The Origin And Source Exploration Of Hydrocarbon Gas In Limu Granite,Guangxi

The Limu mining area is located in the northern part of the Northeast Guangxi depression belt,belonging to the Xianggui plate(south of the Yangtze plate)and close to the contact with the Huaxia plate.The Limu ore-bearing granite,located in the north of Gongcheng syncline of Indosinian tectonic bed,is a syngenetic,contemporaneous and different stage compound granite of Yanshan.Taking Limu granite as the research object,this paper discusses and analyzes the genesis and source of hydrocarbon gas of Limu granite by means of high temperature and high pressure simulation experiment and carbon isotope testing method.The following key results and perceptions were achieved:(1)According to the maps and characteristics of the major elements,trace elements and rare earth elements in Limu granite,Limu granite is a highly differentiated calc-alkaline alkaline peraluminous granite with S-type granite characteristics,and its source rocks were formed in a post-orogenic compression tectonic environment.(2)The diagenetic and ore-forming fluids of Limu granite body are medium and low temperature(160??260?),low salinity(7.69%on average)and low density(0.92g/cm³ on average).The fluid inclusions of Limu granite are mainly composed of H2O in liquid phase,H₂O,CO₂ and CH₄ in gas phase.(3)The hydrocarbon content of Limu granite increases in turn from morning to evening,but the hydrocarbon content of rich Sn ore body decreases obviously,the characteristics of standard curve of rock body in each stage are basically the same,and the characteristics of the third stage rock body and rich Sn ore body curve are close,reflecting the homologous and simultaneous characteristics of Limu granite,and the mineralization of Limu ore area and the third stage granite is most closely related.It is presumed that the ore-forming fluid is the product of hydrothermal differentiation and evolution.(4)The results of carbon isotope testing of hydrocarbon gases show that the value of?¹³C₁ of Limu granite is greater than-30‰,and the carbon isotope is?¹³C₁>?¹³C₂>?¹³C₃>?¹³C₄>?¹³C₅ negative carbon series,indicating that the hydrocarbon gas of Limu granite is of inorganic origin.(5)The Limu granite and the Proterozoic to Early Paleozoic sedimentary rocks have basically the same characteristics of hydrocarbon gas curves,which are different from those of Devonian limestone and Carboniferous limestone.In the high temperature and high pressure simulation experiments,the delta value of hydrocarbon gas appears the phenomenon of"reentry".The Limu granite is S-type granite.It is inferred that the hydrocarbon gas of the Limu granite is partly derived from the Proterozoic-Early Paleozoic sedimentary rocks.