Substitution mechanism controls on compositional variations and spectral responses of white micas in major hydrothermal systems

Abstract

Whether the shorter or longer wavelengths of diagnostic absorption features of white mica serve as a better vector to mineralization remains controversial. This uncertainty stems from a limited understanding of the compositional substitution mechanisms that control spectral variations. This study compiled published datasets of white mica with both available composition and spectroscopy data from five major hydrothermal systems, including porphyry, epithermal, skarn, orogenic, and volcanogenic massive sulfides (VMS) deposits. Three types of white micas, each dominated by distinct substitution mechanisms—interlayer-dominated (Na⁺ ↔ K⁺, and Ca²⁺, Ba²⁺ ↔ 2 K⁺), Tschermak-dominated (^VIAl³⁺ + ^IVAl³⁺ ↔ ^VIFe²⁺, ^VIMg²⁺ + ^IVSi⁴⁺), and illitic-dominated (K⁺ + ^IVAl³⁺ ↔ ^IVSi⁴⁺ + []_interlayer)—have been identified, accounting for the observed inconsistencies in spectral responses. The interlayer substitution is associated with shorter absorption feature (< 2195 nm) arising from the combination of high-frequency Al₂OH stretching (ν) vibration and bending (δ) vibration. Interlayer-dominated white mica is predominantly found in orogenic deposits, where its presence typically indicates proximal mineralization. Tschermak substitution induces continuous wavelength shifts (2195–2215 nm) in the mid-frequency ν region combined with δ vibration. A high-level of Tschermak reaction stimulates low-frequency ν vibration, producing specific longer absorption feature (> 2215 nm) with δ vibration. Tschermak-dominated white mica is the most widespread among diverse systems. Illitic substitution, however, leads to compositional changes that are not detectable in shortwave infrared (SWIR, 1300–2500 nm) regions, thus rarely providing vectoring indicators. This study suggests that data from white mica characterized by different dominant substitution mechanisms should be distinctly evaluated, thereby enhancing the efficacy of SWIR spectroscopy for mineral exploration.