Geological &amp; geochronological framework and mineralization characterization of the Thorn Property, and associated volcanoplutonic complexes of northwestern British Columbia, Canada
Creators: name: Simmons, Adam Thomas
Publisher: name: University of British Columbia
Year: 2010

description: A geological investigation of Late Cretaceous volcanoplutonic rocks and associated hydrothermal systems was undertaken at the Thorn Property, northwestern British Columbia, Canada, to provide a geological framework for future research and exploration, and to characterize a newly recognized belt of Late Cretaceous volcanoplutonic rocks. Emphasis is placed on the Thorn Property in the belt because it contains the thickest sequence of Late Cretaceous volcanic strata in addition to containing a wide variety of hydrothermal alteration and mineralization. Late Cretaceous volcanoplutonic rocks were emplaced into and onto rocks of the Stikine tectonostratigraphic terrane, as part of a continental arc. The Late Cretaceous volcanoplutonic rocks at the Thorn Property form part of the NNW-trending Late Cretaceous volcanoplutonic belt, which extends for at least 300 km from the Yukon Territory into northern British Columbia. Two distinct periods of magmatism have been recognized at the Thorn Property as part of the Late Cretaceous suites. The 87-93 Ma Thorn Suite comprises the oldest components of the belt and are the major host rocks for hydrothermal mineralizing systems. The younger 80-87 Ma Windy Table Suite was emplaced into, and unconformably deposited onto, rock of the Thorn Suite. Four sub-types of Windy Table intrusions are recognized and 1600 m of Windy Table volcanic strata was emplaced over a period of 6 million years in three phases from 80-86 Ma. The oldest phase in composed of welded, dacitic-andesitic lapilli tuffs, which is overlain by rhyolitic flow dome complexes and sporadically deposited variably welded dacitic to rhyolitic lapilli tuffs. Geochronology, field relationships and lead isotopic compositions of sulphides form mineralized systems and feldspar from intrusive rocks suites suggest that fluids for the mineralized systems were sourced from intrusive rocks of the Windy Table Suite. Several styles of mineralization are present at the Thorn Property, including highsulphidation mineral assemblage veins, polymetallic breccia-hosted replacement, porphyry Cu- Mo, skarn and base metal veins. Emphasis was placed on the high sulphidation mineral assemblage veins because they have the highest grade and tonnage potentials of all the systems and provide insights to the potential for high-sulphidation epithermal mineralization in the belt. These veins were deposited in a series of six NE-trending zones of brittle deformation in Thorn Suite rocks. Mineralogy and metal ratios indicate that the fluids for the mineralized vein systems were sourced to the SW, where the fluids may been focused along a previously developed NW-trending fault. Several lines of evidence suggest that the veins were deposited in the sub-epithermal environment at depths greater than 1600 m. Observed advanced argillic alteration (pyrophyllite+dickite+diaspore) at the Thorn Property is characteristic of hot (&gt;250°C) and potentially deep hydrothermal fluids. The distribution of metal ratios, alteration minerals and ore minerals suggests that as the fluids moved along zones of NE-trending brittle deformation from east to west and evolved from high-sulphidation, acidic fluids to more intermediate-sulphidation, neutral fluids with time, distance from source and/or greater fluidwall rock interaction. Field observations indicate that there is a spatial correlation between the presence of both Thorn Suite rocks and Windy Table Suite rocks and the presence of high-sulphidation systems. Two key components of this environment are the presence of non-reactive host rocks, such as the Thorn Suite and the presence of plutonic bodies which have potential to develop a large hydrothermal system (Windy Table Suite rocks). Future studies should focus more on the interaction of the high-sulphidation fluids with the overlying Windy Table volcanic strata to better evaluate the potential of widespread high-sulphidation epithermal systems in the Late Cretaceous volcanoplutonic belt of northwestern British Columbia.