The geochemical evolution of the Catalan potash subbasin, South Pyrenean foreland basin (Spain)

Over the last decade, our studies in ancient evaporitic basins have been based on a detailed study of a single borehole record. The detailed findings in medium- to large-sized evaporitic basins were shadowed with a relevant question: can interpretations from a representative evaporitic record in a single borehole be extended to the whole evaporitic basin? This paper addresses that question; the results obtained are compared with results from another distant point within the basin. The general methodology not only proves its reliability in interpreting the evolution of evaporitic basins from a single borehole but reveals its capability to obtain detailed palaeoenvironmental interpretations.

The chemical evolution of an Upper Eocene evaporitic sequence from the South Pyrenean foreland basin (Spain) has been investigated along the Súria-19 borehole record. Detailed petrographic and mineralogical study, X-ray microanalysis of frozen primary inclusions trapped in halite (Cryo-SEM-EDS), systematic isotopic analysis (δ³⁴S and δ¹⁸O in sulphates) and computer-based evaporation models have been integrated in a multi-proxy methodology. This study revealed that a variable amount of Ca excess is required throughout different parts of the marine Lower Halite Unit (LHU) for sylvite, instead of K–Mg sulphates, to form. This Ca excess is in turn different from that required for the western sector of the same evaporitic basin (Navarrese subbasin). Quick and variable changes in Ca-rich brines or equivalent dolomitization required are explained as internal processes within the basin rather than secular variations in seawater chemistry.

The general hydrological evolution of the Catalan subbasin is explained as a restricted subbasin with a first marine stage in which continental input (up to 50% of total input) had an important control on the geochemistry of the subbasin. A second stage was determined during potash precipitation, in which the subbasin was cut from any seawater input to end up in its last stage as a purely continental evaporitic basin. Coupling evaporation models and analytical results we have obtained the proportions of recycling and their sources, estimated to change from a 100% (total mass of sulphate) Eocene source to 20% Eocene and 80% Triassic (Keuper) towards the latest stage of potash precipitation. The results obtained have been compared with results from the Navarrese subbasin allowing an integrated interpretation of the hydrological evolution of the whole Upper Eocene South Pyrenean basin. Local geochemical variations within the Upper Eocene south Pyrenean basin are explained by the differences in paleogeographical setting of the Navarrese and Catalan subbasins.     

The origin of fluids involved in the formation of gold-bearing skarns of the Andorra granite (Central Pyrenees,Spain): Sulphur isotope data

Summary Arsenopyrite-gold mineralization in the lower Paleozoic series of the eastern Pyrenees is of two types: disseminated in Hercynian metasediments, and veins cross-cutting the main foliation. In the central Pyrenees, gold-bearing arsenopyrite mineralizations have recently been discovered in skarns along the SW contact of the Andorra granite. The ore minerals are similar to those of the veins, with gold also being associated with arsenopyrite, bismuth and sulphides.The late Hercynian Andorra granite intruded Paleozoic metasediments of pre-Caradocian to Carboniferous age. Six types of skarns have developed at or close to the contact between the granite and Devonian limestones. Only three of these, the hedenbergite, pyrrhotite and arsenopyrite skarns, are sulphide-bearing. Gold grades in these skarns range from less than 0.8 g/t in pyrrhotite skarns to 7 g/t in arsenopyrite skarns.The ³⁴S of arsenopyrite from disseminated and vein mineralizations show values from 11 to 15, indicating a sulphur source of crustal origin. Vein sulphides are considered to have formed after a metamorphic remobilization of the sulphide rich disseminations in the Paleozoic series. Sulphur isotopic composition of skarn sulphides is variable: pyrrhotite from pyrrhotite skarns has a ³⁴S +, suggesting a sulphur of magmatic origin. Their gold grades are very low (>0.8 g/t). The heavier values of arsenopyrite from arsenopyrite skarns (³⁴S + 10) and of galena and sphalerite from hedenbergite skarns (³⁴S + 7) correlate with the higher gold grades (7 and 5 g/t, respectively). This is interpreted to reflect a contribution of wall-rock sulphur-rich fluids generated from the thermal metamorphism of arsenopyrite-bearing wall-rock metasediments (³⁴S + 12%o) during skarn formation or by assimilation of country rock by the granitic intrusion during its emplacement. The economic importance of these skarns appears to be related to the amount of crustal fluid involved in skarn formation.

