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The Borba 2 exploration permit consists of four projects and is located 160 km east by road from Lisbon and is approximately 70 Km east of Évora, the district capital. The recognized potential of this sector to host precious-metal and base metal mineralization is enhanced by the presence of abundant Cu and/or Au mineral occurrences in shear zones and in epithermal systems associated with intra-Ordovician volcanism.
The Borba 2 permit covers an area of 328.5 sq. km and represents an elongated polygon with the longest side near 40 km long, and the widest part of approximately 11km. The permit is granted by the State for a total period of five years, including a three-year initial contract, followed by another two-year extension period.
The Rio Tinto Group explored the area for epithermal gold deposits during the early 1980’s. They were followed by Carnon Holdings Ltd, in the early 1990’s, by Auvista Minerals during the period 1996-1998, by Rio Narcea Gold Mines in a JV with Kernow Resources (2005-2007) and more recently by Colt Resources.
The mine was registered in 1866 and has operated intermittently until around 1900. There are records for a total of nine mine levels, down to a total depth of 200m.
Silurian dark-grey shales with intercalations of black cherts, quartzites and graphitic phyllites adjacent to a major NE-SW deep seated fault zone (Messejana fault).
Quartz-cemented breccias emplaced on east branch of the fault zone. Mineralization associated with a 1km long section of the fault zone. Includes three main veins that have been mined in the past:
Dump samples of silicified and sheared acid tuffs from the Bugalho mine area assayed up to 10.97 g/t Au, 5.36% Cu and 20 g/t Ag.
Primary Copper mineralization: chalcopyrite and pyrite and accessory arsenopyrite within quartz carbonate breccia matrix. Supergene Copper mineralization includes malaquite, hematite and skorodite. Gold seems to be mainly associated with arsenopyrite.
Two holes completed by Rio-Tinto. Hole PAM-1 includes an interval of 5.45 m grading 1.53 g/t Au, including 2.47m grading 2.44 g/t Au. The highest individual value obtained came from a gossanous quartz-dolomite altered marble grading 5.77 g/t over an intercept of 0.75m. Banded carbonatization and low-temperature silica alteration occur thoroughly in the hole. Zones of poor recovery are interpreted to represent karst fill features.
Hole PAM-2, located in the same zone includes an intercept of 17m grading 0.5 g/t Au, including an interval of 2 m grading 3.7 g/t Au. The mineralized zone coincides with sections of intense silica-carbonate alteration with several massive gossan zones and localized fresh sulphide dissemination (chalcopyrite and pyrite).
Gold epithermal mineralization occurs in altered volcanics and carbonates and is mainly controlled by an E-W structure, that extends at least 400 metres along strike and is open at depth. The mineralized zone occupies the core of a second order anticline with acid volcanics flanked by Silurian shales, phyllites and graphitic phyllites.
Channel sampling by Rio Narcea Gold Mines in 2016 highlights:
Wide (5-10 meter thick) anastomosing WSW-ESE trending vein system at least 700 m along strike. This shear zone has been historically mined for Cu.