Mineral Inventory of the Algares 30-Level Adit, Aljustrel Mine, Iberian Pyrite Belt, Portugal

Teresa P Silva,João X Matos,João P Veiga,Igor Morais,Luís Albardeiro,Daniel De Oliveira,Pedro Gonçalves

doi:10.3390/min10100853

Teresa P Silva, João X Matos + Show 5 more

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https://doi.org/10.3390/min10100853

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Abstract

Mining activity in Algares (Aljustrel Mine, Portuguese sector of the Iberian Pyrite Belt, IPB) stems prior to Roman times. As the orebody is vertical and relatively thin, mining was carried out mainly along underground adits (galleries). Nowadays, the deposit is considered exhausted and the area is being rehabilitated for a different use. The Algares +30 level adit intersects two volcanic units of the IPB Volcano-Sedimentary Complex. 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 hydrothermal alteration. Along the mine adit, the geological sequence is affected by strong oxidation and supergene alteration, giving rise to the formation of secondary minerals through the oxidation of the sulphides. The most common minerals found were melanterite (FeSO4·7H2O) and chalcanthite (CuSO4·5H2O), forming essentially massive or crystalline aggregates, ranging from greenish to bluish colours. Melanterite from the walls revealed to be Cu-rich by opposition to that from stalactites/stalagmites formed below the old ore storage silo revealing the low-copper-grade ores exploited underground. The mineralogy of the efflorescent salts was used to ascertain the processes involved in their formation, and moreover, the inventory of minerals is presented, as well as their principal characteristics.

Highlights

With more than 90 deposits of massive polymetallic sulphide ores, the Iberian Pyrite Belt (IPB) is one of the most important European metallogenic provinces [1,2]
The mineral characterisation of secondary minerals revealed the presence of alunogen, antlerite, atacamite, characterisation of secondary minerals the melanterite presence of alunogen, antlerite, atacamite, chalcanthite, copiapite, fibroferrite, gypsum,revealed halotrichite, and pickeringite, being chemically chalcanthite, copiapite, fibroferrite, gypsum, halotrichite, melanterite and pickeringite, being chemically aluminium sulphates, hydrated sulphates of iron aluminium and magnesium aluminium, hydrated aluminium sulphates, hydrated sulphates of iron aluminiumchlorides, and magnesium aluminium, hydrated copper sulphates, copper hydroxide sulphates and hydroxide hydrated iron sulphates, iron copper sulphates, copper hydroxide sulphates and hydroxide chlorides, hydrated iron sulphates, hydroxide sulphates with various degrees of hydration and hydrated calcium sulphates
Stereomicroscope images of chalcanthite chlorite/clinochlore

Summary

Introduction

With more than 90 deposits of massive polymetallic sulphide ores, the Iberian Pyrite Belt (IPB) is one of the most important European metallogenic provinces [1,2]. Ten deposits (located in Portugal) can be considered large-sized with regards to their Cu-Zn-Pb contents, including the operating. Pyrite (FeS2 ) is the most common ore mineral. Roman mining practice focused on the exploitation of secondary ores (oxidation zone—iron cap and supergene alteration zone) and, in addition, of primary pyrite ore. The ore was roasted in place to extract the metals for export. Some of the Roman mines were very big endeavours and caused the first events of acid mine drainage (AMD) known in Portugal [3]

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