2016, Vol. 34扩展功能
文章信息
- 周锡强, 遇昊, 黄泰誉, 张力钰, 张恭境, 付勇, 陈代钊
- ZHOU XiQiang, YU Hao, HUANG TaiYu, ZHANG LiYu, ZHANG GongJing, FU Yong, CHEN DaiZhao
- 重晶石沉积类型及成因评述——兼论扬子地区下寒武统重晶石的富集机制
- Genetic Classification of Sedimentary Barites and Discussion on the Origin of the Lower Cambrian Barite-rich Deposits in the Yangtze Block, South China
- 沉积学报, 2016, 34(6): 1044-1056
- ACTA SEDIMENTOLOGICA SINCA, 2016, 34(6): 1044-1056
- 10.14027/j.cnki.cjxb.2016.06.004
-
文章历史
- 收稿日期：2016-05-19
- 收修改稿日期: 2016-07-04
周锡强, 遇昊, 黄泰誉, 张力钰, 张恭境, 付勇, 陈代钊. 重晶石沉积类型及成因评述——兼论扬子地区下寒武统重晶石的富集机制[J]. 沉积学报, 2016, 34(6): 1044-1056.
ZHOU XiQiang, YU Hao, HUANG TaiYu, ZHANG LiYu, ZHANG GongJing, FU Yong, CHEN DaiZhao. Genetic Classification of Sedimentary Barites and Discussion on the Origin of the Lower Cambrian Barite-rich Deposits in the Yangtze Block, South China[J]. ACTA SEDIMENTOLOGICA SINCA, 2016, 34(6): 1044-1056.
重晶石沉积类型及成因评述——兼论扬子地区下寒武统重晶石的富集机制
1. 中国科学院地质与地球物理研究所油气资源研究室 北京 100029
;
2. 中国五矿集团公司 北京 100007 ;
3. 中国科学院大学 北京 100049 ;
4. 贵州大学资源与环境工程学院 贵阳 550025
2. 中国五矿集团公司 北京 100007 ;
3. 中国科学院大学 北京 100049 ;
4. 贵州大学资源与环境工程学院 贵阳 550025
摘要:
重晶石沉积类型丰富，具有多种成因过程。通常，沉积型重晶石可分为生物、热液、成岩和冷泉重晶石四种类型。富钡与富硫酸盐的流体（海水、早成岩孔隙水或热液流体）及其相互作用过程（水柱、热液系统、沉积柱、沉积物-水界面附近）决定了重晶石的沉积环境、宏微观产出方式、同位素组成及相应的地质意义。另外，根据扬子地区下寒武统富重晶石沉积的地质特征，简述了其各种富集机制的适用性及争论。据此建议，结合埃迪卡拉纪-寒武纪转折时期的古海洋背景，对其进行详细的沉积学及地球化学分析，有助于深化成因认识，弥合分歧。
关键词：
重晶石
古海洋
下寒武统
Genetic Classification of Sedimentary Barites and Discussion on the Origin of the Lower Cambrian Barite-rich Deposits in the Yangtze Block, South China
1. Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. China Minmetals Corporation, Beijing 100007, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China
2. China Minmetals Corporation, Beijing 100007, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China
Foundation: National Narural Science Foundation of China,No.41472089,41502117;China Postdoctoral Science Foundation Funded Project,No.2015M581166
Abstract:
Barites are widely distributed in sedimentary records with various geological characteristics and formation processes. In general, sedimentary barites can be subdivided into marine (or biogenic), hydrothermal, diagenetic and cold seeps barites. The sedimentary environments, macro- and micro-occurrences, geochemical features (especially Sr and S isotopes) and corresponding geological implications of these four barite subtypes are controlled by the origin of barium- and sulfate-fluids (seawater, diagenetic porewater or hydrothermal fluids) and related interaction process (in seawater column, sediment column, sediment-water interface or hydrothermal system). In addition, this study further introduces the geological features of the Lower Cambrian barite-rich sediments in the Yangtze Block, South China, and summarizes the enrichment mechanisms having been proposed for these barite deposits and argues on their reconciliation and biases with geological features and processes. Based on the paleo-oceanic context during the Ediacaran-Cambrian transitional period, integrated sedimentological and geochemical researches together would provide better constrains on the origins of the Lower Cambrian barium-rich deposits in the Yangtze Block.
