Linking Short-Wave Infrared Spectral Features of Tourmaline to Compositional Variations: Implications for Porphyry-Skarn Copper Mineralization

Abstract The connection between spectral features and chemical variations in tourmaline supergroup minerals (“tourmaline(s)” hereafter) within porphyry copper deposits (PCDs) remains insufficiently understood, limiting their application as mineralization indicators. This study presents newly obtained short-wave infrared (SWIR) spectral, compositional, and Mössbauer spectroscopy data from the Jiama deposit (1,814 Mt at 0.4% Cu, Southern Tibet) with mineralized and barren samples. Additionally, compositional and spectral data from other published PCDs are integrated to compare compositional differences between tourmaline from mineralized and barren samples and reinterpret past spectral signals. The key absorption features at 2,250 and 2,350 nm in tourmaline are attributed to two distinct anionic sites (O1 and O3) that accommodate hydroxyl groups, with compositional exchanges within metal clusters bonded to these sites shifting the absorption features. The 2,250-nm (2,250W) feature corresponds to Fe2+-Mg exchange, shifting to longer wavelengths with increasing Fe2+ content, whereas the 2,350-nm (2,350W) feature reflects Fe3+-Al exchange, shifting to longer wavelengths with increasing Fe3+ concentration. Tourmaline hosted in mineralized samples from Jiama display longer 2,350W (>2,350 nm), shorter 2,250W (<2,247 nm), and lower 2,250W/2,350W (<0.9570) compared to barren samples, reflecting distinct compositional variations driven by higher Fe3+ content and Fe3+/FeTotal (T) ratios and controlled by elevated oxygen fugacity. Analysis of compositional data from other deposits shows similar features. This study emphasizes the transformation from chemical composition to spectral characteristics in distinguishing tourmaline hosted in mineralized and barren samples, providing a nondestructive and practical tool for field-based exploration. The Fe3+-rich tourmaline, with longer 2,350-nm features, is a reliable marker for mineralized PCDs.