Hydrothermal dolostones in the Shiqi quarry of Wudongde Hydropower Station, Yunnan

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.

Keywords

• shiqi quarry
• hydrothermal dolomitization
• saddle dolomite
• fluid inclusions