Signatures Of Rocks Research Articles

Assessment of Geogenic and Anthropogenic Pollution Sources Using an Aquatic Plant Along the Sonora River Basin: Insights from Elemental Concentrations and Pb Isotope Signatures

Mining is an important activity in Mexico; however, despite its economic benefits, it carries potential environmental risks, including mine spills. On August 6, 2014, ~ 40,000 cubic meters of copper sulfate acid solution was spilled from the Tinajas 1 dam of the Buenavista del Cobre mine in Cananea, Sonora, northwestern Mexico. The solution was directly spilled into the Tinajas creek, which is a tributary of the Sonora River. This event had regional socioeconomic and environmental consequences. That is because the Sonora River provides water for agricultural and livestock activities in the region, as well as to human consumption of products derived from these activities. In an attempt to assess the influence of this spill along the Sonora River, samples of watercress (Nasturtium officinale), an edible aquatic plant known for its ability to uptake metals, were collected along: (a) the 2014 spill route (zones 1 and 2); (b) a zone that experienced a spill in the 1980s (zone 3); and (c) a reference zone (zone 4). The watercress samples were analyzed for concentrations of Cr, Sb, Ba, U, Cu, Cd, Zn, Ni, Fe and Pb to evaluate the effects of the spills along the riverbed. The study was supported with measurements of Pb isotopic ratios to evaluate watercress samples in a two-end-member mixing scenario. The results indicate that no significant statistical differences were detected when concentrations in watercress samples from the 2014 spill route were compared with those from reference zone. Significant statistical differences and relatively higher concentrations for Cu, Zn and Cd were found when comparing watercress samples from the 1980s spill route with those from reference zone. Concentrations of Ba, U and Sb were relatively higher along the 2014 spill route, possibly associated with the highly differentiated Laramide intrusive rocks of the studied area. Concentrations of Cu and Zn along zone 3 exceeded the FAO/WHO values, as well as the geochemical baseline levels of the Sonora River basin. Concentrations of Fe and Pb exceeded the maximum FAO/WHO values in both cases, but did not exceed the geochemical baseline levels of the Sonora River basin. Concentration of Cd in watercress samples exceeded the geochemical baseline, and it was five to nine times higher than the FAO/WHO value. Regarding Pb isotope ratios, a linear arrangement was observed between the two end-members, which were comprised by a geogenic component defined by the Pb isotope signatures of rocks representing the study area, and the other member (anthropogenic) was defined by isotope ratio obtained from a sample collected from the spilled copper sulfate solution in 2014. Watercress samples collected on the spill route yielded Pb isotopes signatures that suggest an influence from the spill of ~ 65 and ~ 42%, for zones 1 and 2, respectively. Pb isotope ratios in watercress samples from zone 3 were closer to the anthropogenic end-member. These Pb signatures reflect a mine spill that occurred in the 1980s, when Buenavista del Cobre mine was operated by a previous company. Finally, in watercress samples from zone 4, the Pb signatures were more likely acquired from the geogenic component.

Slab-derived origin of tremolite–antigorite veins in a supra-subduction ophiolite: the Peridotite Nappe (New Caledonia) as a case study

Hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks (reservoirs involved, fluid/rock ratios, temperature of interaction). Highly depleted harzburgites and dunites of the Peridotite Nappe of New Caledonia are crosscut by fractures, which have been pervasively serpentinized, producing lizardite, brucite, magnetite and minor chrysotile in a near-static environment, probably by sea water circulating in cooling joints. This event is generally referred to as “primary serpentinization”. In a next step, already serpentinized joints were re-opened to produce tension and shear cracks sealed by higher-temperature synkinematic fibrous minerals. Locally, tremolite-bearing veins and pockets, which do not display evidence for void infill, were generated by metasomatic replacement of the wall rock peridotite. Most veins only contain fibrous antigorite but some display tremolite-antigorite intergrowths or even pure tremolite. The latter rocks yield high contents of Ca and incompatible elements, which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source. Whole-rock geochemical and isotopic features (87Sr/86Sr, 18O and H) suggest that antigorite veins, which bear highly radiogenic Sr isotope signatures, were strongly influenced by fluids emitted by the subducted slab and associated sediments. In contrast, the geochemical and isotopic signatures of tremolite-bearing rocks suggest a genetic link with Early Eocene supra-subduction dykes and fluids that leached them. Calcium, strontium and REE-bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation, chromite alteration and high Cr contents in tremolite. The bulk of syn-tectonic fluid-rock interaction associated with shear and tensile fracture development probably occurred at the onset of Eocene intra-oceanic subduction, when the buoyant lower plate (the South Loyalty Basin) obliquely forced its way beneath the nascent Loyalty fore-arc. Mobile elements extracted from supra-subduction dykes temporarily enriched the circulating fluids and generated tremolite, antigorite, Mg-chlorite and magnetite instead of antigorite and magnetite solely. Tremolite crystallization probably ceased due to the exhaustion of Ca-rich fluids in a globally cooling fore-arc environment.

