Noble gas isotopes in fluorite from the Zaghouan Fluorite District (NE Tunisia): constraints on crustal fluid sources in MVT deposits

Fluorite-rich Mississippi Valley–Type (MVT) deposits provide key constraints on the origin and evolution of fluorine-bearing hydrothermal systems in sedimentary basins. Here we present the first He–Ne–Ar elemental and isotopic data obtained from fluid inclusions hosted in fluorite from the Zaghouan Fluorite District (ZFD), northeastern Tunisia. Eighteen samples from eight fluorite-rich MVT deposits, including both stratabound and vein-type mineralization, were analyzed to investigate the sources of ore-forming fluids. Measured 4He/20Ne and 40Ar/36Ar ratios indicate a variable but generally minor contribution from atmosphere-derived fluids. Atmosphere-corrected 3He/4He ratios range from 0.03 to 0.08 Ra (Ra = 1.39 × 10−6), values that are significantly lower than those of the Sub-Continental Lithospheric Mantle (SCLM; 6.1 ± 0.9 Ra) and are characteristic of crustal-derived fluids. Quantitative mixing calculations show that mantle-derived helium accounts for ~1% of the total helium budget. Overall, He–Ne–Ar systematics reveal mixing among three components—crustal, atmospheric, and a negligible mantle contribution—with crustal fluids overwhelmingly dominating the ore-forming system. These results indicate that both stratabound and vein-type fluorite deposits in the ZFD formed in a dominantly crustal environment, supporting basinal brine models for fluorite-rich MVT mineralization. The similarity in noble gas signatures across different mineralization styles further suggests a common fluid reservoir and genetic process, with structural pathways controlling fluid focusing and emplacement rather than fluid source.