The formation of (Ni-Co-Sb)-Ag-As ore shoots in hydrothermal galena-sphalerite-fluorite veins

Unusual hydrothermal native As-sulfide ± native Ag ± arsenide ± antimonide ± sulfosalt ore shoots and their co-genetic sulfide-fluorite-barite-quartz host veins, which are common in the region and in whole Central Europe, were investigated at three localities in the Schwarzwald, SW Germany, to understand the physico-chemical processes governing the change from a normal (= common) hydrothermal to an exceptional ore shoot regime. Based on fluid inclusions, the formation of the gangue minerals is the result of binary mixing between a NaCl-rich brine and a CaCl2-rich brine (both ~ 20 wt% NaCl aq.). This mixing correlation, major and minor fluid composition, formation temperature (~ 150 °C), and δ34S signature are identical (within error) in ore shoots and host veins. Thermodynamic modeling indicates that ore shoot formation must have resulted from a change in redox conditions by a local influx of a volumetrically minor reducing agent, probably hydrocarbons. The elemental content and the mineralogy of each ore shoot locality (Ag-As-rich: Münstertal; Ag–Ni-As-rich: Urberg; Ag–Ni-As-Sb-rich: Wieden) reflect the metal content of the binary mixed fluid, while mineral textures, successions, and assemblages are thermodynamically and, regarding sulfur, kinetically controlled. The formation of vein and ore shoot sulfides requires an addition of sulfide, most probably from the sulfide-bearing host rocks, because thermodynamic and kinetic reasons suggest that the two major vein-forming and metal-bearing fluids are not the source of the sulfur. The final ore shoot textures are influenced by later hydrothermal remobilization processes of As and Ag. This results in a number of sulfosalts, mostly proustite-pyrargyrite. Interestingly, the greater thermodynamic stability of Sb-endmember sulfosalts enables them to form even in As-dominated fluid systems.

Short communication: Inverse isochron regression for Re–Os, K–Ca and other chronometers

Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re/188Os and 187Os/188Os, which often exhibit strong error correlations that may obscure potentially important geological complexity. Using an approach that is widely accepted in 40Ar/39Ar and U–Pb geochronology, we here show that these error correlations are greatly reduced by applying a simple change of variables, using 187Os as a common denominator. Plotting 188Os/187Os vs. 187Re/187Os produces an “inverse isochron”, defining a binary mixing line between an inherited Os component whose 188Os/187Os ratio is given by the vertical intercept, and the radiogenic 187Re/187Os ratio, which corresponds to the horizontal intercept. Inverse isochrons facilitate the identification of outliers and other sources of data dispersion. They can also be applied to other geochronometers such as the K–Ca method and (with less dramatic results) the Rb–Sr, Sm–Nd and Lu–Hf methods. Conventional and inverse isochron ages are similar for precise datasets but may significantly diverge for imprecise ones. A semi-synthetic data simulation indicates that, in the latter case, the inverse isochron age is more accurate. The generalised inverse isochron method has been added to the IsoplotR toolbox for geochronology, which automatically converts conventional isochron ratios into inverse ratios, and vice versa.

Magmatic Material in Sandstone Shows Prospects for New Diamond Deposits within the Northern East European Platform

A detailed study of sandstones recovered from the upper part of the recently discovered KL-01 magmatic pipe in the southern part of the Arkhangelsk diamondiferous province (ADP), containing magmatic material and rare kimberlite indicator minerals, is presented in this paper. Results are compared to the composition of crater samples of the highly diamondiferous Vladimir Grib kimberlite pipe and several poorly to non-diamondiferous ADP pipes. To identify the type of magmatic material admixture, a model of binary mixing between country Vendian sandstones and typical ADP magmatic rocks based on correlations of La/Yb and Zr/Nb ratios and Ni contents is proposed. The modeling results show that the type of magmatic component in the KL-01 samples can be identified as kimberlite, with a maximum admixture of 20 vol.%. Kimberlite indicator mineral geochemistry did not exclude the interpretation that the composition, structure, thermal state and metasomatic enrichment of the lithospheric mantle sampled by the KL-01 pipe were suitable for the formation and preservation of diamonds. The lower boundary of the sampled lithospheric mantle could be in the depth range of 175–190 km, with a diamond window width of 55–70 km. Thus, the sandstones could represent the upper level of the crater of a new kimberlite pipe.

Major ion composition and 87Sr/86Sr of groundwater in the coastal Gujarat alluvial plain, India

<p>Major ions, Sr concentration, and <sup>87</sup>Sr/<sup>86</sup>Sr have been analyzed in groundwater of the coastal Gujarat Alluvial Plain, collected during monsoon, post-monsoon, and pre-monsoon seasons of 2016–2017. The major objective of this study was to understand the regional groundwater salinization mechanism. In the study area, the groundwater is mostly characterized by Na-Cl facies, with few samples of Ca-Cl, Ca-Mg-Cl, Na-Ca-HCO<sub>3</sub>, and Ca-Mg-HCO<sub>3</sub> types. Whereas, the Narmada and the Tapi river water samples are particularly of Ca-Mg-HCO<sub>3</sub> type. The hydrogeochemical facies evolution (HFE) diagram depicts the coastal groundwater freshening irrespective of the season ruling out the lateral seawater intrusion far inland. However, the <sup>87</sup>Sr/<sup>86</sup>Sr and Br/Cl ratios strongly suggest the modern marine influence on the regional groundwater. In the plot of 1/Sr versus <sup>87</sup>Sr/<sup>86</sup>Sr, most of the groundwater samples fall on the binary mixing line between the seepage groundwater and modern seawater endmembers. Therefore, we suspect that the up-coning of recently trapped seawater by groundwater over-extraction is the most plausible reason for the groundwater salinization, which indicate the vulnerability of the coastal Gujarat alluvial plain to the near future sea ingress under the global warming scenario. A few exceptional groundwater samples far north of the Narmada River show more radiogenic <sup>87</sup>Sr/<sup>86</sup>Sr indicative of silicate weathering.</p>

