Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion,Tyndrum, Scotland

Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or ‘refertilised’ mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits.     

The Asama igneous complex, central Japan: An ultramafic-mafic layered intrusion in the Mikabu greenstone belt, Sambagawa metamorphic terrain

The Asama igneous complex comprises layered mafic and ultramafic plutonic rocks exposed over about 500×6000 m in the Mikabu greenstone belt, Sambagawa metamorphic terrain of Mie Prefecture; its margins terminate by faults, and there is no trace of chilled rocks. The exposed layered sequence is about 460 m thick, and includes dunite, plagioclase wehrlite, olivine gabbro and two-pyroxene gabbro. The crystallization sequence of essential cumulus minerals is olivine, followed by plagioclase and clinopyroxene together, and finally the appearance of orthopyroxene. Olivine systematically varies in composition from Fo₈₉ to Fo₇₈ with stratigraphic height in the lower to middle portion of the layered sequence. The composition of clinopyroxene changes from Ca₄₉Mg₄₆Fe₅ to Ca₄₀Mg₄₇Fe₁₃ upward in the layered sequence; cumulus orthopyroxene, which occurs at the top of the exposed layered sequence, has a composition of Ca₂Mg₇₄Fe₂₄. Cumulus chromite occurs as disseminated grains in peridotitic rocks, and tends to increase its Fe³⁺/(Cr+Al+Fe³⁺) ratio with stratigraphic height. The most aluminous chromite [Cr/(Cr+Al) = 0.48] occurs in dunite that crystallized shortly before plagioclase began to separate as an essential phase. The Cr/(Cr+Al) ratio of the most aluminous chromite, coupled with the crystallization order of essential minerals, suggests that the Asama parental magma was moderately enriched in plagioclase and clinopyroxene components in the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. It was similar to a Hawaiian tholeiite and different from the Bushveld and Great “Dyke” parental magmas that were more enriched in orthopyroxene component; it also differed from mid-oceanic ridge basalts that are more depleted in the orthopyroxene component. The Asama clinopyroxene and chromite show characteristically high TiO₂ contents and are also similar to those in Hawaiian tholeiites. The Asama igneous complex probably resulted from the crystallization of a magma of a Hawaiian (oceanic-island) tholeiite composition and formed in an oceanic island regime.     

I-Xe dating: From adolescence to maturity

The I-Xe chronometer is based upon decay of now-extinct ¹²⁹I where the ratio of accumulated daughter ¹²⁹Xe to stable ¹²⁷I reflects the iodine isotopic ratio at closure of the host mineral. Since none of the parent remains, I-Xe is by nature a relative chronometer but, when referenced by a standard mineral of known age, the I-Xe system becomes an absolute chronometer reflecting true closure times. Most iodine hosts are secondary minerals so the I-Xe system is unique in providing details of post-formational chronometry not readily available with other chronometers. The short half-life of ¹²⁹I gives it exceptional precision. However, the secondary nature of iodine host minerals, combined with the inherent precision of I-Xe, were responsible for a large database of “whole-rock” I-Xe ages that were not easily interpreted. As this problem evolved historically, doubts were cast upon the viability of the I-Xe system as a chronometer which persisted until it was tested against other chronometers in single-mineral systems. Properly calibrated, absolute I-Xe ages reflect the true closure time of the host minerals, and sequences of closure times in different hosts provide cooling rates for the parent object.     

鸡西盆地张新地区浅成侵入体对围岩有机质成熟度及变质程度的影响

利用鸡西盆地张新地区西风井揭示的连续剖面,采用镜质体反射率、伊利石结晶度分析测试方法对西风井浅成岩侵入后的上盘围岩有机质成熟度( Ro) 、变质程度进行研究,发现上盘围岩的镜质体反射率随远离辉绿玢岩体的距离增加而减少( 由2. 31%到0. 79%) ; 侵入体对上盘围岩有机质影响最大距离为129. 06 m,是岩体厚度的1. 2 倍; 伊利石结晶度数据显示围岩处于极低级变质程度,围岩的温度随远离辉绿玢岩体的距离增加而降低; 计算得出西风井城子河组地层最大埋藏温度为 259. 24℃,持续时间117. 7 a。最高温度及其持续时间决定围岩的变质程度,是围岩接触变质程度较低的原因。     

