Summary:
The Pilar Mine is located in the southern part of the Caraíba Operations within the Curaçá Valley. The geology of the mine consists of a high-grade metamorphic terrain, composed of gneiss and migmatite of the Caraíba Complex that were intruded by mafic, ultramafic and late granitic rocks. The mafic and ultramafic intrusions are mainly composed of pyroxenite, norite, and gabbro. The melanorite is a term used for logging purpose to describe a host rock, either a gneiss, a gabbro or a norite, that is intruded by several mafic-ultramafic dykes that are too small individually to be logged separately.
The gneiss and migmatites, together with some intrusive bodies are affected by various alteration assemblages including potassic (phlogopite and K-feldspar), sodic (albite), calcsilicate (diopside), carbonate, as well as epidote and lesser garnet. The alteration variably overprints the original texture of the rock units and it obliterates the gneissic foliation where the alteration is more intense. The phlogopite was generally developed later than the k-feldspar and the diopside alteration and is frequently associated with copper mineralization.
Mineralization at the Pilar UG Mine is composed of copper sulphides in the form of chalcopyrite, bornite and rarely chalcocite that occur in four different styles: disseminated, veins, massive, and brecciated. Other sulphide minerals include millerite, pyrite and pyrrhotite. Magnetite is the dominant oxide mineral and occurs intergranular together with chalcopyrite and bornite (Tappert, 2020). The sulphides are heterogeneously distributed in the pyroxenite units in the form of lenses that trend N-S, dip steeply to the west and range from less than 1 meter to 20 meters thick. The known copper mineralization at Pilar extends for 1.2 km N-S, up to 170 m E-W, and has been drilled to a depth of more than 1.6 km along its steep northerly plunge. The mineralized bodies occur in sharp contact with migmatites and at variable angle to the main foliation of the host-rock. In the P1P2 sector of the mine, located in the upper levels of the Pilar UG Mine, mineralization is distributed around an interpreted steeply west dipping tight syncline plunging shallowly to the south. The fold hinge is interpreted as a structural trap for high-grade copper mineralization. Strongly foliated sub-vertical anastomosing shears as well as brittle faults crosscut and locally displace the mineralization.
The Surubim District is located in the central part of the Curaçá Valley. The C12 deposit is located 1.2 km to the SW of the Surubim Mine. The Cercado Velho and Lagoa da Mina deposits are approximately 10 km to the NE of the Surubim Mine whereas the Terra do Sal deposit is situated 8 km to the SE of Surubim Mine. The Surubim Mine and C12 Deposits are hosted in the Surubim gneiss (alternating tonalitic and granorioditic bands with gabbro and diorite bands) whereas the other three deposits of the Surubim District are hosted in the Caraíba gneissic complex (biotite orthogneiss with local migmatite).
The copper mineralization of the Surubim Mine and C12 deposit occurs as lenses that are hosted by phlogopite-altered gabbro injected by pyroxenite dykes to form what is designated as the ultramafic complex. Sulphide minerals are chiefly chalcopyrite and bornite in a ratio of 4:1 that mainly occur as disseminations and veins). Chalcocite, covellite and cubanite also occur as minor sulphides associated with the mineralization surrounding the Surubim Mine. Magnetite and minor pyrite and pyrrhotite are also associated with the mineralization with pyrrhotite being an important sulphide at the Lagoa da Mina and Cercado Velho deposits. The Terra do Sal deposit is characterized by disseminated and veinlets of pyrrhotite, chalcopyrite with minor bornite, pyrite and pentlandite. Specifically:
- At Surubim, the Cu mineralization is hosted in a series of moderately NW-dipping, NNW-SSE striking lenses measuring a few meters up to 45 m thick. Mineralization is known over 300 m in the N-S direction and 100 m E-W and extends to 400 m below surface. The mineralization remains open at depth and to the NNE extension.
- At C12, the Cu mineralization occurs as several interpreted N-S striking lenses measuring a few meters and up to 75 m thick. However, the general trend of the higher-grade Cu mineralization extends for over 170 m to the NW, approximately 50 m in the NE-SW direction, and is known to a depth of 300 m below surface. The mineralization remains open along the NW and NE extensions and at depth.
- The Cu mineralization at Lagoa da Mina corresponds to a series of steep westerly-dipping, northerly- striking lenses associated with melanorite and pyroxenite and measuring from 10 to 50 m thick and up to 110 m long. The deposit covers an area of 200 m N-S by 80 m E-W and is know to a depth of 430 m. The Cu mineralization remains open in its NW-SE strike extensions and at depth.
- At Cercado Velho, the Cu mineralization is hosted in a series of northerly-trending lenses associated with gabbro and norite and measuring from 5 to 30 m thick by up to 110 m long. The deposit covers an area of 455 m N-S by 200 m wide and is known to a depth of 220 m. The Cu mineralization remains open to the north, the south, and at depth.
- The Cu mineralization at Terra do Sal is hosted in several northerly-trending lenses associated with pyroxenite and gabbro and varying between 10 and 50 m thick and reaching up to 120 m in length. The deposit covers an area of 180 m N-S by 130 m in its E-W extension and is known to a depth of 380 m. The Cu mineralization remains open in all directions except to the west.
The Vermelhos District is located 60 km north of the Caraíba Mine. In this district, copper mineralization has been identified over 4.5 km in a NNE direction including the Siriema deposit (N5), the Vermelhos Mine (N7), the N8, N9, and N10 deposits. The Vermelhos Mine area is largely covered by quartz-rich colluvium with rare outcrops occurring along drainages.
The main sulphides of the deposits in the Vermelhos District consist of chalcopyrite (approximately 70 to 75%), bornite (20 to 25%) and minor chalcocite. The chalcopyrite contains low concentration of nickel (Tappert, 2020). Copper sulphides are associated with minor pyrite, pyrrhotite, pentlandite as well as chromite and magnetite. Sulphide textures include interstitial, net-textured, stringer and sulphide-rich matrix breccias mostly concentrated in the mafic-ultramafic units but can also occur as veins and dissemination in the adjacent gneiss. Evidence throughout the Curaçá Valley of sulphide zonation, characterized as pyrrhotite +/- pentlandite zoning to pyrrhotite +/- pentlandite plus chalcopyrite and finally to chalcopyrite plus bornite is more common in the Vermelhos District, both within the Vermelhos UG Mine and at Siriema. High-grade mineralization in the Vermelhos District is often closely associated with phlogopite enrichment. The nickel, cobalt and PGE content tends to be higher in the Siriema deposit than in Vermelhos and the N8 and N9 deposits, but further analytical work is needed to confirm these observations. The detailed textures observed on polished slabs of sulphides show intergrowths of chalcopyrite, pentlandite, chromite, pyrrhotite and bornite. Oxidized mineralization occurs as malachite (Figure 7-39) and chrysocolla within the weathered zone that occurs from 15 m to 40 m depth and, to date, is only associated with mafic-ultramafic rocksin the Vermelhos District.