Laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) Rb-Sr sericite geochronology in orogenic gold deposits: Strategies and significance

Abstract

The challenge of determining the age of orogenic gold deposits has long been an issue due to the lack of suitable dating minerals. In recent years, the development of laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) has made in-situ Rb-Sr dating of K-rich minerals possible. In gold deposits, K-rich minerals such as sericite are commonly present, providing excellent objects for in-situ Rb-Sr dating. However, since sericite can form during various geological processes, selecting the appropriate sericite to represent the age of gold mineralization is a challenging task. This study focuses on the Liba gold deposit in the West Qinling Orogen, hosted in metamorphosed sedimentary rocks, where sericite is widely developed. The sericite in the mining area is classified into three types based on petrographic and geochemical analysis. Type I hydrothermal sericite is large and euhedral, with the characteristic trace element composition of high B and low Ba, Sr, Sc, and V. Type II interfered sericite exhibits trace element contents that are always intermediate between the other two types. Type III metamorphic sericite is small and euhedral, characterized by low B and high Ba, Sr, Sc, and V. Based on differences in petrography and trace elements, Type I hydrothermal sericite can be identified as the end-member of ore-forming hydrothermal fluids. Type III regionally metamorphosed sericite represents the end-member of metamorphic hydrothermal fluids. Type II interfered sericite is a result of the mixing of these two end-member origins. The in-situ Rb-Sr age obtained from Type I hydrothermal sericite is 208.7 ± 4.3 Ma (n = 25, MSWD = 0.98), interpreted as the mineralization age. The age of Type III regionally metamorphosed sericite is 427 ± 44 Ma (n = 22, MSWD = 3.1), indicating the age of regional metamorphism in the strata. The age from Type II interfered sericite has a relatively large MSWD value, and its age ranges from 187 Ma to 485 Ma, indicating that Type II sericite is affected by a combination of hydrothermal and regional metamorphic processes. Type I hydrothermal sericite has a lower Sr isotope ratio of 0.7091 ± 0.0041, suggesting a crust source. Type III regionally metamorphosed sericite (0.7133 ± 0.0077) exhibit higher Sr isotope ratios, indicating stratigraphic source. From the perspective of Rb and Sr content and Rb/Sr ratio, samples with high Rb/Sr ratio (>100) and low Sr content (<20 ppm) are more suitable for dating. When the span of ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr reaches 10 times and 1.5 times respectively, the dating error can reach less than 3 %. The development and application of LA-ICP-MS/MS Rb-Sr dating technology have addressed the challenges of dating gold deposits, providing new avenues for future geochronological research. This technique holds significant potential for advancements in the field of earth sciences.