Petrology最新文献

The Zhaibeishan copper deposit is located at the eastern part of Aqishan-Yamansu metallogenic belt in eastern Tianshan. Zircon U-Pb geochronology, major and trace element, and Sr-Nd isotopic characteristics of Zhaibeishan granite and its relationship with mineralization have been studied. SHRIMP zircon U-Pb dating indicates an Early Carboniferous intrusive time (334.5 ± 2.6 Ma) of the granite. Chemically, Zhaibeishan granites have high silica (71.50–75.06%), aluminum (A/CNK = 1.02–1.23), sodium (Na₂O/K₂O = 0.95–23.83 with 6.89 on average), and total alkalis (Na₂O + K₂O = 6.65–8.43%), and low magnesium (<1%) and titanium (<1%) contents. The Chondrite-normalized REE patterns are characterized by enrichment of LREE relative to HREE (La_N/Yb_N = 4.05–6.85) with moderate negative Eu anomalies (δEu = 0.38–0.73). The Zhaibeishan granites show enrichment of K, Rb, (Large Ion Lithophile Elements), LREE and depletion of Nb, Ta, Ti, and P (High Field Strength Elements), indicating island arc magmatic characteristics. Sr-Nd isotopic data reveal that the I_Sr values range from 0.70473 to 0.70551, while ε_Nd(T) values range from 2.3 to 3.2. We suggest that the Zhaibeishan granites formed in continental arc setting in subduction zone and were probably derived from the product of magma mixing between crust and mantle magmas and experienced subsequent fractional crystallization. Combined with the fluid inclusion and published ore-forming age and isotopic data, we suggest that porphyry mineralization and blind copper orebodies probably exist in the deep part of the Zhaibeishan copper mining area.

The paper presents the first data on the petrography, mineralogy, and geochemistry of jadeitites from the El’denyr massif, Chukotka, Russia, as well as host metalherzolites and amphibolite inclusions in the jadeitites. The jadeitite is composed of an association of jadeite, omphacite, analcime, and pectolite with a Ba−Ti−Si accessory mineral. The host metalherzolite is made of an association of olivine, antigorite, diopside, chlorite, ferrite-chromite, chromium magnetite, and accessory awaruite, heazlewoodite, and pentlandite. The jadeitite contains inclusions with a relict coarse-grained hypidiomorphic-granular texture, which are considered to be relics of the metasomatized protolith of the jadeitite. This protolith was probably high-temperature hydrothermal diopsidite. The inclusions show local recrystallization of primary diopside to aegirine-augite and pseudomorphic development of a fine-grained aggregate of amphiboles (several generations of richterite, actinolite, magnesiokatophorite, K-richterite, and eckermannite), omphacite, pectolite, analcime, phlogopite, accessory maucherite and heazlewoodite after diopside/aegirine-augite and an associated unidentified mineral. The protolith was transformed in several stages before the onset of jadeite crystallization, and these transformations included metasomatic recrystallization and a complete change in its texture. During the last stage, crystallization of the euhedral concentrically zoned jadeite with analcime and pectolite from fluid was accompanied by the recrystallization and dissolution of the last reworked relics of the protolith represented by high-calcium omphacite in microgranular omphacite-jadeite aggregates of jadeitite. The formation of jadeitites and the accompanying metamorphism of the host lherzolites occurred at 500°C and 8.5 kbar, which corresponds to P–T conditions typical of the metamorphism of mantle wedge peridotites in the “warm” subduction regime. The presence of jadeitites in the El’denyr massif and high-pressure metamorphic rocks in the Ust’-Belaya massif, which were studied previously, allows us to consider the Ust’-Belaya terrane as a mélange of a subduction zone active in the Early–Middle Triassic that was deformed and disintegrated during its subsequent exhumation in the Cretaceous.

The paper presents original detailed data obtained by the authors on the Archean Pon’goma-Navolok granulite and charnockite massif in northern Karelia: a geological map of the massif and its surroundings, data on the petrography of the magmatic and metamorphic rocks, and the P–T parameters evaluated for major rock types by the techniques of multimineral thermomabometry and pseudosections. The Pon’goma-Navolok massif is determined to have formed as two intrusive phases at different crustal levels. The first intrusive phase corresponds to the massif of clinopyroxene–orthopyroxene charno-enderbites that crystallized at 8–11.2 kbar and 730–740°C. The second phase comprises dikes of orthopyroxene–biotite charnockites, which formed at 5.6–6.8 kbar and 830–850°C, and biotite granites, which crystallized at 6.8–7.0 kbar and 730–740°C. The dikes most likely correspond to different temperature and water-activity facies. The charnockites and granites were formed by processes of charnockitization and granitization of the charno-enderbites under the effect of saline aqueous solutions. The granulite-facies metamorphism of the metabasite blocks hosted in the charno-enderbite intrusion was of contact nature and was induced by the thermal effect of the charno-enderbites on the roof and wall rocks of the magma chamber. The high metamorphic temperatures of the metabasites (>900°C) and the absence of migmatization aureoles are explained by low water contents in the enderbites.