Key words:
Barite
Paleo-ocean
the Lower Cambrian
0 引言
2.1 生物重晶石
重晶石（BaSO4）是沉积岩（物）里的常见矿物，自太古宙至今时有发育，并具有层状、条带状、结核状、以及弥散状等多种产出形式[1-4]。重晶石具有高密度（~ 4.5 g/cm3）的物理特性，在化工添加剂、油气勘探钻井液等方面具有广泛的工业应用。因此，重晶石的显著富集，具有重要的经济价值。
另一方面，在地质历史时期，重晶石产出丰度具有非均匀的时空分布特征[1, 5-6]，并可能响应了海洋环境的变化[7]。在矿物成分方面，重晶石富含S、O元素，其相关同位素（δ34S、δ18O及δ17O）可用于解译硫循环或古气候特征[3, 8-12]。同时，重晶石具有高Sr、低Rb含量特征，是获取87Sr/86Sr同位素比值的重要矿物[2]。由于分布广、易保存，重晶石的同位素特征已广泛应用于流体演化[13-16]、全球海水Sr、S同位素组分重建等方面的研究[17-19]。此外，重晶石可以作为古生产力指标[20-22]、硫酸盐—甲烷转换带沉积指示物[23-24]或冷泉流体的活动的记录者等[25-27]，具有广泛的地质应用。然而，沉积型重晶石可能源于多种形成过程，并具有不同的成因类型和地质意义。因此对沉积型重晶石的地质特征和成因类型进行对比认识，有助于合理解读其在古海洋、古环境及古地理方面的意义。
本文拟从钡的海洋地球化学循环，并根据重晶石的沉积过程、形成环境、宏微观特征（产出形式、晶体形态）、同位素组成（Sr和S）、以及地质意义等方面对沉积型重晶石进行分类和评述。在此基础上，针对扬子地区下寒武统黑色岩系重晶石富集程度差异、地质特征，对其富集机理进行对比、评述，并指出下一步研究中需注意的问题。
1 海洋钡的循环与分布现今海洋钡的滞留时间较短，约为8000年[28]。海洋水柱和沉积物里，钡可赋存于多种物相，包括碳酸盐岩、有机质、硅质、铁锰氧化物、陆源碎屑和海相铝硅酸盐矿物，以及重晶石等[29]；其中重晶石矿物是固相钡的主要赋存态[30]。通常，重晶石过饱和沉淀主要源于富Ba2+与富SO42-流体的相互作用：Ba2++SO42-BaSO4。由于BaSO4通常具有极低的溶度积（常温常压下约为1×10-10）[31]，因此自然界流体只能独立维持高含量的溶解钡或者硫酸盐。现今海水富含SO42-（平均浓度为28 mM），但亏损Ba2+（平均浓度约为65 nM）。虽然同时存在一定量的硫酸盐和溶解钡，但热力学计算显示现今海洋总体处于重晶石不饱和状态[32-34]，因此制约着它的形成和分布。通常，重晶石微颗粒形成于水柱上部[35-36]，但随后在水柱中下部及沉积物里经历部分溶解[37-38]。因此，海洋水柱同时存在Ba的源和汇，溶解Ba含量总体呈现随深度逐渐增加的趋势。现今开阔海洋钡离子浓度约为20 nM，向深部逐渐增加至约150 nM[39]。在缺氧盆地，水柱里钡离子浓度（如黑海50米以下水柱）可达500 nM以上[40]。此外，地下水和河水具有较高的溶解钡含量，受其影响的滨岸和河口等水域相对更为富集溶解钡[41-42]。另一方面，海底热液流体也不同程度的富集钡离子（浓度可达80 μM以上）[43]，是海洋钡的重要来源之一[44]。
2 重晶石沉积分类海洋里的重晶石可形成于多种地质过程，并最终保存于沉积物里（或富集为沉积型矿床）；在此概称为沉积型重晶石。沉积型重晶石有多种分类方案[2]。根据构造背景和产出特征，Maynard et al. [45]和Maynard和Okita[46]将沉积型层状重晶石矿床分为两大类：①大陆边缘类型，纯重晶石矿床，不伴生Pb、Zn矿床；②克拉通裂谷型，可能伴生Pb、Zn矿床。根据产出形式、流体来源和现今实例，层状重晶石可也分为成岩交代、热液喷流和生物成因类型[1, 47]。根据富钡流体来源及与硫酸盐相互作用过程，最新研究成果将海洋重晶石分为四类：生物重晶石、热液重晶石、成岩重晶石和冷泉重晶石（图 1）[48-50]。各类型重晶石在地层沉积记录里广泛报道。由于形成环境和过程差异，如水柱、热液系统、沉积物—水界面附近、或者沉积物柱孔隙环境等，重晶石具有多种晶体习性和结构、产出形式，以及Sr、S同位素特征（表 1）[48-49, 51]。为了便于科研实践，本文将据此对海洋重晶石沉积类型进行详细介绍。
