Interaction Between Hydrocarbon Fluids And Host Rocks/Evaporites In The Jinding World-class Zn-Pb Deposit:Implication For Ore Genesis

Solid organic matters and hydrocarbons are frequently found in MVT Pb-Zn deposits.It has been suggested that the hydrocarbon accumulation commonly predates mineralization and may be an important source for reduced sulfur or as agents for sulfate reduction.However,merely little research has been conducted on the interaction between hydrocarbons and their host rocks.Its characteristics and implications for later Zn-Pb ore formation remain poorly understood.In this study,the interaction between organic fluids and their host rocks in Jinding Zn-Pb deposit is systematically studied,and the timing,process and products of the interaction and its significance on Zn-Pb mineralization are discussed.Jinding ores are hosted in a dome produced by diapiric migration of evaporites.The ore-hosting rocks include sandstone,limestone clasts-bearing sandstone（defined as Jinding sandstones）and limestone breccias（Jinding breccias）,which contain abundant gypsum/anhydrite.Solid bitumen,interstitial oil and organic inclusions are widely developed in Jinding sandstones and limestone breccias,indicating Jinding dome was a hydrocarbon reservoir.The field observations show that Jinding sandstones were red in color and altered to grey by organic fluids.The Zn-Pb sulfides are mainly in the grey Jinding sandstones with some occurring in the limestone breccias.In limestone breccias,calcite matrix has pseudomorph of gypsum and relatively depleted carbon isotope composition（-21.9‰to-5.4‰）,indicating the interaction between the organic fluids and evaporites occurred in Jinding and formed H₂S and calcite that occurs as the matrix in the limestone breccias.The highly depleted and widely distributed pattern ofδ³⁴S_V-CDT-CDT values of pyrite（-26.0‰to-7.6‰）indicate that the interaction was achieved by bacterial reduction of sulfate（BSR）.Microscopic observations and TIMA analysis show that hematite decreases,and pyrite/marcasite increase in the sandstone matrix in bleached sandstone as compared to red sandstone.Mass balance calculation of whole-rock major elements exibits that Fe₂O_3（T）（+46.2%）and CaO（+46.2%）are gained in the bleached sandstone.Calcite cement in the grey sandstone has relatively depleted carbon isotopic composition（sandstone:-20.8‰～-11.3‰;limestone clasts-bearing sandstone:-9.8‰ to-1.7‰）,indicating that sandstone bleaching was caused by the interaction between organic fluids/H₂S and Jinding sandstones.In order to determine the timing of interaction between organic fluids and host rocks and the mineralization age of Jinding,different types of pyrite were selected to carry out the trace elemental analysis and Re-Os isotopic chronology.In Jinding,pyrite/marcasite occurs in Jinding sandstones,limestone breccias and gypsum-riched part,and was generally thought to be formed at the same stage with sphalerite and galena due to their close spatial relationship.However,throughout the deposit,pyrite/marcasite is much more widespread than sphalerite and galena.Under the microscopic observations,pyrite/marcasite and Zn-Pb sulfides are not completely consistent in spatial distribution.In Jinding sandstone,pyrite/marcasite and sphalerite occur as disseminated without clear crosscutting and replacement relationship（Py-1）.Pyrite/marcasite in gypsum-riched part and limestone breccias is mostly overgrowthed,cut or replaced by sphalerite（pre-ore;Py-2）,with only minor amount of pyrite/marcasite displaying alternate growth with sphalerite（syn-ore;Py-3）.LA-ICPMS trace elemental analysis shows that Py-3 has striking different trace elemental pattern and is enriched in Zn,Pb,Mn and depleted in Mo as compared to other types of pyrite.The Py-1 and Py-2 are suggested to be formed prior to the Zn-Pb mineralization.Re-Os isotopic chronology shows that Py-2 has the isochron age of49.4±3.8 Ma,representing the age of hydrocarbon accumulation and its interaction with host rocks/evaporites in Jinding deposit,while Py-3 has the isochron age of 25.6±3.6 Ma that is the age of Zn-Pb mineralization and consistent with the mineralization ages obtained from paleomagnetic and apatite fission tracks methods by previous studies.This study also suggests that a large amount of pyrite/marcasite was formed in the pre-ore hydrocarbon accumulation stage with only minor amount of pyrite/marcasite formed in the Zn-Pb mineralization stage.The significance of interaction between organic fluids and host rocks on Zn-Pb mineralization includes the following four points:（1）provided a large amount of reduced sulfur for Zn-Pb mineralization;（2）provided replacable calcite for Zn-Pb mineralization;（3）formed a reduced environment which was condusive to hydrocarbon preservation;（4）increased the porosity in the limestone breccias,which improved the quality of oil and gas reservoir in the pre-ore stage,and provided precipitation space for Zn-Pb sulfides.