José Roseiro

The Casas Novas gold deposit is located in the Escoural Gold District of the Ossa Morena Zone (OMZ, SW Iberia) and consists of an orogenic gold system characterized by structurally controlled mesothermal lodes, which are related to a major shear zone. The mineralization shows evidence of two distinct metallogenic phases during regional metamorphism: (i) an early higher-temperature stage marked by Co-Ni-rich loellingite hosting Au-(Ag) alloys accompanied by pyrrhotite, and (ii) a later low-temperature stage with arsenopyrite, gold, maldonite and Bi-Te sulfosalts. Detailed textural analysis documents the evolution from initial Au-Ag alloys enclosed in Co-Ni-rich loellingite to subsequent Au-Bi-Te phases and newly formed arsenopyrite. The results show the thermal and chemical changes in the system, demonstrating the importance of fluid–rock interactions in the redox conditions, which became more oxidized, controlling the gold deposition. The comprehensive overview of the processes that led to the concentration of gold in the Casas Novas deposit provides a valuable contribution to the ongoing studies into the auriferous region of the Escoural Gold District.

In the Iberian Pyrite Belt (IPB), long-term persistence of mine waste piles poses environmental challenges. The present work studies the Trimpancho Mining Complex in northern IPB with exposed mine waste and acidic waters in the proximity to the Chança River, a tributary of the Guadiana international river. A multidisciplinary approach is proposed, using hyperspectral reflectance spectroscopy, portable X-ray fluorescence (pXRF), multispectral Unmanned Aerial Vehicle (UAV) and Sentinel-2 images. Spectroscopic, geochemical and remote sensing methods were applied to characterise the mining area. Comparison of hyperspectral data with spectral libraries were used to validate mineralogy. Multispectral UAV data is used for custom band-ratios and adapted to Sentinel-2 images. Results grouped the samples into four groups. Spectroscopy is indicative of clays (white mica and smectite group), hematite/goethite, jarosite, and arsenopyrite and pyrite (exclusive to the Group 2); iron-rich samples reach maximum reflectance earlier than iron-poor samples. Geochemical studies show an increase in content of heavy metal such as As, Cu, Fe, Pb, and Zn from Group 1 < Group 3 ≈ Group 4 < Group 2, but Group 4 showed elevated Pb and Zn. Custom false colour composition highlighted the groups in UAV and satellite, thus constituting cost-effective tools for finding contamination sources.

The Ossa-Morena Zone (SW Iberian Massif) hosts the largest set of Cambro–Ordovician alkaline magmatic plutons related to the Palaeozoic rifting of the northern Gondwana margin so far described. An organized framework for their classification at different scales is proposed through data-driven ranks based on their distinctive petrological features relative to other rift-related magmatic rocks found throughout western Europe. The classification method aims to enhance geological mapping at different scales, regional- and continental-scale correlations, and, as such, facilitate the petrogenetic interpretation of this magmatism. The hierarchical scheme, from highest to lowest rank, is as follows: rank-1 (supersuite) assembles rocks that have distinctive characteristics from other magmatic units emplaced in the same magmatic event; rank-2 (suite) categorizes the units based on their major textural features, indicating if the body is plutonic, sub-volcanic, or a strongly deformed magmatic-derived unit; rank-3 (subsuite) clusters according to their spatial arrangement (magmatic centres) or association to larger structures (e.g., shear zones or alignments); rank-4, the fundamental mapping unit, characterizes the lithotype (alkaline granite, alkaline gabbro, syenite, albitite, etc.) by considering higher ranks (alkalinity and textural aspects); rank-5 characterizes the geometry of individual plutons (with several intrusions) or swarms; rank-6 (smallest mappable unit) corresponds to each intrusion or individual body from a swarm. Although this classification scheme is currently presented solely for the Ossa-Morena Zone, the scheme can be easily extended to incorporate other co-magmatic alkaline bodies, such as those in the NW Iberian allochthonous units or other peri-Gondwanan zones or massifs, in order to facilitate regional correlations of the rift-related magmatism.

The Santa Eulália Plutonic Complex is a large granitoid ring-complex exhibiting several granite facies, gabbro-diorite rocks and roof pendants from the country rocks. This lithologic variability crossed with the spatial distribution of each lithotype within the complex enables the grouping of the different magmatic rocks in: G0 group (“pink-reddish” granites in an external belt), G1 group ("greyish” granites in the inner part”) and M group (gabbro and diorite as enclaves in the eastern regions of the G0 group). All these groups host facies of interest for dimension stones due to their aesthetic value and, therefore, a systematic thematic mapping has been conducted and refined throughout the years. In this work we verify the applicability of Sentinel-2 multispectral images combined with Random Forest as a classification methodology to distinguish the different gran-itoid groups and support large-scale lithological mapping. The results show overall accurate outline of the facies, with areas corresponding to the G0 group displaying good predictions, areas representing the G1 group moderately good predictions, and areas established for the M group poor predictions, obtaining Producer’s accuracies of ~82%, ~60%, and ~30%, respectively.

