Magnetite and garnet trace element characteristics from the Chagangnuoer iron deposit in the western Tianshan Mountains,Xinjiang,NW China:Constrain for ore genesis
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Located in the eastern Awulale metallogenic belt of western Tianshan Mountains,the large-scale Chagangnuoer iron deposit is hosted in the andesite and andesitic volcaniclastics of the Lower Carboniferous Dahalajunshan Formation,with one lentoid marble as footwall rock beneath the main ore bodies which exhibit as lamellar,stratoid and lenticular.The alteration zonation is similar with typical hydrothermal deposits.According to ore fabric and mineral paragenesis,this deposit can be divided into two ore-forming stages,which are magmatic stage and hydrothermal stage(included prograde sub-stage and quarts-sulfide sub-stage).In the magmatic stage,REE in magnetite is very low,rich in LREE and HREE but depleted in MREE with a U type pattern.In addition,this kind of magnetites has a higher Ti,V,Cr,indicating that Fe might come from the crystallization differentiation of andesitic magma.On the other hand,in the prograde sub-stage,magnetites have a lower REE content,a bit rich in LREE but other REE strongly depleted.Compared with the magnetites in magmatic stage,these magnetites are poor in Ti,V but a bit abundant in Ni,Co and Cu content.Garnets in barren and ore-bearing skarn distribute the same REE patterns,having a relatively high REE content,enriched in HRRE but depleted in LREE,and with a not pronounced positive Eu anomaly,which displays the feature of garnet with metasomatic origin in the calcic skarn.And this hints that the magnetites,which have a paragenesis relationship with ore-bearing garnets,should be also a product of hydrothermal fluid replacement with wall rocks,and most of the mineralizing materials(Fe) probably are derivate from andesitic strata.In combination geological characteristics with trace element geochemistry,we hold that the Chagangnuoer iron ore is probably one polygenetic deposit with the skarn type(predominated) superposition upon the magmatic type.Keywords:
Metasomatism
Paragenesis
Ore genesis
Igneous differentiation
Trace element
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Systematic studies have been carried out on REE compositions of hydrothermai alterated marbles, massive garnet skarns and ores, as well as garnet and quartz formed in multi-stage skarn and deposit formation. The results show that strata-bound skarn and its related Cu deposits were formed in a complex hydrothermai system during multi-sage infiltration metasomatism. The REE characteristics of skarns, however, are similar to those of the marbles. They were not simply inherited from the marble protolith, but were controlled by garnets, and the latter's were determined by the hydrothermai solutions participating infiltration metasomatism. The related hydrothermai fluids characterized with slightly right-hand dip REE patterns, are enriched in LREE and have strong negative Eu anomalies. The uptake of REE from hydrothermai fluid during garnet crystallization resulted in that garnet shared the similar REE patterns. REE in skarns were provided mainly by hydrothermai fluids associated with the emplacement of granodiorite plutons in the mining district. Based on the dimensional variation of REE in skarns combining with the structures of hosting strata, the transporting route and pattern of fluids involved in the formation of skarns and related deposits can be better understood. The present study demonstrates that extensive REE metasomatism can occur in hydrothermai system at shallow to intermediate depths and REE geochemistry are useful in discerning the origin of skarns and skarn-related deposits.
Metasomatism
Protolith
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Felsic
Greenschist
Banded iron formation
Ore genesis
Porphyritic
Metasomatism
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Ore genesis
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The Hongyuntan iron deposit is hosted in pyroclastic rocks of the Lower Carboniferous Yamansu Formation.The ore bodies occur as layers,stratoid bodies or lenses.The principal ore mineral is magnetite,together with minor maghemite,specularite,pyrite and trace chalcopyrite.The gangue minerals include garnet,diopside,actinolite,chlorite,tremolite,epidote,biotite,albite and quartz.The ore structures are mainly of massive and disseminated forms,with occasional banded or veined forms.The ore textures are of subhedral-anhedral granular and metasomatic types.The wall rock alteration shows symmetrical zoning,and the alteration colors change from dark to light from ore bodies outwards.On the basis of observed mineral assemblages and ore fabrics,two periods of ore deposition were recognized,i.e.,skarn period and hydrothermal ore-forming period,which could be further subdivided into four metallogenic stages,namely skarn stage,retrograde alteration stage(main ore-forming stage),early hydrothermal stage and quartz-sulfide stage.Electron microprobe analyses show that the end member of garnet is mainly andradite-grossularite.The composition of pyroxene is mainly diopside-asteroite.The amphiboles is composed mainly of actinolite and tremolite with minor magnesiohornblende.The composition of these skarn minerals suggests that skarn in the Hongyuntan iron deposit is calcic skarn,belonging to metasomatic skarn.The characteristics of main and trace elements suggest that the formation of magnetite was closely related to the skarn.In combination with geological characteristics,the authors suggest that the skarn might have resulted from interaction between Ca-rich pyroclastic and Fe-rich magmatic hydrothermal fluid which was transported along the fault system.The formation of magnetite was hence related to the regressive metamorphism of the skarn.
Metasomatism
Actinolite
Ore genesis
Andradite
Amphibole
Tremolite
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Metasomatism
Ore genesis
Breccia
Magmatic water
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The Chagangnuoer and Zhibo iron deposits occurring in eastern segment of western Tianshan,were hosted primarily by the Late Carboniferous volcanic rocks,which mainly contain basalt,basaltic andesite,trachyte and andesitic tuff.The tectonic setting and age of the volcanic rocks are key elements in the reconstruction of ore genesis.In this paper,representative samples of volcanic rocks from the Chagangnuoer and Zhibo iron deposits were analyzed for major,trace elements and zircon U-Pb dating to validate their tectonic setting and formation ages.Geochemical data indicate that most of the volcanic rocks range in composition from calc-alkaline,high-K calc-alkaline to shoshonitic,with light rare earth element(LREE)and large ion lithophile element(LILE;e.g.,Rb,Th,K)enrichment and almost flat heavy rare earth element(HREE)patterns as well as strongly negative Nb,Ta and Ti anomalies,similar to features of arc volcanic rocks.In the plots of tectonic discrimination,the basaltic volcanic rocks are mainly projected in arc setting regions.LA-ICP-MS U-Pb dating of magmatic zircons from the rhyolite and dacite yield weighted average 206Pb/238U ages of 301.8±0.9Ma and 300.3±1.1Ma,respectively.In addition,weighted average 206Pb/238U ages from 303.8Ma to 305Ma were obtained for two diorites.The similar geochemical characteristics and formation ages of volcanic rocks and diorites indicated that they were sourced from the same parent magma and may formed in the same tectonic setting.Combined with the regional geological data,we therefore suggest that the high-K calc-alkaline to shoshonitic volcanic rocks were probably the products of continental island-arc magmatism during the late stage of subduction.
Trachyte
Lithophile
Dacite
Basaltic andesite
Continental arc
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Stockwork
Ore genesis
Metasomatism
Banded iron formation
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Lile
Diorite
Geochronology
Dike
Lithophile
Fractional crystallization (geology)
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Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Sweden's main iron-source, the 'Kiruna-type' apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish 'Kiruna-type' deposit at Grängesberg have δ(18)O values between -0.4 and +3.7‰, while the 1.90-1.88 Ga meta-volcanic host rocks have δ(18)O values between +4.9 and +9‰. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ(18)Omgt > +0.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ(18)Omgt ≤ +0.9‰), in turn, equilibrated with high-δ(18)O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish 'Kiruna-type' ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.
Felsic
Ore genesis
Iron ore
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