Magmatic-Hydrothermal Superlarge Metallogenic Systems——A Case Study of the Nannihu Ore Field

Located in the Qinling （秦岭） molybdenum metallogenic belt on the southern margin of North China craton, the Nannihu （南泥湖） molybdenum （-tungsten） ore field, consisting of the Nannihu, Sandaozhuang （三道幢）, and Shangfang （上房） deposits, represents a superlarge skarn-porphyry molybdenum （-tungsten） accumulation. Outside the ore field, there are some hydrothermal lead-zinc-silver deposits found in recent years, for example, the Lengshuibeigou （冷水北沟）, Yindonggou （银涧沟）, Yangshuwa （杨树凹）, and Yinhegou （银河沟） deposits. Ore-forming fluid geochemistry indicates that these deposits belong to the same metallogenic system. The hydrothermal solutions were mainly derived from primary magmatic water in the early stage and from the mixture of the primary magmatic water and meteoric water in the later stage, with an obvious decreasing tendency in temperature, salinity and gas-liquid ratio of fluid inclusions. Sulfur and lead isotope data show that the ore-forming substances and related porphyries were mainly derived from the lower crust, and a hidden magmatic chamber is indicated by aeromagnetic anomaly and drill hole data indicate that the Nannihu granite body extends to being larger and larger with depth increasing. The large-scale mineralization was the consequence of lithospheric extension during the late stage of the tectonic regime when the main compressional stress changed from NS-trending to EW-trending.

Crucial control on magmatic-hydrothermal Sn deposit in the Tengchong block,SW China:Evidence from magma differentiation and zircon geochemistry

Magmatic-hydrothermal Sn deposits are commonly associated with high silica magmas,but why most global high silica granites do not bear economic Sn ore grades remains unclear.Two crucial factors controlling magmatic-hydrothermal Sn mineralization,including advanced fractionation and depressurization-induced rapid cooling,were revealed in the case study of the Guyong granitic pluton linked with the Xiaolonghe Sn deposit,in the Tengchong block,SW China.The Guyong granitic pluton comprises three petrological facies:less evolved biotite syenogranite,evolved alkali granite and leucogranite,and highly evolved facies(the protolith of greisenized granite).Similar crystallization ages(~77 Ma)and gradual contact between different petrological facies indicate the Guyong granitic pluton records a continuous fractionation process.Monte Carlo-revised Rayleigh fractionation model suggests the fractionation degree of the Guyong pluton is markedly high(>87 wt.%)that can only be achieved by a high initial water(≥4 wt.%)content in the parent granitic magma revealed by rhyolite-MELTS calculation.Advanced degree fractionation causes the first Sn enrichment but it also significantly increases the viscosity of evolved magmas,suppressing the exsolution and transport of hydrothermal fluids.Hence,it must be compensated by the second critical factor:depressurization-induced rapid cooling,reflected by the occurrence of highly metamict zircons in the greisenized granite.The highly metamict feature,indicated by the large full width at half maximum(FWHM)values of zirconν3(SiO_(4)) peak(>19.5 cm^(-1)),suggests these zircons do not experience thermal annealing but rapidly ascend into a shallow cooling environment.Depressurization-induced rapid cooling facilitates exsolution and transport of hydrothermal fluids,interacting with wall rocks and resulting in Sn mineralization.

Discrepant chemical differentiation and magmatic-hydrothermal evolution of high-silica magmatism associated with Pb–Zn and W mineralization in the Lhasa terrane