Corrected interface flow model for seawater intrusion in confined aquifers: relations to the dimensionless parameters of variable-density flow

Sharp-interface (or interface) flow models with Dupuit-Forchheimer approximation are widely used to assess, to first order, an aquifer’s vulnerability to seawater intrusion (SWI) and to evaluate sustainable management options for coastal groundwater resources at the screening level. Recognising that interface flow models overestimate SWI, corrections have been proposed to account for the neglected mixing and also for the outflow through a finite gap. These corrections, however, were introduced in the context of specific studies and may not be generally applicable as proposed. The interface model is revisited, placing its corrections in the context of variable-density flow (VDF) theory, by expressing them in terms of the dimensionless parameters governing VDF in schematised (aspect ratio?=?thickness/length) homogeneous confined coastal aquifers: the coupling parameter (α), a Péclet number (Pe), and the dispersivities ratio (r_α). Interfaces are compared to the 50%-salinity lines of VDF numerical solutions and regression equations are developed for estimating the outflow gap and for correcting the length of the interface (terminating with a blunted edge); the dispersion correction, which modifies the interface curvature, is restated with a variable exponent. The corrections for dispersion and for the interface length appear to be the most effective; an outflow gap is important only at small α values (strong advection relative to vertical flow due to density differences). These concepts are applied successfully to calculate the interface position in the lowermost confined sub-unit of the Coastal Plain aquifer of Israel, as an estimate of SWI.     

Carbonate-hosted talc deposits in the contact aureole of an igneous intrusion (Hwanggangri mineralized zone,South Korea): geochemistry,phase relationships,and stable isotope studies

Many talc deposits occur in the Hwanggangri Mineralized Zone (HMZ) in dolomitic marbles of the Cambro-Ordovician Samtaesan Formation within 1 km of the contact with the Cretaceous Muamsa Granite. Talc commonly forms fine-grained, fibrous aggregates, or pseudomorphs after tremolite; abundant tremolite is included as impurities in the talc ore. Talc generally was derived from tremolite in calc-silicate rock within the dolomitic marble. Calc-silicate rock, consisting mainly of tremolite and diopside, was generated from silicic metasomatism during the prograde stage, which promoted decarbonation reactions until dolomite was exhausted locally. Hydrothermal alteration of calc-silicate rock to talc is marked by the addition of Mg and Si, and the leaching of Ca; Cr, Co, and Ni were relatively immobile during the retrograde stage. Contact metamorphism related to the granite intrusion generated the successive appearance of tremolite, diopside, and forsterite, or wollastonite-bearing assemblages in the marble, depending on the bulk rock composition. The X_CO2 content of the metamorphic fluids rose initially above X_CO2=0.6, and decreased steadily toward a water-rich composition with increasing temperature above 600 °C in the calcitic marble, while buffered reaction of the dolomitic marble occurred at higher X_CO2 conditions above 600 °C. Talc mineralization developed under metastable conditions with infiltration of large amounts of igneous fluids along a fault-shattered zone during the retrograde stage and is characterized by the loss of Ca²⁺ with the addition of Mg²⁺. Oxygen and carbon isotopic variations of carbonate and calc-silicate minerals are in agreement with theoretical relationships determined for decarbonation products of contact metamorphism. Talc formation temperatures obtained from oxygen isotope fractionation, T–X_CO2 relationships, and activity diagrams range from 380 to 400 °C.     

