IN SITU RB-SR DATING OF HYDROTHERMAL MINERAL DEPOSITS BY LA-ICP-MS/MS

The ages of alteration and mineralization is important for both economic geology research and mineral deposit exploration. K-bearing minerals such as biotite, muscovite, illite, K-feldspar, alunite, and some amphibole are common alteration minerals in many deposit types such as porphyry, epithermal, orogenic, VHMS, skarn, greisen, and pegmatite deposits. Such minerals are currently dated using Ar-Ar or K-Ar dating methods, which is typically time-consuming. With the new generation triple quadrupole ICP-MS, also called ICP-QQQ-MS or ICP-MS/MS, it is now possible to achieve online separation of ⁸⁷Rb and ⁸⁷Sr and thus allow in-situ Rb-Sr dating of these minerals when coupling with a laser ablation system. The advantages of the LA-ICP-MS/MS Rb-Sr dating technique include shorter analytical time and lower cost.

In this study, the accuracy and precision of in-situ Rb-Sr dating were tested on hydrothermal muscovite from the Wolfram Camp W-Mo pegmatite-greisen deposit, northeast Queensland, Australia (WCM-1), and Dajishan granite-hosted W-Ta-Nb deposit, Jiangxi, China (DJS-1). The separation of ⁸⁷Sr from ⁸⁷Rb was achieved using N₂O cell gas. The isotopic ratios were calibrated with Mica-Mg-NP. Data were reduced using an in-house Excel spreadsheet, with the uncertainties in all steps incorporated into the final age uncertainty. The WCM-1 muscovite yields a Rb-Sr isochron age of 305 ± 3 Ma (2σ; MSWD = 2.4), which is statistically indistinguishable from Ar-Ar ages (305 ± 3; 308 ± 3 Ma; 2σ) and Re-Os ages of molybdenite from the same paragenetic stage (306 ± 3; 306 ± 2 Ma; 2σ) [1]. The DJS-1 muscovite produces a Rb-Sr isochron age of 162.2 ± 1.6 Ma (2σ; MSWD = 1.7) that is consistent with the Re-Os molybdenite age of 161.0 ± 1.3 Ma [2]. The accuracy and precision (~1%) of the ages show that the in situ Rb–Sr technique can be a useful complement to the established Ar-Ar method.

[1] Cheng, Y., Spandler, C., Chang, Z., and Clarke, G., 2018, Volcanic-plutonic connections and metal fertility of highly evolved magma systems: a case study from the Herberton Sn-W-Mo Mineral Field, Queensland, Australia: Earth and Planetary Science Letters, v. 486, p. 84-93.

[2] Zhang, S., Chen, Z., Shi, G., Li, L., Qu, W. and Li, C., 2011. Re-Os isotopic dating of molybdenite from Dajishan tungsten deposit in Jiangxi Province. Mineral deposits, 30(6), pp.1113-1121.