Two episodes of Caledonian hydrothermal events contributed to gold mineralization in southeastern Guizhou, Jiangnan orogen, China: Constraints from in situ monazite ages of the Kengtou deposit

The discovery of altered rock−type gold orebodies in southeastern Guizhou (southwestern Jiangnan orogen) highlights the region’s significance as a major gold province. However, the timing and the ore-forming processes remain poorly constrained, hindering a comprehensive understanding of gold metallogenic patterns. The Kengtou gold deposit (>15 tons Au) comprises both quartz vein−type and altered rock−type orebodies. Two types of hydrothermal monazite (Mnz1, Mnz2) were identified in this study. Mnz1, associated with pre-ore ankerite (Ank1) and rutile, occurs within unmineralized metamorphic rocks. In contrast, Mnz2 is spatially associated with auriferous sulfides within gold orebodies. Trace-element analyses show that Mnz1 has significantly higher total rare earth element (ΣREE) concentrations (430,658−583,738 ppm, mean = 565,224 ppm) than Mnz2 (67,779−172,804 ppm, mean = 137,558 ppm), although both exhibit similar hydrothermal-type chondrite-normalized REE distribution patterns. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb dating yielded lower-intercept ages of 443 ± 16 Ma (Mnz1) and 410 ± 13 Ma (Mnz2). These ages constrain two distinct hydrothermal episodes: The earlier episode (ca. 443 Ma) represents a pre-ore stage related to Caledonian regional greenschist-facies metamorphism that triggered widespread sericitization, while the later episode (ca. 410 Ma) marks the main gold mineralization stage during late Caledonian extension, where fluid-rock interaction led to the precipitation of gold-bearing pyrite. Overall, we propose that the recognition of two episodes of Caledonian hydrothermal activity has implications for district-scale gold exploration in the southwestern margin of the Jiangnan orogen.