Abstract
The Middle Permian Lucaogou Formation in the Jimusar Sag, eastern Junggar Basin, NW China, was deposited in a salt lake within an intracontinental rift basin with intense hydrothermal activity. Hydrothermal-sedimentary dolomite in the form of three types of dolostones, namely, analcime-feldspar dolostone (AFD), silicic dolostone (SD) and buddingtonite-albite dolostone (BAD), related to syn-sedimentary hydrothermal activity at lake bottom was discovered. The characteristics and formation mechanism of the dolomite were studied based on micron-scale petrographic and isotopic geochemical research. The syn-depositional formation of these dolostones was indicated by their rock-mineral features and syn-sedimentary deformation stage. The dolomite was composed of relatively poorly ordered proto-dolomite crystals with micron-sized spherical or sub-spherical morphology and coexisted with hydrothermal minerals, including analcime, buddingtonite, albite and chalcedony. Albite clasts were replaced by the dolomite, indicating high-temperature conditions during formation. The remarkably low strontium isotopic compositions of the dolostones (87Sr/86Sr with an average of 0.705687) indicated that mantle-derived materials might have involved in the ore-forming fluid. The dolostones had positive δ13CPDB values (with an average of 6.94‰) and negative δ18OPDB values (with an average of − 8.12‰). Based on the δ18OPDB values, the formation temperatures of the dolomite were at least ~ 25 °C higher than those of the penecontemporaneous dolomite in the Lucaogou Formation in the study area. It is concluded that the dolomite precipitated from hydrothermal fluid erupting at the lake bottom. The possible genetic models are described. We suggest that the hydrothermal-sedimentary dolomite is an important genetic type, and this study may help increase the awareness of this understudied type of dolomite.
Highlights
How dolomite forms under near-surface conditions has been puzzling ever since dolomite was first reported by Dolomieu (1791), and this issue is commonly referred to as the “dolomite problem” (Land 1998; Arvidson and Mackenzie 1999; Machel 2004; Bontognali 2019)
Based on the X-ray diffraction (XRD) analysis results of different parts of the samples extracted with a micro-drill, the mineral contents of the whole rock were estimated by thin section observations and backscattered electron imaging combined with energy-dispersive spectroscopy (EDS)
The sedimentary exhalative rocks that were identified in 26 wells in the Jimusar Sag are the most common geological records of the Permian syn-sedimentary hydrothermal activity in the study area (Li et al 2017a; Liu et al 2018)
Summary
How dolomite forms under near-surface conditions has been puzzling ever since dolomite was first reported by Dolomieu (1791), and this issue is commonly referred to as the “dolomite problem” (Land 1998; Arvidson and Mackenzie 1999; Machel 2004; Bontognali 2019). Modern dolomites precipitating under such conditions have been reported (e.g., Shanks III 2001; Eickmann et al 2009). They are relatively rare, probably due to the scarcity of precursors for dolomitization or proper conditions for direct crystallization. Hydrothermal-sedimentary rocks are composed of ankerite or ferrodolomite microcrystals; low-temperature hydrothermal minerals, such as zeolite, albite, barite and quartz; and a few metal sulfide minerals, such as pyrite, galenite and sphalerite. They are characterized by laminated and mesh vein structures, syn-sedimentary deformation structures and hydrothermal intraclastic textures. It is necessary to investigate this understudied type of dolomite from both scientific and application perspectives
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