Geological And Geochemical Characteristics And Ore Genesis Of The Dongping Gold-Telluride Deposit In The Northern North China Craton

North China Craton（NCC）hosts hundreds of lode gold deposits and has been one of the most important gold provinces in the world.With respect to the genesis of these deposits,numerous studies have been carried out and many significant insights have been acquired.However,a longstanding debate exists whether or not magmatically derived hydrothermal fluids and their mixing with external meteoric water play a crucial role in the genesis of this very important type of deposit.Additionally,tellurides are commonly developed in many lode gold deposits and show a close association with gold mineralization,but the formation mechanism of these tellurides and their relationship with gold enrichment remain unclear.On the other hand,although the ages of gold mineralization in most lode gold deposits in NCC have been precisely constrained,it is not the case for the world-class Dongping Au-Te deposit.In this thesis,an integrated study of field investigation,petrographic observation,geochronology,geochemical compositions and experimental studies to better understand aforementioned issues.The Dongping Au-Te deposit located in the Zhang-Xuan district are primarily hosted in the Shuiquangou alkaline complex with minor ores in the amphibolite-facies to granulite-facies rocks of the Sanggan Group.About seventy orebodies have been delineated at Dongping and they are mainly structurally controlled by NE or NNE-trending faults.The mineralization style consists of high-grade quartz-sulfide veins and low-grade disseminated ores in alternation envelopes.A variety of ore and gangue minerals are observed,mainly including K-feldspar,quartz,garnet,epidote,sericite,specularite,magnetite,pyrite,galena,native gold and tellurides.Minor calcite is also present.Native gold commonly occurs as microfracture-fillings within pyrite or as mineral inclusions within pyrite,magnetite and/or quartz.Tellurides are mostly calaverite,altaite,and petzite etc.These tellurides usually occur together with native gold,and in some cases also chalcopyrite,as polymetallic mineral inclusions within pyrite and/or quartz.Within individual multiphase mineral inclusions,various metallic minerals often show intergrown textures,such as triple joint junctions and mutual curvilinear boundaries.It is noted that each such polymetallic inclusion only contains one or no native gold grain and chalcopyrite is always attached to the inclusion walls.Based on filed and paragenetic relations,three stages of hydrothermal alteration are recognized:pre-ore stage characterized by K-feldspar,milky white and gray quartz,specularite and minor pyrite;syn-ore stage represented by gray quartz,magnetite,sulfide with pyrite and galena in dominance,abundant gold and tellurides;post-ore stage marked by quartz-calcite veins.Note that garnet can be observed in each paragenetic stage.Hydrothermal garnets at Dongping contains 0.80–13.89 mg/kg U,which is predominantly enriched in the Al-rich growth zones of garnet grains.Garnets from pre-,syn-and post-ore veining stages have U-Pb dates of 140±6 Ma,140±4 Ma,and 139±6Ma,respectively,while the varieties from alternation envelopes are dated at 141±4 Ma to 142±5 Ma.These garnet U-Pb dates are similar within uncertainties,and indicate that gold mineralization at Dongping occurred in the Early Cretaceous（～140 Ma）.The U-Pb dating of zircon grains extracted from the syenite of Shuiquangou alkaline complex suggests the such complex was emplaced at 401–390 Ma,whereas Shangshuquan granite has a zircon U-Pb date of 143±1 Ma.Results presented here demonstrate that the gold mineralization at Dongping has no genetic link with the Devoian alkaline magmatism due to their large age gap of～250 Ma.Both garnet and quartz of various stages contain many boiling fluid inclusions.Fluid inclusion study suggests that ore-forming fluids are characterized by Na Cl-H₂O-CO₂and Na Cl-H₂O systems,the temperature ranges of 415–281°C,and the salinities of 1.6–11.7 wt.