|
| 图 1 A. 海洋沉积型重晶石的产出环境；B. 热液重晶石形成模式（修改自[52]），图A中b框细节图；C. 成岩重晶石形成模式，图A中c框细节图；D. 冷泉重晶石形成模式（修改自[53]），图A中d框细节图。注意成岩重晶石与冷泉重晶石的成因联系。 Figure 1 A. The occurrences of sedimentary barites in marine environments. B. Formation of hydrothermal barite (details of inset box b in Fig. 1A),modified from [52]. C. Formation of diagenetic barite (details of inset box c in Fig. 1A). D. Formation of cold seeps barite (details of inset box d in Fig. 1A),modified from [53]. Note the genetic connection between the diagenetic and cold seeps barites. |
表 1 沉积型重晶石成因分类和地质特征
Table 1 Genetic classification and geological features of sedimentary barites
| 重晶石成因类型 | 生物重晶石 | 热液重晶石 | 成岩重晶石 | 冷泉重晶石 |
| 沉积过程 | 生物直接胞内合成，或有机质降解微环境里生成 | 富Ba2+热流体与含SO42-流体混合 | 富Ba2+（或和CH4）孔隙水与含SO42-孔隙水混合 | 富Ba2+（或和CH4）孔隙水与含SO42-海水（或孔隙水）混合 |
| 钡源 | 海水及含钡有机质 | 淋滤洋壳、陆壳及富钡沉积物 | 早期沉积物（原生生物重晶石等） | 早期沉积物（原生生物重晶石等） |
| 硫酸盐源 | 海水 | 海水或热液H2S氧化产物 | 孔隙水（经BSR改造后的残余海水硫酸盐） | 经BSR改造后的孔隙水或海水 |
| 形成地点 | 水柱 | 热液系统（通道、喷口、热液羽） | 沉积物—水界面之下沉积物孔隙环境 | 沉积物—水界面附近孔隙环境及水柱 |
| 产出形式 | 微晶颗粒，散布于水柱和沉积物 | 热液羽微颗粒、烟囱体、结壳、海底热液网脉 | 弥散的重晶石微颗粒，或胶结物、结核、透镜、薄层条带状 | 烟囱体、丘体、海底结壳，胶结物、结核、层状等 |
| 沉积或 构造环境 | 非特定海域（高生产力海区更为显著） | 热液喷流及热液羽区域，常见于洋中脊、弧后盆地等扩张中心、断裂带，陆架边缘断裂带，以及克拉通内部裂谷区等 | 高生产力海区（如陆架边缘上升流地区） | 高生产力海区、陆架边缘陡坡带、构造及断层发育的主动、被动大陆边缘及复杂构造背景的海盆 |
| 生物群落 | 浮游生物、原生动物xenophyophore等 | 热液生物群 | 冷泉生物群 | |
| 代表性伴生矿物 | 无特定伴生矿物 | 多金属硫化物、非晶质硅质沉积等 | 富有机质沉积物，黄铁矿、方解石、白云石等 | 黄铁矿、方解石、白云石等 |
| 粒度 | 亚微米级晶粒（普遍<5μm） | 数十至数百微米，可达毫米级 | 数十至数百微米，可达厘米级 | 数十至数百微米，可达厘米级 |
| 晶体形态 | 常为哑铃形、椭球形晶型，或不具有晶体习性 | 自形、棱柱形、板片状等晶型，放射针束状、扇状自形晶集合体，或者玫瑰花状结构 | 片状板柱状等自形晶型，有时可见交切板状双晶，以及树枝状、玫瑰状集合体 | 螺旋状、菱柱等晶型，以及树枝状和结核环带等生长模式；有时可见交切板状双晶和玫瑰花状结构 |
| S同位素 | 近似同时期海水值 （Δδ34S≈0） | 常介于同时期海水值与热液硫化物值之间（Δδ34S＜0），但有时Δδ34S＞0 | 较同时期海水值不同程度偏高 （Δδ34S＞0，可达+50‰） | 较同时期海水值不同程度偏高 （Δδ34S＞0，可达+50‰） |