High-silica(SiO_(2)>70 wt.%)granites(HSGs)are the main source of W,Sn,and rare metals.However,abundant HSGs,temporally,spatially,and genetically associated with Pb-Zn mineralization,in the Lhasa terrane(LT),provided an ideal opportunity to study the key factors responsible for Pb-Zn enrichment,instead of W-Sn enrichment.Here we contribute to this topic through U-Pb dating of zircon and garnet,and whole-rock and Sr-Nd-Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb-Zn and W deposits in the LT.The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U-Pb ages of 77.3±0.9 Ma and hydrothermal garnet U-Pb ages of 75.7±4.8 Ma,which is 10-15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb-Zn polymetallic mineralization in the LT.The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere,which occurred as a result of the collision of the Indian and Asian plates.These HSGs related to Pb-Zn mineralization,with high totalalkalis and low magnesian contents,are enriched in Ba,Th,and Rb,but depleted in Ti,Eu,Sr,and P.They belong to either the S-type,or I-type granites.The Sr-Nd-Hf isotopic compositions of the Pb-Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input.This was consistent with the postulated source of W enrichment in the LT.The Pb-Zn and W related granites have similar zircon-Ti-saturation temperatures,comparable low whole-rock Fe_(2)O_(3)/FeO ratios,and zircon oxygen fugacity.This indicated that the Pb-Zn-W enrichment in the high-silica magma system could be attributed to a relatively reduced magma.The Pb-Zn related HSGs,abundant quartz and feldspar phenocrysts,and weak fractionation of twin-elements in wholerock analysis,can be used to reconstruct a model of the magma reservoir.We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir,with a loss of magmatic fluids.The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb-Zn related magma;however,different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in wholerock are formed by a magmatic-hydrothermal transition in highly evolved system.As the source and oxygen fugacities of the Pb-Zn and W related magmas are similar,the absence of a giant W-Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation.This implies that the rocks that originated as highly evolved silicaterich parent magmas,with a high degree of magmatic-hydrothermal alteration,would need to be targeted for W-Sn mineral exploration in the LT.In summary,our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb-Zn and W enrichment.This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.

Genesis of the Jiajika superlarge lithium deposit,Sichuan,China:constraints from He–Ar–H–O isotopes

The Jiajika granitic-and pegmatite-type lithium deposit,which is in the Songpan-Garze Orogenic Belt in western Sichuan Province,China,is the largest in Asia.Previous studies have examined the geochemistry and mineralogy of pegmatites and their parental source rocks to determine the genesis of the deposit.However,the evolution of magmatic-hydrothermal fluids has received limited attention.We analyzed He–Ar–H–O isotopes to decipher the ore-fluid nature and identify the contribution of fluids to mineralization in the late stage of crystallization differentiation.In the Jiajika ore field,two-mica granites,pegmatites(including common pegmatites and spodumene pegmatites),metasandstones,and schists are the dominant rock types exposed.Common pegmatites derived from early differentiation of the two-mica granitic magmas before they evolved into spodumene pegmatites during the late stage of the magmatic evolution.Common pegmatites have~3He/~4He ratios that vary from 0.18 to 4.68 Ra(mean1.62 Ra),and their~(40)Ar/~(36)Ar ratios range from 426.70 to 1408.06(mean 761.81);spodumene pegmatites have~3He/~4He ratios that vary from 0.18 to 2.66 Ra(mean 0.87Ra)and their~(40)Ar/~(36)Ar ratios range from 402.13 to 1907.34(mean 801.65).These data indicate that the hydrothermal fluids were shown a mixture of crust-and mantle-derived materials,and the proportion of crustderived materials in spodumene pegmatites increases significantly in the late stage of the magmatic evolution.Theδ~(18)OH_(2)O–VSMOWvalues of common pegmatites range from 6.2‰to 10.9‰,with a mean value of 8.6‰,andδDV–SMOWvalues vary from-110‰to-72‰,with a mean o f-85‰.Theδ~(18)OH_(2)O–VSMOWvalues of spodumene pegmatites range from 5.3‰to 13.2‰,with a mean of 9.1‰,andδDV–SMOWvalues vary from-115‰to-77‰,with a mean of-91‰.These data suggest that the ore-forming fluids came from primary magmatic water gradually mixing with more meteoric water in the late stage of the magmatic evolution.Based on the He–Ar–H–O and other existing data,we propose that the oreforming metals are mainly derived from the upper continental crust with a minor contribution from the mantle,and the fluid exsolution and addition of meteoric water during the formation of pegmatite contributed to the formation of the Jiajika superlarge lithium deposit.