微生物对海水入侵响应特征及指示意义的研究进展

海水入侵是全球性环境地质问题,对沿海城市的供水安全及生态环境造成严重威胁。微生物对环境变化具有高度的敏感性,近年来许多学者开始关注微生物对海水入侵的响应特征,为海水入侵调查研究提供了新的思路与方法。为了充分认识该领域的研究进展,基于Web of Science 核心数据库,利用文献计量学可视化分析方法对海水入侵微生物响应特征研究现状、热点及趋势进行分析。文献分析结果表明：该新兴研究领域的成果集中出现在2011年以后,发文量和引文量呈现上升趋势;中国积极参与海水入侵微生物响应特征研究,其发文量及高引用指数均位居世界第二位,研究成果具有较高的学术影响力;由关键词聚类分析可知,该领域热门研究方向包括地下水微生物群落对海水入侵的响应研究、土壤微生物群落对海水入侵的响应研究、元素地球化学循环与微生物作用研究。研究结果表明：咸-淡水交互区微生物群落演替受盐度、溶解氧、温度、有机碳、pH值等多种因子的共同影响,其主控因子随水文地质条件的不同而变化;海水入侵会影响微生物介导的碳、氮、硫、铁等物质循环过程;在含水层中发现的典型海洋细菌及嗜盐古菌对海水入侵的识别具有重要指示意义,相关技术是海水入侵传统调查方法的有力补充,在古海水入侵、现代海水入侵的辨别应用中具有较大优势和潜力。     

Thermal and energy factors of metal concentration in igneous plutons: an application to metallogenic analysis and exploration

The concentration of metals in intrusive systems is influenced by energy conditions and is associated with maxima of thermal and energy fields. Other factors controlling ore location inside plutons are the regimes of crystallization and of heat flow, and physical-chemical peculiarities of a melt. Ore formation in endocontacts and exocontacts of plutons occurs as a result of fluid filtration in the zones of thermal fracturing. Numerical modelling of thermal processes and the analysis of their influence on metal behaviour give clues that help to forecast promising sites. An example of how genetic models and general regularities can be applied to metallogenic analysis is given.     

The Central-Western Mediterranean: Anomalous igneous activity in an anomalous collisional tectonic setting

The central-western Mediterranean area is a key region for understanding the complex interaction between igneous activity and tectonics. In this review, the specific geochemical character of several ‘subduction-related’ Cenozoic igneous provinces are described with a view to identifying the processes responsible for the modifications of their sources. Different petrogenetic models are reviewed in the light of competing geological and geodynamic scenarios proposed in the literature.Plutonic rocks occur almost exclusively in the Eocene–Oligocene Periadriatic Province of the Alps while relatively minor plutonic bodies (mostly Miocene in age) crop out in N Morocco, S Spain and N Algeria. Igneous activity is otherwise confined to lava flows and dykes accompanied by relatively greater volumes of pyroclastic (often ignimbritic) products. Overall, the igneous activity spanned a wide temporal range, from middle Eocene (such as the Periadriatic Province) to the present (as in the Neapolitan of southern Italy). The magmatic products are mostly SiO₂-oversaturated, showing calcalkaline to high-K calcalcaline affinity, except in some areas (as in peninsular Italy) where potassic to ultrapotassic compositions prevail. The ultrapotassic magmas (which include leucitites to leucite-phonolites) are dominantly SiO₂-undersaturated, although rare, SiO₂-saturated (i.e., leucite-free lamproites) appear over much of this region, examples being in the Betics (southeast Spain), the northwest Alps, northeast Corsica (France), Tuscany (northwest Italy), southeast Tyrrhenian Sea (Cornacya Seamount) and possibly in the Tell region (northeast Algeria).Excepted for the Alpine case, subduction-related igneous activity is strictly linked to the formation of the Mediterranean Sea. This Sea, at least in its central and western sectors, is made up of several young (< 30 Ma) V-shaped back-arc basins plus several dispersed continental fragments, originally in crustal continuity with the European plate (Sardinia, Corsica, Balearic Islands, Kabylies, Calabria, Peloritani Mountains). The bulk of igneous activity in the central-western Mediterranean is believed to have tapped mantle ‘wedge’ regions, metasomatized by pressure-related dehydration of the subducting slabs. The presence of subduction-related igneous rocks with a wide range of chemical composition has been related to the interplay of several factors among which the pre-metasomatic composition of the mantle wedges (i.e., fertile vs. refractory mineralogy), the composition of the subducting plate (i.e., the type and amount of sediment cover and the alteration state of the crust), the variable thermo-baric conditions of magma formation, coupled with variable molar concentrations of CO₂ and H₂O in the fluid phase released by the subducting plates are the most important.Compared to classic collisional settings (e.g., Himalayas), the central-western Mediterranean area shows a range of unusual geological and magmatological features. These include: a) the rapid formation of extensional basins in an overall compressional setting related to Africa-Europe convergence; b) centrifugal wave of both compressive and extensional tectonics starting from a ‘pivotal’ region around the Gulf of Lyon; c) the development of concomitant Cenozoic subduction zones with different subduction and tectonic transport directions; d) subduction ‘inversion’ events (e.g., currently along the Maghrebian coast and in northern Sicily, previously at the southern paleo-European margin); e) a repeated temporal pattern whereby subduction-related magmatic activity gives way to magmas of intraplate geochemical type; f) the late-stage appearance of magmas with collision-related ‘exotic’ (potassic to ultrapotassic) compositions, generally absent from simple subduction settings; g) the relative scarcity of typical calcalkaline magmas along the Italian peninsula; h) the absence of igneous activity where it might well be expected (e.g., above the hanging-wall of the Late Cretaceous–Eocene Adria–Europe subduction system in the Alps); i) voluminous production of subduction-related magmas coeval with extensional tectonic régimes (e.g., during Oligo-Miocene Sardinian Trough formation).To summarize, these salient central-western Mediterranean features, characterizing a late-stage of the classic ‘Wilson Cycle’ offer a ‘template’ for interpreting magmatic compositions in analogous settings elsewhere.     