The high activity of the Klyuchevskoy group volcanoes in the Holocene suggests that considerable volumes of partly solidified magma (cumulates) and mafic–ultramafic intrusions have accumulated in the crust. Together with extensive fluid flow typical the zones of rapid subduction of an old oceanic plate, this provides conditions for the formation of a fluid–magma ore-forming system. Olivine with sulfide inclusions was found in the eruption products of Tolbachik Volcano. Its investigation may provide insight into the composition of crustal fluid of such ore-magmatic systems. The interaction of reduced water-poor fluid with oxidized basaltic melt (NNO + 1.5) containing 2000–3000 ppm sulfur was theoretically modeled. It was shown that at a local fluid content higher than ~1–2 wt %, sulfur in the melt is reduced and sulfide droplets are formed. Sulfur reduction in the melt can also be caused by the dissolution of SO₂, which is the main sulfur species in fluid at log fO₂ ≥ NNO + 1.5. This effect is related to the higher degree of sulfur oxidation (S⁺⁶) in melt, where ({text{SO}}_{4}^{{2 - }}) is the only oxidized sulfur species, compared with SO₂ (S⁺⁴) in fluid. According to calculations, sulfide formation begins after dissolution of approximately 2000–3000 ppm sulfur in the SO₂ form in melt at log fO₂ ≥ NNO + 1. Interaction with fluid with small contents of precious metals (PM) produces sulfide melt droplets with PM contents corresponding to the background values in the melt. According to experimental evidence, Pt and Pd are highly soluble in reduced water-poor fluids in the form of carbonyls, whereas Au is low soluble; in contrast, Au solubility is very high in oxidized fluids (NNO + 1 to NNO + 1.5). Reaction with mineralized fluid containing up to tens of ppm PM produces sulfide melt enriched in Au (oxidized fluid) or Pt (reduced fluid). Interaction of melt with water-poor fluid causes local dehydration and an increase in liquidus temperature, which results in rapid olivine crystallization at high overcooling. The localization of phase transitions at the boundary of fluid bubbles facilitates the entrapment of sulfide droplets by olivine. The rare occurrence of sulfide inclusions in olivine from Tolbachik Volcano can be related to the rapid dissipation of the local effect of magma interaction with small amounts of fluid and dissolution of the precipitated sulfide phase in the melt.

The Norilsk–Talnakh magmatic sulfide Cu–Ni–PGE (platinum-group elements) deposits were formed by the accumulation of metals in immiscible sulfide melt comagmatic with the parental mafic–ultramafic magma. In this study, the main types of magmatic sulfide ores of the Norilsk–Talnakh deposits are considered as manifestations of different stages in the evolution of the initial sulfide melts. In the context of the overall evolution of Norilsk sulfide melts, the earliest ores are Cu-poor pyrrhotite ores with high concentrations of Rh and IPGE (Os, Ir, and Ru), which were discovered at the Talnakh deposit. The second stage of sulfide melt evolution was marked by the formation of most disseminated ores and Cu- and PGE-poor massive pyrrhotite ores. The massive and disseminated ores were formed independently from each other, but generally correspond to the melts with identical compositions. The only exception is low-sulfur PGE-rich ores from the Upper Gabbroid rocks of the differentiated intrusions, which were affected by wall rock assimilation and early magmatic degassing. It has been shown that the concentrations of ore components in the disseminated sulfides, which are examples of in-situ crystallized droplets of immiscible sulfide melt, vary depending on the composition and degree of fractionation of the parental silicate magma. During the final stage, the crystallization of the residual sulfide melts led to the formation of Cu-rich ores with high Pt and Pd contents. The compositions of these main ore types are compared with the compositions (including trace elements) of their base metal sulfides (BMS). All element dependencies in the massive ores follow the fractional crystallization trend of the sulfide melt. PGE in Norilsk ores are concentrated in distinct platinum-group minerals (PGM) and occur as trace elements in BMS. Rhodium and IPGE are concentrated in pyrrhotite, pentlandite, and pyrite; Pt is occasionally found in pyrite; whereas Pd is found predominantly in pentlandite. The concentration of Pd in pentlandite increases from the Cu-poor to Cu-rich ores. Based on a detailed analysis with the application of several methods, the Pd-rich pentlandite (containing 4.84 wt % Pd) from massive primary magmatic Cu-rich MSS–ISS ores is thought to have been formed by a high-temperature mechanism involving a reaction with sulfide melt. Using X-ray absorption spectroscopy (XAS), the oxidation state of Pd in pentlandite (2⁺) and its occurrence in the form of a solid solution, in which Pd apparently replaces Ni in the pentlandite structure, were identified for the first time.

This paper reports new experimental results on the chemical counterdiffusion of major components (SiO₂, Al₂O₃, Na₂O, CaO, MgO, and FeO) and the ({text{CO}}_{3}^{{2 - }}) anion during interaction of basalt and kimberlite melts under upper-mantle pressure. The method of diffusion couples was employed on a BARS split-sphere apparatus at 5.5 GPa and 1850°C. It was shown that the rates of chemical counterdiffusion of all major melt species (SiO₂, Al₂O₃, Na₂O, CaO, MgO, and FeO) and the ({text{CO}}_{3}^{{2 - }}) anion are almost identical during interaction of model basalt and carbonate-bearing kimberlite melts and approximately an order of magnitude higher than the diffusion rates of these components during melt interaction under moderate pressures (100 MPa). The equal diffusion rates of CaO and ({text{CO}}_{3}^{{2 - }}) indicate that molecular CaCO₃ diffusion from the kimberlitic to basaltic melt (model and natural) occurs also at the high pressure. The diffusion patterns are dramatically different during interaction of natural magnesian basalt and model kimberlite, which was observed for the interaction of these melts at moderate pressure. In addition to the molecular diffusion of CaCO₃ into the magnesian basalt, the diffusion rates of other melt species increase significantly. All diffusing components show weak exponential dependence on concentrations approaching D_i = const, similar to that observed during interaction of such melts at moderate pressures.