| Popis: |
Large scale mapping in Prosser Township has revealed an entirely felsic subaqueous volcanic sequence containing both pyroclastic and effusive high silica rhyolites which are exceptionally well preserved. The eruption sequence alternates between thin coulees and tuffaceous units, all of which are covered by two distinct units, a pumice- and spatter-rich tuff breccia and a quartz porphyritic rhyolite. Aphyric coulees are well preserved, having flow banding, perlite, and hyaloclastite, which are indicative of a glassy protolith. In the coulee interior there is no evidence of sea-floor clay alteration; the former glass has been isochemically recrystallized to an inter-grown mixture of quartz and plagioclase during metamorphism. These formerly glassy rhyolites formed thin flows by normal effusive eruption without the intervention of high temperatures or volatile depolymerising agents. The crystal-poor rocks were not erupted near their liquidus temperatures (ca. 980°C) as suggested by earlier work using zircon saturation thermometry, but instead the Ti-in-zircon thermometer demonstrates they were erupted around 830°C. Detailed petrography has allowed the robust determination of a protolith composition which is used to assess mobility of the rare earth and high field strength elements. The rare earth elements are shown to be considerably mobile, approximately by a factor of three within a single, 6 m thick coulee. In contrast the HFSE, with the exception of Pb and Y, have only been mobilized up to 10%, largely as a result of sericitization and chloritization. The Nb/Zr vs. Al2O3/TiO2 diagram reliably discriminates between aphyric and quartz porphyritic rhyolite units which respectively correlate with the Lower and Upper members of the nearby Kidd Creek deposit. Furthermore the Hf-Th-Ta diagram shows a clear magmatic evolutionary trend towards the Hf apex. This is consistent with observed zircon abundances within the aphyric rhyolite coulees. The volcanological, textural and lithogeochemical work allows for the correlation of the Prosser Township rocks to the adjacent Kidd Creek mine stratigraphic sequence. However, the Middle member at the mine, a volcanoclastic unit which separates aphyric and quartz porphyritic lavas, is found as a pyroclastic pumice- and spatter-rich tuff breccia at Prosser Township that has not been previously described in the area. As such it may provide a suitable host rock for hydrothermal massive sulphide mineralization. |