The Apophysis of the Bjerkreim–Sokndal layered intrusion (Rogaland anorthosite province, SW Norway): A composite pluton build up by tectonically-driven emplacement of magmas along the margin of an AMC igneous complex

The Late-Proterozoic Bjerkreim–Sokndal layered intrusion (BKSK) is connected to a foliated, sheet-like igneous body (the Apophysis), that is a potential feeder for the BKSK magma chamber. Field, petrographical, geochemical and structural data are used to demonstrate that the Apophysis is a composite igneous body, constructed by coeval mafic to felsic magmas that were collected in a sub-vertical shear zone. Three liquid lines of descent are distinguished in the main Apophysis component (a felsic series, predominantly quartz mangeritic) and in coeval felsic rocks from the upper part of the BKSK. Minor mineralogical and geochemical discrepancies between these three trends are indicative of distinct sources and crustal contaminants, as well as slight differences in the differentiation mechanisms. Jotunitic to noritic cumulates or crystal-laden magmas, associated with their trapped melts, mingled with the felsic series in two distinct portions of the Apophysis. In one area, this association is dominated by a FTP (Fe–Ti–P-rich) jotunite, interpreted as an accumulation of pyroxenes + Fe – Ti oxides + apatite + plagioclase. In the second area, the melt dominates over the associated cumulate; it is a primitive (MgO-rich and K₂O-poor) jotunite, that was also involved in the genesis of another igneous body in the vicinity of the Apophysis. Magma mixing, in addition to mingling, was also potentially important in the petrogenesis of some jotunite rocks.     

Neoproterozoic mafic-ultramafic layered intrusion in Quruqtagh of northeastern Tarim Block,NW China: Two phases of mafic igneous activity with different mantle sources

