The Borba exploration licence in central-eastern Portugal covers a total area of 636 km. It is located in the Variscan terrain known as the “Ossa-Morena Zone”, the same terrain that hosts Colt’s Boa Fé and Montemor projects, relative to which Borba is located approximately 65km due East.
The region is mostly known for its production of high quality marble dimension stone for the international market, namely from the municipalities of Estremoz, Borba and Vila Viçosa. Small scale underground mines produced copper until the early 20th Century from a number of disseminated deposits, namely Mostardeira, Miguel Vacas, Bugalho and Mociços. Modern opencast mining was carried out to exploit the oxide zone of the Miguel Vacas copper deposit in the period 1980-1990, during which an estimated total of around 895 tonnes of copper metal were produced; upon the interruption of this exploitation the deposit was believed to be open both along strike and (particularly) down dip.
Modern exploration was carried out in the area between 1986 and 2006 by Rio Tinto, Carnon Holdings, Auvista Minerals, Prominas, Rio Narcea Gold Mines. Most of these exploration programs focussed on the gold potential of the area, having led to the discovery of several gold occurrences and anomalies which were poorly explored, such as at Almagreira, Torre and other locations, besides identifying gold contents also at the old copper mines of Bugalho, Mociços and Mostardeira.
The licence area geology encompasses several sedimentary and volcanic formations that extend from the Upper Proterozoic to the Devonian. The copper, copper-gold and gold mineralizations may occur in rocks of distinct ages as disseminations, veins and breccias which are associated with a zone of regional shearing extending for over 30 km in the NW-SE direction within the concession area.