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 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 delta C-13-CO2 values of -23.8 parts per thousand to -3.5 parts per thousand and -16.5 parts per thousand to -3.7 parts per thousand, 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.