Origin of mafic intrusions in the Micangshan Massif, Central China: Implications for the Neoproterozoic tectonic evolution of the northwestern Yangtze Block

The mafic intrusions exposed in the Micangshan Massif preserve important information pertaining to the Neoproterozoic tectonic evolution along the northwestern margin of the Yangtze Block. In this paper, we report new LA-ICP-MS zircon U–Pb ages, Lu–Hf isotopes and whole-rock compositions for mafic sills in the Micangshan Massif to understand their petrogenesis and supplement the regional tectonic evolution. Xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 ± 4 Ma and 858 ± 5 Ma, respectively, suggesting the maximum emplaced age of the mafic sills was at ca. 860 Ma. A granitic dyke sample which intruded into the mafic sills, yields a crystallization age of 799 ± 4 Ma, implying the minimum emplacement of the mafic sills was at ca. 800 Ma. These results indicate that the mafic sills were formed within the span of ca. 860–800 Ma. Geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate FeOtotal and TiO2 contents, as well as moderate to high MgO contents and Mg# values. They display distinct enriched light rare earth element (LREE) patterns, variable remarkable enrichment in large-ion lithophile elements (LILEs; e.g., Cs and Ba), and evident depletion of high-field strength elements (HFSEs; e.g., Nb, Ta and P), resembling signatures of typical subduction-related basaltic rocks. In addition, these sills exhibit comparatively high Nb/Ta, moderate Nb/Yb, as well as low Nb/La, Th/Yb, and Nb/Y ratios, indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source. These observations, together with regional tectonics, suggest that these mafic sills in the Micangshan Massif might be formed in a subduction-related continental arc setting. Therefore, combining our new data with available results from previous studies in the adjacent regions, our study supports the dominant subduction-related tectonics along the northwestern margin of the Yangtze Block during the Early Neoproterozoic time.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Vanadium and its isotope composition of river water and seawater: Analytical improvement and implications for vanadium isotope fractionation

Investigation of redox variations in recent and paleo-oceans has been of particular scientific interest to elucidate the rise and variations of the atmospheric oxygen level by analyses of isotopic signatures of redox-sensitive elements like Fe, Mo, and U. Vanadium is another redox-sensitive metal that has become the target of stable isotope research during the last decade. Research of the oceanic V cycle revealed a rather complex interplay of riverine V as a major V source to the oceans on one side with V deposition in sediments and at hydrothermal vents as major sinks on the other. The balance between these major V pools is sensitive to the ocean water oxygen level and chemistry. The current data set of stable V isotope signatures of seawater is still very small, but indicates already subtle variation of the V isotope signatures in the marine environment. However, the V isotopes of marine sediments and particularly the riverine V isotope composition of dissolved and particulate V, i.e. the major source of V in modern marine environments, has not been constrained at all so far.In this study, we present a new method for efficient V separation from seawater that allows multiple analyses of the V isotope composition of a single sample. To separate V from large amounts (volume ≥2L) of seawater samples, we employ the Bio-Rad® Chelex-100 resin and conventional cation and anion resins to yield a high V recovery of ≥90% from an UV-irradiated sample. Non-irradiated samples were marked by lower V recovery rates of ca. 75%, which was also observed in earlier studies. Further tests however revealed that even such reduced V yields do not incur significant V isotope fractionation within analytical uncertainty. Our δ51VAA value of +0.27‰ ±0.14 (2s.d., n=3) for the NASS-6 seawater reference solution perfectly matched earlier results. In addition, seawater collected in the Wadden Sea at the German North Sea coast is marked by a δ51VAA signature of around +0.02‰ ±0.19 (2s.d., n=17), which is slightly lower than those of the great oceans, and may be related to an influx of river water, bioactivity, or a tide-induced V mobilization.To characterize the V isotope composition of the major V source to the oceans, we determined for the first time V isotope signatures of 13 selected rivers (dissolved and particulate fractions of source water, tributary rivers, and the Yangtze River) in the Yangtze River Basin, China. A large variation of dissolved V (ca. 0.07 to 6.0μg/L) and particulate-bound V (ca. 0.03 to 17μg/L) was found for the sample suite. The obtained δ51VAA values of the dissolved V pool span a range of −0.76‰ (±0.18; 2s.d.) to −0.10‰ (±0.22, 2s.d.), whereas particulate-bound V extends to lower δ51V signatures between −2.13‰ (±0.30, 2s.d.) and −0.11‰ (±0.11, 2s.d.). Notably, dissolved V from the river sources and small tributaries scatters between ca. −0.4‰ to −0.7‰, and agrees well with the predicted average δ51VAA value of −0.6‰ ±0.3 for continental run-off. For the lower Yangtze River, however, the dissolved δ51VAA signatures increase from the Three-Gorges Dam towards the estuary from −0.76‰ to −0.10‰, suggesting V isotope fractionation due to adsorption to abundant particulate Fe oxides, but may also reflect an input of anthropogenic V. The low δ51VAA of particulate V largely follow this trend, and thus indicate ongoing V isotope fractionation during riverine V transport to the ocean.Our first results of stable V isotope investigation of river waters show that V isotope signatures can indeed carry their host rock signature, but are also sensitive to adsorption-driven fractionation in oxidized environments. The latter strongly depends, as predicted from earlier theoretical calculations, on the presence of particulate Fe-(oxyhydr)oxides and highlights gradual V isotope fractionation during riverine V transport to the ocean.

