Tourmaline Mineral Chemistry: A Fertility Assessment and Vectoring Tool for Mineral Exploration in Magmatic-Hydrothermal Ore Systems

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Abstract Tourmaline is a structurally and chemically complex hydrous borosilicate mineral that displays significant compositional variability in both major and trace elements. Its occurrence in a wide variety of ore-forming systems makes tourmaline a powerful provenance indicator because it can record the temporal and spatial evolution of hydrothermal fluid compositions and provide key information regarding metal fertility and exploration vectoring within a system. In this study, tourmaline-bearing rocks were sampled from a diverse spectrum of magmatic-hydrothermal mineralized and barren systems, including porphyry Cu-Mo and Cu-Au deposits and prospects, tourmaline-rich breccia pipes, granite-related Sn deposits, Archean Au lodes, metamorphic terranes, pegmatites, and granite batholiths. Based on 1,948 individual laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) spot analyses of tourmaline grains, tourmaline chemistry can discriminate different types of hydrothermal systems and geologic environments. Pegmatite-related tourmaline has high Al, Li, and Pb contents and low Fe, Mg, and Ti contents, whereas metamorphic tourmaline commonly has high Mn and Zn and low Y and Ba concentrations. High Sr, Ni, and Cr and low Ga, Sn, and Mn/Zn characterize tourmaline from the Red Lake Archean lode gold deposit. Tourmaline from granite-related tin systems is characterized by high Al, Fe/Mg, and Sn and low Sr contents. Tourmalines from porphyry deposits and associated breccia pipes typically have high As, Sb, Sc, V, and Sr, intermediate to high Mn/Zn, and low Fe/Mg. In porphyry-related alteration zones and breccia pipes, tourmaline has compositions that vary systematically from deep to shallow depths. Most transition elements (V, Sc, Zn, and Cr) in tourmaline commonly increase in concentration with increasing distance (or elevation) from mineralization centers. Arsenic, Sb, Pb, and Sr in tourmaline display highest concentrations between 1 and 1.5 km from the deposit center and define geochemical shoulders to the mineralization ore zone. Characteristic trace elements (e.g., As, Sb, Sr, and Y) in tourmaline can potentially be used for fertility assessment to discriminate porphyry deposits from barren granitic batholiths, and potentially distinguish large porphyry deposits from smaller systems. Although there is some overlap in the concentrations of these trace elements relative to the compositional fields defined for tourmalines from mineralized and barren systems, the tourmalines from giant porphyry Cu deposits can be distinctive by having the most variable and highest As (>100 ppm) and Sb (>10 ppm) contents and lowest Sr/Y (<100) ratios. Our study provides a new tool defined by hypogene, low-level geochemical anomalies in tourmaline for mineralization vectoring and fertility assessment in magmatic-hydrothermal systems, especially those associated with porphyry Cu-Mo-Au deposits.

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