Lithium isotopic behaviour during high-temperature fluid-rock reactions of metapelites (>200 °C): A case study from the Baiyun orogenic gold deposit, Liaodong Peninsula, North China Craton

Abstract

Mixing of low δ7Li sediment components (δ7Li(‰) = 1000 × [(7Li/6Li)sample/(7Li /6Li)L-SVEC − 1]) with high δ7Li altered oceanic crusts can explain the MORB-like Li isotopic signature of arc lavas. However, marine sediments with low δ7Li values have rarely been reported. The metasomatism of metasedimentary rocks in mélange zones may induce large Li isotopic fractionation; however, the Li isotopic behaviour of metasedimentary rocks during high-temperature fluid-rock reactions remains poorly understood. In this study, an integrated Li-O-Sr-Nd isotopic investigation was conducted on the Baiyun orogenic gold deposit, which represents a relatively simplified high-T hydrothermal system, to explore the Li isotopic behaviour during high-T metasomatism (>200 °C) of metapelites. The main host rocks in Baiyun are lithologically monotonous schists, with alterations including K-feldspathization, sericitization, and carbonation. The Nd isotopes identified host rock samples that could best represent the protolith of the altered schists in Baiyun. The Sr and O isotopic modelling yielded consistent water/rock ratios of 0.1–7.6 and 0.6–5.9, respectively, suggesting that relatively small volumes of fluids were infiltrated. There is no correlation among Li, Sr, and O isotopes; instead, a negative Li-K2O correlation and a positive δ7Li-K2O correlation of the altered schists are observed, suggesting that potassic metasomatism controlled Li concentrations and Li isotopes. The δ7Li values of altered schists in Baiyun can be classified into Group I, having lower Li concentrations (3.2–22.2 ppm) and δ7Li values (−3.8–0.4‰) than the protolith (31.6–47.9 ppm, 0.9–2.8‰), and Group II, having significantly lower Li concentrations (0.9–2.5 ppm) and overlapped or higher δ7Li values (1.0–4.5‰) than the protolith. Fluid leaching during high-T alteration removes Li and more 7Li than 6Li, which accounts for the lower Li concentrations and δ7Li values of Group I. When there is little Li remaining in rocks owing to progressive fluid leaching, the Li isotopes of the altered schists are equilibrated with heavy fluid Li (fluid buffering), which leads to higher δ7Li values observed in Group II. Based on a case study in Baiyun, this study proposes that similar high-T metasomatism of metasedimentary rocks in mélange zones could result in low δ7Li sediment inputs. Mixing of flux-modified isotopically light sediment inputs with isotopically heavy, altered oceanic crusts could account for the MORB-like Li isotopes of arc lavas.