Hydrothermal overprint on Cenozoic sediments in the Quadrilátero Ferrífero of Minas Gerais: implications for precious metals in cratonic terrains

Cenozoic sediments deposited in graben‐like basins in the Quadrilátero Ferrífero, southern São Francisco craton, Brazil, have anomalous contents of Hg and Sb. Two localities have been studied: the topaz deposit of Antônio Pereira, and the remains of the old opencast workings at Cata Preta, one of the earliest Au mines of Minas Gerais that dates back to about 1690. At Cata Preta, a brecciated wad‐like rock exhibits geochemical fingerprints that point to fractionation of Pt and Pd from Ag. This metal fractionation, reflected in the Au ingots produced in the region, genetically links Cata Preta to the near‐surface bonanza ore of Serra Pelada, which formed as a result of a Late‐Cretaceous hydrothermal overprint in the Amazon craton. Here, we propose that anorogenic hydrothermal activity in cratonic terrains prepared the ground for the enrichment in precious metals that gave rise to two of the most important Au rushes in Brazilian history.     

Moisture recycling and the maximum of precipitation in spring in the Iberian Peninsula

In the semiarid interior of the Iberian Peninsula, the topographic insulation from the surrounding seas promotes the role of internal sources of moisture and water recycling in the rainfall regime. In inland Iberia, the annual cycle of precipitation often has a distinctive peak in the springtime, when evapotranspiration (ET) is the highest, in contrast to the coastal areas, where it is more closely related to the external moisture availability and synoptic forcing, with a maximum in winter-autumn and a pronounced minimum in the summer. In this work we investigate the role of land surface water fluxes in the intensification of the hydrological cycle in the Iberian spring. We used data from 5 km resolution WRF-ARW model simulations over the Iberian Peninsula for eleven months of May (2000–2010). To bring out the effect of ET fluxes, we conducted experiments where ET water over land was removed from the system. Our findings indicate that the impact of ET on precipitation is on average very large (37 % increase). The impact is particularly strong in the interior north and northeast areas where the observed annual rainfall cycle has a peak in May, suggesting that the role of surface water fluxes is very important there. To investigate whether this role is as a water source or to amplify precipitation dynamics, we computed the recycling ratio analytically from the model data. In addition, we developed a procedure to quantify the amplification impact by comparing the recycling ratio and the relative change in precipitation between control and experiments with ET removed. Results show that the role of surface water fluxes on precipitation depends on large-scale forcing and moisture advection. When the synoptic forcing and moisture advection are strong, such as in fronts associated to Atlantic storms, the impact of ET fluxes is small. When there is potential for convection, as is commonly the case of late spring in the Iberian Peninsula, ET fluxes have a significant impact. Surface moisture fluxes moisten the boundary layer and increase moist static energy, strengthening convective processes, and their role goes from being a primary water source for precipitation (recycling) to have mostly an amplification effect as external moisture availability increases. Our findings show that for the eleven simulated May cases over the Iberian Peninsula, the role of ET fluxes in activating recycling is important and explains 27–58 % of their total impact on precipitation, depending on the method used to calculate the recycling ratio. This indicates that the complementary effect of ET on amplifying rainfall from external sources of moisture is comparable in magnitude to the recycling mechanism and important as well.     

Global climate models as forcing for regional ocean modeling: a sensitivity study in the Iberian Basin (Eastern North Atlantic)

This work evaluates the performance of several global climate models (GCMs) as forcing of a regional ocean model configuration centered in the Iberian Basin. The study is divided in two parts. First, the output of nine GCMs is analyzed based on the fields needed to force the ocean model (Regional Ocean Modelling System—ROMS). GCMs differ greatly between them and their performance depends on the field. In the second part, the two GCMs with the worst performances in both extremes of the ensemble are used as forcing for two ROMS simulations, with the purpose of assessing the range of uncertainty comprised in this set of GCMs. Two other ROMS runs are setup: one climatologically forced control run, and one forced with the average of all the nine GCMs—the ensemble mean. Results show that the tendency of overestimation/underestimation of the forcings is reflected in the modeled hydrography, both at the surface and deeper layers down to 500 m. Nevertheless, in terms of circulation, all four runs reproduce the Azores Current, as well as the coastal transition zone seasonality (winter poleward flow and summer upwelling-associated equatorward flow). The CGCMs output performance as forcing depends on the forcing variable: one performs well for one or more variables, but badly for others, and which field is well or badly reproduced varies for each CGCM. Therefore, there is not a single CGCM having the best forcing for all variables. Hence, our results indicate that the most adequate approach consists of using the ensemble mean as forcing rather than using an individual model. This is supported by the general low overall (i.e. for all forcing variables) errors of the ensemble mean regarding the control climatological dataset, and the good comparison of the ensemble-forced ROMS run with the control run.     

