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

Abstract

<p>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 on paleofluids from syn-tectonic felsic and quartz veins associated with the Devonian subduction and Permian collision of the Chinese Altai was conducted. We found that the Devonian fluids were organic alkanes-CO₂-S-Ca-Mg-rich saline fluids with variable CO₂/CH₄ (0.09−5.03) and lower F⁻/SO₄²⁻ (0.02−0.14) and Al³⁺/Mg²⁺ (0−0.11) ratios, whereas the Permian fluids were 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 (0−1.31, mostly <1) and higher F⁻/SO₄²⁻ (0.21−0.76) and Al³⁺/Mg²⁺ (0.10−2.56) ratios. The Devonian and Permian fluids also have similar δ¹³C-CO₂ values of −23.8‰ to −3.5‰ and −16.5‰ to −3.7‰, respectively. These data suggest that both fluids derived mainly from devolatilization and dehydration melting of metasediments; the Permian fluids likely involve more biotite melting in the deeper crust and more mantle-derived components, whereas the Devonian fluids contain more meteoric components. Base metal-dominated Devonian mineralization occurred as deep-sourced organic matter- and S-rich fluids promoted base metal migration, whereas the relatively oxidized fluid conditions inhibited the 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 a more complete picture of how tectono-thermal events fertilize the crust and demonstrate that syn-tectonic fluids can serve as proxies for metallogenic processes during orogenic cycles in general.<br></p>