The evaluation and application of tourmaline crystal chemistry as an indicator of porphyry-style Cu-Mo-Au mineralization

Abstract

As the exploration for porphyry Cu-Mo-Au deposits becomes increasingly challenging, there exists a need for exploration techniques that can detect buried deposits. Tourmaline supergroup minerals (TSM), a chemically and physically robust phase in the surficial environment, is investigated for its indicator mineral potential to identify porphyry Cu-Mo-Au systems. Samples of TSM from mineralized porphyry systems (Cu, ±Au, ±Mo) including Casino (Yukon Territory, Canada), Coxheath (Nova Scotia, Canada), Donoso Breccia-Los Bronces (Chile), Highland Valley Copper (British Columbia, Canada), New Afton (British Columbia, Canada), Schaft Creek (British Columbia, Canada), and Woodjam (British Columbia, Canada) were examined. Results show that TSM are generally an early hydrothermal mineral predating both mineralization and alteration assemblages. They form sub- to euhedral crystals that are exclusively black (in hand sample) and can be found throughout a porphyry system across all zones of alteration. Major-element chemistry of the TSM is unremarkable, with most having NaMg-Al (dravite) compositions (~80 % of grains) with lesser amounts of schorl (Fe). They exhibit consistent concentrations of ~2.0 apfu Mg with most plotting along the oxy-dravite–povondraite trend reflecting the dominance of Al3+ ↔ Fe3+ substitution. An investigation of TSM trace-elements (LA-ICP-MS) revealed key elements (Sr, Pb, Zn, Cu, and Ga) and select element ratios (Sr/Pb and Zn/Cu) that can be used as discriminators of its environment of formation. Utilizing an understanding of both intrinsic and extrinsic controls on TSM chemistry is an effective approach to distinguish TSM forming in porphyry bedrock samples. A decision tree was developed and applied to TSM grains recovered surrounding two known porphyry deposits including Casino-YT and Woodjam-BC to assess its effectiveness. Using both textural and chemical features of TSM from mineralized porphyry systems highlighted above, porphyry- related grains were identified down ice and downstream of both deposits respectively, highlighting the potential for this method to be used in exploration. The crystal chemistry of TSM has been demonstrated to be sensitive to its environment of formation allowing for the discrimination and identification of TSM forming in porphyries from that forming in other geologic environments and ore systems.