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Crowe, D.E., 1990

Geochemistry of volcanogenic massive sulfide and high-grade Au granite-hosted ore deposits, southern Alaska, and, development and application of laser microprobe techniques for analysis of sulfur, carbon, and oxygen isotope ratios

Bibliographic Reference

Crowe, D.E., 1990, Geochemistry of volcanogenic massive sulfide and high-grade Au granite-hosted ore deposits, southern Alaska, and, development and application of laser microprobe techniques for analysis of sulfur, carbon, and oxygen isotope ratios: University of Wisconsin, Madison, Ph.D. dissertation, 209 p., illust.

Abstract

The Prince William Sound district, located in southcentral Alaska, contains innumerable seafloor volcanogenic massive sulfide deposits hosted within volcanic and sedimentary rock sequences of the Late Cretaceous Valdez Group and the Paleocene to Eocene Orca Group accretionary complexes of southern Alaska. Detailed petrologic, whole-rock, stable isotope and fluid inclusion studies indicate that these massive sulfide bodies formed in mid-ocean ridge hydrothermal environments, and are not later epigenetic features related to accretion of the Orca and Valdez Groups to southern Alaska. Geochemical variations, particularly variations in stable isotope and fluid inclusion minimum trapping temperatures, distinguish volcanic rock-hosted deposits (low [delta] 34S and [delta] 18O values) from sediment-hosted deposits (elevated [delta] 34Sand [delta] 18O values). The Estelle pluton, located in the central Alaska Range, contains high-grade Au mineralization related to late-magmatic stage boiling of high-salinity fluids. Detailed petrographic and fluid inclusion studies reveal very-high-salinity fluids spatially associated with concentrically banded, spherical to ellipsoidal mineralized structures. The origin of these structures is problematic, as they resemble 'bubbles' related to hyper-solidus processes (such as immiscibility), yet occur in linear zones that crosscut the host pluton, suggesting subsolidus structural control. Our data are consistent with formation of a late magmatic stage hydrothermal fluid that underwent phase separation, producing an Au-bearing high-salinity phase and a lower-salinity unmineralized phase. The development of the laser microprobe is critical to the study of micro-scale stable isotope variations commonly encountered in ore forming environments. The University of Wisconsin laser microprobe provides high spatial resolution (to 70 [mu]m) with analytical precision of 0.14 per thous for measurement of [delta] 34S in pyrite, pyrrhotite, galena, chalcopyrite and sphalerite. Accuracy is good; corrections of -0.1 to +1.4 per thous are required to correct for mineral specific fractionation related to the process of laser ablation. Analysis of [delta] 18O and [delta] 13C in calcite and dolomite shows promise. Precision is not as good as for sulfides, and mineral specific fractionations are not yet determined, but appear to be necessary.

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