| Sr同位素 | 近似同时期海水值（Δ87Sr/86Sr≈0） | 显著偏离同时期海水值 （通常-0.006＜Δ87Sr/86Sr＜-0.002；或Δ87Sr/86Sr＞+0.002） | 稍微偏离同时期海水值 （通常-0.002＜Δ87Sr/86Sr＜+0.002） | 稍微偏离同时期海水值 （通常-0.002＜Δ87Sr/86Sr＜+0.002） |
| 地质意义 | 记录海水Sr、S同位素组成，重建海洋生产力 | 指示热液活动、并记录热液流体化学特征，指导伴生的沉积喷流矿床的勘探 | 指示SMTZs和下伏甲烷输入，反映沉积速率变化，制约钡古生产力指标 | 指示甲烷释放和冷泉渗流事件，可能影响局部海域Ba（和/或碳）循环，指导层状重晶石勘探 |
| 注：Δδ34S=δ34S重晶石－δ34S同时期海水硫酸盐；Δ87Sr/86Sr=87Sr/86Sr重晶石－87Sr/86Sr同时期海水。 | ||||
生物重晶石是指在生物直接诱导（胞内合成）或间接调控（有机质降解微环境）作用下形成的重晶石[20, 35-36, 54]（图 1A）。重晶石微晶颗粒形成于不饱和海水，普遍认为与生物有关。研究发现，为维持定向或深度而调节身体密度，海洋部分底栖原生动物（如有孔虫类xenophyophore）可直接在细胞内生成重晶石[55-56]。另一方面，培养实验显示，海洋细菌能够提供晶体成核点并促进重晶石晶体的生长[57-58]。目前而言，这类海洋生物报道数量或实例有限，有待进一步探索；它们可能对整个重晶石储库的贡献份额有限。
另一方面，有机质降解时的微环境可促进重晶石的形成。许多海洋浮游生物直接从周围海水吸收钡进入生物结构，较海水相对富集钡[59]。例如，等幅骨虫（Acantharians）壳体由天青石（SrSO4）构成，常含有数千ppm的Ba[60]。因此，它们含有相当规模的活性钡储库，在水柱里降解时可向临近微环境提供钡源，促进重晶石的形成[20, 35, 54]。例如，Ganeshram et al. [61]发现，海岸浮游生物以及实验室培养的硅藻和颗石藻在降解实验中可沉淀重晶石。此外，Dehairs et al. [62]推测，富有机质沉降颗粒里高强度细菌活动可能有助于海洋中层水柱高含量钡颗粒的生成。然而由于现今海洋处于重晶石未饱和状态，浮游生物降解时释放的钡能否被有效维存于微环境，对于重晶石沉淀至关重要[61]。目前认为，粪球粒和有机质团块等，虽然不是重晶石沉淀的必要条件，但能显著促进重晶石的沉淀[20, 61]。理论上只要有有机质集合体的存在，重晶石生成可以发生在任何水深 ，甚至沉积物—水界面附近[63]。
生物重晶石生成于水柱，呈现亚微米级晶粒（普遍<5μm），常为哑铃形、椭球形晶型，或者不具有晶体习性[36, 54, 57]。由于形成于水柱，生物重晶石Sr、S和O同位素主要记录了同时期海水的特征，并可用于重建海水组分的长期演化趋势[17-19, 49, 64]。现今海洋水柱溶解Ba含量与重晶石颗粒和有机碳含量，以及上覆海水的生物生产力具有显著相关性，表明生物或有机质降解控制着水柱Ba循环[21, 65-69]。因此，远洋氧化环境沉积物里生物重晶石与表层生物生产力之间存在显著的相关性[21]，据此可作为重建海洋生物生产力的指标[20, 22, 70-71]。然而值得注意的是，在硫酸盐亏损的缺氧海水或沉积物孔隙水中(由于细菌硫酸盐还原作用)，重晶石可能会经历不同程度的溶解；这种背景下沉积物里的Ba含量并不能有效指示海洋的初始生产力。
2.2 热液重晶石热液重晶石是指深部富钡热流体沿断层或裂隙向海底运移或喷发至水柱过程中，与富硫酸盐流体相互作用而生成的重晶石（图 1B）[50, 72-73]。热液流体里的钡离子可淋滤自洋壳、陆壳基底或长英质岩石（尤其是长石）、或者远洋富钡沉积物[50]。热液重晶石常源于火山岛弧、洋脊、弧后盆地扩张中心的岩浆热源驱动的富硫化物中高温集中流体（150~250℃）或中低温弥散流体（<120℃）的活动，如东北太平洋布兰科（Blanco）断裂带[74]、冲绳海槽JADE热液区[75]、北冰洋洋中脊南部Loki’s Castle热液区[52]、马里亚纳岛弧[72, 76-77]、克马德克岛弧[78-79]、汤加岛弧[80]、巴布亚新几内亚的富兰克林（Franklin）海山[73]、以及加那利群岛亨利（Henry）海山[81]。此外，在构造和减薄地壳导致的高热流背景下，陆架边缘的中低温热液流体也可发育热液重晶石，如南加利福尼亚大陆边缘[50]。