%Na Cl equiv.Garnet grains from quartz veins of various paragenetic stages and the mineralized alternation envelopes have dynamicδ¹⁸O variations of 3.8 to–11.4‰,with the largest intragrain fluctuation up to 5.3‰.These values correspond to calculatedδ¹⁸O values of 6.1 to–9.1‰for the hydrothermal fluids from which the garnet formed.Based on SEM-CL microtextures of quartz veins of various stages,six quartz generations are identified.These generations consistently display small to large ranges inδ¹⁸O values from 13.1 to 0.6‰that correspond to calculatedδ¹⁸O values of 7.5 to–5.8‰for the quartz-forming fluids.The relatively highδ¹⁸O_garnet,δ¹⁸O_quartzand correspondingδ¹⁸O_fluidvalues of various stages presented in this thesis can be best interpreted by the mixing of pulsed magmatically derived fluids and different proportions of meteoric water.Such fluid mixing is throughout the entire fluid history and plays a crucial role in the precipitation of gold.When combined with filed investigation,the calculation of the mass of ore fluids forming the total gold reserves at Dongping,and the large age gap of～2.5 Ga between gold mineralization and the regional metamorphism,ore-forming fluids is unlikely related to the regional metamorphic devolatilization in Neoarchean,and cannot be also metamorphic fluids generated by devolatilization via contact metamorphism of the Sanggan Group and the Shuiquangou alkaline complex in the Early Cretaceous.The heating-quenching experiments at different conditions and in-situ SME heating experiments indicate that the morphology and the spatial distribution of various tellurides and native gold clearly changed after the experiments.Before the experiments,various minerals in single polymetallic inclusions occurs independently and the contact interface amongst these minerals are curvilinear and smooth.However,all tellurides show a post-experiment typical intergrowth texture,and many droplet-like and/or vermiculate nano native gold grans are observed randomly hosted within various tellurides.Additionally,chalcopyrite occurs as mineral inclusions within tellurides in the heating-quenching experiments at 350°C,400°C,and 450°C,instead of closely attaching to the host pyrite.The results presented here demonstrate that tellurides at Dongping formed from the Te-rich melts and such melts are important medias for gold enrichment.Moreover,the changes in morphology and distribution of tellurides and native gold at different temperatures as recorded in three in-situ SEM heating runnings indicate that total homogeneous temperatures of tellurides at Dongping are in the range of 340–400°C.This temperature range is consistent with the relatively high temperatures obtained by fluid inclusion microthermometry,suggesting the ore-forming temperature is suitable for the formation the Te-rich melts.Thus,the experimental study in this thesis allows us to establish the connection of Te-rich melts and gold enrichment.When ore-forming fluid systems were evolved into the conditions of relatively high f（Te）and low f（S）,the Te-rich melts would form as immiscible phases from hydrothermal fluids.Subsequently,these melts could scavenge Au⁰from the fluids until to reach the saturation of Au⁰in such melts.The Au⁰may be pre-existing and/or formed as the results of redox reactions under appropriate physicochemical conditions in the fluid systems.When fluid cooling occurred,the solidification of Te-rich melts started.The morphology of native gold in natural samples was likely the consequence of the gradual growth of single gold crystal nucleus at slow cooling rate.Integrating field investigation,geochronology,isotope geochemistry,and experimental study and also combined with previous data,the Dongping Au-Te deposit is not an alkaline-related gold deposit or orogenic gold deposit as previously proposed.Rather,we clarified this deposit as an oxidized intrusion-related gold deposit emplaced in an extensional setting related to the thinning and destruction of the lithospheric keel beneath the NCC.In the late Mesozoic,the lithospheric mantle of the eastern block in NCC was intensely and extensively extended and weakened,a catastrophic event induced by the westward subduction of the Paleo-Pacific plate.Under such strongly extensional tectonic setting,the deep-seated magma chamber in the crust provided the heat,ore fluids and materials（e.g.,Au and Te）,the critical necessities for the formation of the Donogping deposit in the Early Cretaceous.Subsequently,these ore fluids ascended along brittle fault systems and finally mixed with external meteoric water,leading to the deposition of gold ores and associated mineral assemblages.Note that the Te-rich melts that were formed under the relatively high f（Te）and low f（S）conditions would scavenge Au0 from the ore fluids,leading to the re-enrichment of gold.With the cooling of fluid systems,these Au-rich melts started to crystallize and finally formed various tellurides and native gold.Such scavenging process plays an important role in the refining of ore grades or the formation high-grade gold ores at Dongping.