Lithium isotopic systematics of ore-forming fluid in the orogenic gold deposits, Jiaodong Peninsula (East China): Implications for ore-forming mechanism

Abstract

There are rare studies on the Li isotope systematics of ore-forming fluids in the mesothermal deposits, especially in the orogenic gold deposits. We carried out systematic Li isotopic analyses on whole-rock granite samples, and quartz and pyrite were separated from both the altered-rock type and quartz-vein type gold deposits from the Canzhuang gold deposit of the Jiaodong Peninsula (East China) with purposes of shedding lights on the ore-forming fluid evolution and ore-forming mechanism. Altered granite and unaltered granite yield δ7Li values of 12.6‰ and 1.9‰, respectively, indicating that fluid-related alteration can change the Li isotopic composition of granite significantly and ore-forming fluid is enriched in heavy Li isotope. The δ7Li values of quartz and pyrites from the altered-rock type gold deposits range from 5.5‰ to 21.1‰ and from 0.5‰ to 16.8‰, respectively. The δ7Li values and Li contents of quartz are positively correlated in the altered-rock type gold deposit, with the two figures in the first mineralization stage (5.5–13.7‰ and 0.3–1.1 ppm) being significantly lower than that in the second stage (17.9–20.8‰ and 1.0–10.4 ppm) and the third stage (17.3–21.0‰ and 1.1–4.2 ppm). This is consistent with the lithium isotopic compositions of pyrites of the altered-rock type gold deposits, with δ7Li values of pyrites in the first stage (0.5–2.8‰) being significantly lower than that in the second stage (2.3–16.8‰) and the third stage (5.8–6.2‰). Collectively, these features demonstrate that the Li isotopic compositions of the altered-rock type gold deposits were controlled mainly by interaction between ore-forming fluid and host granites, and that the ore-forming fluids in three stages belong to different pulses of fluid injection and evolve separately. On the contrary, the quartz and pyrite from the quartz-vein type gold deposits yield δ7Li values of 9.4‰ ~33.6‰ and 0.5‰ ~24‰, respectively, which are different from that of the altered-rock type gold deposits. In addition, the lithium isotopes of quartz show no correlation with the lithium contents of quartz, and the δ7Li values of quartz and pyrites decrease gradually from the first to the third metallogenic stage, with δ7Li ranging from 33.6‰ to 9.4‰ for quartz, and from 24‰ to 0.5‰ for pyrite. Based on the fractionation calculation, we suggest that Li isotope variation of the quartz-vein type gold deposits was ascribed to the Rayleigh fractionation in a close metallogenic system. This work demonstrates that the orogenic gold deposits display a significant Li isotopic fractionation and the lithium isotopes of quartz and pyrite are potential in providing insights into the evolution processes of mesothermal hydrothermal systems.