Iodine and bromine contents in the Palaeoproterozoic Hotazel iron formation, Transvaal Supergroup, South Africa: A reconnaissance study

A reconnaissance bulk geochemical study was carried out on ten samples from the Palaeoproterozoic Hotazel iron formation, South Africa, with emphasis on iodine and bromine abundances. Very low-absolute contents recorded for both halogens, i.e. 0.08–0.26 μg/g for I and 0.35–1.23 μg/g for Br, indicate that they should largely have been lost during burial diagenesis. Bulk I/Br ratios (0.08–0.50), however, show marginal variation across the selected sample set and broadly resemble those that typify recent marine sediments containing organic matter. The possibility emerges that both elements are organic matter-sourced and may thus provide a potential proxy for the role of biological processes during deposition and diagenesis of Precambrian iron formations.     

In situ cyclic tests on existing stone masonry walls and strengthening solutions

The present work reports on an in situ experimental test campaign carried out on abandoned traditional masonry houses after the 9th July 1998 earthquake that seriously hit the Faial island of Azores. For the testing purposes, an experimental test setup was developed based on a self‐equilibrated scheme, which is herein described reporting on the advantages and drawbacks of this in situ test setup. Five specimens were tested aiming at characterizing the out‐of‐plane behavior of stone masonry walls and strengthening solutions recommended for post‐earthquake interventions. A detailed comparison between solutions' efficiency is presented including a cost vs benefit analysis. In order to assess the efficiency of the developed test setup for other applications on stone masonry walls, an in‐plane test on an existing URM panel is also presented. Several related issues are discussed, namely the advantages of dealing with the real boundary conditions and the capacity of providing valuable information of the response, as well as a detailed analysis of the obtained results. The authors believe that this work provides an increase in knowledge on the seismic behavior of the existing masonry constructions, resulting from the development of an in situ test setup and the efficiency quantification of strengthening solutions. Therefore, the work is thought to positively contribute for the preservation of architectural heritage and for its seismic vulnerability reduction. Copyright © 2010 John Wiley & Sons, Ltd.     

Inter-laboratory comparison of oxygen isotope compositions from biogenic silica

Several techniques have been introduced in the last decades for the dehydration and release of O₂ from biogenic silica (opal-A) for oxygen-isotope analysis. However, only one silica standard is universally available: a quartz standard (NBS28) distributed by the IAEA, Vienna. Hence, there is a need for biogenic silica working standards. This paper compares the existing methods of oxygen-isotope analyses of opal-A and aims to characterize additional possible working standards to calibrate the δ¹⁸O values of biogenic silica. For this purpose, an inter-laboratory comparison was organized. Six potential working standard materials were analysed repeatedly against NBS28 by eight participating laboratories using their specific analytical methods. The materials cover a wide range of δ¹⁸O values (+23 to +43‰) and include diatoms (marine, lacustrine), phytoliths and synthetically-produced hydrous silica. To characterize the proposed standards, chemical analyses and imaging by scanning electron microscopy (SEM) were also performed. Despite procedural differences at each laboratory, all methods are in reasonable agreement with a standard deviation (SD) for δ¹⁸O values between 0.3‰ and 0.9‰ (1σ). Based on the results, we propose four additional biogenic silica working standards (PS1772-8: 42.8‰; BFC: 29.0‰; MSG60: 37.0‰; G95-25-CL leaves: 36.6‰) for δ¹⁸O analyses, available on request through the relevant laboratories.     

Aalenian to Cenomanian Radiolaria of the Bermeja Complex (Puerto Rico) and Pacific origin of radiolarites on the Caribbean Plate

The study of the radiolarian ribbon chert is a key in determining the origins of associated Mesozoic oceanic terranes and may help to achieve a general agreement regarding the basic principles on the evolution of the Caribbean Plate. The Bermeja Complex of Puerto Rico, which contains serpentinized peridotite, altered basalt, amphibolite, and chert (Mariquita Chert Formation), is one of these crucial oceanic terranes. The radiolarian biochronology presented in this work is mainly based by correlation on the biozonations of Baumgartner et al. (1995) and O??Dogherty (1994) and indicates an early Middle Jurassic to early Late Cretaceous (late Bajocian?Cearly Callovian to late early Albian?Cearly middle Cenomanian) age. The illustrated assemblages contain about 120 species, of which one is new (Pantanellium karinae), and belonging to about 50 genera. A review of the previous radiolarian published works on the Mariquita Chert Formation and the results of this study suggest that this formation ranges in age from Middle Jurassic to early Late Cretaceous (late Aalenian to early?Cmiddle Cenomanian) and also reveal a possible feature of the Bermeja Complex, which is the younging of radiolarian cherts from north to south, evoking a polarity of accretion. On the basis of a currently exhaustive inventory of the radiolarite facies s.s. on the Caribbean Plate, a re-examination of the regional distribution of Middle Jurassic sediments associated with oceanic crust, and a paleoceanographic argumentation on the water currents, we come to the conclusion that the radiolarite and associated Mesozoic oceanic terranes of the Caribbean Plate are of Pacific origin. Eventually, a discussion on the origin of the cherts of the Mariquita Formation illustrated by Middle Jurassic to middle Cretaceous geodynamic models of the Pacific and Caribbean realms bring up the possibility that the rocks of the Bermeja Complex are remnants of two different oceans.