Hydrothermal dolostones in the Shiqi quarry of Wudongde Hydropower Station, Yunnan
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摘要: 云南禄劝乌东德水电站是我国第三座超千万千瓦级水电站，其建设所采用的混凝土人工骨料来源于坝址下游施期料场的中元古代落雪组灰岩。在骨料勘察和实际开采过程中，发现灰岩与白云岩在同一层位产出，其岩性变化界线切割岩层面，该类现象为工程地质研究中所罕见，引起了高度关注。以施期料场灰岩-白云岩过渡带和骨料开采残留的一处白云岩凸起岩块为重点研究对象，开展了野外地质调查、室内岩相学、微区矿物形貌和成分、碳氧同位素以及流体包裹体分析，深入探讨了研究区碳酸盐岩成岩演化和岩相转变过程与机理，提出了新的白云岩化成因模式。结果表明：①矿物形貌和空间分布特征指示与灰岩过渡的白云岩为早期灰岩经过后期白云岩化形成；②白云岩中发育的斑马纹构造、鞍状白云石及其伴生的硫化物和碳质物等，指示其经历了热液白云岩化过程。鞍状白云石中流体包裹体的均一温度（183~215℃）显著高于围岩方解石中流体包裹体均一温度（102~152℃）；③白云岩普遍具有较轻的O同位素值（δ18O=-13.3‰~-7.8‰），而其中鞍状白云石的δ18O最低（-13.3‰），表明它们可能为热液流体直接沉淀的产物，而其他白云岩由于受热液流体影响程度较低而具有较低的包裹体均一温度和较高的δ18O值；④乌东德地区近直立的岩层产状和断裂可能为热液流体运移提供了通道，而其上覆的近水平震旦系灯影组厚层白云岩则起到了封堵作用，使热液流体在不整合面之下与灰岩发生长时间反应；热液流体的运移方向决定了最终的热液白云岩的分布。Abstract: The Wudongde Hydropower Station in Yunan is the third hydropower station in China with a capacity of over 10 million kilowatts.The artificial aggregate used in its construction is derived from the limestone of the Mesoproterozoic Luoxue Formation of downstream the dam site.However, both limestone and dolostone are found in the aggregate survey and mining yard, meanwhile the lithological boundary between cross cuts the stratum.This paper focuses on the limestone-dolomite transition zone and a dolomite bulge remaining in the aggregate mining yard.Detailed geological survey, petrological work and geochemical analysis have been conducted on the samples of the research area.The results show that: ①Morphological features and spatial relationships of the carbonate minerals suggest that the dolostones are formed by dolomitization of limestone precursors. ②The development of zebra structure, saddle dolomite and associated sulfides and carbonaceous material is consistent with hydrothermal dolomitization. ③Fluid inclusions in the saddle dolomite yield homogenization temperatures between 183 and 215 ℃, which are significantly higher than those of fluid inclusions in the surrounding calcite(102-152 ℃).The dolostones are generally depleted in 18O(δ18O=-13.3‰ to -7.8‰), with the saddle dolomites have the lowest δ18O value(-13.3‰), which is consistent with direct precipitation of dolomite from hydrothermal fluids.The other dolomites, which show lower homogenization temperatures and higher δ18O, are probably related to less influence by the hydrothermal fluids. ④The near-vertical strata and the presence of fractures in the Shiqi area might have provided channels for the migration of hydrothermal fluids, while the overlying dolostones of Sinian Dengying Formation probably have acted as a cap layer, allowing prolonged reactions between the hydrothermal fluids and surrounding limestones.The migration direction of the hydrothermal fluids determines the distribution of the hydrothermal dolostones.