Biomarker signatures of Upper Cretaceous Latrobe Group petroleum source rocks, Gippsland Basin, Australia: Distribution and geological significance of aromatic hydrocarbons

Aromatic hydrocarbons extracted from Upper Cretaceous (Maastrichtian and Campanian) Latrobe Group coaly shales and shales from the Gippsland Basin were investigated by gas chromatography–mass spectrometry. A variety of 2-ring to 6-ring aromatic hydrocarbons were identified, including both higher-plant-derived and combustion-derived polycyclic aromatic hydrocarbons (PAHs). These aromatic compounds indicate that the Upper Cretaceous coaly shales and shales were deposited in a fluvial/deltaic environment with significant higher plant input and are at a relatively low thermal maturity stage. Higher plant parameters and angiosperm/gymnosperm indices based on aromatic components suggest that higher land plants, including both angiosperms and gymnosperms, were an important part of the palaeovegetation in the Upper Cretaceous. The aromatic angiosperm/gymnosperm index that includes chrysene- and picene-type PAHs is not as reliable for evaluation of relative angiosperm and gymnosperm contributions as the aliphatic indices due to input of combustion-derived PAHs and the complexity of burning conditions altering the relative contributions. However, combustion-derived PAHs and aromatic angiosperm/gymnosperm indices supplement the information from aliphatic hydrocarbons and provide further evidence to support the hypothesis that palaeoclimate cooled from the Campanian to the Maastrichtian.

Evidence for ancient fractional melting, cryptic refertilization and rapid exhumation of Tethyan mantle (Civrari Ophiolite, NW Italy)

Western Tethyan peridotites exposed in the European Alps show limited amounts of partial melting and mostly fertile compositions. Here we investigate the Civrari Ophiolite (northwestern Italy), which is composed of depleted spinel-harzburgites and serpentinites associated with MOR-type gabbros and basalts. The ultramafic rocks are unique amongst western Tethyan peridotites, showing homogeneous residual compositions after ~ 15% near-fractional melting, lack of pervasive melt percolation and mineral compositions that indicate high-temperature equilibration ≥ 1200 °C. Clinopyroxene chemistry records some of the lowest abundances of Na2O, Ce, and Zr/Hf amongst abyssal peridotites worldwide, suggesting that most abyssal peridotites have been affected by variable degrees of melt retention upon melting or cryptic melt percolation. Locally, cryptic MORB-like melt migration in Civrari peridotites produced orthopyroxene + plagioclase intergrowth around reacted clinopyroxene. These clinopyroxene preserve micron-scale chemical zoning indicating rapid cooling after melt crystallization. 143Nd/144Nd isotopic data indicate that Civrari mantle rocks, gabbros, and basalts are not in isotopic equilibrium. Civrari spinel-peridotites represent a highly radiogenic endmember amongst Western Tethys depleted spinel-peridotites, which together form a partial melting errochron of 273 Ma ± 24 Ma. Ancient near-fractional melting and cryptic melt–rock reaction cause variations in radiogenic εNd and εHf, leading to isotopic heterogeneity of Western Tethys mantle rocks. Such inherited signatures in mantle rocks are most likely to be preserved along (ultra-)slow-spreading systems and ocean–continent transition zones.

