Chemie Der Erde-Geochemistry最新文献

Tourmaline and gold mineralization form an association in the Bakoshi-Gadanya (BAG) Goldfield on the northern West Nigerian Subshield, located in the southern Trans-Saharan Orogenic Belt of West Africa, although the cogenicity of this association remains untested. In this study, we report the results of an integrated study of the boron isotopic and major- and trace-elemental compositions of BAG tourmalines, from which we infer the nature and origin of the hydrothermal parent fluids and their role in the associated gold mineralization. Tourmalines in the BAG Goldfield are of four types: altered granite-related (Tur I), wallrock-hosted (Tur II), Gadanya Tourmalinite (Tur III) and Shanono Tourmalinite (Tur IV). The tourmalines mainly belong to the alkali group with dravite (i.e., Mg-rich) compositions. The MgFe₋₁ and □Al(NaR)₋₁ exchange vectors are the dominant substitution mechanisms for all BAG tourmalines with contributions from deprotonation AlO[R(OH)]₋₁ substitutions. Except for Tur I, which is enriched in Rb (0.02–21.9 ppm) and Cs (0.01–0.63 ppm), due to high fluid-rock reaction, the BAG tourmalines are enriched in Cr (2.0–3908 ppm), Ni (5.0–222 ppm), Co (0.05–27.4 ppm), Sr (41.8–3031 ppm), Sc (1.6–281 ppm), V (32.0–701 ppm), Al (0.01–0.35 apfu), Fe (0.59–1.47 apfu), and Mg (0.67–2.43 apfu), suggesting metasedimentary-derived components. Boron isotopes display bimodal populations from −16.8 to −12.0 ‰ (Tur II to IV) and from −23.0 to −19.0 ‰ (Tur I). We propose that greenschist to amphibolite facies metamorphism during the Pan-African Orogeny devolatilized country-rock metapelites and produced a metamorphic-hydrothermal fluid responsible for BAG tourmalinization. These metamorphic-derived Tourmalines show no relationship to the magmatic-hydrothermal fluid derived BAG gold ore. Our results show that the BAG tourmalines may not serve as indicator minerals in exploration for gold mineralization in the BAG Goldfield in northern West Nigerian Subshield.

Like the paleo-Sutlej River, the paleo-Yamuna River has been hypothesized as a major tributary of the mighty Ghaggar-Hakra (Vedic Saraswati) River and its eastward migration to the modern Yamuna course linked to the deurbanization of the Harappan (Indus) Civilization that peaked ~4.0–3.9 thousand years before present (ka). Here we provide detrital Sr and Nd isotope variabilities in two ~48 m and one ~20 m deep cores drilled on the postulated paleochannel of Yamuna (Y2) and modern-day Yamuna bank, respectively, in the NW Indo-Gangetic Plain. Our isotopic records (⁸⁷Sr/⁸⁶Sr: 0.7358–0.7925, ε_Nd: −14.6 to −21.2) suggest that these sediments were deposited at least since ~88 ka by the west-flowing paleo-Yamuna River, which migrated eastward to its current path shortly after ~18 ka. Therefore, no major fluvial activity was prevalent along the paleo-Yamuna channels during the Early and Mature Harappan phases (5.7–3.9 ka), questioning the widely popular river-culture hypothesis. However, the availability of sufficient water in the relict paleochannels due to intense Indian summer monsoon (ISM) precipitation during the early to middle Holocene, along with a stable landscape not prone to devastating floods because of the migration of paleo-Yamuna to its current course, might have helped the Harappan Civilization to flourish, and subsequently, the pronounced weakening of the ISM might have caused the demise of the Harappan Civilization.

NASA's Stardust mission returned rocky material from the coma of comet 81P/Wild 2 (pronounced “Vilt 2”) to Earth for laboratory study on January 15, 2006. Comet Wild 2 contains volatile ices and likely accreted beyond the orbit of Neptune. It was expected that the Wild 2 samples would contain abundant primordial molecular cloud material—interstellar and circumstellar grains. Instead, the interstellar component of Wild 2 was found to be very minor, and nearly all of the returned particles formed in broad and diverse regions of the solar nebula. While some characteristics of the Wild 2 material are similar to primitive chondrites, its compositional diversity testifies to a very different origin and evolution history than asteroids. Comet Wild 2 does not exist on a continuum with known asteroids. Collisional debris from asteroids is mostly absent in Wild 2, and it likely accreted dust from the outer and inner Solar System (across the putative gap created by a forming Jupiter) before dispersal of the solar nebula. Comets are a diverse set of bodies, and Wild 2 may represent a type of comet that accreted a high fraction of dust processed in the young Solar System.