The Cambrian Series 2 Carbonate Formation from the Alter do Chão Elvas-Cumbres Mayores unit (Ossa-Morena Zone, SW Iberian Massif) is composed of regionally metamorphosed marbles and marlstones that underwent chlorite zone metamorphism and preserve the primaeval limestone ⁸⁷Sr/⁸⁶Sr ratios (0.7083–0.7088). These are consistent with the established Lower Cambrian seawater curve, and therefore used for age constraints in formations lacking fossil contents. The regional mineralogical and Sr-isotopic features of the carbonate rocks are frequently overprinted by the effects of contact metamorphism induced by magmatic bodies emplaced during rift-related and synorogenic events of the Palaeozoic, as well as by post-metamorphic dolomitization processes. The development of calc-silicate minerals due to contact metamorphism is common in the rocks of the Carbonate Formation and apparently results from the interaction of the protolith with fluids of different origin: (i) internally produced fluids released by conductive heating (observed in external contact aureoles) and (ii) external intrusion-expelled fluids that, besides leading to the appearance of distinctive assemblages, also promote an influx of strontium content (observed in roof pendants). Calc-silicate mineralogy varies substantially throughout the region, likely due to the heterogeneous distribution of silicate minerals of the protolith, progression of intrusion-driven fluids, and the irregular effect of thermal gradients. Results suggest that high-grade contact metamorphism (hornblende facies or higher) and dolomitization processes imposed on the Carbonate Formation significantly influence the isotopic signatures of the carbonates, providing limitations in applying Sr-isotopic chronology. Graphical abstract

Old and abandoned mines are testimonials of ancient industrial activities, and as such, they are able to convey environmental concerns. A multidisciplinary approach combining ground measurements—i.e., portable X-ray fluorescence and optical spectroradiometry—with airborne multispectral images—i.e., Sentinel-2 and unmanned aerial vehicles—was conducted to define a baseline for the characterization of areas that constitute environmental burdens. The Mostardeira mine, an old copper mine located in the Portuguese Ossa-Morena Zone, was selected as a case study. The results reveal that the soils have toxic metals—e.g., As (mean = 1239 ppm) and Cu (mean = 435 ppm)—above the defined health thresholds. The spectroradiometry provided insights into the soil characterization using data from the vis-NIR spectral region, allowing us to distinguish agricultural soil, mine waste, and bare soils. The spectra obtained are comparable with the USGS soil spectra standards, namely Clinozoisite Epidote HS299, Hematitic Alt. Tuff CU91-223, and Sand DWO-3-DEL2ar1 no oil. The airborne images considered through the lens of principal components analysis and supervised and unsupervised machine learning techniques (random forest and K-means) are found to be effective tools in creating cartographic representations of the contaminated soils. The collected data are used to construct a baseline for characterizing these environmentally challenging areas, whereas the methodological approach is revealed to be successful for tackling the posed environmental problems, allowing us to map the old mine environment passives.

In SW Iberia (namely in the northernmost domains of Ossa-Morena Zone [OMZ]), NW-SE lineaments of peralkaline igneous massifs are found, known to have been emplaced during the Cambrian-Ordovician rift-related magmatic stage of the Variscan extension in northern Gondwana. In the Portuguese counterpart of the OMZ, these rocks can be found in two distinct tectono-stratigraphic segments, namely the Blastomylonitic Belt (BB) and in the Alter do Chão – Elvas Sector (ACES), intruding Neoproterozoic to Middle Cambrian successions (further extending to the Spanish side). Though the peralkaline magmas are coeval, some contrasting geochemical features allow a well-marked distinction between rocks located in the BB and ACES, which may provide sustained inferences on the petrogenesis and geotectonic framework. The BB rocks composition fit within the phonolite-trachyte spectrum, and show affinities with “within-plate” and A1-type granitoids. On the other hand, rocks from the ACES display trachyte to alkaline rhyolite compositions, chemical features of A2-type granitoids and “anomalous ocean ridge granite” tectonic setting of emplacement. However, within the ACES there are exceptions, as three massifs appear to have the same chemical signatures of the BB. Lithogeochemical data suggest the Ossa-Morena Zone alkaline melts (i) could be extracted from distinct sources, and/or (ii) underwent different degrees of fractionation along with variable crustal assimilation. In addition to lithogeochemistry data, further mineralogical and isotopic studies will be addressed to better understand and provide sustained inferences on the development of the northern OMZ peralkaline magmatism and related ore-forming systems.Acknowledgements: JR acknowledges the financial support provided by Fundação para a Ciência e Tecnologia (FCT) through the PhD grant (UI/BD/150937/2021), and by the Society of Economic Geologists Foundation through the Hugh McKinstry Fund. The authors also acknowledge expenses supported by ICT financed by FCT, under the project UIDB/04683/2020. This work is a contribution to the project “ZOM-3D Metallogenic Modelling of Ossa-Morena Zone: Valorization of the Alentejo Mineral Resources” (ALT20-03-0145- FEDER-000028), funded by Alentejo 2020 through the FEDER/FSE/FEEI.

A Zona de Ossa-Morena, que engloba grande parte do território alentejano, é uma zona paleogeográfica do território nacional. A sua complexa evolução geológica faz com que esta zona apresente uma riqueza significativa no que diz respeito à ocorrência de uma diversidade de matérias-primas, contendo 13 das 27 matérias-primas críticas presentes na lista definida pela União Europeia (não obstante as suas baixas tonelagens), para além de matérias-primas estratégicas como o ouro ou a prata. No presente trabalho, introduz-se os critérios utilizados para a definição de criticidade das matérias-primas, assim como a sua distribuição em Portugal, dando especial enfase às matérias-primas identificadas na Zona de Ossa-Morena.