Hydrothermal apatite SIMS Th[sbnd]Pb dating: Constraints on the timing of low-temperature hydrothermal Au deposits in Nibao, SW China

Abstract

Precise geochronology is a crucial tool for understanding the genesis of low-temperature hydrothermal mineral deposits, but datable minerals are not always available; e.g., mineralization within sedimentary rocks, such as Carlin-type or Carlin-like Au deposits. Here, we demonstrate that hydrothermal apatite associated with Au from the large Nibao deposit in the Yunnan–Guizhou–Guangxi region of South China can be dated to yield the age of Au mineralization. Nibao is a Carlin-like deposit hosted in a Permian carbonate-bearing pyroclastic breccia. The auriferous minerals are predominantly zoned pyrite containing As-, Cu-, and Au-rich rims. Euhedral and subhedral apatite forms a mosaic texture and is intergrown with hydrothermal quartz, sericite, and zoned auriferous pyrite, indicating that the apatite is of hydrothermal origin and was coeval with Au mineralization. Unlike sedimentary and igneous apatite, hydrothermal apatite is relatively depleted in LREEs, enriched in MREEs, and slightly depleted in Eu. Hydrothermal apatite also has high Th/U ratios, negligible U concentrations, and very low concentrations of common Pb. This makes the apatite a potential candidate for ThPb dating. Our SIMS ThPb analyses yield a weighted mean 232Th/208Pb age of 141 ± 3 Ma (N = 23, MSWD = 2.2) for apatite coeval with auriferous pyrite, and this is the best estimate of the timing of Au mineralization. Hydrothermal apatite is also present in similar deposits in the Golden Triangle of South China. Therefore, ThPb dating of hydrothermal apatite potentially allows for the accurate dating of low-temperature hydrothermal deposits in China and worldwide.