秦岭大巴山地区钡矿资源极为丰富,以蕴含世界罕见的层状毒重石矿床为特色.本研究在紫阳毒重石矿床中发现了玫花瓣状结构,经显微镜观察和电子探针分析表明,玫花瓣体由板柱状重晶石呈放射状排列而成,与前人报道的矿物组成明显不同.紫阳灰黑色毒重石矿石有较高的Ⅴ、Ni、Cr、U、Mo含量,较低的Th/U比值(＜0.04),较高的Ⅴ/(Ⅴ+Cr)比值(＞0.8)和Ⅴ/(Ⅴ+Ni)比值(＞0.9),指示寒武纪紫阳毒重石矿床的沉积环境为较还原的环境.紫阳毒重石矿石的C同位素分布在-10.9％ ～-16.0‰,O同位素分布在18.1‰～22.3‰之间,表明毒重石中CO32-是有机质通过氧化作用或脱羧基(生物有机质降解)作用提供的低13C的碳与同期海水提供的高13C的水溶性HCO3-或CO32-的混合来源.综合研究表明,该区毒重石矿床的形成可能与寒武纪早期的海底热液成矿作用有关.%Qinling-Daba Mountains region is extremely rich in mineral resources of barium, characterized by widespread occurrence of the world's unique stratiform witherite deposit. In this study, we found the rose petal-like structure of witherite ores in the witherite deposit at Ziyang, Shaanxi Province. Microscope observation and electron microprobe analysis showed that the rose petal-like structure is composed by slab and column barites. This structure may have formed from pore fluids during early diagenesis. Redox-sensitive trace element analysis indicates high concentrations of V, Ni, Cr, Mo and U in the witherite ores, with low Th/U ratios ( <0. 04) , and high V/(V + Cr) (>0. 8) and V/(V + Ni) ratios ( > 0. 9), suggesting a reduced sedimentary environment for the witherite deposition. Carbon and oxygen isotope analysis show the witherite ores have S C values of - 10. 9‰ ~ - 16. 0‰ and δ18 O values of 18. 1‰ ~ 22. 3‰, indicating a 12C-rich biogenic carbon source possibly resulting from degradation, polycondensation and dehydroxylation of bio-organic matter in sediments at the early stage of diagenesis, mixing with a 13C-rich seawater-derived HCO3 ~ or CO32~. In combination all the previous and present data, we suggest that the deposition of the witherite ores were likely related to the Early Cambrian sea-floor hydrothermal fluids.
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