Consistent experimental investigation of the applicability of Biot-Gassmann’s equation in carbonates

Carbonate formations are characterized by multiscale heterogeneities that control their acoustic response and flow properties. At the laboratory scale, carbonate rocks do not indicate a strong correlation between P- and S-wave velocities and porosity. The velocity disparities between carbonates of similar mineralogy and porosity result from different microstructures derived from their sedimentary facies and subsequent diagenetic transformations. The still-discussed applicability of Biot-Gassmann’s equation for fluid substitution in carbonate rocks remains another key issue. We have developed an integrated experimental workflow that allows a consistent checking of the applicability of Biot-Gassmann’s equation and provides key geologic and microstructural information to understand the petroacoustic signature of carbonate rocks. The defined approach is based on the phase-velocity measurements performed in liquid-saturated conditions using polar and nonpolar fluids. It allows the identification of the whole set of parameters required by Biot-Gassmann’s equation including the bulk modulus of the solid matrix. This approach is implemented on samples representative of two different carbonate formations deposited in lacustrine and marine environments, respectively. The obtained results demonstrate the applicability of Biot-Gassmann’s equation for the two studied carbonate families and indicate the link between their petroacoustic signature and diagenetic history.

Zr/Hf ratio and REE behaviour: A coupled indication of lithogenic input in marginal basins and deep-sea brines

The distribution of dissolved Zr, Hf and Rare Earth Elements (yttrium and lanthanides, hereafter referred to as REE) in the Eastern Mediterranean seawater column was measured in the Kryos basin to evaluate the lithogenic contribution from both Nile River and Sahara and Arabian desert dust. We found dissolved Zr/Hf ratios below the signature of crustal rocks and chondrites; a phenomenon likely driven by the dissolution of the Mn-rich coating of atmospheric dust particles delivered from the desert. In deeper waters, Zr/Hf ratios are clustered close to the signature of crustal rocks and chondrites according to the different Zr and Hf dissolved speciation. The Zr/Hf ratio observed in the deeper 3300 m seawater column corresponds to the value found in the Atlantic Ocean. The REE distribution displays a negative Ce anomaly and a positive La anomaly throughout the shallow waters above a 75 m depth. Zr, Hf, and REE concentrations measured in the anoxic deep-sea brines from Tyro, Medee, and Thetis basins indicate that the dissolution of evaporite within Messinian levels is responsible for the high salt content of these brines and influence Zr/Hf fractionation in the aqueous phase.Hafnium shows stronger affinity relative to Zr for surfaces of salts and Mn–Fe oxyhydroxides. Zr/Hf values and the observed shale-normalised REE features suggest that Zr, Hf and REE are leached from Mn and Fe-rich coatings of desert particles in shallow water layers. This process can be responsible for the observed distribution of studied elements in the studied Eastern Mediterranean oxic seawater. In deep-sea brines, Zr, Hf and REE are released from Messinian evaporites.

Magnetic Mineralogy and Petrophysical Properties of Ultramafic Rocks: Consequences for Crustal Magnetism

AbstractMagnetic properties from the Reinfjord Ultramafic Complex, in northern Norway, which formed as part of a deep magmatic conduit system, have been investigated to determine the magnetic signature of ultramafic rocks now exposed at the surface and deeper in the lower crust. The dominant carriers in these ultramafic rocks are a chrome‐spinel with Fe‐rich exsolution blebs and exsolution lamellae of magnetite in clinopyroxene. Except locally, in a fault zone and in discrete small fractures, these rocks show only minor to no alteration. We infer that the magnetic oxides characterized here are representative of pristine magnetic carriers in similar rocks deeper in the crust. These oxides can be stable in lower crustal, possibly upper mantle, depths when temperatures are below the Curie temperature of magnetite, taking into account pressure effects. These ultramafic rocks are candidates for potential sources of long‐wavelength anomalies.

A gradual transition to plate tectonics on Earth between 3.2 to 2.7 billion years ago

AbstractZircon crystals precipitated from granitoid magmas contain a robust record of the age and chemistry of continental magmatism spanning some 4.375 Ga of Earth history, a record that charts initiation of plate tectonics. However, constraining when exactly plate tectonics began to dominate crustal growth processes is challenging as the geochemical signatures of individual rocks may reflect local subduction processes rather than global plate tectonics. Here we apply counting statistics to a global database of coupled U–Pb and Hf isotope analyses on magmatic zircon grains from continental igneous and sedimentary rocks to quantify changes in the compositions of their source rocks. The analysis reveals a globally significant change in the sources of granitoid magmas between 3.2 and 2.7 Ga. These secular changes in zircon chemistry are driven by a coupling of the deep (depleted mantle) and shallow (crustal) Earth reservoirs, consistent with a geodynamic regime dominated by Wilson cycle style plate tectonics.