由于热流体运移速率差异，热液重晶石具有多种产出形式：①网脉状充填于海底面之下的热液通道[75]，②在海底面之上形成烟囱体、丘体或结壳[72-73, 77, 82]，③以细小颗粒形式随热液羽飘散，并散布于沉积物中[83-85]。洋脊或弧后扩张中心火山热液系统里的重晶石常伴生多金属硫化物（方铅矿、闪锌矿、黄铜矿、黄铁矿等）和非晶质硅质沉积[72-73, 78, 86]。克拉通内部裂谷或者陆架边缘的热液系统沉积的重晶石既可与块状硫化物矿床共生，也可伴生于块状硫化物矿床上部或毗邻区，形成独立的沉积型重晶石矿床，如阿拉斯加地区的Red Dog矿床[87]。热液重晶石呈现自形（如棱柱形、板片状等）晶型，粒度较大，为数十微米至毫米级。它们有时生长于开阔孔洞，构成放射针束状、扇状自形晶集合体[77]，或者玫瑰花状结构[49]。
热液重晶石的生成常源于热液端元流体（热液成因H2S氧化而来的硫酸盐δ34S值为+1‰至+2‰）与海水（现今海水硫酸盐δ34S值约为+20‰）不同程度的混合，因此具有不同程度偏低或类似于同时期海水硫酸盐的δ34S值[49, 88-89]。值得注意的是，如果上覆局限盆地海水或者海底热液系统里的硫酸盐处于相对封闭环境（即硫酸盐补充不足）时，细菌硫酸盐还原作用可使残余硫酸盐的硫同位素值不断增高。在此背景下，部分热液重晶石也可能具有较同时期海水值偏重的δ34S值[52, 75, 81, 90]。由于受到热液活动的影响，热液重晶石87Sr/86Sr同位素值通常不同程度偏离同时期海水值，记录了热液流体淋滤洋壳或者陆壳的特征[91, 92]。 洋脊扩张中心和洋脊翼部的热液重晶石的87Sr/86Sr值通常介于同时期海水值（现今海水87Sr/86Sr = 0.70917）与萃取自洋壳的端元热液流体值（87Sr/86Sr = 0.703 05）之间[51]；而克拉通内部裂谷或者陆架边缘的沉积喷流型热液重晶石则常具有高于同时期海水值的壳源Sr同位素特征[45, 47]。
沉积物里热液重晶石的发育，指示存在热液活动，有助于加深对古海洋的认识。此外，热液重晶石是获取Sr、S同位素及流体包裹体测温的理想对象[75]，可用于解析热液流体地球化学性质。因此，作为海底火山型块状硫化物矿床及沉积喷流型矿床里常见的伴生矿物，热液重晶石有助于认识相应的成矿过程，并促进硫化物矿床的勘探[87, 90]。
2.3 成岩重晶石成岩重晶石是指沉积物—水界面之下，原生生物重晶石在硫酸盐亏损带溶解后形成富钡孔隙水，迁移至硫酸盐—甲烷转换带（Sulfate-methane transition zones，SMTZ）附近与孔隙水残余硫酸盐相互作用而再沉淀的重晶石（图 1C）[24, 38, 93-95]。通常随着深度的增加，沉积物孔隙水里的硫酸盐由于来自上覆海水的扩散补充受限，并且经细菌硫酸盐还原作用而不断消耗[96-98]。随着硫酸盐的完全耗尽，沉积物里有机质在产甲烷细菌作用下进一步发酵产生甲烷，该区带称为产甲烷带（methanogenesis zone）。因此，沉积物—水界面之下一定深度可形成硫酸盐—甲烷转换带。SMTZ之上为硫酸盐还原带，孔隙水含有硫酸盐，重晶石矿物性质稳定。SMTZ之下为产甲烷带，硫酸盐完全亏损，重晶石矿物将逐渐溶解，使得孔隙水具有高含量溶解钡[38, 93, 99]。SMTZ附近甲烷和硫酸盐近完全亏损，存在一个陡变的钡离子梯度（图 1C）。下伏富钡（和甲烷）孔隙水向上扩散迁移，在SMTZ附近与向下扩散的残余硫酸盐汇合，导致形成自生成岩重晶石前锋带[38, 93]。成岩重晶石前锋在现今水深数百米至上千米的海洋沉积物里时有发育，如环太平洋陆架边缘[38]、墨西哥湾[53]、美国东南海岸Blake Ridge[24, 99]、纳米比亚陆架[93]、以及黑海[100]等地。
