Trace Element Characteristics Research Articles

Geology, geochemistry and geochronology of Lindero porphyry gold deposit in the Southern Puna plateau, Argentina

The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca–Na silicate (clinopyroxene + magnetite + K-feldspar + quartz + calcite ± plagioclase), (ii) K-silicate (K-feldspar + quartz + magnetite ± biotite ± anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca–Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style.The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 ± 0.13 Ma (n = 21) and 15.47 ± 0.11 Ma (n = 16), respectively. Individual ages from each unit range by ~1 m.y. and the overall spread of zircon ages is 15.92 ± 0.23 to 14.44 ± 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0–14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 ± 0.16 Ma and 14.93 ± 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units.All of the intrusive units are similar compositionally. They show a fine- to medium-grained porphyric texture (1–4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40–55 vol % of phenocrysts) in a K-feldspar ± quartz microcrystalline (0.02–0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6–61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (87Sr/86Sr = 0.706042 to 0.706607; 143Nd/144Nd = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back-arc volcanic rocks. The Pb isotope ratios of Lindero (206Pb/204Pb = 18.79 to 18.83; 207Pb/204Pb = 15.60 to 15.63; 208Pb/204Pb = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).

Trace element characteristics of pyrite in Dongguashan Cu (Au) deposit, Tongling region, China

The Dongguashan deposit is the largest and deepest Cu (Au) deposit in the Tongling region, eastern China. The presence of skarn and stratiform ore bodies in the same system of the Dongguashan deposit has caused much debate as to whether the mineralization was associated with magmatic-hydrothermal or sedimentary origin. In this study, in situ (LA-ICP-MS and EPMA) elemental compositions of medium-to coarse-grained euhedral pyrite from the stratiform and skarn ore bodies, and pyritization marble are investigated in the Dongguashan deposit. The results of the studied pyrite samples have relatively consistent trace element composition characteristics and several genetically significant elemental combinations. Gold and Ag are mainly of lattice bound appearance in pyrite. The Pb, Cu, Sb and Tl elements are mainly incorporated into the pyrite structure as substitutions. The distinctly trace element distribution correlations (e.g., Mn, Se and Tl) of the studied pyrites indicate that they were formed in a high temperature and oxidizing environment. Low Au (<0.1 ppm), Cu (mainly <10 ppm), Pb (<10 ppm) and As contents (<100 ppm), relatively low Se content (<50 ppm), and high Co contents, high Co/Ni (mainly >1) and Se/Tl (>10) ratios of the studied pyrites imply that the Dongguashan deposit is most likely to be a magmatic-hydrothermal origin.

Trace and rare-earth element characteristics of fine-grained sediments in the Upper Triassic Xujiahe Formation in the western Sichuan Basin, SW China: implications for the provenance and depositional environment

Xujiahe fine-grained sediments (FGSs) are regarded not only as important shale gas targets but also as significant regional hydrocarbon source rocks. To analyze the provenance and depositional conditions of the Xujiahe FGSs, trace elements and rare-earth elements (REEs) from two wells in the western Sichuan Basin were investigated. The results indicate that the Xujiahe FGSs mainly originated from the upper continental crust (UCC), the source rocks gradually changed from mixed influence to relatively pure sedimentary rocks from the second member of Xujiahe Formation (Xu2 member) to the fourth member of Xujiahe Formation (Xu4 member), which was related to the uplift of Longmen Mountain. La–Th–Sc and Th–Co–Zr/10 diagrams indicate an active continental margin environment. The redox condition indices (V/Cr and Th/U) suggest that the Xujiahe FGSs were deposited in an oxic environment, while the paleoclimate indices suggest differences among the members. The Xu3 member shows a stable warm-humid climate, and the Xu2 and Xu4 members alternate between humid and arid climates. The combination of climate and tectonics is the key factor in the formation of the Xujiahe FGSs. The warm and humid climate result in high productivity in the basin, tectonic movement influence the substance supply and create a high sedimentary rate for the Xujiahe Formation; thus, the organic matter in the Xujiahe FGSs can still be effectively preserved even under oxidation conditions.