The giant Kalgoorlie Gold Field revisited

The Neoarchaean Kaigoorlie Gold Field contains the giant Golden Mile and world-class Mt Charlotte deposits,which have been the subject of much research for over 100 years.The Golden Mile deposit is a complex array of ductile to brittle vein and breccia lodes that are predominantly hosted in the highlyfractionated Golden Mile Dolerite sill.The Fimiston lodes comprise an array of narrow lodes that evolved broadly syn- to late-formation of the regional D2 NW-trending foliation.The lodes are characterized by pyrite veinlets and disseminations,quartz veinlets and breccias,and banded quartz-carbonate veins with sericite,carbonate,and pyrite-dominated alteration.Bonanza Green-Leader,or Oroya-style,lodes,with grades in excess of 1000 g/t Au,are similar to the Fimiston-style lodes,but are characterized by abundant visible gold,native tellurium and more abundant telluride minerals within roscoelitebearing alteration zones.The arguably structurally younger Mt Charlotte-style lodes are characterized by a pipe-shaped,coarse-grained quartz,carbonate and scheelite vein-stockwork with distinct verticallyzoned,carbonate-sericite-albite-pyrite ± pyrrhotite dominant alteration assemblages around veins within Unit 8 of the Golden Mile dolerite and porphyry dykes.The network of steep- and gently-dipping extension and shear fracture-fill veins are associated with NE-trending fault se ts that cross cut the regional NW-trend.The deposit area is intruded by swarms of porphyry dykes,including syn-volcanic mafic dykes,early and volumetrically most significant c.2.67 Ga feldspar-phyric porphyry dykes,as well as later c.2.66-2.65 Ga calc-alkaline hornblende-phyric dykes associated with younger c.2.65-2.64 Ga lamprophyre dykes.All post-volcanic dykes have similar orientations to the Fimiston lodes.The feldspar dykes are clearly overprinted by all styles of mineralization,although the relationship between hornblende-phyric and lamprophyre dykes and gold mineralization is more ambiguous.Most agree that gold mineralization was post-peak regional metamorphism of host rocks,although its relative structural timing is controversial.Direct timing constraints on gold mineralization indicate that Fimiston- and Mt Charlotte-style mineralization formed within a relative short period of time around 2.64 Ga,and,as such,support a model of progressive deformation of a Theologically heterogeneous rock package late in the structural history.Fluid characteristics,combined with the structural,metamorphic and absolute timing,support description of gold mineralization at the Golden Mile as orogenic and mesozonal,and this allows direct correlation with orogenic gold deposits worldwide,which classically formed during accretion along convergent margins throughout Earth history.

Evolution of the magmatic–hydrothermal system and formation of the giant Zhuxi W–Cu deposit in South China

The Zhuxi deposit is a recently discovered W–Cu deposit located in the Jiangnan porphyry–skarn W belt in South China. The deposit has a resource of 3.44 million tonnes of WO3, making it the largest on Earth,however its origin and the evolution of its magmatic–hydrothermal system remain unclear, largely because alteration–mineralization types in this giant deposit have been less well-studied, apart from a study of the calcic skarn orebodies. The different types of mineralization can be classified into magnesian skarn, calcic skarn, and scheelite–quartz–muscovite(SQM) vein types. Field investigations and mineralogical analyses show that the magnesian skarn hosted by dolomitic limestone is characterized by garnet of the grossular–pyralspite(pyrope, almandine, and spessartine) series, diopside, serpentine,and Mg-rich chlorite. The calcic skarn hosted by limestone is characterized by garnet of the grossular–andradite series, hedenbergite, wollastonite, epidote, and Fe-rich chlorite. The SQM veins host highgrade W–Cu mineralization and have overprinted the magnesian and calcic skarn orebodies. Scheelite is intergrown with hydrous silicates in the retrograde skarn, or occurs with quartz, chalcopyrite, sulfide minerals, fluorite, and muscovite in the SQM veins.Fluid inclusion investigations of the gangue and ore minerals revealed the evolution of the ore-forming fluids, which involved:(1) melt and coexisting high–moderate-salinity, high-temperature, high-pressure(>450 ℃and >1.68 kbar), methane-bearing aqueous fluids that were trapped in prograde skarn minerals;(2) moderate–low-salinity, moderate-temperature, moderate-pressure(~210–300 ℃and ~0.64 kbar),methane-rich aqueous fluids that formed the retrograde skarn-type W orebodies;(3) low-salinity,moderate–low-temperature, moderate-pressure(~150–240 ℃and ~0.56 kbar), methane-rich aqueous fluids that formed the quartz–sulfide Cu(–W) orebodies in skarn;(4) moderate–low-salinity,moderate-temperature, low-pressure(~150–250 ℃and ~0.34 kbar) alkanes-dominated aqueous fluids in the SQM vein stage, which led to the formation of high-grade W–Cu orebodies. The S–Pb isotopic compositions of the sulfides suggest that the ore-forming materials were mainly derived from magma generated by crustal anatexis, with minor addition of a mantle component. The H–O isotopic compositions of quartz and scheelite indicate that the ore-forming fluids originated mainly from magmatic water with later addition of meteoric water. The C–O isotopic compositions of calcite indicate that the ore-forming fluid was originally derived from granitic magma, and then mixed with reduced fluid exsolved from local carbonate strata. Depressurization and resultant fluid boiling were key to precipitation of W in the retrograde skarn stage. Mixing of residual fluid with meteoric water led to a decrease in fluid salinity and Cu(–W) mineralization in the quartz–sulfide stage in skarn. The high-grade W–Cu mineralization in the SQM veins formed by multiple mechanisms, including fracturing, and fluid immiscibility, boiling, and mixing.

