Mineralogical characterization of the Algares 30-level addit for cultural tourism, Aljustrel mine, Iberian Pyrite Belt

Teresa P. Silvaa*, João X. Matosb, Daniel de Oliveiraa, Igor Moraisb, Pedro Gonçalvesb, Luís Albardeirob, João P. Veigac

aLNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal

bLNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Bairro da Val d´Oca, Apartado 14, 7601-909 Aljustrel, Portugal

cCENIMAT/I3N (Centre for Materials Research), Materials Science Dept., Faculty of Sciences and Technology, New University of Lisbon, 2829-516 Caparica, Portugal 

The Iberian Pyrite Belt (IPB) is one of the most important European metalogenic provinces, with more than 90 deposits of polymetallic massive sulphide ores.  Ten deposits (located in Portugal) can be considered large-sized with regard to their Cu-Zn-Pb contents, including the active Neves Corvo and Aljustrel mines being pyrite (FeS2) the most common ore. Aljustrel consists of six separate ore lenses, one of which is Algares, where mining activity is prior to Roman times, being Vipasca mine exploited during their Empire. Presently the Algares deposit is considered exhausted and the area is being rehabilitated for a different use, e.g. mining and geological tourism. Aljustrel has a vast heritage characterized by old mine structures (cementation tanks, mining headgears, pits, etc.) and underground addits, whose technical, economic, social and cultural legacy must be preserved. The outcropping Algares deposit forms a NW-SE narrow gossan, mapped along 900 m. This structure is classified as a geosite. To promote new educational visits the EDM Company promoted recent renovations inside the Algares +30 level addit (approximately 500 m long). The gallery intersects, from E to W, two volcanic units of the IPB Volcano-Sedimentary Complex, the Megacrystal and the fine Mine (Tufo da Mina). The massive sulphide and related stockwork zone are hosted by the Mine volcanic unit, which shows intense hydrothermal alteration. Along the mine gallery this geological sequence is affected by intense weathering and supergene alteration.

The goal of this study is to characterize the diversity of minerals resulting from the oxidation of the massive and stockwork ores. A detailed geological mapping was done followed by an X-ray diffraction study (XRD), carried out on >100 samples collected from the walls of the addit taking into account both colour and crystal morphology. Fragments were selected at the stereomicroscope and images collected with a digital camera. The first results of the mineral characterization reveal: chalcanthite (CuSO4.5H2O) as the most common mineral on blue samples and melanterite (FeSO4.7H2O) on greenish ones; halotrichite [FeAl2(SO4)4.22H2O], pickeringite [MgAl2(SO4)4.22H2O], fibroferrite [Fe(OH)SO4.5H2O], gypsum (CaSO4.2H2O), antlerite [Cu2(SO4)(OH)4], alunogen [Al2(SO4)3.18H2O], copiapite [Fe5(SO4)6(OH)2.20H2O], hematite (a-Fe2O3), jarosite [KFe3(SO4)2(OH)6] and kaolinite [Al2Si2O5(OH)4] were also identified. The location of each sample was registered for subsequent mapping of the minerals along the gallery walls, to guide future visitors in this mining heritage site.

Acknowledgements: EDM Company and FEDER (Fundo Europeu de Desenvolvimento Regional)/INTERREG POCTEP GEO-FPI - (Ref. 0052-GEO-FPI-5-E).

Abstract presented in the Conference Materiais 2019, 14-17 Abril 2019, Lisboa, Portugal