Studies On The Ore-forming Material Sources Of The MVT Lead-zinc Deposits Occured In Denging Formation In The Northern Margin Of The Sichuan Basin

The Pb-Zn deposits occured in Dengying Formation in the northern margin of the Sichuan basin belong to the Mississippi Valey-type(MVT) Pb-Zn deposits. The main ore minerals include sphalerite and galena, while the gangue minerals consist of barite, fluorite, bitumen, dolomite, calcite and so on. Among gangue minerals, barite and fluorite can enrich and form their own deposits. The MVT lead-zinc deposits and paleo-oil(gas) reservoir coexisted with each other in spatial distribution in this area.They have genetic relationships with each other.Based on the geological setting and various geochemical analysis, some conclusions are drawn as fllows:(1)The contents of the metallogenic elements in the Cambrian Guojiaba Formation carbonaceous mudstone and basement rocks show a high value, so they have a potential to provide ore-forming material sources for the MVT deposits in this area.(2)The ore mineral ’ s REE distribution patterns and various graphs of REE parameters are different to the surrounging rocks. However, it can’t be ignored that the the Cambrian carbonaceous mudstone and basement rocks show a high similarity in all these graphs. Based on these facts, we conclude that the weather denudations of the basement rock contribute to the provenance of the Cambrian carbonaceous mudstone.(3)The Sr isotope data show that the ore mineral’s initial Sr isotope ratios range between 0.709782 and 0.71131, averaging 0.71041, indicating that the Cambrian carbonaceous mudstone and basement rocks are the potential source rocks. The 87Sr/86 Sr of fluorite indicate that their formation are related to the dissolution of the carbonate rocks.The S istope data revel that the value of206Pb/204Pb-207Pb/204 Pb and208Pb/204 Pb are between 17.62- 18.02,15.49- 15.63 and 37.57- 38.35, respectively, reflecting that the Pb isotopic composition is stable and belongs to normal Pb type. The Pbisotopic model age are between 418-662 Ma, indicating that the metallogenic materials are from the Sinian-Cambrian stratum. The relevant plots of Pb revel that the metallogenic materials are mainly of crust source and meanwhile, mixed with some mantle substances.Considering that there are no deep-and-large faults in this region, thus the basement rocks have no access to provide the metallogenic materials for the Pb-Zn deposits. But if the basement rocks haven ’ t supply the metallogenic materials, it seems that some isotope characteristics is unexplainable. However if we put the conclusion that the weather denudations of the basement rock contribute to the provenance of the Cambrian carbonaceous mudstone, and the Cambrian carbonaceous mudstone is the direct source rocks for the Pb-Zn deposit, while the basement rock is the indirect one. The conclusion we draw before and the isotope evidence can prove each other.(4)The sulfur isotopic composition of the ore mineral and barite is enriched in 34 S,similar to the sulfur isotopic composition of oilfield brine.The carbon isotope data of the dolomite and calcite revel that the calcium ions of them derive from the dissolution of the carbonate rocks.Based on the above conclusions, trying to discuss the ore-forming processes of the MVT Pb-Zn deposits in this area, and concluding that the paleo-oil(gas) reservoir has a close relationship with the MVT Pb-Zn deposits.