Thermal History of the Naruo Porphyry Deposit in the Duolong Ore District,Western Tibet:Evidence from U-Pb,^(40)Ar/^(39)Ar and(U-Th)/He Thermochronology

The Naruo porphyry copper deposit containing more than 2 Mt of copper is located in the Duolong ore district in the west of the Bangongco–Nujiang belt in central Tibet.New zircon U-Pb,biotite^(40)Ar/^(39)Ar,zircon(U-Th)/He ages,published age data together with thermal modeling were presented in this paper to investigate the thermal history of Naruo deposit.Thermal modeling reveals a prolonged magmatic-hydrothermal evolution firstly cooling from~700℃to~350℃at 120 Ma,then cooling to 230℃at 106 Ma and maintaining at 200℃from 106 to 90 Ma which is attributed to multiple magmatic events and thermal effect of strike-slip fault.Affected by thrust nappe structure,the sample was consistent with 120℃from 70 to 63 Ma.The Naruo deposit started to experience exhumation at a rate of~0.07 km/Myr since 60 Ma which is related to India-Asia collision.The prolonged magmatic-hydrothermal evolution process might have important influence on the Naruo deposit.The ore-related intrusions preserved in the foot walls of strike-slip fault and thrust nappe structure are the objects of future exploration in the Duolong ore district.

A new type U-Th-REE-Nb mineralization related to albitite:A case study from the Chachaxiangka deposit in the northeastern Qaidam Basin of China

The U-Th-REE-Nb(Ta)-polymetallic mineralization is generally related to either the silica-undersaturated syenites,the silica-oversaturated alkaline/peralkaline granites or igneous carbonatites.In this study,the authors report a new mineralization type,which is related to the magmatic-hydrothermal albitite(with mineral assemblage predominated by albite with volume content>90%),as exemplified by the Chachaxiangka deposit in Qinghai Province of China.The Chachaxiangka deposit is the first albititerelated U-Th-REE-Nb deposit recognized in China and the mineralization can be divided into 3 types:the vein-type,the disseminated veinlet type and breccia type,of which the former 2 are predominant.Three mineralization stages can be identified according to the detailed mineralogical analyses,including the magmatic stage,main hydrothermal mineralization stage and post-ore stage.By comprehensive analyses of the mineralogical,major and trace element compositions,the authors suggest that the albitite vein is magmatic-hydrothermal in origin and both the magmatic evolution and overprint of the hydrothermal fluids play important roles in the formation of the albitite and related polymetallic mineralization.Phase separation between the silicate melt and carbonate/phosphate melt might take place in the magmatic stage,yet the immiscibility between the silicate melt and chloride-dominated fluids is the most important mechanism for the REE mineralization and also causes the Nb-Th re-mobilization and enrichment.The red color of the albitite aplite vein is an eye-catching prospecting mark in the field and more mineralization can be expected at depth and in the surrounding areas.The discovery of the new albitite type U-Th-REENb mineralization give rise to new ideas during future U-Th-REE-Nb exploration,not only in the Qaidam-Altun belt,but also other areas across China.