Behavior of the clinopyroxenes trace elements in spinel-lherzolite xenoliths from Liri (Kapsiki plateau, Cameroon line)

Spinel-lherzolite xenoliths trapped within the alkali basalts flow in the Liri region (Kapsiki Plateau) have a protogranular texture and consist of olivine, orthopyroxene, clinopyroxene and spinel crystals. These xenoliths are residues of partial melting of the primitive mantle, with the low titanium content in clinopyroxene crystals (TiO2 &lt; 0.5 wt.%). The clinopyroxene of the spinel-lherzolite xenoliths from Liri, are divided into two distinct groups according to their trace element characteristics. The variations in the rare earths elements make it possible to classify the different clynopyroxenes in two groups: the first group consisting of the samples of Liri (Liri 1, Liri 02, Liri 3, Liri 05 and Liri 5) rich in light rare earths elements (LREEs), with ratios (Ce/Yb)N normalized which vary between 3.00 and 7.78. It is probably a cryptic metasomatism due to the absence of hydrated minerals (such as amphibole) which caused these enrichments. The second group comprises samples of Liri (Liri 01, Liri 2, Liri 04, Liri 4) depleted in light rare earths elements, with the ratio in (Ce/Yb) N &lt; 1.2. This depletion in rare earths elements results from the extraction of the melting liquid.   Â

Discovery of Ancient Volcanoes in the Okhotsk Sea (Russia): New Constraints on the Opening History of the Kurile Back Arc Basin

Here we present the first radiometric age and geochemical (major and trace element and isotope) data for samples from the Hydrographer Ridge, a back arc volcano of the Kurile Island Arc, and a newly discovered chain of volcanoes (“Sonne Volcanoes”) on the northwestern continental slope of the Kurile Basin on the opposite side of the arc. The 40Ar/39Ar age and geochemical data show that Hydrographer Ridge (3.2–3.3 Ma) and the “Sonne Volcanoes” (25.3–25.9 Ma) have very similar trace element and isotope characteristics to those of the Kurile Island Arc, indicating derivation from a common magma source. We conclude that the age of the “Sonne Volcanoes” marks the time of opening of the Kurile Basin, implying slow back arc spreading rates of 1.3–1.8 cm/y. Combined with published data from the Kurile fore arc, our data suggest that the processes of subduction, Kurile Basin opening and frontal arc extension occurred synchronously and that extension in the rear part and in the frontal part of the Kurile Island Arc must have been triggered by the same mechanism.

Provenance of sub-aerial surface sediments in the Tarim Basin, Western China

Provenance of surface sediments in the Tarim Basin is important for understanding the aridification of the Asian interior and the interplay between the Westerlies and the Asian monsoon. Although the desert sands in the Taklamakan Desert have been studied intensively, there is no consensus regarding their provenance, namely, the dispute exists between various sand sources in different parts of the desert and homogenization of the sands in the entire desert. Moreover, other surface sediments in the basin are poorly investigated. Here we examine the particle-size-specific rare earth element (REE) and trace element characteristics of various surface sediments from different regions of the basin. The results reveal that the <2 µm fraction has multiple sources from Central Asia as well as from the adjoining arid/semi-arid areas to the east, together with the abrasion of coarser particles from multiple regional sources. They are completely homogenized prior to deposition by the action of the Westerlies and the local wind systems. The 2–16 µm fraction of most of the sediments in the basin have a higher proportion of Central Asian dust and a relatively high degree of homogeneity. Westerlies and local wind systems remain a more important transport mechanism than the fluvial activity. In addition, the desert sands of 2–16 µm fraction in the Taklamakan Desert has a substantial contribution from material derived from the basement rocks. The 32–63 µm and >63 µm fraction are dominated by regional-derived materials, mainly from the adjacent mountains (e.g. Pamirs, Kunlun, Kuruktag, Altun, Tianshan Mountains), and the fluvial systems are an important additional transport mechanism. Our findings potentially provide an improved understanding of the dust cycle and atmospheric circulation patterns in Central Asia, and of the provenance of loess in North China.

