Characteristics of Organic Geochemistry of Lower Cambrian in West ern Hunan——Organic-mineralization study on MVT Lead-Zinc ore deposits

Key words: 
• organic matter /
• evolution level /
• mineralization

Abstract: The paper have studied the characteristics of organic geochimestry of Low er Cambrian and relavant mineralization of Lead-Zinc ore deposits in western Hu nan and eastern Guizhou province.The lower Cambrian is located at the passive co ntinental margin of Yangtze landmass in southeast,and divided into three units f rom lower to upper,i.e.,Niutitang Formation,Balong Formation and Qingxudong Form ation,which belong to basin,shelf and ramp to margin reef-band facies of carbon ate platform respectively.Algal limestone,grey-dark muddy limestone and overlyin g oolite limestone are host rocks of Huayuan MVT Lead-Zinc ore deposits in Qingx udong Formation. There are three beds richin organic matter:black shale in lower part of Ni utitang Formation,zebra muddy limestone in lower part and grey-dark muddy limes ton in middle part of Qingxudong Formation,and the black shale is much higher (75%) than other beds (0.2%～0.4%).On the basis of the GC-MS of organic matter t racts and Rock-Eval Py,Py-GC analysis,their characteristics of normal paraffin i s higher carbon number range,single peak at the back,higher carbon number of mai n peak (27～31),weak odd-even predominance,and Pr/Ph＜1，but the charact eristics of biomarker of sterane and terpane are different between them.Ischoles tane is predominant in sterane of black shale(C₂₉ is main(=34.33%),C₂₉ αββ/ααα=2.18) and in grey-dark muddy limestone,cholestane is predominant (C₂₉=42.2%,C₂₉αββ/ααα=0.42).Tricycline terpane is predomenant in terpane of black shale(65.65%),Tm/Ts=1.14,and in grey-dark muddy limestone,ho pane is predominant(=66.2%),Tm/Ts=1.22.Above mentioned characteristics suggest t hat the precursor origin of organic matter among all beds is blue algal and bact eria in Cambrian sea,and deposited on the reduction to weakly redaction environm ents.The evolution level of organic matter is different among them,they have pas sed oil-window and the black shale is higher than other,but it is still not beyo nd the end stage of evolution. Some data evidence that the evolution level of altered grey-dark muddy limestone is not only related to diagenisis but also to hot ore-forming fluid.Therefore,they can result in obvious change of group composits,biomarker,pyrolysis T[ WTBZ〗maxand pyrolysis parameters.In massive muddy limestone,chloroform bit umen “A”=0.004%,saturated hydrocarbon=11.80%～41.1%，aromatic hydrocarbon=15.0 7% ～5.45%,asphaltene=31.00%～50.94%,Zn＜0.1%,but in interbedded rocks,Zn＞1.0%, chloroform bitumen “A”increased to 0.0112%,saturated hydrocarbon rise to 42.18 %～57.06%,aromatic hydrocarbon to 9.7%～21.47%,and asphaltene decrease to 7.36% ～14.55%,pyrolysis Tmaxfrom massive rock's 397℃ increase to interbedded rock's 510℃,and with increase of mineralization there occur the decrease in free hydrocarbon (S₁),pyrolysis hydrocarbon (S₂),hydrogen index (HI),oxygen index (OI) and hydrocarbon index (S₁/TOC) of grey-dark muddy lime stone,All these suggest that there are close relationship between the organic co mposition and mineralization,and some organic composition may be directly precipitate during mineralization.