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Lagerstättenbildende Fluide in den goldführenden Skarnen des Andorra-Granites, Zentralpyrenäen, Spanien: Schwefelisotopen-Daten

Zusammenfassung Arsenkies-Goldmineralisation in altpaläozoischen Serien der ostlichen Pyrenäen tritt in zwei Typen auf: Disseminiert in Hercynischen Metasedimenten, und in Gängen, die die Haupt-Foliation durchschlagen. In den Zentralpyrenäen wurden goldführende Arsenkies-Mineralisationen vor kurzem in Skarnen längs des SW-Kontaktes des Andorra-Granites entdeckt. Die Erzminerale sind ähnlich denen in den Gängen, wobei Gold mit Arsenkies, Wismut und Sulfiden assoziiert ist. Der spät-hercynische Andorra-Granit intrudierte paläozoische Metasedimente von Prä-Caradoc bis Karbon-Alter. Im Kontaktbereich Granit/Devonische Kalke kommen sechs Typen von Skarnen vor. Nur drei davon führen Sulflde, nämlich die Hedenbergit-, Magnetkies- und Arsenkiesskarne. Goldgehalte in diesen Skarnen schwanken von weniger als 0,8 g/t in Magnetkiesskarnen bis zu 7 g/t in Arsenkiesskarnen. Das ö34 S von Arsenopyrit in disseminierten und Gang-Mineralisationen schwankt von 11-15%o, und dies weist auf eine Schwefelquelle in der Kruste hin. Gangsulflde haben sich auch nach einer metamorphen Remobilisierung der Sulfid-reichen Disseminationen in den paläozoischen Serien gebildet. Die Schwefelisotopenzusammensetzung der Skarnsulfide schwankt: Magnetkies aus Magnetkiesskarnen hat ö34 S Werte von +4%o, dies weist auf magmatischen Ursprung hin. Ihre Goldgehalte sind sehr niedrig (<0,8 0,8 g/t). Die schwereren Isotopen von Arsenkies in Arsenkies-Skarnen ( ³⁴S+ 10%o) und von Bleiglanz und Zinkblende aus den Hedenbergit-Skarnen ( ³⁴S+ 7%o) lassen sich mit höheren Goldgehalten (7 und 5 g/t) korrelieren. Dies wird Fluiden zugeschrieben, die reich an Schwefel aus den Nebengesteinen sind und durch die thermale Metamorphose der Arsenkies-führenden Metasedimente (S + 12%o) während der Skarnbildung, oder durch Assimilation von Nebengestein durch den Granit wahrend seiner Platznahme entstanden sind. Die wirtschaftliche Bedeutung dieser Skarne scheint in einer direkten Beziehung zur Menge krustaler Fluide zu stehen.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids und Mineral Deposits, ETH Zürich, March 21–23, 1991.     

Evaluation of trenching,ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization

This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three‐dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above‐surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.     

Multiphase flow and reactive transport model in vadose tailings

Weathering processes affecting pyritic wastes may generate huge amounts of acid waters with high concentrations of potentially toxic contaminants (acid mine drainage). Acid mine drainage is mainly produced in the vadose zone. In the present study, a coupled non-isothermal multiphase flow and reactive transport model of the vadose zone of sulfide mine tailings was developed. The geochemical model included kinetically controlled reactions for Fe(II)-oxidation and for the dissolution of sulfide and aluminosilicate phases and the Pitzer ion-interaction model to describe the behavior of the pore-water solutions. Model results were compared with experimental observations in unsaturated column experiments under strongly evaporative conditions similar to arid or semiarid climates. Evolution trends for temperature, water saturation, evaporation rates, pore-water hydrochemistry and mineral phases observed during the drying experiment were adequately reproduced. The coupled model reproduced the increase of solute concentrations in the column top and the precipitation of a crust of secondary mineral phases. This crust became a barrier for water vapour diffusion to the atmosphere and modified the thermohydraulic behavior of the tailings. Enhanced downward migration of water vapour and its condensation in this colder end of the column were correctly taken into account by the model, which reproduced the dilution observed in the lower part of the column during the experiments.     

