Abstract

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 δ18OVSMOW 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 δ18OVSMOW 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.

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