Research of Fluid Inclusion Gas Composititon in Huayuan Lead-Zinc Deposits——Organic-mineralization study of MVT Lead-Zinc deposits(Ⅱ)

Key words: 
• fluid inclusion /
• gas compositions /
• gaseous hy drocarbon /
• old oil -gas accumulation

Abstract: The fluid inclusion aqueous solutions and gas composition is important material evidence for genesis of ore deposits,which is direct participant of mineralization and closely related to depostional mechanisms.The paper has studied the characters of fluid inclusion gas composition,gaseous hydrocarbon composition and carbon isotope of gas from 18 samples in Huayuan Mississippi Valley-type Lead-Zinc deposits which occured in algal limestone oolitic-limestone and muddy limestone of Lower Cambrian Qingxudong Formation.The characters are that: 1.There are a little organic inclusion in ores which has been comfirmed with fluorescence microscope that a few circular organic inclusion or organic remains give out bright yellow or green-blue fluorescence. 2.Most analyzed inclusions consist of H₂O with CO₂ (average 5.0mol%),gaseus hydrocarbon (average 1.1mol% ) and smaller amounts of CO and,H₂,etc.The content of H₂O is stable and the content of CO₂and hydrocarbon is various in inclusion.Gas composition of inclusion is the same between sulfides and gangue minerals,but in sulfide with different characters. 3.Methane (CH₄) is the main part in hydrocarbon composition of the most inclusionsbut the content is notgreater than 95% (average 84.89%),in inclusion of sphalirite at different host rocks,which is difference,the methane content in inclusion of spharilith at light gray limestone (averge 92.65%) is more than the spharilite at gray muddy limestone (average 84.89%). 4. δ¹³C of methane in inclusion of main mineralization stage ranges from -27.97‰ to -26.13‰,average -27.08‰ (PDB),δ¹³C of CO₂ in inclusion of main mineralization stage can be distinctly divided into two parts,the chief one in the main mineralization stage ranges from -19.43‰ to -14.5‰,average -17.28‰(PDS),the second one at the same time and that in the late mineralization stage are rather large (-8.12‰～3.0‰,average -3.71‰), suggesing that the main origin of CO₂ in inclusion is related to the thermal degradation of organic matter. Above-mentioned characters of inclusion gas composition suggest that the organic matter really occurred in the gas composition of inclusion,the gaseous hydrocarbon and CO₂ is organic origin in fluid inclusion;the hydrocarbon possibly has some source bed,and the depositional mechanism is possibly related to mixture of aqueous solutions with a pre-existing old oil-gas accumulation at the sites of deposition.