Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including z...Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while STSr/86Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in S7Sr/S6Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320℃. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are： （1） dolomite host rock →sphalerite-galena-barite-fluorite; （2） dolomite host rock →saddle dolomite →quartz; （3） dolomite host rock →saddle dolomite→bitumen; （4） dolomite host rock →saddle dolomite →barite. The mean chemical composition of the host dolomite matrix and HTD didn＇t change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation ＆ expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement. The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%-5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis （Z2-C1） and the later one to the Emei taphrogenesis （D2-T2）. But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.展开更多
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 do...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 positiveδ13CPDB values(with an average of 6.94‰)and negativeδ18OPDB values(with an average of-8.12‰).Based on theδ18OPDB values,the formation temperatures of the dolomite were at least^25ºC 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.展开更多
基金funded by National Basic Research Program of China (Grant No. 2012CB214805)
文摘Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while STSr/86Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in S7Sr/S6Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320℃. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are： （1） dolomite host rock →sphalerite-galena-barite-fluorite; （2） dolomite host rock →saddle dolomite →quartz; （3） dolomite host rock →saddle dolomite→bitumen; （4） dolomite host rock →saddle dolomite →barite. The mean chemical composition of the host dolomite matrix and HTD didn＇t change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation ＆ expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement. The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%-5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis （Z2-C1） and the later one to the Emei taphrogenesis （D2-T2）. But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.
基金This study was supported by the National Natural Science Foundation of China(Grant Nos.41572086,41802120).
文摘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 positiveδ13CPDB values(with an average of 6.94‰)and negativeδ18OPDB values(with an average of-8.12‰).Based on theδ18OPDB values,the formation temperatures of the dolomite were at least^25ºC 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.
