Mineralogical and Geochemical Vectors Within and Around Advanced Argillic-Altered Rocks of British Columbia

Abstract Large zones of advanced argillic alteration are unusual features in many porphyry deposits in British Columbia but occur, typically <10 km2, in several districts. In this study, mineral assemblages and compositions across zones that include advanced argillic alteration in three British Columbia mineral properties are characterized: Tanzilla, the Alunite Ridge, and Kemess North. Petrographic and short-wave infrared results show alteration zoning of mineral assemblages within and around zones of advanced argillic-altered rocks from distal green muscovite-chlorite to pale-green muscovite and to white muscovite-clay (kaolinite, dickite) assemblages proximal to a porphyry deposit. Similar zoning occurs from deep to shallow parts of a porphyry deposit. Aluminum-rich minerals such as andalusite, corundum, diaspore, and topaz, commonly with pyrophyllite and/or muscovite-kaolinite-dickite assemblages, occur within the central zones of advanced argillic alteration, with respect to the main upflow zone. The green and pale-green muscovites typically have phengitic compositions (Fe and Mg rich), whereas the white muscovite is Al rich. The alteration zonation is also mapped by whole-rock geochemical data using new K-Mg-Al molar and K/Rb-Al diagrams. An Na-Ca depletion index correlates with the mapped alteration mineralogy. Cathodoluminescence indicates that disseminated, residual, and recrystallized quartz within more central parts of advanced argillic alteration displays red luminescence and typically occurs with blue luminescent pyrophyllite and clays. Two trace element indices, MPIx (MDRU porphyry index, Mineral Deposit Research Unit) and MPIx-Lateral, compare trace elements occurring above porphyry deposits to those concentrated at depth or distally to provide vertical and lateral mineralization footprints. These observations provide a tool set that can compare prospects in a district, identify the size and level of exposure of the mineralized centers, and aid rapid and cost-effective exploration vectoring for porphyry copper and related epithermal mineralization within and around advanced argillic-altered rocks in British Columbia and elsewhere.