Geochemistry of cretaceous fine-grained siliciclastic rocks from Upper Mundeck and Logbadjeck Formations, Douala sub-basin, SW Cameroon: Implications for weathering intensity, provenance, paleoclimate, redox condition, and tectonic setting

The chemical composition of the Upper Mundeck Formation (UMF) and Logbadjeck Formation (LF) from the Douala sub-basin (Cameroon) has been investigated to determine their provenance, the influence of the weathering, the paleoclimate, and the redox condition upon the source rock signature, as well as to decipher the tectonic setting of these sediments. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and various paleoweathering proxies suggest that these rocks underwent high degree of chemical weathering in the source region. The intensity of weathering was higher for LF relative to UMF samples. The ACNK plot, coupled to the mineralogical assemblage, dominated by the occurrence of illite, indicates that post-depositional K-enrichments may have affected the bulk composition of the studied rocks. The Post Archean Australian Shale (PAAS)-normalized patterns of the investigated rocks show mostly moderate to high LREE/HREE ratios (up to 20.75) and negative to positive Eu anomalies, suggesting a provenance from felsic rock with negligible contribution of mafic components. The Rb/Sr ratio for UMF (0.43 ± 0.1) and LF (0.56 ± 0.31) suggests a generally warm condition during their deposition. The V/Cr and U/Th ratios of UMF (3.29 ± 4.94 and 0.24 ± 0.10, respectively) and LF (1.67 ± 1.34 and 0.17 ± 0.03, respectively), and the authigenic uranium (AU) content of both formations indicate an oxic depositional environment. The Sr/Ba ratio of UMF (0.24 ± 0.21) and LF (0.33 ± 0.43) suggests an overall low salinity of the depositional environment. The tectonic discriminant-function-based diagram revealed a rift or a collisional setting for the studied rocks, which is inconsistent with the general geology of the Douala sub-basin, revealing probably the complex history of the source rock.

Geochemistry of the Monte Filau orthogneiss (SW Sardinia, Italy): insight into the geodynamic setting of Ordovician felsic magmatism in the N/NE Gondwana margin

The Mt. Filau orthogneiss is an Ordovician orthogneiss outcropping in the External zone of SW Sardinia chain. It consists of dark, biotite-rich facies, a leucocratic coarse-grained facies and a leucocratic fine-grained facies with igneous andalusite. Coarse- and fine-grained leucocratic orthogneiss have comparable major elementcontents, being slightly enriched in SiO2 and depleted in Fe2O3, MgO, TiO2 and CaO as compared to the biotite-bearing orthogneiss. Bt-bearing orthogneiss shows higher Sr and Ba concentrations than leucocratic ones, whereas Rb content is higher in leucocratic orthogneiss as compared to the Bt-bearing ones. Zr content shows a progressive decrease from biotite-bearing orthogneiss, coarse-grained leucocratic ones, to fine-grained orthogneiss. In the spider diagram the Mt. Filau orthogneiss shows the typical signature of calc-alkaline rocks, with negative anomalies of Ba, Nb, Sr and Ti, and positive anomalies in U, K. REE patterns of Bt-bearing and coarse-grained leucocratic orthogneisses are characterized by a moderate LREE fractionation, flat HREE and negative Eu anomaly. Fine-grained leucocratic orthogneiss shows flatter patterns, stronger Eu anomalies and slight HREE enrichment. The geochemical features suggest a clear evolution trend, from the less evolved Bt-bearing orthogneiss to the more differentiated fine-grained leucocratic orthogneisses which likely represent aplite bodies deriving from the most acidic residual melt. Selected trace and REE elements of Mt. Filau are compared with other Ordovician orthogneiss outcropping in the Axial zone of Sardinia Variscan belt. Besides, their geochemical features are also compared with metavolcanics of External and Nappe zone. The geochemical affinity of orthogneisses and metavolcanics from Variscan Sardinia, together with the geochronological data, allows to state a clear cogenetic relationship between the bodies belonging to the calc-alkaline Ordovician magmatic cycle. Our results suggest that the early Paleozoic basement of Sardinia might represents the witness of an early Paleozoic subduction-accretionary complex recording convergence along the N/NE Gondwana margin.

