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
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
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DOI：
10.2113/econgeo.111.3.695
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发表时间：
2016-05
期刊：
影响因子：
5.8
通讯作者：
Lingang Xu;B. Lehmann;J. Mao;Wei Zheng;Huishou Ye;Hongying Li
中科院分区：
文献类型：
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作者：
Lingang Xu;B. Lehmann;J. Mao;Wei Zheng;Huishou Ye;Hongying Li
扬子克拉通北方缘沿着秦岭-大坝地区早寒武世黑色页岩和硅质岩中含有重要的高品位层控重晶石和毒重石矿床。重晶石为主的矿床发生在北方冲断域，而毒重石为主的矿床发生在西南几十公里的冲断域。作者研究了锶同位素（87 Sr/86 Sr）和稳定同位素组成对神仙台和赤岩两个典型钡矿床中的重晶石和毒重石的δ 34 S、δ 13 C和δ 18 O进行了测定，并与前人报道的Sr、S、C、δ 34 S、δ 13 C和δ 18 O进行了比较，扬子克拉通早寒武世裂陷边缘沿着地层位置相似的多种钡矿床的氧同位素资料。秦岭-大坝地区大部分重晶石和毒重石矿床的87 Sr/86 Sr比值较一致，平均值为0.70845 ± 0.00017（1 σ，n = 53），处于早寒武世海水的0.7081 ~ 0.7090范围内。重晶石δ 34 S和δ 18 OVSMOW值分别为8.0 ~ 76.1‰（平均40.6 ± 13.2‰，1 σ，n = 35）和9.9 ~ 24.7‰（平均19.0 ± 3.0‰，1 σ，n = 103），普遍高于早寒武世海水硫酸盐的δ 34 S和δ 18 OVSMOW值。我们建议，海水来源的钡沉淀在缺氧水柱和早期成岩作用。与同期海水硫酸盐相比，细菌硫酸盐还原（BSR）和部分硫酸盐限制下的再氧化使成岩期重晶石的δ 34 S和δ 18 O值较高。钡碳酸盐矿物（毒重石和钡方解石）的δ 13 C值在−10.8至−23.6‰之间（平均值为−14.1 ± 3.2‰，1 σ，n = 84），表明氧化有机质中的碳参与其中。虽然重晶石和毒重石的87 Sr/86 Sr比值、δ 34 S和δ 13 C值非常接近，但重晶石的δ 18 OVSMOW值略高（平均22.0 ± 1.6‰，1 σ，n = 19）（平均18.0 ± 2.5‰，1 σ，n = 84），可能揭示了富CO2冷渗漏在以毒重石为主的矿床形成过程中的作用。神仙台存款中重晶石的Sr同位素组成与其它重晶石矿床不同，具有较低的87 Sr/86 Sr比值，表明成岩流体与下伏基性火山岩相互作用时，放射成因Sr的参与程度较低。
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 ( δ 34S, δ 13C, and δ 18O) 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 (1 σ , n = 53), in the range of Early Cambrian seawater from 0.7081 to 0.7090. Barite δ 34S and δ 18OVSMOW values vary from 8.0 to 76.1‰ (avg 40.6 ± 13.2‰, 1 σ , n = 35) and from 9.9 to 24.7‰ (avg 19.0 ± 3.0‰, 1 σ , 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 δ 34S and δ 18O values of the diagenetic barite compared to contemporaneous seawater sulfate. The δ 13C values of Ba carbonate minerals (witherite and barytocalcite) vary from −10.8 to −23.6‰ (avg −14.1 ± 3.2‰, 1 σ , n = 84) and indicate the involvement of carbon from oxidized organic matter. Although the 87Sr/86Sr ratios, δ 34S, and δ 13C values of barite and witherite are very close, barite has slightly higher δ 18OVSMOW values (avg 22.0 ± 1.6‰, 1 σ , n = 19) than witherite and barytocalcite (avg 18.0 ± 2.5‰, 1 σ , 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.