include cubanite, arsenopyrite, tetrahedrite, and secondary marcasite. Silicate gangue is comprised predominantly of quartz and chlorite, with minor sericite, talc, and calcite. Primary sedimentary features preserved within massive sulfide are ubiquitous within the deposits and include planar bedding, crossbedding, and graded bedding.Fluid inclusion microthermometry of quartz gangue from the massive sulfide deposits is consistent with hydrothermal fluid temperatures found in genetically related modern seafloor hydrothermal environments. Minimum trapping temperatures of fluid inclusions in quartz from the stockwork feeder zone of the volcanic-hosted deposits generally range from 210 degrees to 260 degrees C. Fine-grained anhedral quartz gangue in massive sulfide layers also contains sparse primary fluid inclusions; typically the homogenization temperature is lower in the sediment-hosted deposits. Hydrothermal fluids in the volcanic-hosted settings exited the volcanic pile directly into seawater and were rapidly quenched, thus fluid inclusions only record high-temperature fluid conditions. Similar hydrothermal fluids in the sediment-hosted settings exited the volcanic pile into warm, seawater-saturated sediments and were cooled more slowly, in an environment where lower temperature fluid inclusions were formed and preserved.Sulfur and oxygen isotope data support a submarine origin for the deposits. Sulfur isotope values range from delta 34 S GDT = 0.8 to 5.9 per mil for volcanic-hosted deposits, and from 2.7 to 9.2 per mil for sediment-hosted deposits. The higher values for sediment-hosted ores reflect increased amounts of reduced seawater sulfate in the hydrothermal fluid, with sulfate reduction occurring in the seawater-hydrothermal fluid mixing zone within the sediment pile below the seawater-sediment interface. Oxygen isotope values for quartz gangue associated with massive sulfide also reflect the lower temperature deposition that is characteristic of the sediment-hosted deposits: delta 18 O quartz = 7.0 to 7.8 per mil (SMOW) for volcanic-hosted deposits and delta 18 O quartz = 9.6 to 16.9 per mil for sediment-hosted ores.Whole-rock geochemical analyses of the basalts and related igneous rocks from both the Valdez and Orca Groups show that these rocks are consistent with formation in midocean ridge and/or seamount environments. The geochemistry of the Valdez Group rocks suggests that sediment contamination of basaltic magma has occurred, imparting a primitive arclike signature. Discrimination diagrams show that the rocks from both groups have normal midocean ridge basalt, enriched midocean ridge basalt, and low K arc tholeiite affinities, and plot along the tholeiitic-calc-alkaline boundary.The fluid inclusion, stable isotope, and whole-rock data collected in this study are all consistent with the hypothesis that the massive sulfide deposits in the Prince William Sound district are sea-floor volcanogenic massive sulfide deposits that formed in both sediment-starved and sedimented midocean ridge settings.