Sedimentary facies characteristics and organic matter enrichment mechanism of lower Cambrian Niutitang Formation in South China

The purpose of this study was to examine the sedimentary facies characteristics of lower Cambrian Niutitang Formation (∈1n) in South China, to reveal the mechanism of organic matter enrichment, and to guide exploration of shale gas. Macro investigation and experimental analyses were used to assess the lithology in detail, total organic matter mass fraction w(TOC), mineral composition, and trace element characteristics of ∈1n. The influencing factors of organic matter enrichment were discussed extensively, and a sedimentary facies mode was suggested. In the early stage of ∈1n, the locations of Well E’yangye 1, Well Ciye 1, Well Changye 1, and Well Anye 1 respectively develop, platform inner sag, outer shelf, Jiangnan slope belt, and South China detention basin. In the late stage of ∈1n, the sedimentary facies evolve with decreasing sea level. The study area presents a complete three-step basin in the Early Cambrian. In the early stage of ∈1n, the first step is the Yangtze carbonate platform, the second step is the outer shelf and slope, and the third step is the deep-water basin. From the Yangtze carbonate platform to the deep-water basin, w(TOC) and the mass fraction of quartz gradually increase, the mass fraction of carbonate mineral decreases, and the mass fraction of clay mineral is higher in the second step. The sea level fluctuation results in a higher w(TOC) vertically in the lower ∈1n shale, and the paleogeographic (provenance) conditions lead to better horizontal development of organic matter in the outer shelf, slope and detention basin. Trace elements are abundant in the lower ∈1n, and w(TOC) is correlated positively with many trace elements. In the outer shelf, slope, and adjacent areas, hydrothermal activity and upwelling current bring nutrient-rich material and promote organic matter enrichment under a strong reducing condition. Deep-shelf, slope and deep-water basin are the best facies for the formation and preservation of organic matter, especially deep-water basin facies. It remains necessary to strengthen the exploration of shale gas in the deep-water basin of ∈1n in central Hunan, China.

Inclusion and Trace Element Characteristics of Emeralds from Swat Valley, Pakistan

Inclusion and Trace Element Characteristics of Emeralds from Swat Valley, Pakistan

Ecological and health risk assessment of trace elements in surface soil in an arid region of Xin Jiang, China

Trace element pollution in soil has become an increasingly common issue and potentially harms the environment and human health. In this study, the contamination levels and ecological and health risk indices of trace elements in surface soils in Bai Cheng, Xin Jiang, were investigated in order to examine if the land usage changed from pasture to agriculture has influenced the behaviours of these trace elements. In this study, descriptive analysis, normality test, the Tukey-HSD method and non-parametric methods were used to investigate the characteristics of trace elements. The geo-accumulation pollution and ecological risk indices were used to analyse the degree of contamination by trace elements. Correlation and principle analyses were used to identify the sources of trace elements and a health risk assessment was used to analyse human exposure to trace elements of the soil. Cadmium was the main pollutant in the 0–5 cm soil layer in both agricultural and pastoral areas. Thus, it contributed most to the Geo-Accumulation Index (Igeo) and the comprehensive potential ecological risk index (RI) of the surface soils. The levels of RI for the topsoil layer of agricultural areas fell into moderate to high (II-III) ecological risk categories, and those of pastoral areas belonged to low to moderate risk categories (I-II). Both values of Igeo and RI of the surface soils in agricultural areas were higher than in pastoral areas. The carcinogenic risk of Cr through hand-to-mouth intake and the total risk of trace elements in soils for humans were higher than the safety reference level. Chromium was the greatest contributor to total carcinogenic risk. Trace element concentration levels in surface soils in pastoral areas were relevant to geochemical characteristics and atmospheric deposition, whereas trace element concentration levels in surface soils in agricultural areas were derived not only from geochemical characteristics but also from human activities. The change in land use from pasture to agriculture increased the trace element concentration level in surface soils. There were both ecological risks and human health implications for residents exposed to such contaminants of soils in the studied region.

