PETROLOGY AND GEOCHEMISTRY OF INTRUSIVE ROCKS OF THE SINYUKHINSKOYE DEPOSIT (GORNY ALTAI)
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Abstract:
New data on phase relationships, petrology and geochemistry of the Sinyukhinskoye rocks and dikes of the same deposit are presented. Five phases of rock intrusions formed in a homodromic-antidromic sequence are found. The Sinyukhinskoye rocks fall in the oxidized type. In the depth, a source of melting was garnet lherzolites, and, to a lesser extent, spinel lherzolites of an enriched mantle source of the lithospheric slab with a degree of partial melting from 0.2 to 0.4 and low pressure. The formation of intrusive rocks of the deposit took place in the post-collisional environment. The magmatite fluids of the massif and dikes were characterized by high partial pressures of such volatile components as H2O, Cl, F, CO2, which were the main carriers of metals. The rocks exhibit the tetrad effect of the W-type REE fractionation. The dependences of the values of the latter and the concentrations of copper and gold in igneous rocks and fluids were determined by different regimes of acidity and alkalinity of the medium. The deposit belongs to a complex family of ores of the copper-gold-skarn-porphyry type. In the Cheryomukhovaya Sopka and Chir areas, this is also confirmed by the presence of the copper-gold-porphyry type in the ore field, in addition to the gold-copperskarn mineralization.Keywords:
Massif
Dike
Metasomatism
Metasomatism
Tourmaline
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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Diopside
Andradite
Actinolite
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In the Yerington district of western Nevada, large bodies of skarn formed in a limestone unit of Triassic age adjacent to a Jurassic batholith. This batholith is dominantly granodiorite but contains quartz monzonite stocks and dike swarms that host major porphyry copper deposits. The units enclosing the limestone, an andesitc tuff and a silty limestone, were also extensively metasomatized, as were outlying intrusions of granodiorite.Skarn formation is divided into an early skarnoid stage and a late metasomatic skarn stage. The skarnoid stage formed massive fine-grained granditc garnet in the andesitc tuff, limestone, and silty limestone, with subordinate amounts of pyroxene. The iron content of garnets increases significantly away from the granodiorite, whereas pyroxenes maintain a relatively constant diopsidic composition.Granodiorite apophyses, which occur in the Triassic rocks up to 0.5 km from the main batholith contact, are extensively altered to endoskarn. The main alteration type consists of massive, anhedral granditc garnet with a significant titanium component. The distribution of endoskarn suggests that proximity to limestone was a key factor in endoskarn formation and that one hydrothermal fluid altered both sedimentary and igneous rocks simultaneously.The second stage of alteration most dramatically affected the limestone, forming coarse-grained skarn. Two skarn types are recognized: (1) a magnesium-rich type in dolomitized marble, dominated by pyroxene, and (2) an iron-rich type in calcite marble, dominated by andradite. Age relations between the two are ambiguous, but constraints inferred from T-X (sub CO 2 ) stability relations suggest that magnesium-rich skarn formed first and at high temperatures, followed by iron-rich skarn. During a late stage, intermediate granditc garnet veined both skarn types, and actinolite (+ or - salite) formed locally at the andradite-marble contact.In detail, magnesium-rich skarn nearest the granodiorite is zoned from diopside through serpentine (replacing diopside) + calcite, then from clinohumite + calcite + dolomite to calcite + dolomite marble. Additional phases include monticellite, spinel, magnetite, ludwigitc, and szaibelyite. Farther from the batholith, clinohumite is absent, tremolite separates serpentine from pyroxene, and periclase occurs locally.Pyroxene and serpentine are enriched in iron with distance from the granodiorite, the pyroxene changing from nearly pure diopside to intermediate smite a kilometer away. This compositional variation is consistent with an increase in mu Fe /mu Mg , with time and with distance from the batholith. The presence of magnetite pseudomorphs after hematite and the variation in magnetite composition indicate that the oxidation state decreased with time near the granodiorite.Iron-rich skarn consists of andradite garnet, which on the outer skarn contact directly replaced calcite. Inclusions of wollastonite noted in garnet cores from one sample suggest that initially a wollastonite zone was locally present at the skarn margin. Veins of actinolite and magnetite cut garnet skarn; late actinolite at the garnet-marble contact, locally a site of sulfide deposition, suggests a reversal in the trend toward increasing mu Fe /mu Mg . At the Douglas Hill mine, apatite, quartz, and sulfides replaced andradite.Mineral assemblages and fluid inclusion data indicate that both magnesium- and iron-rich skarn formed at low X (sub CO 2 ) , generally less than 0.1. Temperatures of formation of magnesium-rich skarn are inferred to have been in the range 650 degrees to below 400 degrees C. Temperatures of formation of iron-rich skarn are not constrained by observed assemblages. Fluid inclusion data show that late apatite replaced andradite at temperatures between 120 degrees and 200 degrees C.
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There are many bedded skarns occurring as continuous beds are conformable with the wall rocks in which there are no zonings of typical contact metasomatism skarns.There are not characteristics of contact metasomatism skarn in the bedded skarn,which is mainly composed of garnet,diopsite,tremolite and chlorite.It's proved that the formation of the bedded skarn is not due to the contact metasomatism between the magmatic intrusion and the carbonate rocks,but the superimposed metasomatism in which the sedimentary rock or submarine volcanic sedimentary rock(basalt) of the Indo-Chinese epoch is reformed by the granitic intrusion of the Yanshanian epoch.
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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.
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Skarns, ores and hydrothermally metasomatic rocks associated with some major skarn iron deposits in China contain abundant volatile components, such as F, Cl and H2O. Alkaline (sodic or potassic) metasomatism is obviously evident in the magmatic and other alumo-silicate wall rocks. They may serve as important ore-searching indicators. In this paper, the probable source of iron fluids, transport forms of iron and conditions of precipitation of magnetite are also discussed. From the studies of major skarn iron deposits in China, the authors hold that volatile components, such as F, Cl, H2O, etc., and alkaline (K, Na) metasomatism play a very important role in the formation of this type of iron deposits[1, 2, 3].
Metasomatism
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Iron ore
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Metasomatism
Scheelite
Phlogopite
Pegmatite
Hornfels
Amphibole
Ore genesis
Diopside
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Metasomatism
Greisen
Scheelite
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