Diagenesis And Metallogenesis Of The Erdaohe Pb-Zn-Ag Deposit,Inner Mongolia

The Erdaohe Pb-Zn-Ag polymetallic deposit is one of the few large deposits in the Middle Great Xing’an Range.Conducting a research in depth shall theoretically support the regional exploration.But the fact is that much previous knowledge were acquired based on data before mining,hence is outdated from the present mining progress,especially,in aspects of the major ore-related intrusion,mineralization age,and detailed metallogenic process.Moreover,the lack of dynamic background hinders the foundation of the metallogenic model and its application to regional comparison.This study is carried out on the basis of solid field survey,and involves researches from different aspects,including petrology,petro-geochemistry,isotopic geochemistry,mineral chemistry,fluid inclusion and isotopic geochronology.We have attained new comprehension on the deposit geology,origin and dynamic setting of the major ore-forming intrusion,detailed diagenesis and metallogenesis of the major mineralization epoch and the ore-forming ages.A metallogenic model was established and offered guidance for further regional exploration.Most ore bodies in the Erdaohe deposit is hosted in the skarn,which is developed along the contact zone between the monzodiorite and the Luohe Formation marble.More than ten other lodes are hosted a few kilometers away from the skarn in the Luohe Formation sedimentary and Tamulangou Formation intermediate-basic volcanic rocks.The widespread skarnization is zoned in the following pattern: the epidote-dominant skarn is developed in the vicinity of the monzodiorite;it gradually transforms into garnet-clinopyroxene skarn as the distance grows farther away from the monzodiorite;rhodonite-dominant skarn is observed only in the distance as veins.A total of three epochs of mineralization are recognized.The major mineralization epoch is genetically related to the monzodiorite and can be further divided into three stages,as the early skarn stage,the late skarn stage,and the sulfide stage.There are two other superimposed mineralization epochs.One is attributed to the diorite porphyry veins,whose F-rich hydrothermal fluids altered,extended,and expanded the skarn ore in depth;and also formed shallow fluorite-Cu-Zn lodes around the diorite porphyry veins.The other is linked to the latest granite porphyry veins,which formed the As-rich mineralization within the fracture zones and superimposed on previous ores,inducing a secondary mineralization enrichment to various degrees.The monzodiorite belongs to metaluminous high-K calc-alkaline series rock.It is low in silica,with enrichment in phosphorus,alkali,large ion lithophile elements(LILE)and light rare earth elements(LREE)but depletion in Nb and Ta.The weighted average zircon U-Pb age of 225.7 ± 1.7 Ma determines an intrusion age in the Late Triassic.Sr-Nd-Hf isotopic composition and other geochemical features suggest a primitive magma originated from the partial melting of a mixed source of juvenile lower crust and mantle-derived material.A comprehensive comparison with other Triassic magmatic rocks within or around the Xing’an Block was conducted,which leads to a intimate relationship between the magmatism and the southward subduction of the Mongol-Okhotsk Ocean.Besides the subduction model of the southward subduction of the Mongol-Okhotsk Ocean was updated:(1)The initiation of the low-angle subduction occurred from the Late Permian to the Middle Triassic.During this period,the Erguna-Central Mongolia Block was in an active continental arc setting,while the Xing’an Block under the liminal basin spreading setting.(2)In the middle Late Triassic,the prolonged low-angle subduction triggered the uplift of the overriding Erguna Block,the continental crust thickening,and slab roll-back.Beneath the Xing’an Block,the long-lasting subduction slab front reached the condition of eclogitization that favors the density increase,which finally caused the slab break-off.And that is when the major mineralization epoch occurred.(3)The onset of the next stage of subduction took place in the late Late Triassic.A steeper subduction angle may hamper its advance and in turn caused an temporarily insignificant impact on the Xing’an Block,but was still manifested as coeval active continental arc magmas in the Erguna Block.The Late Triassic diagenesis and metallogenesis can be further divided into three parts,on which systematic mineralogy,mineral chemistry and fluid inclusion studies were conducted.A brief conclusion of each stage is summarized here.(1)The crystallization of hornblende-plagioclase-apatite from the magma began at around894℃.At this point,the relative oxygen fugacity is above the nickel-nickel monoxide buffer(NNO),with an average ΔNNO of +0.9.Calculation on the volatiles yielded water content of 2.7–3.6 wt.%,F content of around 0.11 wt.% and Cl content of around0.15 wt.%.As the temperature dropped below 798℃,iron-titanium oxides,K-feldspar,biotite and zircon started to crystallize.In the meantime,the exsolution of fluids is accompanied by a significant increase of relative oxygen fugacity from ΔNNO +0.9 to+3.1,which favors the pre-metallogenic enrichment of chalcophile elements.(2)The outwards migration of fluids results in a systematic change in mineral assemblage.The co-crystallized andradite and hematite rim the epidote endoskarn,which record a high temperature(469.2–>520℃),mid-high salinity(8.81–12.62 wt.