In this paper we report zircon U–Pb age, chemical compositions of rock-forming minerals, and whole-rock elemental and Sr–Nd isotopic data for the No. II mafic-ultramafic intrusive complex (N2MC) in the Quruqtagh area at the northeastern margin of the Tarim Block, northwestern China to evaluate its petrogenesis and tectonic significance. The N2MC with an exposure area of ca. 12 km² has a funnel-shaped cross-section and intruded the Paleoproterozoic basement. U–Pb zircon dating gives a crystallization age of 760 ± 6 Ma. Rock types of the N2MC include lherzolite, pyroxenite, gabbro and minor diorite. Major elements geochemistry of these rocks exhibits a tholeiitic trend with a wide range of SiO₂ contents (38.8–60 wt.%). On the other hand, they are systematically enriched in LILE, LREE and depleted in HFSE and HREE, thus leading to low HFSE/LREE ratios (e.g., Nb/La ≈ 0.3). Isotopically, the studied rocks are characterized by negative whole-rock εNd(t) values (? 7.6 to ? 2.8) and variable high (⁸⁷Sr/⁸⁶Sr)_i (0.7095–0.7059). These features, together with chemical compositions of the rock-forming minerals and the presence of the primary phlogopite and hornblende, suggest that N2MC was likely formed via crystal fractionation/cumulation (with negligible crustal contamination) of a tholeiitic magma derived from a metasomatized subcontinental lithosphere mantle (SCLM) in an extensional environment. The enrichment of the mantle source could be ascribed to the metasomatism by subducted-slab-released fluids before partial melting. Overall, reported Neoproterozoic igneous rocks throughout the Tarim Block constitute two major phases of Neoproterozoic igneous activities, i.e., ca. 825–800 Ma and ca. 780–745 Ma, respectively. Similar to that of many other Rodinian continents, this feature is interpreted to be related to the two phases of Neoproterozoic mantle plume activity under the Rodinia. Furthermore, there exist two types of mafic-ultramafic complex at Quruqtagh, i.e., the ca. 820 Ma carbonatite-bearing and the ca. 760 Ma tholeiitic, which could reflect the presence of two different mantle sources.     

Rate of seawater intrusion estimated by geophysical methods in an arid area: Al Khabourah, Oman

Seawater intrusion and its spreading rate are the challenging problems in over-pumped coastal aquifers of arid zones like the Batinah region of Oman. The study delineates the saline plume, identifies saline/freshwater zones, and estimates the migration rate of the plume in the Al Khabourah area of the Batinah coast. Time domain electromagnetic surveys, aided by vertical electrical sounding surveys, and seismic refraction methods have defined the locus of the saline/freshwater interface in the area. The current (2007) interface position, when compared with that determined during 2002, indicates a prominent recession in the saline plume and suggests an average annual recession rate of 120 m. This recession may be attributed mainly to the recharging dam of Wadi Al Hawasinah, constructed in 1995, and the enforcement of new water resources legislation. This study reveals the shielding role of the recharging dam to counter advancing saline intrusion.     

Soil microbial community response to seawater intrusion into coastal aquifer of Donghai Island,South China

Due to the sea level rise and the excessive exploitation of freshwater, seawater intrusion is becoming a critical issue. To clarify the degree of seawater intrusion in Donghai Island and the microbial community structure and functional response to seawater intrusion, groundwater samples and sediment samples were collected at profiles A, B; the profiles were along the direction of groundwater flow, perpendicular to the coastline, a hydrogeochemical survey and soil microbial community analysis were also performed. The hydrogeochemistry analysis showed that the chemical type of groundwater was Na–Cl, brackish water was dominant in the area, and coastal groundwater was strongly affected by seawater intrusion. The effect of seawater intrusion on structural and functional diversity of soil microbes was analyzed from soil samples of the study area, by polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE). The results of DGGE patterns and phylogenetic tree show that the extent of seawater intrusion has directly influenced soil microbial community structure. The changes of microbial community structure might be related to the major elements’ concentrations in groundwater. Phylogenetic affiliation indicated that γ-proteobacteria were dominated in the profile A, while β-proteobacteria were mainly appeared in the profile B. The Flavobacteriaceae was only appeared at the shrimp ponds nearby.     

The Skaergaard intrusion: from icon to precious metal deposit

Ever since Wager and Deer's classic memoir in 1939 the Skaergaard intrusion has been (or should have been) a central part of every student's education in igneous petrology. Seventy years after the initial fieldwork it is still a major target for academic research, seeking to understand the processes of magmatic diversification. Consequently, a huge literature has accumulated on the subject. Recently, attention has turned to the Skaergaard as a precious metal exploration target.     