The Ahualulco Volcanic Complex (AVC) is situated in the north-central part of the San Luis Potosí Volcanic Field (SLPVF) that is found in the southern portion of the Mesa Central (MC). The Cúcamo, AVC is mainly composed of mafic and intermediate volcanic rocks. The present study focuses on understanding the evolution, origin, and magmatic processes and petrogenesis of mafic and intermediate rocks in the Cúcamo, AVC. The Quaternary mafic rocks have porphyritic textures with the mineral assemblage of olivine, and clinopyroxene. These volcanic rocks display high K calc-alkaline basaltic compositions with enrichment in light rare earth elements (LREEs) and incompatible elements. Geochemical modeling reveals that mafic magmas were derived through a partial melting process of a spinel lherzolite source at low degrees of melting (~2 to 15 %) in an extensional regime. The intermediate volcanic rocks show porphyritic and glomeroporphyritic textures with matrix formed by randomly oriented microlites. The main mineral assemblage consists of plagioclase, K-feldspar, and clinopyroxene. These volcanic rocks are characterized by calc-alkaline basaltic andesitic and andesite compositions with enrichment in light rare earth elements and incompatible elements. Geochemical modeling suggests that intermediate rocks were derived from high ratios of assimilation and fractional crystallization processes between mafic melts and continental crust in an extensional environment.

The Jurassic mafic to felsic magmatism affecting the older Ediacaran-to-Cambrian basement of the Sanandaj-Sirjan Zone of Iran has been traditionally interpreted as the product of arc and/or back-arc magmatism related to the early stages of Neo-Tethys subduction beneath Iran in the early Jurassic. Recent works and new compositional and geochronological data have started challenging this commonly accepted model in favor of scenarios involving continental rifting, mantle plume activity, and/or passive margin formation. In the Hamedan area of the central sector of the Sanandaj-Sirjan Zone, the Jurassic Cheshmeh-Ghasaban gabbro (ca. 165 Ma) is a key formation to better understand the tectono-magmatic framework of the whole area. Our new data, combined with the existing literature, suggest a transitional to alkaline OIB-like compositional character for this gabbro similar to the nearby but slightly younger (ca. 145 Ma) Panjeh and Ghalaylan basaltic complexes (in the Songhor-Ghorveh area). When integrated with the existing geochemical data of Jurassic mafic rocks from the central Sanandaj-Sirjan Zone, our results point to a scenario of intracontinental rifting, possibly involving the upwelling old metasomatized (by Proto-Tethys subduction?) mantle or mantle-plume activity.

Nitrogen (N) stable isotope ratio (δ¹⁵N) in coal organic matter (OM) provides information on the N source and dominant mechanisms affecting isotopic fractionation during coalification. However, published data on δ¹⁵N distribution in coal is rare. The present study is one of the first reports on the δ¹⁵N composition of peat, lignite, sub-bituminous and anthracite coals in India and one of the first attempts to understand the processes influencing δ¹⁵N composition at different stages of coalification from peat to anthracite. Peats were collected from the western coast of North Andaman Islands and Lake Loktak in Manipur. Plant samples were collected from the peat sampling locations. Cenozoic lignites were collected from Panandhro, Matanomadh, Umarsar and Tadkeshwar mines in Gujarat and Neyveli in Tamil Nadu. Cenozoic sub-bituminous and Permian anthracite coals were collected from Assam and Sikkim, respectively. Variation of δ¹⁵N in plants is attributed to the differences in rainfall, plant type and N sources. Lower δ¹⁵N values in peats (mean 1.19) compared to the plant samples (mean 2.77) indicate a nonlinear response of δ¹⁵N to the relative enrichment or loss of N during peat formation in Lake Loktak and decomposition of OM under anaerobic conditions leading to selective preservation of ¹⁴N in the Andaman Islands. The δ¹⁵N composition of the studied peat (−1.4–1.6), lignite (−1.4–1.8) and coals (−2.8–5.0) retains their OM source signature. Overall higher δ¹⁵N values of Cenozoic lignites compared to the Cenozoic sub-bituminous coal reflects regional differences in climate. Higher δ¹⁵N (1.3–5.0) values in Gondwana anthracites reflects the tectonic influence of Himalayan orogeny.

The widely distributed Late Hercynian–Indosinian granites in the West Qinling Orogenic Belt are keys to better understand the tectonic–magmatic evolution of the West Qinling Orogenic Belt. In this paper, granitoids from the Zhacanggou area of Guide Basin in the western section of the West Qinling Orogenic Belt were studied on petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopes. Zircon U–Pb dating yield granitoids in the Zhacanggou area were emplaced at 228.3 ± 4.4 Ma, 265.2 ± 2.2 Ma and 277.8 ± 2.5 Ma, respectively. The whole-rock geochemical compositions of these granitoids belong to the weakly peraluminous calc-alkaline to metaluminous high-K calc-alkaline series, and exhibit different degrees of negative Eu anomalies, loss of high-field strength elements (Nb, P and Ti), and enrichment of large ion lithophile elements (Rb, Th, U and Sr). The ε_Hf (t) values of Early–Middle Permian and Late Triassic granitoids range from −12.0 to 5.3 and –10.6 to −5.0, and corresponding two-stage model ages (T_DM2) of 956 to 2051 Ma and 1581 to 1936 Ma, respectively. A summary of geochronology for granitoids formed in the West Qinling Orogenic Belt during the Late Hercynian–Indosinian indicating magmatic activities concentrated in the Late Triassic (210–230 Ma) and Permian–Middle Triassic (235–277 Ma). These granitoids were both formed by partial melting of ancient crust, which then mixed with enriched lithospheric mantle, and the former has a higher mantle contribution than the latter. The early granitoids were associated to northward subduction of the Mianlue oceanic slab, while the late granitoids were formed in the transition from collision to extension.

