Advances in Micro-Beam Monazite U-Th-Pb Dating for the Metallogenic Geochronology of Sediment-Hosted Disseminated Gold Deposits

Sediment-hosted disseminated gold deposits are a crucial global source of gold. However, accurately determining the timing of hydrothermal mineralization has long been a challenge, which has hindered our understanding of ore genesis and ore-forming processes. Monazite is a common accessory mineral in sediment-hosted disseminated gold deposits. With high U and Th contents, extremely low common Pb, and a high closure temperature for Pb, monazite is an ideal mineral for U-Th-Pb isotopic dating. Recently, monazite U-Th-Pb geochronology has been successfully applied to constrain the timing of mineralization in sediment-hosted disseminated gold deposits. Nevertheless, monazite is highly susceptible to late-stage fluid metasomatism and frequently undergoes dissolution and reprecipitation. The complex compositional variations and zoning textures of monazite often prevent traditional analytical methods from obtaining accurate geochemical data. This paper focuses on the monazite U-Th-Pb isotopic system and compares four major dating methods. Electron probe microanalysis (EPMA) is suitable for identifying micrometer-scale compositional zoning, but its age precision and ability to correct common Pb are limited. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) has high analytical efficiency and is suitable for batch in situ micro-beam age analyses, but it has a relatively large spot size and requires correction for isotopic fractionation and matrix effects. Secondary ion mass spectrometry (SIMS) combines relatively high spatial resolution and analytical precision, although it is strongly affected by matrix effects and is costly. Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) provides the highest age precision, but it cannot distinguish different domains within single grains with complex internal structures. Based on previous studies, this paper reviews the effects of monazite textures, structures, and chemical compositions on age results, and indicates that high-spatial-resolution SIMS U-Th-Pb dating with a spot size of ＜5 μm can be effectively applied to constrain the mineralization ages of sediment-hosted disseminated gold deposits. Before U-Th-Pb dating, genetic mineralogy investigations, including mineral assemblages, ThO₂ contents, backscattered electron (BSE) images, and elemental mapping, should be conducted to selectively analyze hydrothermal monazite domains related to mineralization. Matrix-matched standards should also be used to correct matrix effects and obtain more reliable age information.