Orange Pickeringite from the Algares 30-Level Adit, Aljustrel Mine, Iberian Pyrite Belt, Portugal

Teresa P Silva,Luís Albardeiro,Pedro Gonçalves,João X Matos,Ugur D Menda,Fernanda Carvalho,Igor Morais,Daniel De Oliveira,João P Veiga

doi:10.3390/min11101115

Teresa P Silva, Luís Albardeiro + Show 7 more

Open Access

https://doi.org/10.3390/min11101115

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Abstract

The sheltered environment of the Algares +30 level adit (underground mine gallery) contributes to the preservation of secondary water-soluble minerals formed on the tunnel walls. The massive sulphide and related stockwork zone are hosted by the Mine Tuff volcanic unit and are exposed in the walls of the gallery, showing intense oxidation and hydrothermal alteration. Minerals from the halotrichite group were identified on the efflorescent salts, typically white fine-acicular crystals but also on aggregates with dark orange/brownish colour. Mineral characterization was performed using several methods and analytical techniques (XRD, XRF-WDS, SEM-EDS, DTA-TG), and the chemical formulas were calculated maintaining the ratio A:B ≅ 1:2 in accordance with the general formula of the halotrichite group, AB2(SO4)4·22H2O. This methodology allowed the assignment of the orange colour to the presence of trivalent iron on iron-rich pickeringite in partial substitution of aluminium.

Highlights

The Aljustrel Mine is located in the Portuguese sector of the Iberian Pyrite Belt (IPB), one of the most important European metallogenic provinces, with more than 90 deposits of polymetallic massive sulphide ores [1,2]
Were used in this study to characterise crystal aggregate samples belonging to the halowere used in this study to characterise crystal aggregate samples belonging to the halotrichite group, collected from the walls of the Algares 30-level adit in the Aljustrel mine trichite group, collected from the walls of the Algares 30-level adit in the Aljustrel mine (Algares deposit, IPB massive sulphides) during a rehabilitation intervention for its val(Algares deposit, IPB massive sulphides) during a rehabilitation intervention for its orisation as a mining and geological heritage site
Of the 25 fragments selected from the 19 samples, the majority were identified as whitish pickthe 25 fragments selected from the 19 samples, the majority were identified as whitish eringite, along with two iron-rich pickeringite and one white halotrichite

Summary

Introduction

The Aljustrel Mine is located in the Portuguese sector of the Iberian Pyrite Belt (IPB), one of the most important European metallogenic provinces, with more than 90 deposits of polymetallic massive sulphide ores [1,2]. The mine site contains six ore lenses, namely, Estação, Feitais, Algares, Moinho, São João and Gavião. Feitais and Moinho are being exploited by the Almina Company. Deposit mining activity occurred prior to and during the Roman Empire’s occupation of the Iberian Peninsula. As the Algares orebody is vertical and relatively thin, mining was never developed in a large open pit setting [7]; the exploitation was mainly for copper, lead and zinc, with gold and silver as by-products. The outcropping Algares deposit forms a NW-SE orientated narrow gossan, mapped along 900 m [9,10,11]

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Discussion

Conclusion