In situ LA-ICP-MS analyses of mica and wolframite from the Maoping tungsten deposit,southern Jiangxi,China

The Maoping tungsten deposit is located in the Nanling W-Sn metallogenic belt in South China.Greisen and quartz vein types of mineralization developed in this deposit.Protolithionite occurs in the granite.Zinnwaldite is occurs mainly in greisen and wolframite-quartz veins whereas phengite is found in the underground quartz veinlets.In granite and greisen,protolithionite,and zinnwaldite are partly replaced by Li-phengite.LA-ICP-MS trace element analyses of micas and wolframite are employed to characterize the ore-forming source and evolution of ore forming fluids.Micas show compositional variation trend in vertical directions with a decrease of W,Sn,Nb,and Ta and an increase of MgO,V,Ni,and Co.Wolframite in greisen has higher Mo,Sn,Nb,Ta,and REEs than those in quartz veins.All wolframites show similar REE patterns with enrichment of HREE.Wolframites in greisen and quartz veins have negative Eu anomalies,while wolframites in quartz veinlet display positive Eu anomalies.Compositions of mica and wolframite from different mining levels of the Maoping deposit suggest that the ore-forming fluids are dominated by magmatic hydrothermal fluids in the deep with a slight addition of meteoric water in the shallow.Brittle fractureinduced depressurization and fluid mixing controlled the evolution of ore-forming fluids and possibly lead to the wolframite deposition.

A Discussion on the Two-stage Mineralization of the Baguamiao-Shuangwang Gold Orefield, Shaanxi Province, China

The gold orefield studied is located on the south border of the underthrust-collision zone of the Qinling microplate and the North China microplate in the Indosinian epoch. The main ore deposits localized in the area where the WNW-trending compression-shear type fault of the Indosinian epoch intersected the NE-trending tenso-shear type fault of the Yanshanian epoch. The orebody appeared in a chambered or wedged form. The mineral composition is relatively complex. On the southeastern border of the orefield there have developed intermediate-acid anatectic magmatites of the Mesozoic Era. Three-phase inclusions (Lco2, Vco2 and LNaCl-H2O) comprise over 50%, associated with vapor phase consisting of H2O, CO2, CO, CH4, N2 and H2. The ore-forming fluids can be divided into 2 stages (the early and the late). The samples are projected in the area of mixture of initial magmatic and meteoric water on the δD-δ18O diagram, suggesting two types of mineralization, i.e. the re-equilibrated magmatic-hydrothermal type and the circulating geothermal water type of a meteoric water source. The mineralization occurring in this orefield might be a superimposition of 2 tectono-magmatic activities (the Indosinian and Yanshanian movements). Therefore, it is considered a superimposed B-S type gold orefield.

Qiyugou auriferous breccia pipes: A low-sulphidation epithermal system linked with transition of tectonic regime from collisional compression to extension, Xiong ershan, Henan, China

The Xiong’ershan area,as a part of a metallogenic province in cetral China,contains substantial Leimengou porphyry molybdenite deposit,Qiyugou breccia gold deposits and vein gold deposit occurred in fault zone,as well as previously refer to as orogenic gold deposit.Around or nearby the Mesozoic porphyry granite,the typical porphyry deposit,much more breccia pipes and breccia gold deposit are developed in the area.In this paper we focus on reporting preliminary results obtained from field,petrographic work on the Leimengou porphyry molybdenite and the Qiyugou breccia gold deposit,as well as laser Raman microspectroscopy and microprobe analyses carried out on samples from the Au-bearing Qiyugou breccia pipes.An adularia-calcite assemblage is reported in these breccia gold deposits.Two stages of hydrothermal alteration,pervasive and vein,are identified.The first stage consists of the alteration of the breccia clasts and rock flour by actinolite,green biotite,epidote,chlorite and minor sericite.This hydrothermal activity is likely to relate to the molybdenite mineralisation episode.The second stage is characterised by quartz veining and adularia and calcite filling open-spaces.Vein ores associated with quartz,sericite,and minor calcite alteration occur in NE-trending fault that cut the first stage of alteration.The adularia-calcite alteration is genetically associated with gold mineralization.The adularia-calcite assemblage in the area suggest that the Qiyugou pipes are of low-sulphidation epithermal in nature.The Qiyugou breccia pipes are coeval and spatially associated with the nearby Leimengou porphyry Mo deposit,which together conform to an alkalic mineralising system.We propose a model for the development and evolution of the breccia pipes in conjunction with a model that describes the genesis of the porphyry Mo and auriferous breccia pipes as a unified magmatic-hydrothermal system,which is linked with transition from collisional compressional to extensional geodynamic regime.

