W–Au skarns in the Neo-Proterozoic Seridó Mobile Belt,Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit

The Seridó Mobile Belt (SMB) is located in the Borborema Province in northeastern Brazil and consists of a gneiss basement (Archean to Paleo-Proterozoic), a metasedimentary sequence (marble, quartzites, and schists), and the Brasiliano igneous suite (both of Neo-Proterozoic age). In this region, skarns occur within marble and at the marble–schist contact in the metasedimentary sequence. Most of the skarn deposits have been discovered in the early 1940s, and since then, they have been exploited for tungsten and locally gold. Recently, the discovery of gold in the Bonfim tungsten skarn has resulted in a better understanding of the skarn mineralization in this region. The main characteristics of the SMB skarns are that they are dominantly oxidized tungsten skarns, with the exception of the Itajubatiba and Bonfim gold-bearing skarns, which are reduced based on pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. In the Bonfim deposit, pressure estimates indicate that the skarns formed at 10- to 15-km depth. The mineralized skarns present the prograde stage with almandine, diopside, anorthite, and actinolite-magnesio-hornblende, and titanite, apatite, allanite, zircon, and monazite as accessory minerals. The retrograde stage is characterized by alkali feldspar, clinozoisite–zoisite–sericite, calcite, and quartz. Scheelite occurs in four ore-shoots distributed within the marble and at the marble–schist contact. The main ore body is 5–120 cm wide and contains an average of 4.8-wt.% WO₃, which occurs in the basal marble–schist contact. Fold hinges appear to control the location of high-grade scheelite. The late-stage gold mineralization contains bismite (Bi₂O₃), fluorine-bearing bismite, native bismuth, bismuthinite (Bi₂S₃), and joseite [Bi₄(Te,S)₃], and also chlorite, epidote, prehnite, chalcopyrite, and sphalerite. This gold–bismuth–tellurium mineralization exhibits a typical late character and occurs as a black fine-grained mineral assemblage controlled by conjugate brittle-ductile faults (and extensional fractures) that crosscut not only the banding in prograde skarn but also the retrograde alkali feldspar and clinozoisite–zoisite–sericite assemblage. The Au–Bi–Te-bearing minerals are intergrown with retrograde epidote, prehnite, chlorite, chalcopyrite, and sphalerite, indicating that gold mineralization at Bonfim is linked to a late-stage skarn event. The polymetallic nature of the Bonfim deposit can be used as an important guide for the exploration of this type of skarn deposit in the Borborema Province, which potentially contains significant new, undiscovered gold and polymetallic deposits.     

A chronology of the Little Ice Age in the tropical Andes of Bolivia (16°S) and its implications for climate reconstruction

Dating moraines by lichenometry enabled us to reconstruct glacier recession in the Bolivian Andes since the Little Ice Age maximum. On the 15 proglacial margins studied, we identified a system of ten principal moraines that marks the successive positions of glaciers over the last four centuries. Moraines were dated by performing statistical analysis of lichen measurements based on the extreme values theory. Like glaciers in many mid-latitude mountain areas, Bolivian glaciers reached their maximal extent during the second half of the 17th century. This glacier maximum coincides with the Maunder minimum of solar irradiance. By reconstructing the equilibrium-line altitude and changes in mass-balance, we think the glacier maximum may be due to a 20 to 30% increase in precipitation and a 1.1 to 1.2 °C decrease in temperature compared with present conditions. In the early 18th century, glaciers started to retreat at varying rates until the late 19th to early 20th century; this trend was generally associated with decreasing accumulation rates. By contrast, glacier recession in the 20th century was mainly the consequence of an increase in temperature and humidity. These results are consistent with observations made in the study region based on other proxies.     

Origin of groundwater salinity and hydrogeochemical processes in a confined coastal aquifer: Case of the Rhône delta (Southern France)