Petrogenesis of early syn-tectonic monzonite-granodiorite complexes – Crustal reprocessing versus crustal growth

The 563.7 ± 6.1 Ma old, early-syntectonic Mon Repos complex is a predominantly metaluminous, magnesian, calc-alkalic granodioritic to granitic intrusion. Major and trace element variations imply that the rocks evolved through fractional crystallization processes involving amphibole, biotite, Fe-Ti oxides, zircon, and apatite. Rocks less evolved than granodiorite show evidence of hornblende accumulation. Initial Sr (87Sr/86Sr: 0.7090–0.7111) and Nd (εNd: −5.3 to −12.3) isotopic compositions of the granodiorites and granites are highly heterogeneous and vary with SiO2 contents indicating that assimilation of older crustal components occurred. Initial 206Pb/204Pb (16.65–17.65) and 207Pb/204Pb ratios (15.52–15.60) are similar to other mafic-intermediate complexes from the Damara belt. One monzodiorite and two quartz monzonites are K2O-, LILE-, and HFSE enriched and have similar SiO2 contents as the granodiorites. These samples are similar to post-collisional mafic magmas from elsewhere in the world. Their ε Nd values (−3.8 to −4.3), 87Sr/86Sr ratios (0.7051–0.7073) and trace-element characteristics imply that their unexposed parental melts are derived from a lithospheric mantle source that was contaminated and metasomatized by crustal material during ancient subduction processes. The data are explained by assuming a “flat” subduction model where melting predominantly involves ancient continental mantle lithosphere and crust with limited, if any, melting of the underlying asthenospheric mantle. During flat subduction, a sliver consisting of buoyant oceanic lithosphere was amalgamated with the base of the ancient continental lithosphere of the overriding plate. The oceanic mantle lithosphere and the oceanic crust dehydrated (but did not melt) and these fluids lowered the solidus of the overlying continental mantle lithosphere and crust. This scenario can explain the occurrence of rare K2O- and LILE-enriched monzodiorites and quartz monzonites with crustal-like isotopic compositions observed in this study as well as some more alkaline rocks in the belt. Because the metasomatized continental lithospheric mantle has lower solidus temperatures than peridotitic mantle, it is very likely that such metasomatized rocks may melt early during flat subduction. Thus, K2O-enriched monzodiorites to monzogranites are not restricted to late stages of the evolution of an orogen but may already form at the onset of an orogeny.

Geochemical characteristics of trace elements in size-resolved coastal urban aerosols associated with distinct air masses over tropical peninsular India: Size distributions and source apportionment

Trace elements in atmospheric particulate matter play a significant role in air quality, health and biogeochemical cycles. The present study reports on geochemical characteristics of size-resolved trace elements in PM10 aerosols collected under different air masses over a coastal urban location in peninsular India. A contrast in elemental distribution was observed for the particle size above 7.0 μm and below 1.1 μm under the influence of northeasterly air masses as characterized by Al > Fe > Zn and Fe > Al > Zn, respectively. The concentrations of the crustal elements (Al, Fe, Ti, P, Ba, Co) were high and illustrated by a unimodal size distribution with a peak in coarse mode (>2.0 μm) during northwesterly air masses. On the other hand, combustion-derived metals (Cu, Zn, Cd, Sb, and Pb) were maximized under northeasterly air masses, characterized by unimodal size distribution with a peak in fine mode (<2.0 μm). The enrichment factor (EF) analysis reveals the contribution of anthropogenic emissions to Cd, Sb, Pb, Zn, Cu, Cr, Ni, As, and Sn metals, particularly to the high enrichment of trace metals in fine mode. These results suggest that crustal emissions are major sources of trace metals in coarse mode aerosols; whereas combustion derived anthropogenic emissions contribute to the fine mode aerosols. The positive matrix factorization (PMF) analysis revealed that crustal sources (52–90%) were most abundant for particles >7.0 μm, whereas combustion related emissions such as vehicular and traffic sources are predominant for particles <1.1 μm. The present study demonstrates that trace metals in coastal urban aerosols are affected by changes in emission sources/strengths and regional transport of air masses originated from the northeasterly and northwesterly parts of the tropical Indian subcontinent.

