Characteristics Of Prehnite-Quartz Geode And Its Hydrothermal Evolution In Puge Basalt,Southern Sichuan,China

Predecessors have carried out a lot of research on the mineral morphology,color mechanism,and mineral origin in the basalt prehnite-quartz crystal cave of Emei Mountain in southern Sichuan,but the relationship between the mineral combination in the crystal cave and the hydrothermal evolution is lacking.This paper takes the prehnite-quartz geode minerals in the basalt in the Puge area of southern Sichuan as the research object.With the aid of optical microscope,laser Raman spectrometer,cold and hot table and other instruments and equipment,the mineral combination and distribution characteristics in the geode are analyzed and tested.The composition and homogenization temperature of fluid inclusions in quartz were analyzed,and the trace elements of prehnite were tested by inductively coupled plasma mass spectrometer.The evolution characteristics of the hydrothermal fluid in the geode are preliminarily discussed.The main minerals of prehnite-quartz geodes are quartz and prehnite,and the secondary minerals include wollastonite,epidote,chlorite,hematite,etc.,most of the minerals can be seen in different generations,basalt on the wall of the cave Epidote is obvious.From the geode wall to the center of the geode,the order of mineral formation is:first generation light purple quartz+hematite→second generation quartz+first generation prehnite→third generation quartz+second generation prehnite+first generation 1st generation wollastonite→4th generation quartz+2nd generation wollastonite+epidote+chlorite.Different generations of quartz show a certain degree of stratification in the interior,and a large number of gas-liquid inclusions can be seen in different growth layers,and the volume of gas in the inclusions accounts for about 5%to 10%.The hydrothermal fluid in the geode belongs to the low salinity（8.28%～10.61%Na Cleqv）,low temperature（141.7～188.2℃）type hydrothermal fluid.The density of the hydrothermal fluid is0.88～0.93g/cm³.The changing trend of rising and falling.The initial hydrothermal fluid in the geode contains volatile components such as H₂O,CO₃^2-,and SO₂.In the later period,organic components such as CH₄ and C₃H₆ are gradually added,resulting in the change of the properties of the hydrothermal fluid.The changes in the composition and temperature of fluid inclusions in quartz in different generations,especially the changes in the composition of sulfur dioxide and methane in the hydrothermal fluid,and the results of previous studies on the sulfur isotopes of basalts in this area,infrared spectroscopy and carbon isotopes of asphalt,indicate that grapes The hydrothermal fluid in the rock-quartz geode is closely related to the Lower Permian carbonate rock underlying the basalt.The overall change trend of the trace elements of prehnite in the two generations is the same,indicating that the prehnite in the geode has homologous evolution characteristics,but the content of trace elements in the prehnite of different generations has obvious changes,indicating that there is a material change in the process of mineral crystallization.The characteristics of V/Cr value and V/（V+Ni）value indicate that both generations of prehnite were formed in an oxidizing environment.The Ta/Hf ratio indicates that the hydrothermal fluid in the prehnite-quartz geode is mixed with mantle fluid.According to the trace element content of prehnite in geode and the characteristics of trace elements and carbon,hydrogen and oxygen isotopes of basalt and related mineral deposits in the area,the hydrothermal fluid in geode mainly comes from basalt and its underlying Lower Permian carbon Some of the acid rocks may come from the upper mantle.Multi-source hydrothermal fluids cause the alternation of the geode environment from oxidation→reduction→oxidation→reduction to form a combination of prehnite,quartz,wollastonite,epidote,chlorite and other minerals of different generations.