% Na Cl equiv.)fluid,as well as a relative oxygen fugacity above the hematite-magnetite buffer(HM).Clinopyroxene and garnet are commonly paragenetic in the widespread exoskarn.In this stage,the fluid is of high temperature and observed in immiscibility.The salinity of the gas-rich two-phase,liquid-rich two-phase,and halite included multi-phase inclusions are 6.45–8.55,7.73–11.10,and 31.87–40.44 wt.% Na Cl equiv.respectively.The relative oxygen fugacity is right between the HM and FMQ(fayalite-magnetite-quartz)buffers,which decreases along with the temperature.The fluid inclusions in rhodonite is of mid-low temperature(218.5–330.0℃),mid-low salinity(0.35 –9.98 wt.% Na Cl equiv.).(3)The fluid in the sulfide stage is featured with mid-low temperature(171.4–407.2℃),mid-low salinity(0.35–14.87 wt.% Na Cl equiv.)and immiscibility.A systematic change in mineral composition and assemblage indicates a sequential precipitation of Fe,Zn,Mn,Pb,Bi,Te,Ag,Sb from the hydrothermal.The phase equilibria suggests an increase of both sulfur and tellurium fugacity is followed by a more obvious decrease of tellurium over sulfur fugacity.However,the phase transition from manganese silicate to manganese sulfide is controlled by a faster decrease of oxygen over sulfur fugacity.Besides,compared to the fluid in the major sulfide mineralization,the one from the arsenic-rich superimposed mineralization is apparently higher in temperature(268.6–409.5℃)and salinity(8.28–13.18 wt.% Na Cl equiv.).Meanwhile,the phase equilibria indicates a sulfur fugacity drop for decreasing temperature,which is opposite to the one in the major sulfide stage.Two types of titanite were recognized as crucial mineral for geochronology.The first type(Ttn-1)co-crystallized with other altered minerals,as actinolite and epidote,replacing original magmatic minerals,such as hornblende and Fe-Ti oxides.It is enriched in LREE with highly fractionated LREE and MREE,meanwhile varies greatly in Eu/Eu*,Sr contents,Nb/Ta and Th/U ratios.All these evidence indicates that the crystallization of Ttn-1 is mainly decided by conditions of the fluid fractionated from the monzodiorite,but also affected by heterogeneous dissolution/precipitation of local minerals.The second type(Ttn-2)are observed in strongly deformed skarn,together with concomitant hydrothermal apatite and garnet replacing original garnet.Ttn-2 is enriched in MREE with higher degrees of fractionation among HREE and between Lu and Hf.And the concomitant garnet incorporates significantly more HFSE than original garnet.A secondary F-rich fluid is interpreted responsible for the crystallization of Ttn-2.From Ttn-1 we acquired an U-Pb age of 234.0 ± 5.7 Ma for skarn formation,which is widely consistent with the intrusion age of the monzodiorite.The U-Pb age of 144.4 ±3.9 Ma is interpreted to represent superposition time of the F-rich fluid,which is coherent with intrusion age of the diorite porphyry veins.Combining with other ages from previous studies,a geochronological framework containing three mineralization epochs were set up.The metallogenic model was established for the Erdaohe deposit.(1)Impacted by the southward low-angle subduction of the Mongol-Okhotsk Ocean,the slab break-off occurred in the Late Triassic beneath the distal Xing’an Block.In this extensional environment,the monzodiorite magma intruded into the Luohe Formation,especially along the interface between the marble and slate.During cooling and crystallization,the magma gradually became more and more oxidized,and fractionated a volatile-and metallogenic elements-rich fluid.The fractionated fluid altered the monzodiorite,then thermally driven by the hot intrusion,migrating outwards and forming the widespread exoskarn.This is a dynamic procedure involving hydrofracture promoted pressure decreasing and crystallization—fractures filled by crystallization prevents fluid migration—accumulated pressure once again induced hydrofracture,during which the physical-chemical condition of the fluid constantly changes and forms a spiral forward process for fluid migration and crystallization.As an overall decrease of temperature,oxygen fugacity but an increase of sulfur fugacity,the earliest sulfides began precipitation.Continued mixture with meteoric water via fractures played an important role in the prevalent sulfide precipitation,which contributed to the formation of most Pb-Zn-Ag ore bodies.(2)The subduction direction shifting of the Paleo Pacific Ocean beneath the NE China caused a regional extension.The diorite porphyry and granite porphyry successively intruded into the ore district.The exsolution of a fluorine-rich and arsenic-rich hydrothermal superimposed on the original skarn ores and eventually formed the large deposit.We also proposed several ore-controlling factors.The Luohe Formation is considered the favorable wall rock which is also a likely source for metallogenic materials.NE trending faults should be noted as main ore-controlling structures.And multiple magmatic events have certain potential for mineralization.Several prospecting indications were put forward with respect to gossan,alteration,mineral assemblages and so on.By analyzing known geological information within the ore district,we think the extension and expansion of skarn ores in the southwest end,and the shifting of the strike from northeast to northwest point to concealed ore bodies and Late Jurassic intrusions.On the basis of collected geological,geophysical and geochemical data around the deposit,we also delineate a metallogenic prospect.