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Key words:
- Shiqi quarry /
- hydrothermal dolomitization /
- saddle dolomite /
- fluid inclusions
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图 1 扬子地块西缘前寒武纪地质简图(a)和研究区地质简图(b)(据文献[21]修改)
Figure 1. Precambrian geological map of the western margin of the Yangtze block (a) and simplified geological map of the study area (b)
图 2 施期料场落雪组地层划分
三级阶梯分别对应于高程为1 015, 1 025, 1 055 m的3个平台; Pt2l1.落雪组一段; Pt2l2.落雪组二段; Pt2l3.落雪组三段
Figure 2. Stratigraphic division of the Luoxue Formation in Shiqi quarry
图 3 施期料场落雪组白云岩野外特征
a.陡倾的白云岩层；b.白云岩中的断层面(箭头所示)；c.白云岩中的岩层破碎和扰动现象；d~e.白云岩中的斑马纹构造和溶孔(箭头所示)；f.白云岩中发育的溶孔(晶洞)，孔壁充填鞍状白云石；g.含碳质物的条纹状杂色白云岩突出岩块；h.砂糖状白云石颗粒(浅色)及粒间充填的碳质物(黑色)，样品采自图g中箭头处
Figure 3. Field characteristics of the dolostones in Luoxue Formation
图 4 白云岩-灰岩过渡带露头和手标本(16WDD10)
a, b.过渡带露头野外照片及对应的素描图, 图中数字代表样品编号的位置。c~e.过渡带样品的3个侧面；f~h.分别为c~e对应的素描图(D为白云岩；L为灰岩)
Figure 4. Outcrop and hand specimen of the dolomite-limestone transition zone(16WDD10)
图 5 白云岩-灰岩过渡带样品(16WDD10)经茜素红-S染色后镜下照片
a.低倍镜下照片拼图(单偏光，右下角数字为用于拼接的照片编号，箭头表示拼接方向)；b.含方解石区域；c.方解石中的白云石呈“漂浮自形晶”(箭头所示); 红色为被染色的方解石；灰黄色为未被染色的白云石; a.单偏光；b, c.正交光
Figure 5. Photomicrographs of the sample from the dolomite-limestone transition zone(16WDD10)
图 6 鞍状白云石镜下特征
a.低倍镜下照片，显示白云石颗粒呈自形颗粒(单偏光，染色片)；b, c.鞍状白云石，具有环带结构，可见晶面和解理纹弯曲(箭头所示，单偏光)；d.鞍状白云石呈波状消光(正交镜)；e, f.鞍状白云石颗粒中充填的黄铁矿(Py)(e.单偏光；f.反射光)
Figure 6. Charateristics of photomicrographs of saddle dolomites
图 7 过渡带样品(16WDD10)不同区域二次电子图像(a~c)和矿物EDS谱线(d)
a.白云石含量低于方解石的区域；b.白云石含量与方解石相近区域，注意小颗粒菱形白云石(2和3)包裹在方解石(1)内部；c.纯白云石区域；a, b, c图中十字位置表示EDS分析点位；Dol.白云石；Cc.方解石；Qz.石英；Ms.白云母
Figure 7. Secondary electron images (a-c) and mineral EDS spectra (d) in different regions of the transition zone sample(16WDD10)
图 8 过渡带样品(17WDD10-6)SEM-EDS-拉曼光谱协同分析结果
a.样品岩片扫描图，红框为EDS面扫(c~h)的区域；b.SEM背散射图像，红框为拉曼光谱面扫(i~p)的区域；c~h.EDS面扫元素分布图；i~q.拉曼光谱图和谱线。i.反射光照片，其中彩色区域为拉曼面扫范围；j~q.拉曼光谱面扫获得的黄铁矿、有机质、白云母、金红石、白云石、石英、方解石、黏土矿物分布图；o.上述矿物叠加的拉曼光谱图(蓝色为石英；红色为有机质；蓝绿色为白云母、金红石、白云石；绿色为方解石；黄色为黄铁矿；紫色为黏土矿物)；r.各矿物的拉曼光谱线
Figure 8. Correlated SEM-EDS-Raman analysis of the transition zone sample(17WDD10-6)
图 10 流体包裹体平均温度折线图(a)和主矿物及脉体均一温度频率直方图(b)
Figure 10. Diagram showing the variation of average homogenization temperature of fluid inclusions(a) and histograms showing homogenization temperatures for the host minerals and veins(b)
图 11 流体包裹体均一温度(Th)-盐度图
Figure 11. Homogenization temperature (Th) of fluid inclusions vs.salinity diagram
图 12 施期料场残留白云岩凸起岩块处C-O同位素特征(均为VPDB)
Figure 12. C-O isotope compositions (VPDB) of dolomites at the remnant outcrop in Shiqi quarry
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