Geochemical signatures of waste rocks around Sarcheshmeh porphyry copper mine dumps, southeastern Iran: Implications for exploration, economic by-products and the environment

The Sarcheshmeh porphyry copper deposit (1.7 Gt @ 0.65% Cu, 0.02% Mo, 1.22 ppm Ag, 0.06 ppm Au) is one of the giant Miocene continental arc porphyry ore systems in the Tethyan metallogenic copper belt. The supergene sulfide ore zone (6 Mt. @ 2.43% Cu, 0.022% Mo, 1.55 ppm Ag and 0.114 ppm Au) is mainly capped by a hematitic gossan. This hematitic gossan is a possible surficial exploration target for unexposed porphyry copper ores. The total tonnage of waste rocks is about 2.6 G tonnes of gossanized as well as sulfide-bearing waste rocks with an average modal content of 10% pyrite-chalcopyrite. The gossanized waste rocks show boxwork texture and are composed of hematite, goethite, jarosite, malachite, chalcanthite, brochantite and delafossite. In order to assess the exploration, economic and environmental aspects of chalcophile and precious elements in the waste rocks, a total of 86 waste rocks and 10 residual gossan samples were collected by a systematic random sampling method, analysed by XRF and ICP-MS methods for major oxides and 50 minor and trace elements at ALS Minerals Lab, Ireland. The highly anomalous elements in immature residual gossans are Au (0.775 ppm), Ag (13.4 ppm) and Mo (122 ppm). The gossanized waste rocks are also enriched in Au (0.608 ppm), Ag (87 ppm) and Mo (2960 ppm). These geochemical signatures could be used as possible exploration guides for concealed porphyry copper deposits. The enrichment of Au, Ag and Mo is caused by the lower mobility of these elements in the immature gossans and gossanized waste rocks. The elemental enrichment factors normalized to the Sarcheshmeh ore zone are Au (2.50), Pd (1.05), Ag (4.67), Pb (4.6), Zn (17.34), Sn (2.64), W (1.47), Re (22.22), Se (5.38), Te (5.75), Tl (4.1), Bi (3.29), Ga (1.15), In (11.5), La (1.2), Ce (1.2) and Y (4.1), respectively. The mean contents of the most valuable by-product elements in the gossanized waste rocks include Cu (3168.2 ppm), Mo (180.22 ppm), Au (0.085 ppm), Ag (5.7 ppm), W (42.76 ppm), Re (0.01 ppm), Bi (10.87 ppm) and Se (13.45 ppm). The S-NNP diagram highlights that the sulfide-bearing waste rocks have also a negative impact on the environment, are prone to generate acid mine drainage (AMD) and release the potentially harmful metal (loids): Pb (107 ppm), Zn (365.3 ppm), As (83.53 ppm), Sb (10.24 ppm), In (0.44 ppm), and Se (13.45 ppm) into the surrounding area. These elemental values occurred at concentrations about 4–269 times greater than the crustal abundances and natural background of fresh granodiorite. As such, the heavy metal-rich gossans and sulfide-bearing waste rocks around the Sarcheshmeh mining areas may be considered as an exploration guide for valuable by-product elements as well as environmental concern.

Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts

The isotopes of chlorine (37Cl and 35Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ37Cl (‰) = [(37Cl/35Clsample/37Cl/35ClSMOC) − 1] × 1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14‰ (Apollo 15), +10 to +19‰ (Apollo 12), +9 to +15‰ (70017), +4 to +8‰ (MIL 05035), and +15 to +22‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al. (2015) and Barnes et al. (2016), as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust.

Early Cretaceous trachybasalt-trachyte-trachyrhyolitic volcanism in the Nyalga basin (Central Mongolia): sources and evolution of continental rift magmas

As shown by geological, mineralogical, and isotope geochemical data, trachybasaltic-trachytic-trachyrhyolitic (TTT) rocks from the Nyalga basin in Central Mongolia result from several eruptions of fractionated magmas within a short time span at about 120 Ma. Their parental basaltic melts formed by partial melting of mantle peridotite which was metasomatized and hydrated during previous subduction events. Basaltic trachyandesites have high TiO2 and K2O, relatively high P2O5, and low MgO contents, medium 87Sr/86Sr(0) ratios (0.70526-0.70567), and almost zero or slightly negative εNd(T) values. The isotope geochemical signatures of TTT rocks are typical of Late Mesozoic basaltic rocks from rift zones of Mongolia and Transbaikalia. The sources of basaltic magma at volcanic centers of Northern and Central Asia apparently moved from a shallower and more hydrous region to deeper and less hydrated lithospheric mantle (from spinel to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The geochemistry and mineralogy of TTT rocks fit the best models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic magmas. Mass balance calculations indicate that trachytic and trachydacitic magmas formed after crystallization of labradorite-andesine, Ti-augite, Sr-apatite, Ti-magnetite, and ilmenite from basaltic trachyandesitic melts. The melts evolved from trachytic to trachyrhyodacitic and trachyrhyolitic compositions as a result of prevalent crystallization of K-Na feldspar, with zircon, chevkinite-Ce, and LREE-enriched apatite involved in fractionation. Trachytic, trachyrhyodacitic, and trachyrhyolitic residual melts were produced by the evolution of compositionally different parental melts (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively), which moved to shallower continental crust and accumulated in isolated chambers. Judging by their isotopic signatures, the melts assimilated some crustal material, according to the assimilation and fractional crystallization (AFC) model.