成岩重晶石的富集程度取决于SMTZ在特定深度的持续时间及溶解钡的输入情况。这又受到沉积速率[101]、下伏甲烷输入量[24]、沉积物里原生海洋生物重晶石含量等因素的影响[99]。通常，高生产力海区沉积物里可积累较丰富的生物重晶石，其大量溶解后有助于提升孔隙水钡离子含量，促进成岩重晶石的形成[38, 94]。因此，成岩重晶石在富有机质岩层里更为常见。它们以弥散状重晶石晶粒、厘米至分米级重晶石结核或者薄层重晶石条带等多种形式产出[7, 102-103]。由于具有相对稳定的生长条件，成岩重晶石常形成片状、板柱状等自形晶型，有时可见交切板状双晶，粒度常为数十至数百微米，有时可达厘米级[48-49]。它们可呈现树形生长、玫瑰状集合体等形态结构[38]。由于SMTZ里发育甲烷厌氧氧化作用（AOM），成岩重晶石可能也伴生黄铁矿、方解石、白云石等矿物[95, 104-105]。
成岩重晶石形成于沉积物，主要记录了孔隙水Sr、S同位素特征。孔隙水环境里，细菌硫酸盐还原作用（BSR）优先将富32S硫酸盐还原为H2S，导致残余硫酸盐逐渐富集34S（Rayleigh分馏效应）[97, 106-107]。由于记录了孔隙水残余硫酸盐重硫同位素特征，成岩重晶石较同时期海水不同程度的富集34S（Δδ34S可达+50‰）[49, 103]。另一方面，沉积柱中孔隙水主要继承或扩散自上覆海水，二者 87Sr/86Sr值通常比较接近。但如果存在早期海洋沉积物、富放射性成因Sr的陆源物质及亏损87Sr的洋壳物质等的改造时，孔隙水Sr同位素将不同程度的偏离同时期海水值[27, 108]。因此，成岩重晶石的87Sr/86Sr反映了孔隙水与各种沉积物质的相互作用，接近或小幅度偏离同时期海水值（Δ87Sr/86Sr介于±0.002）[49, 51]。
成岩重晶石及其富集情况能够指示烃类输入和SMTZ的位置[24, 93, 95, 109-111]，以及沉积速率显著变化[101]等地质过程。此外，成岩重晶石的发育，表明沉积物里的生物重晶石经历了溶解、迁移和再沉淀，这将制约钡含量在古生产力指标方面的应用[20, 30, 94]。值得注意的是，成岩重晶石及相关矿化作用在前寒武纪仅有少量报道[112-113]，至显生宙其丰度和规模则有显著增加[16, 90, 101-103, 105, 111, 114-115]。因此最近研究认为，这种情形可能响应了埃迪卡拉纪-寒武纪转折时期古海洋硫酸盐浓度及生态系统的转变[7]。
2.4 冷泉重晶石冷泉重晶石是指富钡（或/和烃类）孔隙水沿裂隙运移至沉积物—水界面附近（冷泉渗流地），与富硫酸盐孔隙水或海水相互作用而形成的重晶石（图 1D）[25-27, 51, 116]。冷泉重晶石与成岩重晶石在流体来源、沉积过程、同位素组成等多方面具有相似性。在断层、侧向挤压构造作用或者高沉积速率形成的沉积加载下，孔隙水可获得超压并垂向向上迁移。这将导致位于沉积物SMTZ里的成岩重晶石前锋带迁移至沉积物—水界面附近，转化为冷泉重晶石体系（图 1C和D）[26, 51, 53]。冷泉流体的钡源与成岩重晶石类似，主要为沉积物里堆积的活性生物重晶石[51]；因此冷泉重晶石常出现在高生产力背景下的富有机质沉积环境。冷泉重晶石在现今海洋时有报道，如加尼福尼亚陆架[27, 116]、秘鲁陆架上升流地区[117]、阿拉斯加附近阿留申海槽[118]等汇聚型大陆架边缘，墨西哥湾[25, 119]等被动大陆边缘，以及鄂霍兹克海[26]、日本海[120]等复杂构造背景的大陆边缘。此外，墨西哥、中国南方、美国阿拉斯加和内华达地区古生代大型层状重晶石矿床[51]，以及非洲西北部Taoudéni Basin新元古代盖帽碳酸盐岩里的重晶石沉积等也被认为与冷泉活动有关[121]。
冷泉重晶石的产出形式受制于重晶石前锋带的深度（图 1D），而这又受控于冷泉流体渗流速率[23, 53]：低渗流速率时，重晶石沉积于沉积物—水界面附近的孔隙环境，形成微晶胶结物或结核（类似成岩重晶石）；高渗流速率时，重晶石沉积于海底面或水柱，形成结壳、多孔丘体以及大型烟囱体等典型的冷泉渗流结构[25-27, 116, 119]。冷泉重晶石呈现螺旋状、菱柱状等晶型，可见交切板状双晶；有时形成玫瑰花状、树枝状和结核环带等生长模式[26, 119-120]。由于常伴生AOM作用，冷泉重晶石沉积常可共生碳酸盐岩及少量黄铁矿。其中，重晶石和碳酸盐岩的相对含量受控于孔隙水里甲烷与钡离子含量的比例[23]：低甲烷/钡离子比值条件下，以重晶石沉淀为主；高比值条件下，硫酸盐受到AOM的消耗，以碳酸盐岩沉淀为主。因此，冷泉重晶石和冷泉碳酸盐岩普遍共生，但也可以相互独立而存在[51, 90]。值得注意的是，现今海底冷泉重晶石沉积区也可能伴生相应的冷泉生物群，如微生物席，以及贝类、管状蠕虫、腹足等大型生物[25-27, 118]。