Lithium Isotope Analytical Methods and Implications for Rare-Metal Mineralization in Granite-Pegmatite Systems:An Overview

The origin of highly-fractionated granite-pegmatite systems and their associated rare metal mineralization has been widely studied,but there is still ongoing debate.Prevailing hypotheses suggest that pegmatite formation and the associated rare metal mineralization are closely related to aqueous fluid processes.Lithium(Li)isotope analysis has been widely applied to trace granite-pegmatite evolution.This is because lithium is widely present in various minerals(e.g.,mica,tourmaline)that record the melt and fluid compositions,and lithium isotopes are sensitive to magmatic-hydrothermal processes.We briefly review the methodology of Li isotope analyses,the mechanisms of Li isotopic fractionation,and,in particular,Li isotope fractionation in granite-pegmatite system based on Li isotope data we have collected and the latest developments in Li isotope geochemistry.With the development of analytical technology,high-precision measurement of the Li content and isotopic compositions have facilitated a series of scientific breakthroughs in understanding the magmatic-hydrothermal evolution of rareelement ore deposits.Li isotope analyses on bulk mineral separates have demonstrated their ability to trace various hydrothermal processes.In situ Li isotope analysis methods has been enhanced by the development of new,homogeneous mineral reference materials.In situ SIMS and LA-MC-ICP-MS Li isotope measurements on minerals(e.g.,tourmaline)will likely become more important in studying the fluid-rock interactions in magmatic,metamorphic,and hydrothermal processes,as well as on pegmatite petrogenesis and rare-metal mineralization.

Role of Magmatism and Related-Exsolved Fluids during Ta-Nb-Sn Concentration in the Central Eastern Desert of Egypt:Evidences from Mineral Chemistry and Fluid Inclusions

The rare metals of Abu Dabbab area in the Central Eastern Desert of Egypt have been investigated for their mineralogy and conditions of precipitation using combination of EMPA and fluid inclusions studies,in order to delineate the source,mechanism of formation and evolutionary model for these economic metals.The(Ta-Nb-Sn)-bearing minerals at the Abu Dabbab area include columbite group minerals(CGMs),wodginite and cassiterite.In both granitic intrusion and its enclosed quartz veins,most of zoned CGMs and cassiterite grains are commonly characterized by a well-developed twostage texture.Hence,columbite-(Mn)(CGM-Ⅰ)represents the early formed phase of CGMs that is characterized by high Mn#values(0.64-0.92)with quite low Ta#values(0.13-0.49).It was invaded by Ta-rich phases including tantalite-(Mn)(CGM-Ⅱ;Ta#=0.13-0.49)and wodginite,which contain high Ta_(2)O_(5)and SnO_(2)(17.91 wt.%).In regard to cassiterite,there are distinct compositional differences between the early-phase cassiterite(Cst-Ⅰ)and the late-phase one(Cst-Ⅱ),where the latter is enriched in Ta_(2)O_(5),Nb_(2)O_(5)and FeO.The chemistry and textural criteria of the early stage CGM-Ⅰand Cst-Ⅰ,all are indicative of magmatic origin.While,the latter CGM-Ⅱ,wodginite and Cst-Ⅱwere influenced by the late magmatic Ta-rich fluids.Fluid inclusions microthermometry shows criteria of phase separation represented by both boiling and fluid immiscibility.The initial fluid was supposed to be of magmatic origin(magmatic CH4),that was consequently influenced by fluid mixing/dilution with post-hydrothermal/meteoric water with respect to the decompression process during uplift.Isochore construction gave rise to an estimate P-T conditions(T=330-370℃,P=22-50 MPa).The fluid inclusions’microthermometry supports a transition between magmatic and late to post-hydrothermal activities in addition to surface-derived fluid(meteoric fluid?)in a part as main source for the polymetallic deposits.

Iron Isotopes and Trace Element Compositions of Magnetite from the Submarine Volcanic-Hosted Iron Deposits in East Tianshan,NW China:New Insights into the Mineralization Processes