What is ZTEM seeing over these tropical porphyry deposits?

A ZTEM survey over the Cobre Panama mine lease, which highlights the known porphyry copper deposits in the cluster, is credited with the discovery of a sixth deposit that has been included in the mine plan. The source of the ZTEM response is within the fresh rock below weathered saprolite and extends to a depth of many hundreds of metres, as does the orebody. The response is at least partly due to the sulphide content of the orebody, both pyrite and chalcopyrite. However, the sulphide percentage of all the deposits is quite low, to a maximum of 3% in significant volume. A shallow airborne TEM survey also detects a near-surface, fresh rock signature associated with the ZTEM response of each deposit. Thus the TEM response should probably be attributed in part to the associated alteration, mainly sericitic, and this will contribute to the intensity of the ZTEM signature as well. Inversion modelling in 2D and 3D indicates the ZTEM is detecting deeper parts of the orebodies, with the correct gross geometries, including the main sulphide mineralisation. These conclusions are supported by analysis of multiple deposits in the cluster.

Multistage mineralization at the hypozonal São Sebastião gold deposit, Pitangui greenstone belt, Minas Gerais, Brazil

São Sebastião is a recently discovered Archean gold deposit at the northwesternmost part of the Quadrilátero Ferrífero (QF), southern São Francisco craton, Brazil. The gold ore is strata-confined in two levels of banded iron formation (BIF) of the basal stratigraphic unit in the Pitangui greenstone belt (2.8 Ga). Pitangui is correlated to the widely known Rio das Velhas greenstone belt, which hosts several orogenic gold deposits such as the world-class Cuiabá. The gold mineralization at São Sebastião is associated to replacement-style sulfidation over magnetite, where the main developed phases are: pyrrhotite, chalcopyrite, arsenopyrite, pyrite and arsenian-pyrite. Native Au occurs as inclusions in pyrrhotite formed at dilatation domains (e.g., breccias, fold hinges) during compressional events (Rio das Velhas Orogeny at 2.8–2.75 Ga), but mainly as inclusions in late- to post-kinematic pyrite and arsenopyrite, as fracture infills in arsenopyrite and in contact with gangue minerals. Electron-microprobe analyses in arsenopyrite reveal a 465–560 °C precipitation temperature interpreted from As atomic percentage and indicate that high-temperature fluids were active after the deformational event. The temperature increase caused the incorporation of LA-ICP-MS-detected lattice-bound Ni and Co in arsenian-pyrite and arsenopyrite, related to the mafic-ultramafic signature of boundary rocks. High temperature conditions are supported by pyrite and ISS (intermediate-solid solution in the Cu-Fe-S system) formation at 600 °C and melting-derived textures resulting from the precipitation of Bi- and As-melts, typical of amphibolite-facies environments with high fS2. Whole-rock geochemical assays of drill core samples display two clear trends on Ag, Bi, Cu vs. Au binary plots and on principal component analysis results. The trends are a product of the heterogeneous spatial distribution of melts and high-temperature fluids. Melting features and superposition of mineralization styles are common in hypozonal gold deposits experiencing higher heat influx. At São Sebastião, the thermal effect is likely related to the 2.7 Ga granitic intrusions that surround the Pitangui belt. São Sebastião shows distinct characteristics than those described for the mesozonal gold deposits in the Rio das Velhas greenstone belt and may depict the deeper roots of the QF gold system.

