Marble-hosted sulfide ores in the Angouran Zn-(Pb–Ag) deposit, NW Iran: interaction of sedimentary brines with a metamorphic core complex

The Angouran Zn-(Pb–Ag) deposit, Zanjan Province, NW Iran, is located within the central Sanandaj-Sirjan Zone of the Zagros orogenic belt. The deposit has proven and estimated resources of 4.7 Mt of sulfide ore at 27.7% Zn, 2.4% Pb, and 110 g/t Ag, and 14.6 Mt of oxidized carbonate ores at 22% Zn and 4.6% Pb. It is hosted by a metamorphic core complex that is unconformably overlain by a Neogene volcanic and evaporite-bearing marine to continental sedimentary sequence. The sulfide orebody, precursor to the significant nonsulfide ores, is located at the crest of an open anticline at the contact between Neoproterozoic to Cambrian footwall micaschists and hanging wall marbles. ⁴⁰Ar–³⁹Ar data on muscovite from mineralized and unaltered footwall micaschists suggest a rapid Mid-Miocene exhumation of the metamorphic basement (∼20 Ma) and yield an upper age constraint for mineralization. The fine-grained sulfide ore is massive, replacive, often brecciated, clearly postmetamorphic and dominated by Fe-poor sphalerite, with minor galena, pyrite, anhydrite, quartz, muscovite, dolomite, and rare calcite. Sphalerite contains Na–Ca–Cl brine inclusions (23–25 mass% total dissolved solids) with homogenization temperatures of 180–70°C. Fluid inclusion chemistry (Na–K–Li–Ca–Mg–Cl–Br), ore geochemistry, S, and Pb isotope data suggest that the Angouran sulfide ore formed by the interaction of modified, strongly evaporated Miocene seawater and the lithotypes of an exhumed metamorphic core complex. Minor contributions of metals from Miocene igneous rocks cannot be excluded. Mineralization occurred in a collisional intra-arc setting with high heat flow, probably during the transition from an extensional to a compressional regime. The Angouran deposit may represent a new type of low-temperature carbonate-hosted Zn–Pb ore that is distinct from Mississippi Valley type and sedimentary-exhalative deposits.Editorial handling: B. Lehmann     

Three-dimensional finite element modelling of stress relaxation tests in anisotropic clayey medium: direct problem and back analysis

This paper deals with a new strategy for initial stress identification by stress relaxation methods, coupled with finite element calculation, and applied to the overcoring test. The back-analysis of such a test uses an inversion method which consists in the minimisation, with a gradient-based algorithm, of a cost functional of least-squares type, which quantizes the difference between measured and computed strains. The computed strains are assessed by three-dimensional finite element modellings of the overcoring test. The inversion methodology is applied to a recent in situ overcoring test performed at Mont Terri laboratory, Switzerland. The inversion gave good results and allows us to validate the inversion methodology. The constitutive law considered for this application is transverse isotropic elasticity but the inversion method developed is applicable to most constitutive laws and every kind of in situ test.     

Establishing the Database of Inundation Potential in Taiwan

The structural measure was the major solution for flood defense in Taiwan. However, the measure is always limited to the design standard and cannot prevent the damages when floods exceed certain scale. Therefore, non-structural measures for flood mitigation are the indispensable complements to structural solutions. The study introduces the establishment of inundation potential database that provides required information for the non-structural measures in Taiwan. The database was built by numerical simulations, based on different rainfall scenarios, and has been applied by the local governments of Taiwan for land use managements, flood warning systems, emergency responses, and flood insurance programs to reduce the flood damages and impacts.     

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J₀, J₁, J₂, and J₃, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J₀ formed during an early event. Petrographic observations, geochemistry, and δ¹⁸O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J₁, J₂, and J₃ jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J₀ jasperoids caused them to have higher δ¹⁸O values than J_1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ¹⁸O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ¹⁸O values are associated with the largest deposits. The 28‰ range of jasperoid δ¹⁸O values is best explained by mixing between an ¹⁸O-enriched fluid and an ¹⁸O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ¹⁸O composition of jasperoids indicates that gold was introduced by the ¹⁸O-enriched fluid. The lowest calculated δ¹⁸O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the ¹⁸O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the ¹⁸O-enriched ore fluid is not constrained. The δ³⁴S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H₂S and sulfate were derived from sedimentary sources. Likewise, the δ¹³C and δ¹⁸O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO₂ was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.     