冷泉重晶石的Sr和S同位素主要记录了孔隙水与海水的混合信号。若海水混合比例较低时，冷泉重晶石与成岩重晶石具有相似的孔隙水Sr、S同位素特征，即与同时期海水值相比具有相近的87Sr/86Sr比值（Δ87Sr/86Sr介于±0.002），以及显著偏重的δ34S值[15, 49, 51]。若海水混合显著时，冷泉重晶石的Sr、S同位素将更为接近同时期海水值。需要指出的是，冷泉重晶石的δ34S/δ18O比值可提供渗流速率和硫酸盐还原速率信息：该值大于4：1（代表微生物硫酸盐还原作用斜率值）被认为指示了低渗流速率背景下高微生物硫酸盐还原速率（硫酸盐相对封闭体系），反之则指示高渗流速率背景下低微生物还原速率（硫酸盐相对开放体系）[25]。
冷泉是陆架边缘重要的地质过程，其释放大量低温富钡（和甲烷流体）流体，可能显著影响半封闭海洋盆地[116, 122]，甚至地质历史时期全球海洋钡（和碳）循环[28]。此外，地质历史里的冷泉重晶石也直接指示甲烷输入和冷泉渗流事件[109, 123-125]，并可能是古生代层状重晶石矿床的成因机制[15, 51, 126]。
3 扬子地区下寒武统重晶石华南扬子地区下寒武统黑色页岩—硅质岩地层普遍富含重晶石，具有显著的高钡含量特征（常高于1 000 ppm，有时甚至可达数万ppm）[127-128]。其中，扬子地区湘黔相邻区牛蹄塘组和秦岭大巴山地区洞河群发育大型层状重晶石成矿带，是沉积型重晶石矿床的典型代表[1, 47, 129-132]。重晶石在沉积物里异常富集，通常需要特殊的地质条件（如高生产力背景下生物重晶石或者富钡流体的大量输入等）[133]，而这通常反映了重要的地质事件。在此，我们以扬子地区下寒武统重晶石为例，分析其沉积特征，讨论富集机制和地质意义。
3.1 地质特征扬子地区下寒武统富重晶石岩系在地理图上呈带状分布[129, 132]。基于主微量等地球化学特征，目前普遍认为下寒武统富重晶石的黑色岩系主要沉积于缺氧环境[132, 134-137]。重晶石矿床的硫酸钡含量一般为85%~95%，并伴生毒重石、钡解石、黄铁矿、钡冰长石、斜钡钙石、闪锌矿、硫钒铜矿和胶磷矿等矿物[130, 138]。重晶石呈现弥散、层状、透镜状、条带状、结核状、玫瑰花状等多种沉积结构。重晶石围岩里可见藻类、海绵骨针、管状生物等化石[139]。同位素地球化学方面，层状重晶石δ34S值一般为+25‰至+60‰[132, 138, 140-143]，其中重晶石结核δ34S值可达74‰[103]；较早寒武世海水（35‰~40‰）[144]显著富集34S。扬子地区下寒武统重晶石87Sr/86Sr分布较窄，普遍介于0.708 0至0.709 0之间[132, 145, 146]，总体接近早寒武世海水值（0.708 2~0.709 0）[147]。少量重晶石样品可能受陆源杂质的影响，87Sr/86Sr比值可达0.709 5~0.716 7[146]。
3.2 富集机制根据下寒武统重晶石富集的地质特征，研究者们提出了多种成因机制，如生物富集[1, 148]、富钡冷泉流体释放[15, 51]、或者热液活动[47, 129-130, 142, 145, 149]等地质作用。
生物富集模式支持者认为，早寒武世扬子地区陆架边缘上升流发育，富营养水体促进生物繁盛，高生产力背景下生成大量海洋生物重晶石[1, 148]。它们可能直接导致层状重晶石的沉积，或者在缺氧海盆或沉积物里经溶解、迁移再沉淀形成富重晶石沉积。该模式得到重晶石及其围岩（黑色页岩—硅质岩）富含有机质、Si和P元素证据的支持，暗示沉积时水柱处于高生产力背景或者缺氧环境[137, 149]。然而，重晶石的Sr、S同位素特征均不同程度偏离同时期海水值，这显著不同于水柱直接生成的生物成因重晶石。同时，单一的生物作用模式难以解释扬子地区下寒武统重晶石富集的形态、规模和品位，且缺乏古今实例。
