Acta Geochimica最新文献

Hydrothermal alteration with bleaching of host rocks is the most important prospecting indicator for gold deposits in the Jiangnan Orogen Belt. The alteration has been identified as pre-ore carbonate (siderite)-sericitization and the Fe of siderite in the alteration zone is derived from the host rocks rather than fluids. In addition, such alteration decreases in intensity and width with depth and gold mineralization also occur in the non-bleached rocks, casting doubt on the reliability of the prospecting indicator. Detailed petrographic work and SEM analysis on the Wangu deposit indicate that there are two types of siderites, i.e., Sd1 and Sd2. Among them, Sd1 grains are relatively small and distributed along the planes of unaltered host rocks, while Sd2 grains, only occurring in the altered slates, are commonly larger. Both types of siderites were altered by auriferous fluids, producing porous cores and minerals such as pyrite, quartz, and ankerite. Compared with unaltered parts, the altered parts have lower Fe, but higher U, Pb, and REE. In addition, Sd1 and Sd2 are similar in Mn, Na, V, and Sr concentrations but different in Fe and Mg. The occurrence and geochemical compositions of both siderites indicate that Sd1 could be transformed into Sd2 by pre-mineralization alteration through dissolution-reprecipitation. Chlorite is another important Fe-bearing mineral in the host rocks, and EPMA analysis suggests that it is ripidolite with relatively high Fe contents. Consequently, chlorite can also provide Fe to form the pre-ore carbonate(siderite)-sericitization. Geochemical modeling demonstrates that both ripidolite and siderite can result in sulfidation and therefore gold precipitation. As a result, this study demonstrates that pre-ore alteration with characterized bleaching is not a prerequisite for gold mineralization despite of its prominent features. Due to the presence of Fe-bearing Sd1 and chlorite, gold mineralization could still occur through sulfidation in the unaltered rocks.

Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes, such as magma differentiation, magma recharge and mixing, and crustal contamination. The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province (LIP) show complex textural and compositional zoning patterns, e.g., normal, reverse, oscillatory, and patchy zoning patterns. Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern (Pl-A) with euhedral high-An cores (An = 76–78%, in mole fraction) and low-An rims (An = 68–72%), indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores. Some phenocrysts have a core–rim reverse zoning pattern (Pl-B) with irregular ovaloid cores, characterized by extremely low An (60–61 mol%) and Ba (84–88 ppm) contents and extremely high ⁸⁷Sr/⁸⁶Sr ratios (0.7120–0.7130). The rims of the Pl-B have relatively high An (69–72 %), Ba (~160 ppm) contents, and low ⁸⁷Sr/⁸⁶Sr_i (~0.7056). These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas. Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern (Pl-C) with multiple oscillations of An (70–80 %), Ba (88–147ppm) from core to rim, revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions. ⁸⁷Sr/⁸⁶Sr ratios of the Pl-C do not vary significantly (0.7050–0.7054). A small portion of phenocrysts has patchy patterns in the cores (Pl-D), where the low-An patches (72–75 %) in form of elliptical or irregular elongated shapes were enclosed by the high-An domains (80–87 %). These features can be attributed to crystal dissolution and regrowth processes during the reaction between early-formed low-Cumulates and recharged hot primitive melts. The cores, mantles, and rims of different types of plagioclase phenocrysts (except the core of Pl-B) commonly display nearly constant Sr isotopic compositions, implying insignificant wall-rock assimilation at shallow-level magma reservoir(s) during the growth of these plagioclase phenocrysts. In conclusion, the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP.

At the beginning of the Cenozoic, the atmospheric CO₂ concentration increased rapidly from ~2000 ppmv at 60 Ma to ~4600 ppmv at 51 Ma, which is 5–10 times higher than the present value, and then continuous declined from ~51 to 34 Ma. The cause of this phenomenon is still not well understood. In this study, we demonstrate that the initiation of Cenozoic west Pacific plate subduction, triggered by the hard collision in the Tibetan Plateau, occurred at approximately 51 Ma, coinciding with the tipping point. The water depths of the Pacific subduction zones are mostly below the carbonate compensation depths, while those of the Neo-Tethys were much shallower before the collision and caused far more carbonate subducting. Additionally, more volcanic ashes erupted from the west Pacific subduction zones, which consume CO₂. The average annual west Pacific volvano eruption is 1.11 km³, which is higher than previous estimations. The amount of annual CO₂ absorbed by chemical weathering of additional west Pacific volcanic ashes could be comparable to the silicate weathering by the global river. We propose that the initiation of the western Pacific subduction controlled the long-term reduction of atmospheric CO₂ concentration.

