Strontium, Sulfur, Carbon, and Oxygen Isotope Geochemistry of the Early Cambrian Strata-bound Barite and Witherite Deposits of the Qinling-Daba Region, Northern Margin of the Yangtze Craton, China

Early Cambrian black shale and chert in the Qinling-Daba region along the northern margin of the Yangtze (South China) craton host important high-grade strata-bound barite and witherite deposits. Barite-dominated deposits occur in a northern thrust domain, whereas witherite-dominated deposits occur in a thrust domain a few tens of km to the southwest. The authors studied the strontium isotope (⁸⁷Sr/⁸⁶Sr), and stable isotope composition (δ³⁴S, δ¹³C, and δ¹⁸O) of barite and witherite from the Shenxiantai and Chiyan deposits, two representative Ba deposits in the region, and compared these data to previously reported Sr, S, C, and O isotope data on a variety of Ba deposits in a similar stratigraphic position along the Early Cambrian rifted margin of the Yangtze craton.

Most barite and witherite deposits in the Qinling-Daba region have consistent ⁸⁷Sr/⁸⁶Sr ratios with an average value of 0.70845 ± 0.00017 (1σ, n = 53), in the range of Early Cambrian seawater from 0.7081 to 0.7090. Barite δ³⁴S and δ¹⁸O_VSMOW values vary from 8.0 to 76.1‰ (avg 40.6 ± 13.2‰, 1σ, n = 35) and from 9.9 to 24.7‰ (avg 19.0 ± 3.0‰, 1σ, n = 103), respectively, generally higher than those of seawater sulfate in the Early Cambrian. We propose that seawater-derived barium was precipitated in the anoxic water column and during early diagenesis. Bacterial sulfate reduction (BSR) and reoxidation under partial sulfate limitation produced higher δ³⁴S and δ¹⁸O values of the diagenetic barite compared to contemporaneous seawater sulfate. The δ¹³C values of Ba carbonate minerals (witherite and barytocalcite) vary from −10.8 to −23.6‰ (avg −14.1 ± 3.2‰, 1σ, n = 84) and indicate the involvement of carbon from oxidized organic matter. Although the ⁸⁷Sr/⁸⁶Sr ratios, δ³⁴S, and δ¹³C values of barite and witherite are very close, barite has slightly higher δ¹⁸O_VSMOW values (avg 22.0 ± 1.6‰, 1σ, n = 19) than witherite and barytocalcite (avg 18.0 ± 2.5‰, 1σ, n = 84), possibly revealing the role of CO₂-rich cold seeps under sulfate-limited sedimentary-diagenetic conditions during the formation of the witherite-dominated deposits. The Sr isotope composition of barite in the Shenxiantai deposit is different from the other barite deposits and has lower ⁸⁷Sr/⁸⁶Sr ratios, which suggests the involvement of less radiogenic Sr in diagenetic fluids which interacted with underlying mafic volcanic rocks.