Germanium isotopic variations in igneous rocks and marine sediments

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICPMS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of ⁷⁴Ge/⁷⁰Ge ratios are expressed as δ⁷⁴Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2‰ for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4‰ in δ⁷⁴Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of ⁷⁴Ge/⁷⁰Ge, ⁷³Ge/⁷⁰Ge and ⁷²Ge/⁷⁰Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of δ⁷⁴Ge in igneous rocks is only 0.25‰ without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a δ⁷⁴Ge of 1.3 ± 0.2‰ can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher δ⁷⁴Ge values between 2.0‰ and 3.0‰. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the δ⁷⁴Ge of the seawater to +3.0‰.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have δ⁷⁴Ge values ranging from 0.7‰ to 2.0‰. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.     

Political ecology in North America: Discovering the Third World within?

Political ecologists working in many other parts of the world are now heading north, or simply going global, posing a series of important questions related to theory, methodology, politics, and policy along the way. This special issue, contains papers originally delivered at a conference held at Rutgers University in 2003 that addressed this phenomenon. The papers collected offer case studies that reveal the First World as subject to a host of processes that can be insightfully understood via a political ecology perspective. First, globalized production and consumption regimes have created new linkages that demand synoptic analyses of often far-flung research sites. Second, the painful coincidence of deindustrialization and a radical restructuring of agricultural credit and price support systems have devastated North American and European heartlands, effectively producing “Third World” conditions in many depressed rural areas. Third, migration streams originating in Latin America, Africa and many parts of Asia have brought sizable Third World populations into the spatial heart of capitalism. Fourth, the belated recognition of some indigenous claims to resources, especially in Canada, and fierce opposition to others, have reopened questions of (internal) colonial domination. Finally, we see the burgeoning First World political ecology literature as the culmination of what Louise Fortmann has called “the long intellectual journey home” for many scholars who originally carried out research on/in the Third World. All of these factors have combined to help political ecologists discover suitable analytical terrain in the First World.     

Estimation of clay content in soil based on resistivity modelling and laboratory measurements

The determination of clay content in near‐surface formations is crucial for geotechnical, hydrogeological and oil‐contamination studies. We have developed a technique for estimating clay content that consists of the minimization of the difference between the theoretically calculated and measured soil resistivities as a function of water salinity. To calculate the resistivity, we used a model that takes into account the electrochemical processes in the clay micropores. The experimental measurements of soil resistivity were performed on soil samples, completely saturated by brines at different concentrations of NaCl salt in the range 0.6–100 g/l, to obtain the resistivity versus salinity curve. The parameters obtained with this curve inversion are the clay content, the total porosity and the cation exchange capacity. To verify the new technique, we determined clay concentrations of artificial mixtures of calibrated sand and clay. The relative mean error in the clay content does not exceed 20% for a 5% fitting error of the resistivity versus salinity curves. Such evaluations allow the correct separation of the main lithological groups (sand, sandy loam, loam, and light, medium and heavy clay). We applied this technique to estimate the petrophysical parameters of soils (clay content, porosity and cation exchange capacity) at various sites in Mexico. The results improved the interpretation of the vertical electrical soundings, the lithological soil characterization and the delineation of oil‐contaminated areas.     

Recent Trends in Satellite-Derived Chlorophyll and Aerosol Optical Depth Together with Simulated Dimethylsulfide in the Eastern China Marginal Seas

The biogenic compound dimethylsulfide(DMS)produced by a range of marine biota is the major natural source of re-duced sulfur to the atmosphere and plays a major role in the formation and evolution of aerosols,potentially affecting climate.The spatio-temporal distribution of satellite-derived chlorophyll_a(CHL)and aerosol optical depth(AOD)for the recent years(2011-2019)in the Eastern China Marginal Seas(ECMS)(25°-40°N,120°-130°E)are studied.The seasonal CHL peaks occurred during late April and the CHL distribution displays a clear zonal gradient.Elevated CHL was also observed along the northern and western coastlines during summer and winter seasons.Trend analysis shows that mean CHL decreases by about 10%over the 9-year study period,while AOD was higher in south and lower in north during summertime.A genetic algorithm technique is used to calibrate the key model parameters and simulations are carried out for 2015,a year when field data was available.Our simulation results show that DMS seawater concentration ranges from 1.56 to 5.88 nmol L^?1 with a mean value of 2.76 nmol L^?1.DMS sea-air flux ranges from 2.66 to 5.00mmol m^?2 d^?1 with mean of 3.80mmol m^?2 d^?1.Positive correlations of about 0.5 between CHL and AOD were found in the study region,with higher correlations along the coasts of Jiangsu and Zhejiang Provinces.The elevated CHL concentration along the west coast is correlated with increased sea-water concentrations of DMS in the region.Our results suggest a possible influ-ence of DMS-derived aerosol in the local ECMS atmosphere,especially along the western coastline of ECMS.