Geochemistry and geochronology of A-type basement granitoids in the north-central Aravalli Craton: Implications on Paleoproterozoic geodynamics of NW Indian Block

The basement granite gneisses from the north-central Aravalli Craton in NW India were investigated for geochemistry and geochronology. In a peneplain terrain, the granite gneiss outcrops are scanty and samples were collected mainly from two small hills and several ground-level exposures in the Sakhun – Ladera region. Well-foliated granite gneiss is the dominant lithology that also hosts dark, lenticular enclaves, and is in turn, intruded by mafic dykes. The granite gneiss has silica content ranging from 61.37 wt.% to 68.27 wt.% that marks a slight overlap with the enclaves (54.32 wt.% to 62.17 wt.%). Both groups have a high K2O/Na2O (~2 or higher) ratio. Geochemically, the granite gneiss classify as granite – granodiorite, and enclaves as granodiorite - diorite. The In-situ LA-ICP-MS zircon U–Pb geochronology of granite gneiss has yielded a statistically valid 1721 ± 9 Ma age that we interpret as the emplacement age for the granitic protolith. Geochemical characteristics of granite gneiss underline fractional crystallization of an I-type melt as the main process, and continuity of trends in enclaves underlines their mutual genetic link. The genetic association is further verified by a consistency in the trace element characteristics and REE patterns. The Nd-isotope signatures define a single grouping for both granite gneiss and enclaves, with εNd(t) values ranging from −6.38 to −6.61, further substantiating a common source. The geochemical tectonic discrimination schemes consistently point toward an extensional setting and A-type characteristics for granite gneiss and enclaves. These are analogous to the coeval (1.72–1.75 Ga), A-type granitoids from the Khetri and Alwar basin in the North Delhi Fold Belt, implying a much larger areal extent for the Paleoproterozoic anorogenic magmatism in the northern segment of the Aravalli Craton. The Paleoproterozoic age for the presumed ‘Archean’ basement in this region offers tacit evidence that the BGC – II is a stratigraphically younger terrane as compared to the Archean age, BGC – I.

40Ar/39Ar Geochronology and New Mineralogical and Geochemical Data from Lamprophyres of Chompolo Field (South Yakutia, Russia)

The alkaline igneous rocks of the Chompolo field (Aldan shield, Siberian craton), previously defined as kimberlites or lamproites, are more correctly classified as low-Ti lamprophyres. The emplacement age of the Ogonek pipe (137.8 ± 1.2 Ma) and the Aldanskaya dike (157.0 ± 1.6 Ma) was obtained using 40Ar/39Ar K-richterite dating. The Chompolo rocks contain abundant xenocrysts of mantle minerals (chromium-rich pyropic garnets, Cr-diopsides, spinels, etc.). The composition of the mantle xenocrysts indicates the predominance of spinel and garnet–spinel lherzolites, while the presence of garnet lherzolites, dunites, harzburgites, and eclogites is minor. The Chompolo rocks are characterized by large-ion lithophile element (LILE) and Light Rare Earth Element (LREE) enrichments, and high field strength element (HFSE) depletions. The rocks of the Ogonek pipe have radiogenic Sr (87Sr/86Sr (t) = 0.70775 and 0.70954), and highly unradiogenic εNd(t) (−20.03 and −20.44) isotopic composition. The trace element and isotopic characteristics of the Chompolo rocks are indicative of the involvement of subducted materials in their ancient enriched lithospheric mantle source. The Chompolo rocks were formed at the stage when the Mesozoic igneous activity was triggered by global tectonic events. The Chompolo field of alkaline magmatism is one of the few available geological objects, which provides the opportunity to investigate the subcontinental lithospheric mantle beneath the south part of the Siberian craton.

Geochronology and geochemistry of the Tongjige granodiorites in the Jinshajiang suture zone, SW China: Constraints on petrogenesis and tectonic evolution of the Palaeo‐Tethys Ocean