冷泉作用模式认为，扬子地区下寒武统大型层状重晶石沉积可能受益于陆架边缘富钡（和烃）的冷泉活动[15, 51, 132]。扬子地区下寒武统重晶石的Sr同位素比值在同时期海水值附近小幅度波动[146]，表明可能源于受沉积物改造的孔隙水。它们的硫同位素较同时期海水显著富集34S值[140]，可能记录了沉积物孔隙水里残余硫酸盐（富集34S同位素）的信号。因此，下寒武统重晶石的Sr和S同位素特征与现今成岩或冷泉重晶石相似[51, 132]。此外，下寒武统重晶石富集的冷泉模式也得到其他地质证据的支持：①重晶石具有成岩和海底生长特征[139]，表明随着孔隙水流体向上运移强弱的变化，重晶石前锋带深度不断波动；②围岩里的化石疑似冷泉生物群；③牛蹄塘组早成岩重晶石—黄铁矿结核显示沉积物里发育冷泉流体[103]；④重晶石未见伴生的大规模的贱金属硫化物沉积；⑤富重晶石沉积带状分布，表明受控于断层相关的冷泉流体通道；⑥现今大陆边缘可见冷泉重晶石显著富集的实例[51, 117]。值得注意的是，扬子地区下寒武统部分重晶石（毒重石）也不同程度经历了早成岩作用的富集[103, 132]。
热液活动模式认为，前寒武纪—寒武纪转折时期扬子地区广泛发育海底热液活动[128, 150]，并促进了下寒武统硅质岩和Ni-Mo多金属的沉积[151]。在此背景下，下寒武统重晶石富集也可能源于热液活动，并得到大量地质证据的支持：①重晶石具有网脉结构及热水喷流沉积结构[130]；②硅质岩、黑色页岩等重晶石围岩的地球化学特征显示有热液流体的影响[142, 152]；③重晶石Sr同位素值偏离同时期海水值，反映了热液流体的贡献[47, 129, 145-146]；④伴生少量黄铜矿、环带钡冰长石等热液矿物[135, 153-154]；⑤流体包裹体数据揭示中等温度条件[131]；⑥围岩里的化石疑似海底热液生物群[139]；⑦重晶石成矿带线性排列，表明似乎受控于断层相关的热液流体通道。
3.3 讨论由此可见，扬子地区下寒武统重晶石的各种富集机制均能得到地质证据不同程度的支持。然而，有些证据并非特定成因模式的必要条件。比如，重晶石较海水显著较重的δ34S值，显示了在硫酸盐亏损环境里、微生物硫酸盐还原的分馏作用。这种环境可能是海洋生物富集模式里硫酸盐供给不足的局限海盆环境[129, 135, 140, 143]，也可以是冷泉模式里海洋沉积物中的孔隙水环境[15, 132]。同时，重晶石Sr同位素相对于早寒武世海水值一定程度的偏移，也分别被解释为热液[135, 145]，或者海水来源的证据[132]。另一方面，扬子地区下寒武统重晶石富集带的线性分布特征，既可能是受控于陆架边缘沿岸上升流高生产力条件，也可能受控于断层相关的冷泉或热液流体通道。此外，重晶石富集带是否伴生硫化物矿床、是否发育烃类流体及相关的碳酸盐岩沉积、渗（喷）流流体温度特征及Sr同位素解释等，也可以是冷泉模式和热液模式的重要分歧[51, 91, 151]。
因此，扬子地区下寒武统重晶石的显著富集可能涉及各种成因过程。例如，高生产力背景下，陆架边缘沉积物里初步富集海洋生物重晶石。早成岩阶段，这些重晶石不断溶解，提升孔隙流体钡含量，一方面可迁移至SMTZ附近再沉淀而不断富集，另一方面也可为冷泉或热液重晶石提供钡源。反之，大量富钡冷泉或热液流体释放至海水，也可能促进水柱里海洋（生物）重晶石的生成。值得注意的是，埃迪卡拉纪—寒武纪转折时期海洋生态（后生动物及粪球粒的到来）[155-156]，以及海洋地球化学（硫酸盐浓度的增高）[157]的转变，可能是理解下寒武统普遍具有高钡含量及富集重晶石的一个新思路[7]。总之，扬子地区下寒武统黑色岩系富重晶石沉积的成因机制仍然悬而未决。结合该时期古海洋背景，在详细的沉积学基础上，综合地球化学特征，有助于深化对该层段重晶石的沉积类型和成因机制的认识，并弥合分歧。
4 结论沉积型重晶石在地质历史里分布广泛、类型丰富，具有重要的经济和地质意义。基于形成环境、宏微观沉积特征、Sr和S同位素组成等特点，沉积型重晶石可分为生物、热液、成岩和冷泉重晶石四个端元类型。通过地质特征，识别重晶石成因类型，是合理解读相应地质意义的关键。扬子地区下寒武统重晶石不同程度体现了生物、冷泉（及成岩）和热液重晶石的地质特征，相关研究有待进一步深化。
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