MVT Pb‐Zn Deposits in Paleo‐Hydrocarbon Reservoir on the North Margin of Sichuan Basin, China

Abstract

The Pb-Zn deposits in Nanjiang-Mayuan area on the north margin of Sichuan Basin chiefly occurred in dolomites of Sinian Dengying Formation. Host rock of deposits distributed both on the two wings of composite anticline of Micangshan, the Mid-Late Proterozoic Huodiya group and Jinning-Chengjiang period magmatic rock located in the cores of the composite anticline. The Dengying Formation in the study area could be divided into four lithologic Member: the first Member (Zd 1 ) was mainly made up of rhythm of several botryoidal dolomite →snowflake dolomite→microlitic-minicrystal dolomite; the second Member (Zd 2 ) consisted of the interbed of hoary-gray microlitic dolomite, gray algal-laminated dolomite and dolarenite; sandstone, siltstone mingled with blue gray mudstone constituted the third Member (Zd 3 ); the fourth Member (Zd 4 ) chiefly included microlitic-finely crystalline dolomite, dolarenite and algal dolomite. The geological and geochemical evidences suggested that there existed two obvious paleo-karst interfaces between Zd 2 and Zd 3 , between Cambrian and Zd 4 . Dissolved pores and karst caves well developed in the dolomite under the interface of paleo-karst and some of them were infilled with galena, blende, fluorite, barite, dolomite, quartz and bitumen. Geological Characteristics of DepositsDeposits such as Pb-Zn deposits, fluorite deposits and barite deposits has been found in Dengying Formation in the study area, among which, MVT Pb-Zn deposits developed best. Mineralization characteristics of fluorite deposit and barite deposit are similar with that of MVT Pb-Zn deposits; therefore, they were grouped into a subclass of MVT Pb-Zn deposits (Rowan et al., 1996).Study showed that paleo-karstification played a significant role in the mineralization of Pb-Zn deposits, fluorite deposits and barite deposits, and all these deposits developed under the paleo-karst interface between Cambrian and Zd 4 . The Pb-Zn deposits often occurred in layers; the ore mineral mainly consists of blende and/or galena, and gaugue minerals mainly consist of dolomite, pyrite, quartz and bitumen. Sometimes zebra structure was formed among galena, blende and dolomite. Pb-Zn deposits, fluorite deposits and barite deposits can adjacently occurred and form independent cystic or stratified ore body respectively. The galena, blende, fluorite and barite etc. mainly infilled in fissure, dissolved pores or karst cave formed by supergene karstification, which indicated that dissolved pore and karst cave were both the channel of metallogenic hydrothermal migration and the room for metallogenic elements accumulation and deposit. Meanwhile, the distribution of Pb-Zn deposits, fluorite deposits and barite deposits were controlled by paleo-karst landform and metallogenic elements only concentrated and deposited on karst highland in the paleokarst belts selectively.Research indicated that MVT Pb-Zn deposits were superimposed with the paleo hydrocarbon reservoirs of the Dengying Formation with each other in the space. Based on investigation of bit...

Genetic Relationship between Mississippi Valley-Type Pb–Zn Mineralization and Hydrocarbon Accumulation in the Wusihe Deposits, Southwestern Margin of the Sichuan Basin, China

“…The Ediacaran Dengying Formation is not only the main host of MVT Pb-Zn deposits [7,8,30,36,[38][39][40] but also an important commercially productive source of natural gas in the Sichuan Basin [41][42][43][44]. Many MVT Pb-Zn deposits have been found in the relict paleo-oil/gas reservoirs in the Dengying Formation located in the periphery of the Sichuan Basin [7,8,45]; the Wusihe Pb-Zn deposits located in the southwestern margin of the Sichuan Basin is one representative example of these deposits. The gas reservoirs in the Dengying Formation, such as the Gaoshiti and Moxi gas reservoirs, are mainly distributed in the deeply buried traps located in the central Sichuan Basin [41][42][43][44], and sporadic sphalerite and galena can be observed in these commercially productive natural gas reservoirs.…”

“…Bitumen is a common byproduct within matured or over-matured oil/gas reservoirs. The bitumen in the Dengying Formation is pyrobitumen [41,[44][45][46][47], and it formed during in situ transformation from paleo-oil reservoirs to paleo-gas reservoirs; therefore, the location of the bitumen retained today is equivalent to the location of the paleo-oil/gas reservoirs. According to the abundance and distribution of bitumen in dissolved pores and karst cavities, two relict paleo-oil/gas reservoirs are preserved along the paleokarst interface (Figure 2).…”

“…Bitumen is a common byproduct within matured or over-matured oil/gas reservoirs. The bitumen in the Dengying Formation is pyrobitumen [41,[44][45][46][47], and it formed during in situ transformation from paleo-oil reservoirs to paleo-gas reservoirs; therefore, the loca-…”

U–Pb Dating of Fibrous Dolomite in the Hydrothermal Dolostone of the Dengying Formation, Central Sichuan Basin, and Its Response to Supercontinent Breakup

“…Indeed, this thermal effect has been observed to extend to the regions of Sichuan, Yunnan, Guizhou, and the Qinghai-Tibet Plateau. Hydrothermal lead-zinc mineralization within the dolostone of the Dengying Formation in the Sichuan-Yunnan-Guizhou region serves as evidence of fluid activity triggered by thermal events [16][17][18].…”