The Contribution Of The Bitumen To The Mineralization Of The Non-stratabound Metal Deposit

The Erdaokan Ag-Pb-Zn deposit is one of the newly discovered large silver-lead-zinc deposits in the Dubaoshan metallogenic belt.A large amount of natural bitumen found in the ore which is proved to be closely related to the mineralization,and is very important in research point of view about the genesis of this deposit.To date,many aspects such as the genesis of the deposit and more importantly,the role of bitumen in metal precipitation has not been studied.After conducting detailed field geological and the petrographic observations,we carried out multi-parametric analysis,such as mineralogy,petrology and organic geochemistry,the main trace and rare earth elements,infrared spectrum,C-H-O-S-Pb-Rb-Sr-Sm-Nd isotopic study to reveal the information on the natural mineralogical characteristics of the bitumen and other minerals,ore-forming age,source of ore forming fluids and genesis of the deposit.Combining all the data,we discussed the contribution of bitumen to the precipitation of the metals in the Erdaokan Ag-Pb-Zn deposit.Two ore-bodies were found in the Erdaokan Ag-Pb-Zn deposit.The host rock is dominated by structural breccia and pyroclastic rocks,and the ore is seriously altered,mainly by silicification,pyritization,magnetization,hematite and carbonation.The Ag-bearing minerals in the Erdaokan Ag-Pb-Zn deposit are mainly occurred in quartz veins or siliceous cement in the form of visible silver minerals(argentite and pyrargyrite)and metallic sulfide minerals with invisible silver.The homogenization temperature of these silver-bearing minerals during precipitation is 79?-191?.The?salinity is 0.35wt%24.lwt%,and the density is 0.89g/cm3?1.1g/cm3,which are characterized by low temperature and low density.The hydrogen and oxygen isotope results of quartz showed that ?D and ?18Owater values range from-109.7‰ to-119.1‰,and-4.64%o to 1.98‰,respectively.The ore_forming fluid is characterized by mixture of meteoric and magmatic waters.The sulfur and lead isotope data of pyrite and galena showed(?34S:-0.97‰to 9.31‰)that the metal components and sulfur were derived from magma.The geochronological results showed that the Sm-Nd age(234.6±1.2Ma?234.9±1.4Ma)of the bitumen,the Rb-Sr age(232.9±2.3Ma)of the sulfide(pyrite and galena)and the U-Pb age(234.2±2.8 Ma)of the zircon in the adjacent wall rocks were highly consistent.These results show that the geological age of the deposit is 232.9±2.3Ma?234.9±1.4Ma,which belongs to the late Triassic.This age is consistent with the time of the subduction of the Mongolia-Okhotsk Ocean.It is thought that the region experienced large-scale magmatic activity in the Triassic period which results in the formation of the dacite porphyry,diorite porphyrite and other igneous rocks found around the deposit.During the upward migration of magma,the ore-forming hydrothermal fluid migrated to the tectonic fracture zone,and where it interacted with organic matter rich strata such as limestone and carbonaceous shale.Marine limestone and carbonaceous shale are the main source rocks for the input of organic matter.The organic carbon contents of these rocks are 0.771%,3.17%-3.37%,respectively,which can effectively provide material for the formation of the bitumen.At the same time,biomarker analyses reveal that bitumen,limestone and carbonaceous shale all show low TAR values,without significant parity advantage and nC24-nC35 did not have carbon dominance index,indicating marine genesis of the organic matter.The carbon isotope results showed that ?13C values of bitumen,limestone,carbonaceous shale and siderite are-18.91‰?-19.33‰,-0.13‰?-3.7‰,-2.18‰?-3.81‰and-2.52‰?-4.85‰,respectively,which indicate that the source of organic matter of the bitumen is closely related to limestone and carbonaceous shale with a marine organic origin,which has no direct genetic relationship with inorganic carbon deep in the earth.Thermochemical sulfate reduction and thermochemical oxidation of organic matters induced by high-valence metal(Mn and Fe)are the main mechanisms of metal precipitation in the Erdaokan Ag-Pb-Zn deposit.There are a large number of siderophile elements(Ag,Cu,Pb,Zn,Sb and As)brought by ore-forming fluids in bitumen,and the lack of oxyphilic elements(K,Na,Mg,Al and Ca)which are abundant in limestone and carbon shale,well verifies the reaction process between ore-forming fluid and bitumen.Consequently,the fluids metasomatised the organic matter bearing wall rocks and a part of organic matter entered in the ore forming fluids which triggered a series of chemical reactions occurred in the reduction environment,resulting in the precipitation of bitumen,galena,magnetite,sphalerite,pyrite,pyrargyrite,siderite,rhodochrosite and other ore minerals.