GSA Connects 2021 in Portland, Oregon

Paper No. 184-10
Presentation Time: 2:30 PM-6:30 PM


HUANG, Shiqiang, Department of Geology and Geological Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, CHANG, Zhaoshan, Department of Geology and Geological Engineering, Colorado School of Mines, Center for Advanced Subsurface Earth Resource Models (CASERM), 1500 Illinois Street, Golden, CO 80401 and GARBE-SCHÖNBERG, Dieter, Institute of Geosciences, CAU Kiel University, Ludewig-Meyn-Strasse 10, Kiel, D-24118, Germany

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.