Long-term lead contamination and isotopic source identification in Northern Hungary

<p>Lead is one of the extensively distributed PTEs (potentially toxic elements) in the environment due to wide-scale anthropogenic activities (e.g., mining, vehicular emissions, industries, etc.), its geochemical feature, and natural abundance. The World Health Organization (WHO) defined Pb as 2<sup>nd</sup> most dangerous element for health, with particular concern for mental and physical disorders in adults and children. Salgótarján and Ózd cities (Northern Hungary) are two main former heavy industrial cities, with the smelter, steel industry, coal-fired power plant, coal mines, etc., supplied the country with coal and iron and steel products for centuries. The main aim of the research is to obtain a lead distribution map of the region and identify the potential sources by use of stable lead isotopes.</p><p>Urban soil samples were collected from each km<sup>2 </sup>of both cities. Additionally, a soil as geochemical background, as well as local slag and coal as suspected major pollution sources were collected. Lead content and stable Pb isotopes of all samples were analyzed by ICP-MS spectrometry.</p><p>Obtained results indicate heterogeneous distribution and high lead enrichment in both cities, where the Pb concentration ranged from 8.5 to 1692 ppm in Salgótarján and from 6.6 to 1674 ppm in Ózd. The average lead isotopic ratio in soil samples ranged from 1.146 to 1.240 (<sup>206</sup>Pb/<sup>207</sup>Pb) for Salgótarján and from 1.084 to 1.240 for Ózd. Total Pb concentration and isotopic ratios of slag and coal samples depicted notable differences as isotopic ratios for Salgótarján and Ózd coals are <sup>206</sup>Pb/<sup>207</sup>Pb:1.175 and <sup>206</sup>Pb/<sup>207</sup>Pb:1.256, respectively. Meanwhile, the lead content in the Salgótarján fly-ash slag (from the coal-fired power plant) was identified as 14 ppm (<sup>206</sup>Pb/<sup>207</sup>Pb:1.175, similar to coal). However, the Ózd smelter slag was characterized by high lead concentration (202 ppm) and lowest isotopic composition (<sup>206</sup>Pb/<sup>207</sup>Pb=1.118).</p><p>To calculate the relative contribution of anthropogenic sources, suggested binary mixing models were used. It is revealed that in Salgótarján soils average 34 % of Pb enters from industrial sources, 43 % from coal, and 23 % from the natural environment. In contrast, in Ózd, the proportion of anthropogenic lead is estimated on average by 53 % from industries (slag), 38 % from coal, and only 9% from natural input. The proportion of coal and slag in the soil samples was proved by thorough microscopy observations and SEM analysis as well.</p><p>In conclusion, based on the comprehensive analysis, local smelter and steel-iron industries were the dominant Pb contamination sources in both cities.</p><p> </p><p><strong>Keywords: </strong>Lead pollution, isotopic ratio, source identification, binary mixing model</p>

Short communication: Inverse isochron regression for Re–Os, K–Ca and other chronometers

Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re / 188Os and 187Os / 188Os. 188Os is usually far less abundant, and is therefore measured less precisely, than 187Os and 187Re. This causes strong error correlations between the two isochron ratios, which may obscure potentially important geological complexity. Using an approach that is widely accepted in 40Ar / 39Ar and U–Pb geochronology, we here show that these error correlations are greatly reduced by applying a simple change of variables, using 187Os as a common denominator. Plotting 188Os / 187Os vs. 187Re / 187Os produces an inverse isochron, defining a binary mixing line between an inherited Os-component whose 188Os / 187Os-ratio is given by the vertical intercept, and the radiogenic 187Re / 187Os-ratio, which corresponds to the horizontal intercept. Inverse isochrons facilitate the identification of outliers and other sources of data dispersion. They can also be applied to other geochronometers such as the K–Ca method and (with less dramatic results) the Rb–Sr, Sm–Nd and Lu–Hf methods. The generalised inverse isochron method has been added to the IsoplotR toolbox for geochronology, which automatically converts conventional isochron ratios into inverse ratios and vice versa.

Circulation timescales of Atlantic Water in the Arctic Ocean determined from anthropogenic radionuclides

The inflow of Atlantic Water to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss of sea ice, and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the two long-lived anthropogenic radionuclides 129I and 236U together with two age models to constrain the pathways and circulation times of Atlantic Water in the surface (10–35 m depth) and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit from the unique time-dependent tagging of Atlantic Water by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of Atlantic Water between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current), and up to 20 years in the Canada Basin, reflecting the pathways of Atlantic Water through the Arctic and their exiting through the Fram Strait. In the mid-depth Atlantic layer (250–800 m), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 55 years, while the mode ages representing the most probable ages of the TTD range between 3 and 30 years. The estimated mean ages are overall in good agreement with previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift in the mode age towards a younger age compared to the mean age also reflects the presence of a substantial amount of lateral mixing. For applications interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages obtained in this study suggest that changes in the properties of Atlantic Water will quickly spread through the Arctic Ocean and can lead to relatively rapid changes throughout the upper water column in future years.