The Jinshajiang suture zone (JSZ) is one of the most significant tectonic zones in the eastern Tethys area. A large number of plutons are regarded as the result of tectono‐magmatic activities associated with the evolution of the Palaeo‐Tethys Ocean. The Tongjige pluton in the central part of JSZ can indicate the magmatism and its geodynamic evolution. The Tongjige pluton (225.6 Ma) consists mainly of granodiorite with abundant coeval mafic microgranular enclaves (MMEs) (226.1 Ma). The host granodiorite is calc‐alkaline and typical I‐type, with SiO2 contents ranging from 65.75 to 66.91 wt%. The MMEs have relatively high MgO (4.00–4.07 wt%) and Fe2O3T (9.27–9.40 wt%) abundances and low SiO2 (53.59–53.87 wt%). Both MMEs and host granodiorites are characterized by the enrichment of light rare earth elements and depletion of heavy rare earth elements. They are also negative in Eu and enriched in Th and U, which are consistent with the trace element characteristics of typical island‐arc magmatite. The zircon Hf(t) values (−3.48 to −0.31) are negative, with the model ages of 1.3–1.2 Ga. These petrographic and geochemical characteristics indicated that the Tongjige pluton was derived from the melting of an ancient lower crust and underwent the process of mafic‐felsic magmatic mixing in the post‐collision background. The Triassic granitoids in the JSZ show a temporal–spatial relationship with the subduction zone, which constrain the evolution of the Palaeo‐Tethys Ocean. The mixing mechanism of the Tongjige pluton is associated with the break‐off of a subducted slab, which triggered subsequent upwelling of asthenosphere beneath accreted arc fragments and induced underplate of lithospheric mantle‐derived magmas and mixed with the lower crust‐derived felsic magma. In general, the Late Triassic pluton recorded significant crustal growth and continental development, marking the final stage of subduction‐related magmatism in the JSZ region, after which the region entered into a stage of post‐collision evolution.

Partial Melting of the Lower Oceanic Crust: Implications for Tracing the Slab Component in the Source of Mid‐Ocean Ridge Basalts

AbstractThe potential role of the lower oceanic crust in the source region of mid‐ocean ridge basalts has been examined by a series of partial melting experiments on three natural pyroxenite xenoliths entrained in alkali basalts well representing the lower oceanic crustal material at 1 GPa, 1100–1300°C. Layered experiments were also performed to simulate the reaction between lower oceanic crust‐derived melts and surrounding dunite and have successfully enabled the melt infiltration. Melt compositions are consistently basaltic and vary significantly as a function of melt fraction, and these lower oceanic crust‐derived melts are characterized by high Eu/Eu*, Nb/Ta, and Ba/Th values, especially for experiments conducted at 1300°C. No reaction zone is observed in layered experiments, and our results demonstrate that the compositional variations caused by the reaction with dunitic mantle in the lower oceanic crust‐derived melts are relatively minor at 1 GPa. Our melting experiments show that melting of the lower oceanic crust can provide trace element characteristics such as positive Sr and Eu anomalies and consistent Nb/Ta and Zr/Hf ratios, but it cannot account for the wide major element compositional variations (e.g., high FeOT and TiO2 contents) in mid‐ocean ridge basalt (MORB). Some component other than the surrounding dunite in the upwelling mantle flux is needed to substantially modify the major element compositions of the lower oceanic crust‐derived melts. It is quite likely that these lower oceanic crust‐derived melts could be extracted from the upwelling mantle while still preserving their trace element and isotopic imprintings.

The impacts of diagenesis on the geochemical characteristics and Color Alteration Index of conodonts

The conodont Color Alteration Index (CAI) has been widely used to determine the maximum temperature in carbonate rocks, despite recognition that conodont colour can be affected by other factors, such as diagenesis. Measurements of trace element characteristics in conodonts of varying CAI (1.5–6.0) from the Canadian Cordillera show that those specimens with the most anomalous CAI with respect to independent estimates of maximum temperature also have the highest concentrations of iron (> 3000 ppm). The adsorption of transition metals such as iron onto bioapatite crystals and permineralization by their oxides in conodont elements are herein proposed as mechanisms for the modification of CAI during diagenesis. Furthermore, the trace element characteristics of conodonts, primarily those of the lanthanide, or rare earth element (REE) series, have frequently been used as a proxy for palaeoceanographic conditions, including anoxic events. However, the impact of early diagenetic processes post-burial obscures this marine signal, and instead, the trace element characteristics of conodonts likely reflect the characteristics of pore waters or diagenetic fluids. Several geochemical ‘tools’ have been proposed to test for such overprinting of palaeoceanographic information, including Y/Ho vs. ΣREE, MREE/MREE*, U concentration, and La/Yb. However, REE and Raman structural characteristics in a suite of conodonts from the Canadian Cordillera indicate that these ‘tools’ cannot be systematically applied. The majority of conodont specimens analysed in this study appear to have been affected by post-burial diagenetic alteration using one or more of the ‘tools’, but seem to be unaltered when investigated using other ‘tools’. Additionally, several specimens which appear to be diagenetically unaltered when using the metrics of the geochemical ‘toolbox’ exhibit diagenetically induced structural changes. The geochemical ‘tools’ are thus unable to discriminate between diagenetic alteration induced before and after burial, and they should therefore be accompanied by structural analyses if REE characteristic of conodonts are used to infer palaeoceanographic conditions.

