Mineralogical and Geochemical Insights into Formation of the Muji Carbonic Springs, NW China

The Muji carbonic springs on the northeastern margin of the Pamir Plateau provide a natural window into tectonically controlled CO2 degassing within a continental collision zone. Through mineralogical and geochemical analyses, this study constrains the formation mechanisms and regional geological significance of carbonic spring systems. The formed deposits are dominated by calcite and aragonite, with minor dolomite, quartz, and gypsum. The compositions of major elements are consistent with the observed mineral assemblages, reflecting that the carbonate deposition was mainly governed by CO2 degassing intensity and associated kinetic effects under cold-spring conditions. Carbon isotopes of the deposits are consistently enriched in heavy carbon with δ13C values of +3.5‰ to +9.1‰, indicating a persistent contribution of deep-sourced CO2, most likely derived from metamorphic decarbonation of the crustal carbonates. Calcite exhibits moderate δ13C values due to rapid precipitation limiting isotope enrichment, whereas aragonite records higher δ13C signatures under subdued degassing and stable hydrodynamic regimes. The narrow δ18O range (−10.7‰ to −12.6‰), closely matching that of the spring waters, indicates that the tufas record the δ18O of the spring waters through DIC-water oxygen exchange. Trace element distributions (Sr–Ba–U) reveal systematic enrichment in deep-sourced fluids and progressive downstream geochemical alteration driven by spring–river mixing. The HD springs show high Sr and δ13C values, indicating minimal dilution of ascending CO2-rich fluids, while MJX and MJXSP groups record variable degrees of shallow mixing. Collectively, the Muji system exemplifies a coupled process of “deep fluid input–shallow mixing–precipitation kinetics.” Its persistent heavy δ13C and trace-element enrichments demonstrate persistent metamorphic CO2 release through fault conduits under ongoing compression. These findings establish the Muji springs as a key non-volcanic analogue for deep CO2 degassing in continental collision zones and provides new insights into crustal carbon recycling and tectonic–hydrochemical coupling at plateau margins.