Alteration, mineralization, and genesis of the Lietinggang–Leqingla Pb–Zn–Fe–Cu–Mo skarn deposit, Tibet, China
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Metasomatism
Andradite
Titanite
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The Dunde iron deposit,discovered in recent years,is one of the large-scale iron deposits in the Awulale ore belt in Xinjiang. The ore body is hosted in the metamorphosed volcaniclastic-sedimentary sequence of Dahalajunshan Formation of Lower Carboniferous,and is obviously controlled by fracture and closely associated with skarn. Detailed study on main skarn minerals such as garnet,diopside and magnetite by means of electron microprobe analysis and microscopic observation show that the skarn from the Dunde iron deposit can be divided into two types,i. e. volcanic metamorphic skarn and hydrothermal metasomatic skarn. Volcanic metamorphic skarn is composed of garnet( mainly grossularite) and augite( mainly diopside) and was formed in a reductive environment. Hydrothermal metasomatic skarn is composed of garnet( mainly andradite) and augite( mainly diopside),and was formed in a oxidizing environment. Volcanic metamorphic stage magnetite contains high Al2O3,TiO2and low MgO,belonging to magmatic origin type. Hydrothermal metasomatic stage magnetite contains high Al2O3,MnO,low MgO,TiO2,belonging to hydrothermal metasomatic type. The skarn has similar total REE content to that of the andesitic tuff in the stratum,and both show similar patterns. The above characteristics indicate that the skarn in the Dunde iron deposit might be resulted from thermal metamorphim and hydrothermal metasomatism. The iron ores and skarn spatially coexist and have experienced similar development and evolution.
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Abstract The Aghbolaq skarn deposit is located in the Urumieh‐Golpayegan plutonic belt, NW Iran. The garnetite skarn (stage I) has been intensely cross‐cut by the magnetite‐garnet skarn (stage II) which were, in turn, cut and offset by the ore‐hosting quartz veins/veinlets (stage III). The predominance of andradite (Adr 82.5–89.1 ) and its high Fe 3+ /Al ratio (up to 1685) apparently supports the high f O 2 , salinity and prevalence of magmatic/hydrothermal fluids involved, rather than meteoric waters, during the magnetite‐garnet skarn formation. Two major groups of fluid inclusions, namely aqueous (LV, LVS) and aqueous–carbonic (LV C , LL C V C ), were recognized in garnet and quartz veins that, especially in growth zones and along intra‐granular trails, better display fluid inclusion assemblages (FIAs) than those in clusters. The prograde magnetite‐garnet skarn was formed by the metasomatic fluid at relatively high T h (209–374°C), under a lithostatic pressure of ∼200 bars. The retrograde mineralized quartz veins were formed at temperatures ranging from 124°C to 256°C, by dilute and less saline (2.57–11.93 wt% NaCl eq.) hydrothermal fluids under a hydrostatic pressure of ∼80 bars. The fluid evolution of the Aghbolaq skarn began with an earlier simple cooling of metasomatic fluid during the prograde stage, followed by the later influx of low salinity meteoric fluids during the retrograde stage.
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This paper presents in situ microanalysis data of garnets form the Jinshandian iron skarn deposit, Hubei Province. Studies show that garnets from the Jinshandian iron skarn deposit can be divided into two stages, i.e., early Al-rich garnets mainly with grossular and grossular-andradite series, and late Fe-rich garnets dominated by andradite, with the variation implying the increase of the fluid oxidation. Compared with the early garnets, the late stage are rich in large ion lithophile elements and high field strength elements as well as REE. Early grossular shows typical HREE-enrichment and LREE-depletion features, while smaller fractionation between HREE and LREE characterizes grossular-andradite series. Total REE content, δEu and HREE/LREE fractionation degrees of Fe-rich garnet samples vary from sample to sample, and are even different in different parts of a single garnet grain, suggesting that the process of its formation was not stable, and fluid properties changed greatly, possibly due to the addition of evaporate minerals from the wall rocks. Garnet in situ microanalysis research also suggests that the evaporate minerals added into the Jinshandian skarn system had features of heterogeneity and periodicity.
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The Huanggang deposit is the most important and largest skarn Fe–Sn polymetallic deposit in the Southern Great Xing’an Range of Northeast China. Cassiterite, magnetite, and other metal minerals are related to the garnets within skarn systems. The zoned garnets from various skarn stages are able to record numerous geological and mineralizing processes including variations in physicochemical conditions and hydrothermal fluid evolution. In this contribution, we present the mineralogy, systematic major, trace, and rare earth element (REE) concentrations of zoned garnets from the Huanggang Fe–Sn polymetallic skarn deposit. The in situ analytical results of garnets in the prograde skarn stage from andradite core (Grt I) to grossular rim (Grt II) reveal that core sections were formed from a fluid that was generally LREE-rich, with relatively high ∑REE, high LREE/HREE ratios, and weak negative Eu anomalies, whereas rim sections were crystallized from a fluid that was typically HREE-rich, with relatively low ∑REE, low LREE/HREE ratios, and obviously negative Eu anomalies. The garnets of the retrograde skarn stage from Fe3+-rich andradite core (Grt III) to andradite rim (Grt IV) demonstrate that the core sections have a flat trend with high ∑REE and obvious negative Eu anomalies, whereas rim sections were formed from a fluid with relatively low ∑REE, HREE-rich and obviously negative Eu anomalies. The garnets from the prograde skarn stage principally display relatively lower U and higher Y and F concentrations than those from the retrograde skarn stage. Based on optical and textural characteristics, REE patterns, Eu anomalies, and trace element variations in zoned garnets, it can be shown that, during skarn formation, Huanggang hydrothermal fluids shifted from near-neutral pH, oxidizing conditions, and high W/R ratios with relatively low LREE/HREE ratios characteristics to acidic, reducing conditions, and low W/R ratios with relatively high LREE/HREE ratios characteristics. We infer that variations in fluid compositions and physicochemical conditions may exert major control on formation and evolution of garnets and skarn hydrothermal fluids.
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