The Aqishan-Yamansu metallogenic belt(AYMB)in East Tianshan hosts abundant sub-marine volcanic-hosted iron deposits.Although there is agreement with the magmatic source of the ore-forming materials and the role of hydrothermal replacement in iron ore formation,the mineraliza-tion processes of these iron deposits remain uncertain.Three ore types are identified on the basis of the geological occurrences of minerals and the sequence of mineral in ores.The typeⅠores are characte-rized by magnetite,diopside,amphibole with a few pyrite,and chalcopyrite.The type II ores are mainly composed of magnetite,garnet,chlorite with a few pyrite,while the type III ores are mainly composed of magnetite,quartz,calcite with a few pyrite.In order to constrain the mineralization processes of these ore types,we performed iron isotopes and trace element compositions of magnetite from three typical iron deposits(Yamansu,Duotoushan and Luotuofeng).Trace element and Fe isotope investiga-tions of the three ore types reveal two major groups.The groupⅠconsists of analyses of the typeⅠandⅡores,with both showing a narrow range of positiveδ56Fe values(+0.08‰to+0.22‰for typeⅠores and+0.15‰ to+0.22‰ for typeⅡores)and plotting in the range of the ortho-magmatic field.In contrast,the group 2 is composed merely of the typeⅢores,showing a wider range of negativeδ56Fe values(-0.49‰ to-0.01‰),which is similar to the features of Fe-skarn magnetite.As shown in the binary dia-grams of magnetite trace elements and a fractionation of the Fe isotopes,different ore types were likely produced during gradually changing ore-forming stages from magmatic to hydrothermal.Collectively,the submarine volcanic-hosted iron deposits in the East Tianshan are likely the results of a continuous magmatic-hydrothermal mineralization process.

Petrology,Geochemistry,and Sr-Nd-S Isotopic Compositions of the Ore-Hosting Biotite Monzodiorite in the Luanjiahe Gold Deposit,Jiaodong Peninsula,China

The Jiaodong Peninsula is one of the most important Au ore provinces in China.There is an ongoing debate on the correlation between ore formation and magmatism in this province,because few intrusive rocks exhibit a clear association with ore deposits.A mineralized biotite monzodiorite(BM)stock,with disseminated ore,pervasive phyllic alteration,and no deformation,was found in a borehole in the footwall of the Zhaoping fault within the Luanjiahe Au deposit,which may shed light on this debate.The biotite monzodiorite contains explosion breccias,miarolitic cavities,skeletal and den-dritic quartz,and late-stage evolved aplite dikes,and the in-situδ34S values of the disseminated pyrite which is associated with Au mineralization are-1.7‰ to 7.3‰(mean=3.5‰),indicative of a magmatic-hydrothermal system.These findings,combined with the reported age of 123 Ma,show that the intru-sion has close spatial,temporal,and geochemical relationships with Au mineralization in the area.The biotite monzodiorite is metaluminous,high-K calc-alkaline and shoshonitic,with enrichment in light rare earth elements(REEs)and large-ion lithophile elements(LILEs),depletion in high-field-strength elements(HFSEs),and enriched Sr-Nd isotopic compositions.The intrusion may be the product of par-tial melting of enriched lithospheric mantle with a small lower crustal component.The hydrous,Au-bearing,enriched mantle source,and the strongly oxidized magma that was generated,created fa-vorable conditions for Au mineralization.

Gold solubility in silicate melts and fluids:Advances from high-pressure and high-temperature experiments

The solubility of Au in silicate melts and fluids governs the enrichment and migration of Au during the formation of magmatic-hydrothermal Au deposits.Large Au deposits require vast amounts of Au to migrate from the upper mantle-lower crust to the shallow crust,and high Au solubility in magma and hydrothermal fluid facilitates the formation of Au-rich magma and fluid in the crust and mantle source and efficient transport.This paper reviews recent high-pressure and high-temperature experimental studies on Au species in magmas and hydrothermal fluids,the partitioning behavior of Au between silicate melts and fluids,and the effects of temperature,pressure,oxygen fugacity,sulfur fugacity,silicate melt composition,and volatiles(H2O,CO2,chlorine,and sulfur)on the solubility of Au in magma.We show that the solubility of Au in magma is largely controlled by the volatiles in the magma:the higher the content of reduced sulfur(S2-and HS-)in the magma,the higher the solubility of Au.Under high-temperature,high-pressure,H2O-rich,and intermediate oxygen fugacity conditions,magma can dissolve more reduced sulfur species,thus enhancing the ability of the magma to transport Au.If the ore-forming elements of the Au deposits in the North China Craton originate from mantle-derived magmas and fluids,we can conclude,in terms of massive Au migration,that these deep Au-rich magmas might have been generated under H2 O-rich and moderately oxidized conditions(S2-coexists with S6+).The big mantle wedge beneath East Asia was metasomatized by melts and fluids from the dehydration of the Early Cretaceous paleo-Pacific stagnant slab,which not only caused thinning of the North China Craton,but also created physicochemical conditions favorable for massive Au migration.