The Rhône delta, South of France (Camargue, 750 km²) is a coastal saline wetland located along the Mediterranean Sea. The confined aquifer of this delta shows high values of electrical conductivity rising from the north (4 mS/cm) to the shoreline (58 mS/cm). This work aims to identify the origin of groundwater salinity and the geochemical processes occurring in this coastal confined aquifer according to the degree of salinity. A natural tracing approach is considered using monthly sampling in 8 piezometers for chemical and isotopic analyses (¹⁸O, ²H, ¹³C_TDIC). Ionic and isotopic ratios demonstrate that strong salinities are due to a simple mixing between Mediterranean seawater and freshwater; seawater contribution reaches up to 98% at 8 km from the shoreline. Seawater intrusion induces a particular groundwater chemistry which varies with the degree of seawater contribution: (1) In the less saline part of the aquifer (seawater contribution <20%), the intrusion induces an increase of Na⁺ in groundwater leading to Ca²⁺/Na⁺ exchange processes. The δ¹³C_TDIC analyses show that matrix exchange processes most likely occur for the less saline samples. (2) In the saline part of the aquifer (seawater contribution >20%), the intrusion induces SO₄ reduction which is confirmed by highly depleted δ¹³C_TDIC values (up to −19‰). The δ¹³C_TDIC also reveals that methanogenesis processes may occur in the most reductive part of the aquifer. Due to SO₄ reduction, the intrusion induces a shift in carbonate equilibrium leading to supersaturation with respect to dolomite and/or magnesian calcite. Thus carbonate precipitation may occur in the area strongly influenced by seawater.     

Hydrogeochemical modeling of organo-metallic colloids in the Nsimi experimental watershed,South Cameroon

This paper presents a hydrogeochemical modeling code HYDROS, which combines the multi-component transport model with equilibrium speciation module MINTEQA2. The processes of adsorption, aqueous speciation and mineral precipitation/dissolution are represented in the model. The numerical model uses a sequential iterative approach for solving the solute transport and the equilibrium geochemistry modules. Further the transport part is solved using an operator split approach wherein a finite volume method is used for solving the advective equations while a classical finite difference method is employed for solving the dispersive equations. The model performance is evaluated by comparing it with MINTOX for a literature problem. HYDROS is then applied to the case study of the transfer of transition metals with organic colloids in the swamp groundwater system of the experimental Nsimi watershed, representative of the humid tropical ecosystem of the South Cameroon Plateau. Field observations at the site swamp system suggest that the carbon is mainly transferred as organic colloids (i.e., dissolved organic carbon) produced by the slow biodegradation of the swamp organic matter. Using HYDROS, the behaviour of Al(III) and Fe(III) elements in the base flow system is simulated during inter rain events of a short rainy season (May–June 1996). The elemental time-series for Al, Fe, Cl, pH compare well with the simulation results. The colloids are found to have a strong impact on the mobilization and transfer of Al(III) and Fe(III), which are considered to have low mobility in weathering environment.     

Chronology and palaeoenvironmental implications of the ice‐wedge pseudomorphs and composite‐wedge casts on the Magdalen Islands (eastern Canada)

The Magdalen Islands are a valuable terrestrial record, evidencing the complex glacial and periglacial history of the Gulf of St. Lawrence. Thirteen structures interpreted as ice‐wedge pseudomorphs or composite‐wedge casts were observed at four sites on the southern Magdalen Islands and testify to the former presence of permafrost under periglacial conditions. These features truncate Carboniferous sandstone or Last Glacial Maximum (LGM) glacial and glaciomarine diamicts, both overlain by subtidal or coastal units. Six optically stimulated luminescence (OSL) and four radiocarbon ages were obtained from both host and infilled sedimentary units. These ages provide the first absolute chronological data on these structures, shedding new light on the relationships between glacial and periglacial phases. Our chronostratigraphic data suggest that, after the deglaciation and the emersion of the archipelago, thermal contraction cracks grew during the cold period of the Younger Dryas (11–10 ka; 12.9–11.5 cal. ka BP). The Younger Dryas, which is well documented in the Maritime Provinces of Canada, occurred after a pedogenesis phase associated with the Allerød warm period evidenced by the well‐developed palaeopodzol ubiquitous on the Magdalen Islands.     

Suitability of CLIGEN precipitation estimates based on an updated database and their impacts on urban runoff: a case study of the Great Lakes Region,USA

ABSTRACT

Weather generators rely on historical meteorological records to simulate time series of synthetic weather sequences, the quality of which has direct influence on model applications. The climate generator CLIGEN’s database has recently been updated to comprise consistent historical records from 1974 to 2013 (updated CLIGEN database, UCD) compared to the current database in which records are of different lengths. In this study, CLIGEN’s performance in estimating precipitation using UCD (eight stations) and the subsequent impact on urban runoff simulations (371 stations) were evaluated in the Great Lakes Region, USA. Generally, UCD-based precipitation could replicate observed daily precipitation up to the 99.5th percentile, but maximum precipitation was underestimated. Results from the Long-Term Hydrologic Impact Assessment model using UCD-based precipitation showed about 0.57 billion cubic meters more (14.9%) average annual runoff being simulated compared with simulations based on the current CLIGEN database. Overall, CLIGEN with the updated database was found suitable for providing precipitation estimates and for use with modeling urban runoff or urbanization effects.