Miguel Vacas Mine, Nossa Senhora da Conceição e São Bartolomeu, Vila Viçosa, Évora, Portugali

Regional Level Types
Miguel Vacas Mine	Pit (Flooded)
Nossa Senhora da Conceição e São Bartolomeu	Civil Parish
Vila Viçosa	Municipality
Évora	District
Portugal	Country

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- A former producing open-pit Cu mine that extracted and treated oxide copper. The deposit was exploited from 1925 to 1986 by various companies. A drilling program was initiated in 2015.

- Vein-like to lenticular chalcopyrite, native copper, and malachite in brecciated quartz veins. Known for excellent ibethenite and other secondary copper phosphates.

-Three contiguous mining concessions: no. 1550 Fonte da Moura, no. 1551 Monte d'El-Rei, and no. 1552 Miguel Vacas.

Notes:
- Due to an administrative merger in January 2013, Miguel Vacas is now part of a new parish called 'Nossa Senhora da Conceição e São Bartolomeu.'

- There are numerous libethenite specimens found in private collections labeled as originating from places like "Estremoz," "Alandroal," and "Mostardeira," but it is highly likely that all of them actually come from the Miguel Vacas mine.

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This mine is often referred to in various documents as being located at Monte dos Vieiros, Pardais.

Miguel Vacas (also locally known as Mina dos Vieiros, Cobre dos Vieiros, etc.) was a copper mine actively exploited between 1925 and 1986. Its mining activity possibly dates back to the Roman period, but there is no strong evidence to confirm it. In its initial phase, ore extraction was carried out through tunnels and galleries. Its open-pit mining (by Minargol) began in 1979.

Occurrences of copper, independent of igneous activity, disseminated.
Occurrences of copper, independent of igneous activity, veins.

Description: Brecciated quartz veins, sub-concordant with the regional structure, covering an extension of 1,600 meters with a NNW trend, a dip of 70°E, and an average thickness of 0.80 meters; post-Hercynian mineralization. The tectonized surrounding schists sometimes show significant impregnations of copper minerals.

Main mineralizations: chalcopyrite, native copper, malachite.

Morphologies: Vein, lenticular.

Host rocks: Limestones, schists.

Local geology: Silurian, NE flank of the large Sousel-Estremoz-Alandroal anticline.

Regional geology: Silurian.

LNEG/Siorminp information.

Historical notes on prospecting and exploration (1):
The Miguel Vacas mine has been the subject of several prospecting and mining exploration works since ancient times, with one of the earliest identified works being a small open-pit mine attributed to the Romans. From 1925 to the present, several companies have conducted studies and exploration in the concession areas:

The English company G.F. Norton & Co., from 1925 to 1929, conducted significant reconnaissance and mine development works (including the opening of a transverse access gallery and drainage, and over 1,000 meters of reconnaissance galleries were opened where a vein was located). Three shafts were also dug for extraction and ventilation purposes. The company did not commence mining operations, possibly due to the economic depression of 1929-1931.

In 1948/1949, the American company American Smelting and Refining conducted a simplistic and summary study of the deposit's structure in its outcrop, and to verify the continuity of mineralization at depth, three close-spaced boreholes were drilled, intersecting the vein structure at 151 and 240 meters. The company eventually abandoned the works.

Two modest national mining companies conducted some small-scale mining from 1950 to 1953, selling the concentrates obtained to CUF. Through gravity beneficiation techniques (jigging and tables), 369.568 tons of concentrate with an average copper content of 24.9% were obtained from 6,000 tons of extracted ROM with an average copper grade of 2.0%, according to annual reports from the Technical Directorate.

From 1956 to 1979, these concessions were held by EMIL - Empresa de Mineração, SARL. In 1965, EMIL entered into an agreement with the Canadian company Mining Exploration International (MEI), conducting the most important investigation and exploration campaign at Miguel Vacas, which included the following works:

. Geochemical surface prospecting along the outcrop line;
. Geological mapping of the outcrop zone;
. Surface reconnaissance through trenches in the central zone of the deposit;
. Twenty drillholes totaling 4,093 meters, reaching depths between 60 and 330 meters;
. Geological mapping of underground workings, collecting samples for average grade determination;

After the completion of these works, the company abandoned the project, apparently considering it economically unviable.

In 1979, EMIL was acquired by MINARGOL - Sociedade Mineira de Argozelo, SARL. In the same year, the delineation and development of a mining project for the extraction of the oxidized portion of the Miguel Vacas deposit began, with enriched copper grades due to supergene enrichment. The ore extraction was carried out through open-pit mining, using a step-cut approach. Ore beneficiation was performed through heap leaching, followed by electrolytic recovery, a method facilitated by the oxidized state of the ore. The mine was in operation until 1986, and a total of 215,000 metric tons of ore were extracted, yielding 481.3 metric tons of copper metal.

Around 1990, the English company Carnon Holdings Limited conducted various prospecting, research, and evaluation works in the Vila Viçosa - Alandroal area, with particular attention to the Miguel Vacas deposit. The company analyzed and reevaluated the existing data regarding the deposit, estimating the unexplored copper resources. For Miguel Vacas, total geological resources of approximately three million metric tons of ore were calculated, with an average copper grade of 1.42% (including oxidized and sulfide ore).

Between 1996 and 1998, Auvista Minerals NL conducted three drillings, with the objective of one of them being to replicate drill MV-7 (one of the drillings carried out by EMIL). It was performed 60 meters away from MV-7, with the same orientation, aiming to intersect the possible extension of the mineralization to the surface indicated by drill MV-7. This same drilling intersected the mineralized breccia of Miguel Vacas between 154 and 182 meters. The company suspects that the volcanic rock intersected from 185 meters acted as a source of mineralization.

Between 1998 and 2000, the company Prominas - Geologia e Minas, Lda., conducted a metallogenic interpretation through detailed geological observation of the open pit at the Miguel Vacas mine. They concluded the existence of two generations of mineralization: 1) older stratabound mineralization, with folded lithologies as the host rock; 2) more recent Cu ± Au mineralization confined to a 20 to 30-meter zone formed by granular breccia, cryptocrystalline chalcedonic quartz with an external "stockwork" structure. It consists of a "gossan boxwork" with varying amounts of iron and secondary copper minerals. This structure is located in a shear zone.

Between 2005 and 2007, Rio Narcea Gold Mines, S.A., conducted several prospecting and research works (including lithogeochemical analysis, channel sampling, and reanalysis of drillings) in the area of the Miguel Vacas mine. However, based on the results, they decided not to proceed with further prospecting and research in the area of the Miguel Vacas mine.

(1) Gustavo Pereira Fernandes: Copper Mineralizations of the Miguel Vacas Mine: Petrographic and Geochemical Characterization. Master's Degree in Economic Geology, University of Lisbon, Faculty of Science, Department of Geology. Lisbon, 2012.

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- Mindat Articles:
Portuguese Ossa-Morena Zone Mining - By Martins da Pedra http://www.mindat.org/article.php/1631
Miguel Vacas Mine - By Martins da Pedra http://www.mindat.org/article.php/1740

Commodity List

This is a list of exploitable or exploited mineral commodities recorded at this locality.

Mineral List

46 valid minerals.

Rock Types Recorded