Numerical simulation of stress distributions and displacements around an entry roadway with igneous intrusion and potential sources of seam gas emission of the Barapukuria coal mine, NW Bangladesh

This paper uses two-dimensional boundary element method (BEM) numerical modeling to analyze the deformation and failure behavior of a coal seam and to understand the nature of gas flow into a roadway entering the Barapukuria coal mine in Bangladesh. The Barapukuria basin contains Permian-aged Gondwana coals with high volatile B bituminous rank. Three models (A, B, and C) are presented here. Model A assumes horseshoe-shaped geometry, model B assumes trapezoid-shaped geometry, and model C assumes horseshoe-shaped geometry coupled with a roof fall-induced cave generated by the break-up of rock materials along the vertical dimension of an igneous dyke. The simulation results show that there is little difference in strata deformation between models A and B. In model A, there is no horizontal tensional stress and the overall horizontal stress patterns are compressive, while the distribution and magnitude of vertical stress show higher tensional stresses on the immediate rib sides and floor. In model B, both horizontal and vertical stress distributions indicate low to medium tensional stresses on the immediate roof, floor, and rib sides, but compressive stresses are prominent toward the interior of the coal seam. Deformation vectors indicate that failure extends laterally to about 7.5 m around the excavation geometry.On the contrary, for model C, the distributions and magnitudes of horizontal and vertical stress show higher tensional stresses in both rib sides of the roof fall zone. The deformation around the dyke-induced perturbation zone affects a large volume of coal. The deformation vectors with high magnitudes are nearly horizontal and propagate laterally up to 30 m; whereas, low-magnitude deformation vectors extend about 25 m toward the roof and 20 m toward the floor. The vertical tensional displacement, which is concentrated in the floor and the left and right hand sides of the roof, propagates about 30 m on both sides and about 22 m in the floor. From these simulation results, it is thought that the extension of the dyke-induced perturbation zone toward the roof, floor, and rib sides of the entry roadway initially creates small tensional cracks that gradually grow into large-scale tensional features. These features could also be responsible for high concentrations of gas, which are emitted into the mine from fractured coals due to insufficient mine ventilation and low atmospheric pressure.     

A new approach to the classification of igneous rocks using the basalt-andesite-dacite-rhyolite suite as an example

The obvious need to be able to predict, on a routine basis, the name of an igneous rock from its chemistry is examined in the light of some multivariate statistical methods. Discriminant analysis is ruled out for general use, due to its inability to distinguish an unknown that does not belong to any of the groups set up by the analysis, and the complexities of the decision-making logic with large number of groups. The best method appears to be classification procedure which predicts the probabilities of an unknown belonging to a particular group. The two methods are illustrated using samples of basalts, andesites, dacites and rhyolites.     

Thallium isotope variations in an ore-bearing continental igneous setting: Collahuasi Formation, northern Chile

Thallium is a highly incompatible element and a large fraction of the bulk silicate Earth Tl budget is, therefore, expected to reside in the continental crust. Nonetheless, the Tl isotope systematics of continental rocks are essentially unexplored at present. Here, we present new Tl isotope composition and concentration data for a suite of 36 intrusive and extrusive igneous rocks from the vicinity of porphyry Cu deposits in the Collahuasi Formation of the Central Andes in northern Chile. The igneous lithologies of the rocks are variably affected by the hydrothermal alteration that accompanied the formation of the Cu deposits.The samples display Tl concentrations that vary by more than an order of magnitude, from 0.1 to 3.2 μg/g, whilst ε²⁰⁵Tl ranges between −5.1 and +0.1 (ε²⁰⁵Tl is the deviation of the ²⁰⁵Tl/²⁰³Tl isotope ratio of a sample from a standard in parts per 10⁴). These variations are primarily thought to be a consequence of hydrothermal alteration processes, including metasomatic transport of Tl, and formation/breakdown of Tl-bearing minerals, which are associated with small but significant Tl isotope effects. The Tl abundances show excellent correlations with both K and Rb concentrations but no co-variation with Cu. This demonstrates that Tl displays only limited chalcophile affinity in the continental crust of the Collahuasi Formation, but behaves as a lithophile element with a distribution that is primarily governed by partitioning of Tl⁺ into K⁺-bearing phases. Collahuasi samples with propylitic alteration features, which are derived from the marginal parts of the hydrothermal systems, have, on average, slightly lighter Tl isotope compositions than rocks from the more central sericitic and argillic alteration zones. This small but statistically significant difference most likely reflects preferential retention of isotopically heavy Tl in alteration phases, such as white micas and clays, which formed during sericitic and argillic alteration.     