The application of GPR and ERI in combination with exposure logging and retrodeformation analysis to characterize sinkholes and reconstruct their impact on fluvial sedimentation

This work illustrates the practicality of investigating sinkholes integrating data gathered by ground penetrating radar (GPR), electrical resistivity imaging (ERI) and trenching or direct logging of the subsidence‐affected sediments in combination with retrodeformation analysis. This mutidisciplinary approach has been tested in a large paleosinkhole developed during the deposition of a Quaternary terrace on salt‐bearing evaporites. The subsidence structure, exposed in an artificial excavation, is located next to Puilatos, a village that was abandoned in the 1970s due to severe subsidence damage. Detailed logging of the exposure revealed that the subsidence structure corresponds to an asymmetric sagging and collapse paleosinkhole with no clear evidence of recent activity. The sedimentological and structural relationships together with the retrodeformation analysis indicate that synsedimentary subsidence controlled channel location, the development of a palustrine environment and local changes in the channel pattern. GPR profiles were acquired using an array of systems with different antenna frequencies, including some recently developed shielded antennas with improved vertical resolution and penetration depth. Although radargrams imaged the faulted sagging structure and provided valuable data on fault throw, they did not satisfactorily image the complex architecture of the fluvial deposit. ERI showed lower resolution but higher penetration depth when compared to GPR, roughly capturing the subsidence structure and yielding information on the thickness of the high‐resistivity alluvium and the nature of the underlying low‐resistivity karstic residue developed on top of the halite‐bearing evaporitic bedrock. Data comparison allows the assessment of the advantages and limitations of these complementary techniques, highly useful for site‐specific sinkhole risk management. Copyright © 2016 John Wiley & Sons, Ltd.     

Relative equilibria of the restricted three-body problem in curved spaces

We use a formulation of the N-body problem in spaces of constant Gaussian curvature, \({\kappa }\in \mathbb {R}\), as widely used by A. Borisov, F. Diacu and their coworkers. We consider the restricted three-body problem in \(\mathbb {S}^2\) with arbitrary \({\kappa }>0\) (resp. \(\mathbb {H}^2\) with arbitrary \({\kappa }<0\)) in a formulation also valid for the case \({\kappa }=0\). For concreteness when \({\kappa }>0\) we restrict the study to the case of the three bodies at the upper hemisphere, to be denoted as \(\mathbb {S}^2_+\). The main goal is to obtain the totality of relative equilibria as depending on the parameters \({\kappa }\) and the mass ratio \(\mu \). Several general results concerning relative equilibria and its stability properties are proved analytically. The study is completed numerically using continuation from the \({\kappa }=0\) case and from other limit cases. In particular both bifurcations and spectral stability are also studied. The \(\mathbb {H}^2\) case is similar, in some sense, to the planar one, but in the \(\mathbb {S}^2_+\) case many differences have been found. Some surprising phenomena, like the coexistence of many triangular-like solutions for some values \(({\kappa },\mu )\) and many stability changes will be discussed.     

Annama H chondrite—Mineralogy,physical properties,cosmic ray exposure,and parent body history

The fall of the Annama meteorite occurred early morning (local time) on April 19, 2014 on the Kola Peninsula (Russia). Based on mineralogy and physical properties, Annama is a typical H chondrite. It has a high Ar‐Ar age of 4.4 Ga. Its cosmic ray exposure history is atypical as it is not part of the large group of H chondrites with a prominent 7–8 Ma peak in the exposure age histograms. Instead, its exposure age is within uncertainty of a smaller peak at 30 ± 4 Ma. The results from short‐lived radionuclides are compatible with an atmospheric pre‐entry radius of 30–40 cm. However, based on noble gas and cosmogenic radionuclide data, Annama must have been part of a larger body (radius >65 cm) for a large part of its cosmic ray exposure history. The ¹⁰Be concentration indicates a recent (3–5 Ma) breakup which may be responsible for the Annama parent body size reduction to 30–35 cm pre‐entry radius.     

Petrographic and geochemical evidence for multiphase formation of carbonates in the Martian orthopyroxenite Allan Hills 84001

Martian meteorites can provide valuable information about past environmental conditions on Mars. Allan Hills 84001 formed more than 4 Gyr ago, and owing to its age and long exposure to the Martian environment, and this meteorite has features that may record early processes. These features include a highly fractured texture, gases trapped during one or more impact events or during formation of the rock, and spherical Fe‐Mg‐Ca carbonates. In this study, we have concentrated on providing new insights into the context of these carbonates using a range of techniques to explore whether they record multiple precipitation and shock events. The petrographic features and compositional properties of these carbonates indicate that at least two pulses of Mg‐ and Fe‐rich solutions saturated the rock. Those two generations of carbonates can be distinguished by a very sharp change in compositions, from being rich in Mg and poor in Fe and Mn, to being poor in Mg and rich in Fe and Mn. Between these two generations of carbonate is evidence for fracturing and local corrosion.     