Melt-Lithosphere Interaction Controlled Compositional Variations in Mafic Dikes from Fujian Province, Southeastern China

Late Mesozoic magmatism in southeastern China has been widely considered to be related to the subduction of the Paleo-Pacific Plate. However, it remains controversial whether mafic rocks are derived from the lithosphere or the asthenosphere. Here we present a comprehensive study on mafic dikes from Fujian Province in southeastern China, aiming to understand their source. Two types of mafic rocks have been recognized based on their trace-element features. Type-I rocks show arc-like trace-elemental characteristics, while type-II rocks are distinguished by their relatively flat patterns in primitive-mantle-normalized trace-element diagram. Despite such differences between two types of rocks, these mafic dikes show two trends in the plots of 87Sr/86Sr(i) versus La/Nb, which can be explained by the influences of crustal contamination and melt-lithospheric mantle interaction, respectively. 87Sr/86Sr(i), La/Nb, Sr/Y and Zr/Y ratios of type-I rocks are significantly correlated to the thickness of the underlying lithosphere, and the signals of lithosphere are clearer with increasing lithospheric thickness. This highlights the important influences of melt-lithosphere interaction during their formation. Such observations also indicate that these mafic rocks are more likely to have been originated from the asthenosphere rather than the lithospheric mantle.

Palaeoenvironmental analysis of the Lower Silurian Longmaxi Formation in the Zhaotong area of the Sichuan Basin, South China: Implications for organic matter accumulation mechanisms

Geochemical data have been used as the most effective proxies for determining palaeoclimate and palaeoenvironmental conditions. In this article, we conduct a systematic review on a variety of geochemical data of sediments that are considered proxies for tracing palaeoclimate and palaeoenvironmental information. Comprehensive studies have been carried out on the palaeoproductivity, redox conditions, palaeoclimate, and the acquired data on the enrichment mechanism of organic matter have been summarized. The general mechanisms of the geochemical data regarded as the palaeoclimate and palaeoenvironmental archives include the characteristics of mineral composition, major and trace elements, and rare earth elements. The palaeoproductivity index, including the Ba and P, indicate that the organic‐rich shale was deposited in a high‐productivity water column. The trace elements revealed that the Longmaxi units recorded variable redox conditions from anoxic to aerobic. The elements V, U, Th, Ni, and Co together with the U/Th, V/Cr, Ni/Co, and V/(V + Ni) are essential indicators for defining the redox conditions. The NASC‐normalized REE + Y distribution indicates that the Longmaxi Formation was slightly altered by detrital materials in reduced and acidic environments. The Sr/Cu and MgO/CaO all reveal that the palaeoclimate was gradually transformed from humid to dry heat. The Sr/Ba ratio implies that the water depth fluctuates frequently at the early stage of sedimentation. All geochemical data integrated with the sedimentary characteristics show that anoxic conditions, high palaeoproductivity, and relatively humid palaeoclimates contributed to the accumulation of organic‐rich shale. We propose that jointly utilizing various geochemical and sedimentary indices can expand the outlook of the palaeoenvironment and provide a better understanding of the organic matter accumulation mechanisms.

