Geological and geochemical constraints on the origin of the Early Cambrian Kalaizi Fe–Ba deposit in Western Kunlun, NW China

The Kalaizi deposit in the Bulunkuole Group of Taxkorgan area, Western Kunlun (NW China) is a unique regional Fe–Ba deposit accompanied with sporadic Fe–Pb–Zn sulfide mineralization within a series of iron deposits. All the ore bodies are stratabound and hosted by the Early Cambrian metamorphosed argillaceous clastic rocks. Here, we analyzed C–S–O–Sr isotopes and trace elements in individual minerals (e.g., magnetite, barite, ankerite) from the Kalaizi deposit to investigate the origin of this deposit for the first time. The S isotope fractionation (average of 21‰) between sulfate and sulfide minerals provided clear evidence that the deposit was formed in a semi-closed marine depositional environment with moderate replenishment of seawater sulfate. The distinct Post-Archean Australian Shale (PAAS)-normalized positive Eu anomalies (2.02–11.03) and lower Y/Ho ratios (27.2–33.4) relative to modern seawater in all individual magnetite minerals separated from magnetite–barite ores and the δ18O isotope signatures (10.0–13.0‰) in individual sulfate minerals suggested that hydrothermal activity played an important role in the formation of the deposit. Barium was likely derived from the dissolution of marine barite with a contribution from more radiogenic terrigenous sediments considering the slight deviations (within ±0.0013) of 87Sr/86Sr ratios of Kalaizi barites from the Early Cambrian seawater. The sulfate and sulfide minerals in the ore deposit should evolve from coeval seawater containing residual SO42− and resultant H2S by means of bacterial sulfate reduction process because (i) δ34S values (37.8–42.8‰) of anhydrite and barite minerals were slightly higher than those of penecontemporaneous seawater, (ii) pyrite and galena minerals owned δ34S values (16.3–23.5‰) were lower than coeval seawater, and (iii) 13C isotope values (−10.0 to −12.1‰) of single ankerite minerals were depleted. To summarize, the Kalaizi bedded Fe–Ba deposit is interpreted as a cold seep barite deposit overprinted by hydrothermal fluids.