Mineralogical and geochemical constraints on the origin of ferromanganese crusts from the Rivera Plate (western margin of Mexico)

Marine hydrogenetically precipitated ferromanganese crusts are widespread in the Pacific Ocean. They occur as pavements coating volcanic or sedimentary hard-rock substrates, mainly on the slopes of seamounts, plateaus and hills in ocean basins and continental margins. We studied three ferromanganese crusts from one dredge haul from the Rivera Plate (western margin of Mexico), which are up to 15 mm thick and grow directly on a substrate of pillow basalt fragments. They consist of laminated, botryoidal, porous aggregates mostly of Fe and Mn oxyhydroxides and up to 10% silicates. XRD analysis showed the predominance of poorly crystallized mineral phases in the crusts that include Mn-feroxyhyte and vernadite, and an authigenic smectite-like clay. Detrital minerals probably derive from granodiorites of the eastern wall of the neighboring Middle America Trench. Scattered barite grains occur on the crust surface and suggest plume fall-out derived from hydrothermal vents, although a possible pelagic source cannot be dismissed. Ratios between major (Fe, Mn, Si) and trace (Co, Ni, Cu) elements reveal that such crusts are predominantly hydrogenetic in origin, although they show a hydrothermal influence that increases in the outer layers. Iron contents range from 16.2 to 25.2 wt.%, and manganese from 3.4 to 14.5 wt.%. The Fe/Mn ratio ranges from 1.6 to 7.0. The SiO₂/Al₂O₃ ratio ranges from 4.3 to 6.6, indicating the presence of biogenic silica (radiolarians). The concentrations of copper (up to 383 ppm) and cobalt (up to 534 ppm) are significantly below those of the typical hydrogenetic crusts, whereas Ni (up to 1320 ppm) is about the same or slightly lower. REE normalized profiles and ΣREE values (486 to 732 ppm) match those reported for hydrogenetic crusts, but suggest a hydrothermal contribution for the later crust layers. The inferred crust growth rates using the Co-chronometer (44 and 229 mm/Myr) are higher than those reported for pure hydrogenetic crusts (mostly 1 to 6 mm/Myr), thus indicating an increasing hydrothermal influence. The highest growth rate (229 mm/Myr, corresponding to the outer crust layers) suggests a regional hydrothermal input of iron and manganese to seawater. The intensification of regional submarine hydrothermal activity began about 13,000 yr ago, and may be related to the tectonic activity in the complex junction of the Rivera, Cocos, Pacific and North America plates.     

Dependence of sediment sorting on bedload transport phase in a river meander

It is widely recognized nowadays that there are at least two different phases of bedload sediment transport in gravel‐bed rivers. However, the transition between these phases is still poorly or subjectively defined, especially at bends in rivers, where cross‐stream sediment transport can strongly influence changes in the texture of the transported sediment. In this paper, we use piecewise models to identify objectively, at two points in the cross‐section of a river bend, the discharge at which the transition between bedload transport phases occurs. Piecewise models were applied to a new bedload data set collected during a wide range of discharges while analysing the associated changes in sediment texture. Results allowed the identification of two well‐differentiated phases of sediment transport (phase I and phase II), with a breakpoint located around bankfull discharge. Associated with each phase there was a change in bedload texture. In phase I there was non‐dominance in the transport of fine or coarse fractions at a particular sampling point; but in phase II bedload texture was strongly linked to the position of the sampling point across the channel. In this phase, fine particles tended to be transported to the inner bank, while coarse sizes were transferred throughout the middle parts of the channel. Moreover, bedload texture at the inner sampling point became bimodal while the transport of pebble‐sized particles was increasing in the central parts of the river channel. It is suggested that this general pattern may be related both to secondary currents, which transfer finer particles from the outer to the inner bank, and to the progressive dismantling of the riverbed surface layer. Copyright © 2018 John Wiley & Sons, Ltd.