Syn-tectonic fluids decoding effects of tectono-metamorphic cycles on regional metallogenic evolution of the Chinese Altai, central Asia

Despite their close temporal and spatial relationships, the effects of tectono-thermal events on ore formation remain obscure. To better understand this process, a comprehensive geochemical investigation was conducted on syn-tectonic pegmatite and quartz veins associated with the Devonian subduction and Permian collision of the Chinese Altai. We found that the Devonian fluids were organic alkanes-CO₂-S-Ca-Mg-rich saline fluids with variable CO₂/CH₄, lower F⁻/SO₄²⁻ and Al³⁺/Mg²⁺ ratios, whereas the Permian fluids are immiscible fluids including CO₂-C₄H₁₀-CO-rich oxidized gas bubbles and CH₄-C₃H₈-C₂H₆-Ca-Na-K-Al-S-Cl-F-rich reduced saline fluids with lower CO₂/CH₄ (mostly <1), higher F⁻/SO₄²⁻ and Al³⁺/Mg²⁺ ratios. The Devonian and Permian fluids also have similar δ¹³C-CO₂ values of −3.5~−23.8‰ and −3.7~−16.5‰, repressively. These data suggest that both fluids derived mainly from devolatilization and dehydration melting of metasediments but the Permian fluids likely involve more muscovite dehydration and biotite melting in the shallower and deeper crust, respectively. Besides, the Devonian fluids contain more meteoric components whereas the Permian fluids contain more mantle-derived components. Base metal-dominated Devonian mineralization occurred as the deep-sourced organic matter-S-rich fluids promote base metal migration whereas the relatively oxidized fluid conditions inhibited mineralization of many other metals. By contrast, the more reduced and F-rich Permian fluids with more mantle contributions facilitated the extraction of Au and uptakes of rare metals from reworked metasediments and promoted their mineralization. These findings provide more complete pictures of how tectono-thermal events fertilize the crusts and demonstrate that syn-tectonic fluids can serve as proxies for metallogenic processes during orogenic cycles in general.