Mineralogy and chemistry of tourmaline in the Woodjam porphyry deposits, British Columbia

Abstract

Tourmaline is a common accessory and gangue mineral in a diverse range of hydrothermal ore-forming systems covering a broad spectrum of pressure, temperature and chemical conditions, reflecting tourmaline's wide stability range. Previous studies, dating back into the 1960s, have shown that tourmaline can very closely track changes in physicochemical conditions within the fluid from which it is precipitated. Tourmaline's refractory and resistant physical properties mean that the mineral is able to preserve this information through numerous subsequent phases of metamorphism, alteration and weathering; in many cases tourmaline may be the only robust remaining record of original hydrothermal processes. Although tourmaline's utility in mineral exploration has been demonstrated, very few studies have attempted to measure concentrations of the ore metals themselves. In general, these limitations have been imposed by the common use of EPMA alone to interrogate mineral chemistry at high spatial resolution. In many porphyry-related deposits of the South American Cordillera, tourmaline occurrences are abundant and extensive, and have attracted much interest for use during mineral deposit exploration. However, owing to various factors including differences in their tectonic settings and magma chemistries, porphyry-related deposits of the Canadian Cordillera rarely report significant tourmaline among their mineral inventory. Consequently, studies of tourmaline associated with Canadian deposits are limited, and almost no data exist on tourmaline occurrences associated with alkalic or hybrid alkalic - calc-alkalic systems. In this study we report mineralogical and LA-ICP-MS trace element analyses of tourmaline minerals from within the hybrid alkalic and calc-alkalic Woodjam porphyry Cu-Au deposits of central British Columbia. These minerals display significant variation in both their major and trace element compositions, reflecting both temporal and spatial evolution of the magmatic- hydrothermal fluids from which the deposit formed. Tourmaline associated with high-temperature potassic alteration has greater schorl (i.e., Fe-rich) component and trace element abundances characteristic of the core of a porphyry hydrothermal system, while tourmaline recovered in association with distal, lower-temperature, albite-rich alteration has an increased dravitic (i.e., Mg-rich) component and a distinct trace element suite. Tourmaline recovered in intimate association with chalcopyrite mineralization has a pronounced blue colour and increased copper content. Hence, within the Woodjam and other similar deposits it may be possible to use tourmaline mineralogy and chemistry as a vector to mineralization. In addition, careful analysis of tourmaline recovered from surficial samples (till, soil, etc.) may provide detailed information on the type and abundance of any concealed mineralized body.