Granitoids are the most important component of continental crust, yet there has been debate regarding the classification and petrogenesis of peraluminous I- and S-type. As a result of fractional crystallizing and crustal contamination, whole-rock geochemistry sometimes fails to accurately reflect the type of primitive magma. Recent studies, however, suggest that the accessory mineral compositions can shed light on the character and petrogenesis of their primitive magma. In this contribution, we use apatite and zircon as indicators to explore the distinctions between peraluminous I- and S-type granitoids, and the petrogenesis of typical peraluminous I-type granitoids (Baoshan granitoids). Apatite trace elements indicate that their initial magma was mafic I-type, even though whole-rock compositions appear to be the hybrids of I- and S-type granitoids. Additionally, we propose that the assimilation and fractional crystallization processes are responsible for the decoupling between the compositions of whole-rock and accessory minerals. The compositions and isotopes of zircon can also reveal the components of the magma source region. The zircons ε_Hf(t) values of the Baoshan granodiorite porphyry and lamprophyre have comparable _Hf(t) values (−9.5 to −6.2 and −12.5 to −6.2, respectively). Based on the spotting of ~900 Ma inherited zircons and enriched ε_Hf(t) values, we propose that the granitoids were formed by the partial melting of felsic Paleoproterozoic crust and a little of Neoproterozoic mafic juvenile crust, while lamprophyre was generated by the cooling of upwelling magma from the same source region as granitoids. According to the apatite trace element ratios (Sr/Th vs. La/Sm), the source region of the Baoshan intrusion is identified to been metasomatized by slab-derived fluid. Our data, in conjunction with previous studies, suggest that the paleo-Pacific slab roll-back triggered the high-temperature asthenosphere mantle upwelling, while the assimilation and fractional crystallization occurring along with the rising melts in route to the surface.

The Kömürlükdere and Göçükdibi Cu-Zn ores are located in the Central Pontide orogenic belt, where Besshi-type deposits (such as Hanönü, Zeybek, Sayyayla [Kastamonu]) have been discovered in last two decades. The Göçükdibi and Kömürlükdere ores are hosted in metabasites and quartz schist that belong to the Middle Jurassic Kunduz metamorphic rocks in the accretionary complex in the Central Pontides. The ore bodies show stratiform features, parallel to schistosity, within an alternation of metabasite and quartz schist succession. The average thicknesses of the ore zone and ore levels within the ore zone are 12 m and 3 cm, respectively. The ore bodies contain chalcopyrite, sphalerite, and magnetite, with dominant pyrite formed during the ore formation phase. Meanwhile, hematite, covellite, malachite, and goethite minerals formed during the supergene processes.

Microprobe studies showed that the Fe and Cd content of sphalerite varies from 0.08 to 1.12 wt%, indicating Fe-poor sphalerite, and 0.08–0.27 wt%, respectively. The Zn/Cd ratios (average 274.4 for Kömürlükdere, 288.6 for Göçükdibi) of the sphalerite are comparable to those of the worldwide volcanogenic-massive sulfide (VMS) ore system related to andesitic-basaltic source rocks. The Co/Ni ratio (mean 3.2) of pyrite shows volcanogenic pyrite.

The average δ³⁴S of the stratiform pyrites is 3.9 ‰ (ranging from 2.06 ‰ to 5.34 ‰), indicating that the sulfur comes from a large homogeneous source, possibly magmatic. The average homogenization temperature (Th) and salinity of fluid inclusions obtained from quartz and sphalerite are 313 °C and 7.7 % equiv. NaCl, respectively, and similar to those of the global VMS deposits.

The metabasites contain an average of 164 ppm Cu, 127 ppm Zn, and 2.2 ppm Sb, which shows enrichment several times greater than the background value of the basalts. Meanwhile the ore levels contain an average 0.23 % and 0.83 % Cu and Zn values, respectively. The trace metal enrichment of associated metabasites and the ore zones is evident for elements such as Cu, Zn, Cd, As, and Sb, indicating that they both precipitated in the same basin as syngenetic, and that there was repeated pulses of metal-rich fluids exhaled into the basin.

Field observations and analytical data (mineral chemistry, sulfur isotope, fluid inclusion, etc.) show that the Kömürlükdere and Göçükdibi Cu-Zn ore bodies occurred as Besshi-type deposits within the Middle Jurassic Central Pontide Supercontinent, which developed in the marine environment appropriate to the accretionary complex located along the southern margin of Eurasia.