Being a newly discovered largescale Ag(Sb) deposit, the Songxi deposit is occurre d at the margin of Zhaigangshang volcanosedimentary basin, which belongs to sou t hern section of the Yongan—Meixian Depression. Most of previous researchers t ho ught that its genesis might have a close relationship to the volcanic activi ty , and several genetic opinions, such as submarine basic volcanic exhalation and lowmedium temperature volcanic hydrothermal fluid filling, have been proposed. In this work, a high vacuum quadrupole gas mass spectrometer system has been use d to analyses the compositions of organic gases in fluid inclusions in minerals, and nine light hydrocarbons have been recognized. The light hydrocarbons are mo stly composed of C1～4 saturated alkanes, while the contents of C2～4  unsaturated alkenes and aromatic hydrocarbons are very low, suggesting that magmatic activit ies might have not been involved in the metallogenic processes. C1/C1～2 ratios of the hydrocarbons show that they are mixture of organic gases produced by microo rganism activity and thermal cracking of type Ⅱkerogens(kukersite) in sedimenta ry host rocks. According to the compositions of light hydrocarbons, the tectonic setting of the Songxi deposit should be a rift. From one aspect, the analytical da ta support the newly proposed genetic model by the authors that the Songxi depos it may belong to a sedimentary hot brine transformed deposit.