Geochemical Characteristics And Genesis Of Lacustrine "White SmokeType" Hydrothermal Sedimentary Rock In Qingxi Sag, Jiuquan Basin

Hydrothermal sedimentary rock is becoming one of the research hot spots in the geoscience field.However,little achievement has been done with the hydrothermal sedimentary rock formed in the lacustrine basin.The dark argillaceous dolomite and dolomitic mudstone in the Xiagou Formation of Lower Cretaceous in Qingxi sag, Jiuxi basin have been identified as infrequent lacustrine white smoke type hydrothermal sedimentary rocks.This is the particular subject dissertation.The subject is supported by Natural Science Foundation(NO.40672073)and Ministry of Education university special scientific research fund for doctor (NO.20060616014).Based on rock-mine identify,carbon and oxygen isotope,Sr isotopic,trace elements,rare earth elements,solution inclusion and other experimental tests,combining with the latest research achievements home and abroad,some results of the lacustrine white smoke type hydrothermal sedimentary rock are got as follow:1)The hot water minerals in the lacustrine white smoke type hydrothermal sedimentary rock of Xiagou Formation are mainly albite and ankerite,quartz,analcite, barite,dickite and fluorite are its secondary mineral components,and pyrite,galena, chalcopyrite,zinc blende occur in the rock occasionally.Seven paragenetic associations of hot water minerals in the hydrothermal sedimentary rocks of the Xiagou Formation are recognized.The hot water minerals represented as albite and ankerite are cyclopean and cumulate of hypautomorphic-automorphic structure.It shows that crystallization and precipitation are the mainly chemical sedimentation of the hydrothermal sedimentary rock of Xiagou Formation.The hydrothermal sedimentary rock is of micritic-aphanitic texture,and shows laminated structure mainly,with hot water clastic texture,hot water breccia texture,network vein structure,vortex structure and deformation bedding only partially in the rocks.The hot water clastic and hot water breccia texture can be identified seven types as different minerals associations,some of which are rare and of special significance.2)It is proved that the micrite ankerite of Xiagou Formation is precipitated in a relatively closed environment with a high temperature mineralizing solution from the carbon and oxygen isotopes analyses.The solution interchanges carbon and oxygen isotopes with the surrounding rocks in the rising and circulating,as a result the hot solution gets heavy carbon as to has a higher value ofδ¹³C.The characteristic of carbon and oxygen isotopes of Xiagou Formation may be caused by the hot solution, which is formed in the process that supracrustal rocks are leached and extracted by descending lacustrine water in the surface crust.3)Through Strontium isotope test,the ⁸⁷Sr/⁸⁶Sr ratio of the laminated hot water sedimentary rock only has minor changes.It can represent homogenization ⁸⁷Sr/⁸⁶Sr ratio in the hot brine pool.The composition of Sr-isotope of hydrothermal breccia and cementation are almost consistent,which shows that both of them are the same period and the same resource eruptive flow sediments.The ⁸⁷Sr/⁸⁶Sr ratio of hydrothermal sedimentary rock in the study area is much higher than the ⁸⁷Sr/⁸⁶Sr ratio of seawater and basalt of corresponding period and a little higher than the ⁸⁷Sr/⁸⁶Sr ratio of the algal limestone,which represents the strontium isotopes composition of lake water in early cretaceous,but lower than the ⁸⁷Sr/⁸⁶Sr ratio of underlying silicon-aluminum rock.It shows that the thermal solution is neither related with seawater nor simplex lake water or mantle magmatic water.It is primarily speculated that early Cretaceous sublacustrine hydrothermal solution in the area is the mixed hydrothermal solution which is constituted of deep circulating lake water and a little of mantle-derived magmatic water and full of high radiogenic Strontium of underlying silicon-aluminum basement rock.4)It was shown that the hydrothermal sedimentary rock of Xiagou Formation has basal