Jorge Figueiras

Geodynamic evolution of the South Variscan Iberian Suture as recorded by mineral transformations

New structural, petrographic, mineralogical and geochemical data from the Beja-Acebuches Ophiolite Complex (BAOC) are presented, and reviewed together with data published elsewhere. The new data obtained shed light on questions such as: 1) the relative importance of the obduction event; 2) its geological record in the deep levels of BAOC; 3) the nature and intensity of the Variscan metamorphism and deformation during subsequent continental (arc) collision; 4) the age relationships between BAOC and the Beja Igneous Complex; and 5) by means of numerical modelling, the thermal metamorphism of the Ossa-Morena autochthonous terranes induced by the ophiolite obduction. The emerging picture is that of a fairly simple overall geological evolution for BAOC, seamlessly integrated within the evolution of the southern branch of the Iberian Variscides. Obduction of BAOC is a relatively minor early event in the general NE–SW convergence that gave rise to the orogen as seen regionally and is recorded by an anisotropic, high-temperature, metamorphic fabric at the gabbro levels and by subtle features of the chemical composition of primary minerals at the underlying peridotite level; it caused chilling of the obducted ophiolitic slab and no significant metamorphism on the autochtonous rocks of the Ossa-Morena Zone. BAOC underwent most of its deformation and (amphibolite facies) metamorphism during a later collisional event, that took place as the most primitive rocks of the Beja Igneous complex were being intruded, and whose waning stages are responsible for extensive serpentinisation of peridotites and for important aquocarbonic fluid discharges along the semibrittle–brittle shear zones meanwhile developed.

Base-Metals and Organic Content in Stream Sediments in the Vicinity of a Landfill

Landfill facilities are essential to modern societies as repositories for municipal solid wastes (MSW). However, irrespective of the regulations necessary to their construction there is always a certain risk of environmental impact. For this reason monitoring should be essential to their operation. In the present work, the environmental impact of such a landfill is studied, whose operation has been running for only 3 a. The approach used was to sample a series of stream sediments along two contaminated run-off water paths. These sediments were chemically analysed, and their mineralogy was also studied. Their most important feature is the existence of some contamination in Cu, Zn, and Pb above background levels in the first 200 m in the direction of water flow, whose concentration gradually decreases until background levels are reached. Although some artificial alloys and sulphides (sphalerite) were shown to be present, they could not account for the analysed concentrations, and cannot explain the negative gradient concentration by themselves. Sequential metal extraction methods together with solid phase extraction for the isolation of organic content, showed that metals adsorbed on the sediments are mostly associated with organic compounds. Although monocarboxylic organic acids were among the most important organic compounds present both in the contaminated waters and sediments, there is no clear correlation between these and base metal concentrations in the sediments. A stronger correlation exists if the total concentration of organic compounds is considered, which suggests that base metals can be easily partitioned into the sediments by the rapid adsorption onto their surfaces by organic compounds near the pollution source. The organic content identified decreases down stream as do base metal concentrations. This can be explained by the formation of complexes with organic acids as is extensively reported in natural and laboratory systems. The overall results point to the limited capacity for pollutant retention in this system, and suggest that in case of serious failure the contaminated plumes are likely to disperse rapidly into the environment.

Roman and modern slag at S. Domingos mine (IPB, Portugal): compositional features and implications for their long-term stability and potential reuse

S. Domingos ores have been exploited for Cu and S from antiquity to 1966. Slag is composed of Fe-, Ca-, Zn- and Pb-rich silicate glass, fayalite, hedenbergite, Fe-spinel, and blebs of Fe-, Cu-, Zn- (and Pb-)sulphides, minor sulphosalts and Cu-Zn-Pb-iss with evidence for low-T exsolution and reactions. Silicates and oxides often crystallised far from equilibrium out of undercooled melts. Weathering leads to Fe-(hydro)oxide + digenite ±Fe-, Cu- and Pb-sulphate, Ca, Zn and Pb loss from glass and acid drainage. The slag is an excellent aggregate raw material but its chemical instability calls for preliminary processing, which might lead to economic metal recovery.

Fully oxidized chromite in the Serra Alta (South Portugal) quartzites" chemical and structural characterization and geological implications

Abstract Several quartzite bodies outcrop along the Ferreira-Ficalho Thrust Fault (South Portugal), a major accident of the Iberian Variscan Orogen. The sediment is a very pure quartz sandstone, with trace amounts of ultraresistant heavy minerals and chromite. Chemical characterization (microprobe analyses and Mössbauer spectroscopy) showed the chromite to be unique: besides being Zn-rich, complexly zoned and a cation deficient spinel, all the iron was found to be fully oxidized to Fe3+. Structure refinement of single-crystal X-ray diffraction intensities unambiguously identifies the mineral as a chromite and the Mössbauer data are consistent with tetrahedrally coordinated Fe3+ in the spinel structure. Current geodynamical models see the Ferreira-Ficalho Thrust Fault as a first-order suture resulting from a complex collision of two distinct continental blocks with partial obduction of the intervening oceanic crust. The chromite grains could be envisaged as remnants of an early erosion of this obducted oceanic crust, but its unique chemical character does not allow any definite conclusion. Yet, the complete quartzite heavy mineral contents and its petrographic features are not consistent with their deposition within a continental collision situation.