The Mamfe Basin has been the subject of many studies, but some debates persist, especially concerning the stratigraphic nomenclature, corundum origin, and marine transgression. The aims of this work are (1) to propose a new lithostratigraphic nomenclature of the Mamfe Basin formation based on new field observations, (2) to determine the source rock distribution and the origin of gem deposits and (3) to correlate the Cameroon section of the Mamfe Basin with the Nigerian deposits. The main results show that the name Manyu River Group is more appropriate as the Manyu River crosses all the facies in the Mamfe sedimentary Basin belonging to the Manyu Division. According to the facies analysis, the age of deposition, the mineralogic and geochemical data such as the V vs. Al₂O₃ and ∑REE vs. Al₂O₃, TiO₂ MgO, Na₂O, P₂O₅, and CaO diagrams, this Group is composed of at least five Formations, including four Cretaceous Formations, from bottom to top: the Etoko Formation (alluvial and fluvial channel to fluvio-lacustrine deposits), the Nfaitok Formation (lagoonal deposits), the Bachuo Ntai Formation (fluvial braided channels or fluvio-deltaic environment) and the Eyumojock/Nsanaragati Formation (fluvio-lacustrine deposits) and the new Cenozoic Formation named Bakebe Formation (fluvio-lacustrine deposits). The gem minerals such as corundum, rutile, or tourmaline in the Cretaceous deposits of the Mamfe Basin are mainly detrital as indicated by the presence of worn shapes and fragments of these minerals. The presence of sapphire in the Albo-Cenomanian deposits indicates a Precambrian age of the felsic source rock, likely the plutonic rocks such as granite or pegmatite as indicated by the abundance of tourmaline and high LREE/HREE ratios (14.81–34.29) and slightly negative and positive Eu anomalies (0.85–1.15). This marine incursion in the Mamfe Basin was probably from West Nigeria, according to the geographic location of the Mamfe Basin and the general palaeogeographic evolution of the Benue Trough.

Individual hydrocarbons identified to be macrocyclic alkanes in a torbanite from the Sydney Basin (Australia) were successfully isolated from its extracts using preparative gas chromatography and analyzed by NMR. Saturated cyclic structures were confirmed by single peaks in the NMR ¹H and ¹³C spectra indicating single forms of H and C atoms exist in these biomarker molecules. This is consistent with the methylene unit in a ring system assignment of the macrocyclic alkanes and accounts for a formula of (CH₂)_n. The unusual molecular structures of these compounds are consistent with those that were identified from previous GC retention index data and co-injection with a standard supports the previous research. The mass spectral fragmentation behaviors of representative cyclic alkanes were further investigated by comparing them with the mass spectra of isolated individual macrocyclic alkanes. The characteristic fragment ions in the macrocyclic alkanes of (M–28)⁺ and base peaks of m/z 97, 111, 125, etc., can be assigned as being generated by simple α-/i-cleavage and hydrogen rearrangement. These fragmentation pathways combined with retention indices should assist in differentiating these compounds from monounsaturated alkenes and alkylated monocyclics having similar mass spectral characteristics in other geological samples.