characteristics of hydrothermal depositional formation of LREE>HREE, with high abundance∑REE,exceptional distribution pattern of REE,positiveδCe and negativeδEu,which is different from characteristics of marine exhalative rock,but is highly similar to the characters of alkalescent tholeiitic basalt in the same layer,the hydrothermal sedimentary rock of Xiagou Formation was considered to be closely related to the origin of basalt in the same horizon.The Eu/Sm-La/Ce simulation in the mixing process of hot solution and lacustrine water shows that the deep hot water sedimentation of Xiagou Formation is in hot brine pool which is in the condition of reducing and alkalescence.5)It is concluded that the hot solution of Lower Cretaceous in Qingxi sag is relevant with volcanic eruption from solution inclusion,C-O-Sr isotope,trace elements geochemistry and rare earth elements tests.The hot solution,which is descending lacustrine water mixed with magmatic water,is moderate-to-low temperature(90～200℃),moderate-to-low salinity(8～22wt%NaCl),and moderate density(0.97～1.2g/cm³)strongly alkaline hot brine water.It is short of metal sulfides, rich in CO₃^2-,SO₄^2-,Cl^-,F^-,Ca²⁺,Mg²⁺,Fe²⁺,Ba²⁺,Si⁴⁺,Al³⁺,Na⁺ and other major elements and anionic complexes,and contains Zn,Rb,Mo,As,Ni,Co,Sb,Ba,Ag and other deep source gas-liquid or mantle source trace elements associations.The temperature,salinity,density and pressure of the mineralizing hot solution of hydrothermal sedimentary rocks in different location decreases outwards by the order: main exhalative-channel—branch exhalative-channel—exhalative mouth—near exhalative mouth—distal basin.6)There are three driving forces for hot solution movement:gravity driving force,compaction driving force and magma heat driving force.The three forces cause the circulation of hot solution together,and extract metallogenetic chemistry materials from surrounding rocks.There may be four kinds of mechanisms of overpressure hot solution overbalance returning movement,three of the mechanisms are structural uplift pump,decompression in the shallow fault and supercharge of surging volcanic magma hot water,and they cause hot solution moving upward to breakout against overlying sediments with boiling and bursting along faults.It is mixed with the lower tempreture and salinity lacustrine water,and cooled suddenly.Then the chemical balance of the hot solution is broken and chlorine complex compounds are liberated. As a result,hydrothermal minerals are precipitated forming a series of lacustrine white smoke type hydrothermal sedimentary rock of different kinds.7)The lacustrine white smoke type hydrothermal sedimentary rock of Xiagou Formation of Lower Cretaceous in Qingxi sag,Jiuxi basin can be divided into four genetic types.By consulting available data about hydrothermal sedimentary rock published in China and abroad,this kind hydrothermal sedimentary rock belongs to rare lacustrine orthodolomite,which has particular genetic features and identification signs.8)The sedimentary model of the lacustrine white smoke type hydrothermal sedimentary rock of Xiagou Formation of Lower Cretaceous in Qingxi sag is founded: main exhalative mouth→branch exhalative-channel connecting to the main exhalative-channel→near main exhalative mouth→near branch exhalative mouth or further brine pool→distal basin.In the main exhalative mouth,hot water breccia hydrothermal sedimentary rock(aluminum silicate minerals + sulfate minerals + trace metal sulfide minerals association)develops;In the branch exhalative mouth,network vein hydrothermal sedimentary rock(carbonate minerals + aluminum silicate minerals + sulfate minerals)develops;Near main exhalative mouth,clastic hydrothermal sedimentary rock(carbonate minerals + aluminum silicate minerals + sulfate minerals) develops;Near branch exhalative mouth or in further brine pool,sedimentary type hydrothermal sedimentary rock(carbonate minerals + aluminum silicate minerals association)develops;At distal basin diffuse type hydrothermal sedimentary rock (only carbonate minerals association)develops.