Control of sea-water intrusion by salt-water pumping: Coast of Oman

A shallow alluvial coastal aquifer in the Batinah area of Oman, with sea-water intrusion that extends several kilometres inland, has been studied experimentally, analytically and numerically. The water table is proved to have a trough caused by intensive pumping from a fresh groundwater zone and evaporation from the saline phreatic surface. Resistivity traverses perpendicular to the shoreline indicated no fresh groundwater recharge into the sea. Using an analytical Dupuit-Forchheimer model, developed for the plain part of the catchment, explicit expressions for the water table, sharp interface location and stored volume of fresh water are obtained. It is shown that by the pumping of salt water from the intruded part of the aquifer, this intrusion can be mitigated. Different catchment sizes, intensities of fresh groundwater pumping, evaporation rates, water densities, sea level, incident fresh water level in the mountains and hydraulic conductivity are considered. SUTRA code is applied to a hypothetical case of a leaky aquifer with line sinks modeling fresh water withdrawal and evaporation. The numerical code also shows that pumping of saline water can pull the dispersion zone back to the shoreline.     

Differentiation and crystallization conditions of basalts from the Kerguelen large igneous province: an experimental study

Phase relations of basalts from the Kerguelen large igneous province have been investigated experimentally to understand the effect of temperature, fO₂, and fugacity of volatiles (e.g., H₂O and CO₂) on the differentiation path of LIP basalts. The starting rock samples were a tholeiitic basalt from the Northern Kerguelen Plateau (ODP Leg 183 Site 1140) and mildly alkalic basalt evolved from the Kerguelen Archipelago (Mt. Crozier on the Courbet Peninsula), representing different differentiation stages of basalts related to the Kerguelen mantle plume. The influence of temperature, water and oxygen fugacity on phase stability and composition was investigated at 500 MPa and all experiments were fluid-saturated. Crystallization experiments were performed at temperatures between 900 and 1,160°C under oxidizing (log fO₂ ~ ΔQFM + 4) and reducing conditions (log fO₂ ~ QFM) in an internally heated gas-pressure vessel equipped with a rapid quench device and a Pt-Membrane for monitoring the fH₂. In all experiments, a significant influence of the fO₂ on the composition and stability of the Mg/Fe-bearing mineral phases could be observed. Under reducing conditions, the residual melts follow a tholeiitic differentiation trend. In contrast, melts have high Mg# [Mg²⁺/(Mg²⁺ + Fe²⁺)] and follow a calk-alkalic differentiation trend at oxidizing conditions. The comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre-eruptive conditions of these magmas. The pre-eruptive temperature of the alkalic basalt was about 950–1,050°C. The water content of the melt was below 2.5 wt% H₂O and strongly oxidizing conditions (log fO₂ ~ ΔQFM + 2) were prevailing in the magma chamber prior to eruption. The temperature of the tholeiitic melt was above 1,060°C, with a water content below 2 wt% H₂O and a log fO₂ ~ ΔQFM + 1. Early fractionation of clinopyroxene is a crucial step resulting in the generation of silica-poor and alkali-rich residual melts (e.g., alkali basalt). The enrichment of alkalis in residual melts is enhanced at high fO₂ and low aH₂O.