The hydrothermal system architecture related to the formation of the contemporaneous Au-bearing Horne and Quemont volcanogenic massive sulfide (VMS) deposits was visualized by employing kriging methods to map whole-rock oxygen isotope compositions, zones of silica addition and loss, and water contents in two- and three-dimension. Zones of alteration were mapped in three-dimensions in the vicinity of the steeply dipping Horne deposit, to depths of as much as 2 km. In all, nearly 300 samples were analyzed for oxygen isotopes and supplemented by previously published whole-rock analyses. Contents of SiO2, H2O, MgO, Al2O3, and S from chemical analyses of nearly 5,000 samples within the two- and three-dimensional study regions were used separately, and in combination with the oxygen isotope data, for modeling and hydrothermal mapping purposes.
The Horne and Quemont deposits formed within a similar time frame, but in different magmatic-hydrothermal systems, distinguished by their mapped hydrothermal architecture. The Quemont deposit appears to be centered on the Powell pluton, which intruded late into an apparent volcanic-filled, rift-graben structure. Although structural complexities are apparent, we infer mineralizing high-temperature upflow in the footwall of the Quemont deposit to have emanated from a reaction zone above the Powell pluton (and its precursors), beneath a zone of extensive silicification. Faulting on the Andesite fault and Horne Creek faults, plus erosion, has removed evidence of the upflow zone in the hydrothermal system of the Horne deposit.
Areas of silicification correspond, in general, with isotopic evidence of lower temperature alteration. Such alteration east of the Quemont deposit signaled the waning of hydrothermal activity. The suggested cooling, for the most part, promoted the precipitation of silica. In the case of the Horne deposit, mixing of metalliferous hydrothermal fluid with cold seawater in the permeable footwall rocks, in an apparently relatively stratigraphically stable and long-lived hydrothermal system, evidently led to marked footwall silicification. The silicified footwall may have contributed to an increased efficiency of sulfide precipitation in the Horne deposit. Continued intrusion and some post-VMS hydrothermal activity is recorded in the hanging-wall section to the Horne deposit. Our data suggest that deposition of the 10 million ounces (Moz) of Au within the Horne deposit was syngenetic, and not the product of subsequent hydrothermal activity.