PORTUGUESE OSSA-MORENA ZONE MINING


The present work focuses on the most significant small-scale mining operations of base metals, performed in the past within this zone in Portugal.




























The Ossa-Morena Zone is a southern geotectonic belt of the Iberian Variscides which begins with a Precambrian metamophic formations followed by Cambrian and Silurian formations, and ends with a late Devonian period. In contact with the Central Iberian Zone formed a important shear zone that extends from o Porto to Cordoba in Spain. This metamorphic belt is named Blastomylonitic Belt. In south, Ossa-Morena Zone contact with South Portuguese Zone through Pulo do Lobo formations.
Concerning to magmatism, the NE sector has a predominance of granitic rocks. The south, the basic calcalki character of the intrusions has rocks like: diorites, gabbros, serpentinites anorthosites, porphyries etc.
The most important mineral occurrences with base metals are associated with the Cambrian-Ordovician formations. Precious metals are related with the Precambrian formations.
Concerning to non-metallic minerals, this geotectonic belt is rich in ornamental rocks, particularly marbles in the anticlinal of Estremoz.









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A - NORTH ALENTEJO ZONE
In reference to geological typology and aspects of regional mineral occurrences, this area is divided into three Belts:

(A-a) - Arronches – Campo Maior Belt
(A-b) - Alter do Chão – Elvas Belt
(A-c) - Sousel – Barrancos Belt



(A-a) - Arronches – Campo Maior Belt
This belt consists mainly of Late Precambrian formations belonging to Tomar-Badajoz in Blastomylonitic belt. Copper mineralization with old mining are represented with Tinoca/Azeiteiros mines. Lead mineralization is rarer and is represented by former mining with the Baloço mine. The high metamorphic formations related to important tectonic accidents resulting in hydrothermal activity, contributed significantly to a concentration with mineralization of pyrite, chalcopyrite, pyrrhotite, magnetite and, more rarely galena with some Ag. This Blastomylonitic belt in Extremadura province – Spain, has important mineralization with old mines.


(A-b) - Alter do Chão – Elvas Belt
This belt is located in Cambrian formations. Particulary in the area of Alter do Chão there are occurrences of Cu, Pb, Zn and Ba and are related with emissions of volcanic episodes in this formations.





(A-c) - Sousel – Barrancos Belt
This belt basically corresponds to Cambrian, Ordovician, Silurian and Devonian formations.
The mineralization has essentially cooper vein structures and also stratiform and disseminated mineralisations.
The vein structures are subconcordant or completely discordant from the formations in which they are embedded.
The most important sub-concordant veins with old mines, are Miguel Vacas and Moçicos mines, situated at the base of the Silurian, and in basic metavolcanics of Mid Cambrian formations.


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The most important of discordant veins with old mines are Minancos and Bugalho mines in Silurian formations and Zambujeira mine, in Ordovician formations.






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The Mostardeira, Aparis and Botefa mines are also located in discordant veins and are situated on the SW flank of the Terena syncline on Devonian formations.
Concerning to stratiform mineralised structures, these are related to a Cambrian-Ordovician discordance with main mineralisations of pyrite and rare chalcopyrite, sphalerite and galena. Mineralisations of this minerals also occur in dolomitic limestones of Cambrian formations.
The disseminated mineralisations formed essentially of pyrite and chalcopyrite are associated to sub-volcanic structures and constitute the cement of eruptive breccias in Silurian formations.










B - ÉVORA - BEJA MASSIFS
In reference to geological typology and aspects of regional mineral occurrences, this area is divided into three massifs Belts:

(B-a) - Arraiolos – St. Aleixo Belt
(B-b) - Magnetitic – Zinciferous Belt
(B-c) - Porphyry Belt
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(B-a) - Arraiolos – St. Aleixo Belt
Tis belt has various occurrences of copper in vein structures with old mining explorations, some on a small scale (Azaruja, Monte do Trigo, Reguengos and S. Aleixo). These mineralisations are related with granodiorite intrusions and basic volcanics.





(B-b) - Magnetitic – Zinciferous Belt
This belt basically corresponds a Cambrian, Ordovician and Silurian formations with introductions of Hercynian granitic massifs such as Pias and Pedrogão where fits mineralization of magnetite and have been explored with old mines like Orada and Azenhas mines. This belt begins in Montemor-o-Novo with cooper mineralizations with old mines like Courela do Conde, Safira, Courelinha, Gouveia, Alcainha, Sobral/Gatonheira and Alvito.










The most important deposits in this belt are Algares (Cu, Pb, Zn) and Balsa (Pb, Zn) in Portel region, Enfermarias (Zn, Cu, Pb) and Preguiça/Vila Ruiva (Zn, Pb) with old mines, in Moura and Sobral da Adiça regions respectively, and other occurrences of Pb, Sb, Ag, Zn in the Ficalho-Moura-Vale de Vargo triangle.
The Balsa and Preguiça deposits are associated with dolomitic limestone; those at Algares and Enfermarias are related with an volcanic episode which occurred at the base of the Early Cambrian, contemporaneous with a carbonated sedimentation, with which an important hydrothermal system is associated.



(B-c) - Porphyry Belt
This belt covers a vast area in which volcanic rocks like rhyodacites, felsites, spilites and basalts are surrounded to the NE and SW by diorites, gabbros and ultrabasic rocks. The most significant mineral occurrences are Corte Pereiro, Caeirinha, Alcáçovas, Peroguarda and Asseiceiras.

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Martins da Pedra

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