The Dulong deposit, located in the Laojunshan area of southeastern Yunnan, China, is an important polymetallic deposit due to its high reserves of tin, zinc, and indium. The occurrence state of indium is critical for understanding its supernormal enrichment mechanism. Previous studies investigated the occurrence state of indium (including the valence state) based on the indium content in sphalerite and the correlation between metal concentrations. However, more evidence is needed to better constrain indium occurrence at the micro-, nano-, or even atomic scale. In this study, EPMA-FIB-SEM-TEM and XPS techniques were employed to investigate the indium distribution characteristics and occurrence state in sphalerite from the Dulong Sn–Zn–In polymetallic deposit. The maximum concentration of indium in the indium-rich sphalerite samples is 0.37 %, and the results of the EPMA analysis showed a relatively homogeneous distribution of indium in sphalerite. The FIB-SEM-TEM results demonstrated that the lattice stripes of sphalerite were periodically and continuously distributed at the nanoscale, confirming that sphalerite in the deposit was an excellent single crystal structure, and the peak heights of the various characteristic peaks of indium in the EDX spectra were relatively close to each other, with no distinct peaks of high indium content. In addition, the XPS results indicate that the element valence state of indium in sphalerite is In³⁺, and it combines with S²⁻ to form a bond. These results indicate that indium in sphalerite of the Dulong deposit is uniformly distributed at both the micro- and nanoscale, and there is no indium-independent mineral. In³⁺ enters the crystal lattice of sphalerite by replacing Zn²⁺ in the form of isomorphic substitution.

The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area, Bastar craton, Central India. The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses. Compositionally zincian chromite shows a range of Cr₂O₃ (39.69 to 51.66 wt%), Al₂O₃ (05.30 wt% to 08.71 wt%), FeO (21.74 wt% to 27.51 wt%), Fe₂O₃ (10.19 wt% to 19.36 wt%) with higher ZnO content ranging from 1.73 wt% to 4.08 wt%. Accordingly, their Cr# [Cr/(Cr + Al)] varies in a narrow range from 0.83 to 0.85. Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds, meteorites, and association with any sulfide-rich mineralised belt. This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite. The empirical thermometric calculation from chromite, amphibole, and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism (~ 700 °C). The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration, i.e., serpentinisation. The olivine is a major repository for Mn, Zn, and Co in peridotite/ultramafic; these elements get mobilised during the metamorphism and serpentinisation. This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic. As a result, chromite-rich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange. It converts common chromite into zincian chromite, as reported in altered ultramafics elsewhere.

The Suizhou meteorite is a heavily shocked and melted vein-containing L6 chondrite. It contains a minor amount of diopside with a (Ca_0.419Mg_0.466Fe_0.088)SiO₃ composition, and a shock-metamorphosed diopside grain associated with ringwoodite and lingunite was found in a melt vein of this meteorite. Our electron microprobe, transmission electron microscopic and Raman spectroscopic analyses revealed four silicate phases with different compositions and structures inside this shock-metamorphosed diopside grain, termed phase A, B, C and D in this paper. Phase A is identified as orthorhombic (Ca_0.663Mg_0.314)SiO₃-perovskite which is closely associated with phase B, the vitrified (Mg_0.642Ca_0.290Fe_0.098)SiO₃ perovskite. Phase D is assigned to be (Mg_0.578Ca_0.414)SiO₃ majorite which is associated with phase C, the vetrified Ca-rich Mg-perovskite with a (Mg_0.853Ca_0.167)SiO₃ composition. Based on high-pressure and high-temperature experiments, the diopside grain in the melt vein of the Suizhou meteorite would have experienced a P–T regime of 20–24 GPa and 1800 – > 2000 °C. Such P–T conditions are high enough for the decomposition of the diopside and the formation of four different silicate phases. The orthorhombic (Ca_0.663Mg_0.314)SiO₃ perovskite found in the Suizhou L6 chondrite might be considered as the third lower-mantle silicate mineral after bridgmanite and davemaoite after the detailed analyses of its crystal structure and physical properties being completed.

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 (mean 1.62 Ra), and their ⁴⁰Ar/³⁶Ar 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 0.87 Ra) and their ⁴⁰Ar/³⁶Ar 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 crust-derived materials in spodumene pegmatites increases significantly in the late stage of the magmatic evolution. The δ¹⁸O_H2O–VSMOW values of common pegmatites range from 6.2‰ to 10.9‰, with a mean value of 8.6‰, and δD_V–SMOW values vary from − 110‰ to − 72‰, with a mean of − 85‰. The δ¹⁸O_H2O–VSMOW values of spodumene pegmatites range from 5.3‰ to 13.2‰, with a mean of 9.1‰, and δD_V–SMOW values 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 ore-forming 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.