Petrological and Geochemical Constraints on the Origin of Early Cretaceous Volcanic Rocks in the Central‐East Asia: Implications for Crustal Growth and Evolution

AbstractNew petrological and geochemical data for Early Cretaceous volcanic rocks in the North China Craton (NCC) combined with published data of cotemporaneous volcanic rocks in the NCC and eastern Central Asian Orogenic Belt (ECAOB) are used to understand their petrogenesis and crustal growth and evolution. Early Cretaceous volcanic rocks in the NCC show characteristics typical of continental crust, with negative Nb and Ta anomalies, and enrichment of K and Pb. The εNd of Early Cretaceous volcanic rocks in the NCC correlate with indices of differentiation (e.g., Mg#), implying that mantle‐derived magmas were modified by open‐system processes. The high‐Mg clinopyroxene and melt inclusions hosted by high‐Fo olivine in Early Cretaceous volcanic rocks in the NCC have lower 207Pb/206Pb and 208Pb/206Pb ratios than low‐Mg clinopyroxene and melt inclusions hosted by low‐Fo olivine, which can be explained by the interaction between lower crust and basaltic magmas (magma mixing or assimilation). Unlike contemporaneous counterparts in the NCC, Early Cretaceous volcanic rocks in the ECAOB have high εNd values. Such features can also be produced by crustal assimilation and/or magma mixing, given the juvenile lower crust beneath the ECAOB. The “adakitic” signature of Early Cretaceous volcanic rocks is most likely inherited from the lower continental crust. Large‐scale partial melting of the lower crust may have been related to continuous magmatic underplating in an extensional environment. This work suggests that Early Cretaceous volcanic rocks in the Central‐East Asia represent a nonnegligible contribution to crustal growth, despite having “crust‐like” trace element and isotopic characteristics.

Differentiation mechanisms of the early Hadean mantle: Insights from combined 176Hf-142,143Nd signatures of Archean rocks from the Saglek Block

Positive 142Nd anomalies in Eoarchean rocks provide evidence for early (4.40 ± 0.03 Ga; Morino et al., 2017) depletion of Earth's mantle. This model age lies within errors of the Pb-Pb “age of the Earth” (Connelly and Bizzarro, 2016), and is similar to model ages inferred for crystallization of the lunar mantle (McLeod et al., 2014), implying that this large-scale event may reflect crystallization of a magma ocean following the Moon-forming impact. However, differentiation mechanisms responsible for the formation of this early depleted mantle reservoir and the depth at which it formed cannot be constrained from the Sm-Nd isotope system alone, because the magnitude of Sm/Nd fractionation during partial melting or fractional crystallization shows little dependence on pressure-controlled changes in mantle mineralogy. In contrast, the Lu-Hf isotope system is highly dependent on mineralogy, notably the presence or absence of garnet, an upper mantle phase, and thus may be used to constrain the pressure of fractionation. This study provides the first 176Lu-176Hf isotopic results on mafic and ultramafic rocks belonging to the Eoarchean (Nulliak) and Mesoarchean suites of the Saglek Block (northern Labrador, 3.2–3.9 Ga). The 176Lu-176Hf dating confirms the distinction between these two groups of rocks and provides ages consistent with those obtained from 147Sm-143Nd dating. The whole rock 176Lu-176Hf errorchrons yield ages and initial epsilon values of 3766 ± 140 Ma, ε176Hfi = 6.0 ± 2.5 and 3023 ± 390 Ma, ε176Hfi = −0.3 ± 2.5 for the Nulliak suite and the Mesoarchean suite respectively. The time-integrated 176Lu/177Hf for the sources of the Nulliak and the Mesoarchean suites considering a time of differentiation at 4.40 ± 0.03 Ga are estimated to be 0.047 ± 0.005 and 0.033 ± 0.005, respectively. For the Mesoarchean samples, the combined 146,147Sm-142,143Nd and 176Lu-176Hf data are consistent with a near-chondritic mantle source. On the other hand, Nulliak ultramafic rocks were derived from a mantle reservoir with superchondritic Lu/Hf and Sm/Nd. The Nulliak parent reservoir, however, does not plot on the ε176Hf-ε143Nd mantle array defined by modern oceanic basalts. Instead, the Nulliak source more likely belongs to a distinct array defined by Eo- and Meso-Archean komatiites. These results are interpreted in the framework of a simple model of crystallization of a primordial magma ocean. It appears that the fractionation observed in the mantle source of Nulliak was most likely generated by crystallization of a garnet-bearing assemblage in the shallow mantle, above the transition zone rather than by perovskite fractionation in the lower mantle. To preserve this depleted reservoir from the rest of the hot and vigorously convecting mantle, the Nulliak mantle source may have been isolated either at the top of the mantle in a buoyant lithosphere or near the core-mantle boundary, with the latter setting being more consistent with the komatiitic nature of the erupted rocks. Given that the garnet signature argues for differentiation of the Nulliak source at relatively shallow depth (few hundred kilometers), its isolation in the deep mantle would require a cumulate overturn following crystallization of the magma ocean.