Evaluating the application of texture and chemistry of detrital tourmaline as an indicator of porphyry Cu mineralization: A case study from the Casino porphyry Cu-Au-Mo deposit, Yukon, Canada

Abstract

Tourmaline is an effective indicator mineral for porphyry Cu-Au-Mo exploration because the compositional and morphological features of tourmaline in such systems are distinct from those found in non-mineralized environments. The texture and chemistry of detrital tourmaline was investigated using local stream sediment samples in the vicinity of the Casino calc-alkalic porphyry Cu-Au-Mo deposit (Yukon, Canada) to identify prospective grains by comparing them to grains in bedrock samples from the deposit. Bulk stream sediment samples were collected up to 18 km downstream of the Casino deposit and as far as 30 km away in a drainage basin that does not contain any known porphyry Cu mineralization. Overall, a method combining the physical and chemical characteristics is effective in discriminating detrital tourmaline, including morphology (i.e., grain clusters), inclusion populations (i.e., lack of inclusions), crystal chemistry (i.e., Na-dominant X-site, Mg/Fe dominant Y-site, and OH⁻/O²⁻ dominant W-site), and trace-element compositions (i.e., high Sr/Pb values and relatively low Zn/Cu values). Detrital tourmaline was evaluated on the basis of these parameters. Detrital grains are light brown to brown, sub- to euhedral, and range in texture from individual isolated grains to clusters of radiating aggregates. Tourmaline in the Casino deposit compositionally ranges from Fe-rich; 0.03–2.54 apfu, avg. 0.61 to Mg-rich, avg. 2.00 apfu, with a minor povondraite (Fe³⁺) component. Detrital tourmaline is similar in composition with Fe-rich; 0.03–3.73 apfu, avg. 1.04 to Mg-rich, 0.14–2.93 apfu, avg. 1.83 to tourmaline in bedrock at Casino. In both bedrock and detrital tourmaline, alkali- and alkaline-earth element concentrations are similar with Na > Ca > vacancy. The most relevant distinctions between tourmaline groups comes from grain morphology (i.e., clustered versus individual grains) and trace-element chemistry. A significant abundance of prospective porphyry Cu tourmaline grains (14 % of total recovered tourmaline in one sample) can be observed up to 18 km downstream from the Casino deposit (the current limit of sampling), while in drainages where there are no known porphyry Cu systems, tourmaline fitting these criteria is absent and reflect grains sourced from other geologic environments (e.g., pegmatitic, granitic, and metamorphic). This method provides a new addition to the indicator mineral toolbox for the exploration of porphyry Cu systems.