Nature and mechanism of ilmenite alteration: a Mössbauer and X-ray diffraction study of oxidized ilmenite from the Beja-Acebuches Ophiolite Complex(SE Portugal)

Abstract Ilmenites from the least-altered rocks of the Beja-Acebuches Ophiolite Complex (SE Portugal), with low Ti values and excess Fe, despite rare optical evidence of hematite exsolution, were studied by 57Fe Mössbauer spectroscopy and X-ray diffraction. According to single-crystal XRD the sequence of alternate layers characteristic of the ideal ilmenite structure is preserved, the excess Fe being accommodated in the Ti layers. No superparamagnetic oxides were detected by 57Fe Mössbauer spectroscopy. The typical spectra of bulk αFe2O3 and of Fe3+-containing ilmenite, in the paramagnetic state above 49 K and magnetically ordered at 6 K, are observed. The average degree of oxidation of the ilmenites, estimated from the chemical analysis assuming ideally stoichiometric full cation site occupancies, is also confirmed by 57Fe Mössbauer data. Since our crystal chemistry study gave no evidence of crypto-exsolution textures within the ilmenite with the observed compositions, fast cooling from magmatic temperatures and decomposition of ilmenite in supergene conditions is suggested.

Initial stages of pollutants dispersion around municipal waste disposal facilities: a case study in Northern Portugal

The combined use of several geological, geochemical and geophysical observations during six years enabled the detection of a very faint halo of chemical contamination induced by the operation of a young waste disposal facility in Northern Portugal. The halo does not classify as pollution by any standards, but is observed to be intensifying and thus signals a potential hazardous situation in future years. The approach used shows that a comprehensive and diverse monitoring programme, although relatively inexpensive and easy to carry out, may detect concealed leaks long before any environmental concern is justified, and thus may help avert future complex remediation needs caused by the uncontrolled growth of undetected underground pollution plumes.

A case study of two and three-dimensional inversion of dipole-dipole data: the Enfermarias Zn-Pb (Ag, Sb, Au) Prospect (Moura, Portugal)

The Enfermarias Zn-Pb(Ag, Sb, Au) prospect, located in the Moura-Ficalho region (Alentejo, Portugal), was intensively explored during 1980s. A drilling programme was successful, intersecting massive and disseminated sulphide mineralizations. The geological and geophysical data set available for Enfermarias is quite vast and comprises 13 borehole geological logs, as well as gravity, magnetic and resistivity data. In this paper, the dipole-dipole resistivity surveys are re-examined and interpreted using both 2D and 3D inversion to provide an interesting comparison of the two different approaches. Results from gravity data modelling and data from borehole logs are used as independent validity tests on the quality of the results. The 2D and 3D models indicate a conductive overburden covering more resistive bedrock, which appears at a depth of 70–100 m in the western part of the profiles. Results obtained from 3D inversion, however, show a very clear separation of the most resistive domains in the two different bodies, whereas this separation does not appear in the 2D models. The 3D model also shows a higher degree of spatial continuity than the partial 2D models, which are, in general, richer in shortwavelength anomalies, and show a higher variation from profile to profile. Comparison of the resistivity results with gravity in two coincident profiles reveals an excellent agreement between the main shallow features of both 2D and 3D models. This suggests that the resistivity model does indeed satisfy the main features of the available geological and geophysical data.

57Fe Mössbauer spectroscopy study of the correlation between the Fe3+ content and the magnetic properties of natural Cr-spinels

Both the Fe oxidation degree and the magnetic properties determined by 57 Fe Mossbauer spectroscopy were used to characterize the Cr-spinels from the Beja-Acebuches Ophiolite Complex (SE Portugal). Two different types of Cr-spinel were observed. The first one has an average Fe 3+ /(Fe total) ≈ 25%, and corresponds to unaltered Cr-spinel grains which remain paramagnetic down to 6K. The second one results from the alteration of the first type, has an average Fe 3+ /(Fe total) ≈40% and order magnetically between 222 K and 78 K. Fe 3+ /(total Fe) increases with the magnetic ordering temperature. The grain population ordering at the highest temperatures, in the range 202–222K, has the highest Fe 3+ /(total Fe) ≈ 58%. Quantitative Fe site distributions can be obtained from room-temperature Mossbauer data if the different recoilless factors for tetrahedral Fe 2+ and octahedral Fe 3+ are considered. The observed second-order Doppler shifts are consistent with Mossbauer temperatures of 330K and 605K, reported in the literature for tetrahedral Fe 2+ and octahedral Fe 3+ in other oxide spinels. The differences in the magnetic ordering temperatures allowed the characterization of both types of Cr-spinels using a single Mossbauer absorber, thus avoiding the need to separate them physically, which, in this case, would be very difficult. The results are compared to those obtained for other Cr-spinels and, for the first time in the case of natural Cr-spinels, the increase of their magnetic ordering temperatures with their oxidation degree, resulting from natural processes, is discussed in terms of the magnetic exchange interactions between the Fe cations.