Illite Rb-Sr isochron dating constrains the timing of Carlin-type gold mineralization: insights from the Yata deposit, South China

Abstract

Precise dating of Carlin-type gold deposits (CTDs) remains challenging owing to the scarcity of suitable, widely distributed minerals for reliable isotopic dating. Illite from widespread argillization in CTDs has been underutilized for precise dating due to mixing of diagenetic and hydrothermal varieties. Here we integrate petrographic, mineralogical, and geochemical analyses with grain-size resolved Rb-Sr isochron dating of illite from the Yata deposit in the Gold Triangle (South China) to clarify the formation processes of distinct illite populations and evaluate their potential to constrain timing of Carlin-type gold mineralization. Two texturally distinct illite populations were separated and verified as nearly pure phases using scanning electron microscope (SEM), electron microprobe analysis (EMPA), X-ray diffraction (XRD), and short-wave infrared (SWIR) spectroscopy. Ilt_1 occurs as large isolated sheets (>5 μm) with undeformed morphology and sharp boundaries against adjacent minerals. Their initial 87Sr/86Sr ratios (∼0.7085) match Middle Triassic seawater, supporting a diagenetic origin from sedimentary pore fluids. Ilt_2 forms fine scaly aggregates (<5 μm) infilling pores and enveloping auriferous pyrite, indicating a hydrothermal origin associated with gold mineralization. Both populations have broadly similar Sr isotopic signatures and geochemical compositions, indicating that Ilt_2 largely inherited its chemical signature from pore fluids recorded by Ilt_1, with minor Ca, Fe, and K variations likely reflecting interaction between ore fluids and pore fluid within the sedimentary rocks. Rb-Sr isochrons yield ages of 237.7 ± 9.8 Ma for Ilt_1, consistent with the diagenetic age of the host Middle Triassic Xuman Formation, and 216.7 ± 3.8 Ma for Ilt_2, coinciding with regional Indosinian metallogenic event (∼210 Ma). Mechanical mixing during illite separation cannot be excluded, so the older age represents a lower limit for sediment diagenesis of the host Xuman formation, while the younger age provides an upper limit for gold mineralization at Yata. Given the ubiquity of argillization in CTDs, illite Rb-Sr dating provides a robust method for constraining CTD mineralization globally, although the chemical composition of ore-related illite may not reliably reflect that of the primary ore-forming fluids.