Chemical composition of tourmaline in orogenic gold deposits

Tourmaline from 18 orogenic gold deposits and districts, hosted in varied country rocks and metamorphic facies, was investigated by EPMA (electron probe micro-analyzer) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) to establish discriminant geochemical features to constrain indicator mineral surveys for gold exploration. Such tourmaline most commonly belongs to the alkali group, with a dravitic composition. LA-ICP-MS results were investigated with binary plots and PLS-DA (partial least square-discriminant analysis). PLS-DA suggests that the major element composition of tourmaline from orogenic gold deposits is buffered by the hydrothermal fluid, whereas trace element composition is strongly controlled by the composition and the metamorphic facies of the country rocks. Contents of Sn, Ga, Ti, rare earth elements (REE), Zr, Hf, Nb, Ta, Th, and U vary with the metamorphic facies of the country rocks. Tourmaline from orogenic gold deposits has high contents of Sr, V, and Ni and low Li, Be, Ga, Sn, Nb, Ta, U, and Th compared to tourmaline from other deposit types and geological environments. Binary plots such as Sr/Li vs. V/Sn, Sr/Sn vs. V/Nb, Sr/Sn vs. Ni/Nb, and Sr/Sn vs. V/Be, as well as PLS-DA, discriminate tourmaline from orogenic gold deposits from that of other settings. Binary plots highlight a transitional variation in the trace element composition of tourmaline from metamorphic, to magmatic-hydrothermal, to magmatic environments.

Automated summary

Tourmaline from 18 orogenic gold deposits was analyzed using EPMA and LA-ICP-MS to establish geochemical features for indicator mineral surveys in gold exploration. The major element composition of tourmaline is influenced by hydrothermal fluids, while trace elements are controlled by country rock composition and metamorphic facies. Tourmaline from orogenic gold deposits has distinct elemental compositions compared to other deposit types.