Genesis of Cretaceous igneous rocks and its related large scale porphyry Cu-Au mineralization in Chating, the Middle-Lower Yangtze River Metallogenic Belt: The geochemical constrains

The newly discovered large Chating porphyry Cu–Au deposit is associated with early Cretaceous intrusive rocks in the Middle-Lower Yangtze River Belt (MLYRB), eastern China. Two types of intrusive rocks in the Chating area have been identified, i.e., the quartz-diorite porphyry, related to the Cu–Au mineralization, and the diorite porphyry that is ore-barren. Zircon LA–ICP–MS U–Pb dating yields 145–136 Ma for the Chating intrusions. They both belong to High-K calc-alkaline series, with enrichments of large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletions of high field-strength elements (HFSEs) and heavy rare earth elements (HREEs), and no or slightly negative Eu anomalies. The quartz-diorite porphyry shows high Sr contents and Sr/Y ratios; low MgO, Cr and Ni contents; medium (La/Yb)N and Mg# (34.6–54.6) and low K2O/Na2O (<1) values; decoupling of Sr/Y and (La/Yb)N; and low Th/U (3.49–6.08) values, thus exhibiting characteristics of slab-derived adakitic rocks. In addition, it has slightly enriched Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.70655–0.70717, εNd(t) = −6.9 to −8.8, and εHf(t) = −7.6 to −10.6) and high radiogenic Pb isotopes ((206Pb/204Pb)i = 18.22–18.70, (207Pb/204Pb)i = 15.63–15.66 and (208Pb/204Pb)i = 38.49–38.97), reflecting the involvement of subducted sediments in the magma source. Therefore, the fertile quartz-diorite porphyry was most likely generated by the partial melting of the slab-derived, subducted sediments and a partial degree of metasomatized mantle-derived magma with crustal assimilation. As to the barren diorite porphyry, it has similar Sr–Nd–Pb isotope and trace element features as the fertile adakitic samples. However, it shows relatively lower SiO2 contents; high contents of MgO (3.89–4.06 wt%), Cr and Ni (avg. 47.6 ppm and avg. 15.01 ppm); high Mg# (51.2–69.8); and more enriched Hf isotope characteristics (−8.5 to −12.9), which imply that this barren diorite porphyry could be generated from the partial melting of enriched metasomatised lithospheric mantle assimilated with old continental crust materials. The δ34S values of the pyrite and chalcopyrite of the ores in Chating indicate a magmatic-derived source, resembling other porphyry-skarn Cu–Au deposits in the MLYRB. The fertile Qtz-diorite porphyry adakitic rocks are characterized by higher oxygen fugacities than the barren diorite porphyry, which are closely related with the partial melting of the subducted oceanic slab in an oxidizing environment, thus making them favorable for Cu–Au mineralization.

Chemical compositions of sulfides in the porphyry Cu ores, Yangla Cu deposit, Yunnan, China: implication for ore genesis

The Yangla Cu deposit is the largest ore deposit in the Jinshajiang polymetallic metallogenic belt, northwest Yunnan, China. There is no consensus on the genesis of the ore deposit owing to the limited studies on the chemical compositions of sulfides. This study used an electron probe micro-analyzer to constrain the chemical compositions of pyrite, chalcopyrite, molybdenite, and sphalerite in the porphyry Cu ore of the Yangla Cu deposit and compared them with the chemical compositions of sulfides in the skarn Cu ore. The trace element contents and their occurrences were used to estimate the metallogenic temperature and infer the genesis of the Yangla deposit. The results show that the sulfides in the porphyry Cu ores have variations of ore element concentrations relative to their theoretical values. Pyrite is depleted in S but elevated in Fe; chalcopyrite is depleted in Cu, Fe, and S; and molybdenite and sphalerite are enriched in S whilst depleted in Mo and Zn. The concentrations of the main metallogenic elements Cu, Fe, Mo, Zn, and S in the porphyry are generally lower than those in skarn, suggesting that the porphyry ore was formed in a moderate to moderate-high temperature metallogenic environment. The formation time may also be slightly later than that of the skarn Cu ore. Elements such as As, Co, Cu, Pb, Zn, Mo, Cd, and Ni mainly exist as isomorphic replacements and mineral inclusions in the sulfides of both porphyry and skarn Cu ores. The trace element features of sulfides in the two ore bodies show that the Yangla Cu deposit may be a composite super imposed ore deposit, and its formation has undergone the process of exhalative-sedimentary to skarn-porphyry mineralization.