The Jaguarari or Caraiba Cu orebody is located 370 km NNW of Salvador in Bahai, Brazil.
Mineralisation was first recorded in the Caraiba district in 1874, although its potential was not recognised until investigated by the National Department for Mining Productions (DNPM) in 1944. Exploration and feasibility studies commenced in 1969, before being taken over by the National Bank of Economic and Social Development (BNDES). Caraíbas Metals S/Apen commenced the Céu Aberto open pit operations at Cariaiba in 1979, followed by the initiation of underground mining in 1986. Privatisation was complete by 1994, and the operating company became Caraíba Mining S.A. In 2010, the mining of the Surubim deposit, 33 km to the north began, and in 2013, production at the Angico deposit, 42 km north of Caraiba began.
The Caraiba deposit is situated on the northern margin of the São Francisco Craton craton, within the Curaçá high-grade granulite facies granite gneiss terrane that hosts numerous sill-like mafic-ultramafic intrusive bodies. This gneiss terrane is part of the Salvador-Curaçá orogen, which is, in turn, the northern extension of the north-south trending Atlantic Coast granulite belt (Mascarenhas et al., 1984; Oliveira and Tarney, 1995) that has been interpreted as a continental magmatic arc (Figueiredo, 1980) of Neoarchaean age (2.4 Ga, Barbosa, 1990) cutting across the craton, separating the Serrinha and Mairi Archaean blocks to the east and west respectively (Barbosa, et al., 1996, Barbosa, 1996).
The Serrinha Block is composed of Archaean gneisses and migmatites, Cr-bearing mafic-ultramafic bodies, Palaeoproterozoic greenstone sequences and granites. The Itiúba syenite, which separates the Curaçá gneiss terrane and Serrinha Block, was intruded, deformed and metamorphosed at amphibolite facies conditions, at ~2.0 Ga (Rb-Sr; Figueiredo, 1976; Conceiçáo, 1990).
The Mairi Block, to the west, comprises Archaean gneisses and migmatites, cut by a major Cr-bearing mafic-ultramafic intrusion that is unconformably overlain by Palaeoproterozoic metasedimentary and volcanic rocks of the Jacobina Group, all intruded by a Meso- to Palaeoproterozoic granite (Inda and Barbosa, 1978; Silva, 1996).
The country rocks of the Curaçá gneiss terrane are interpreted to comprise a metamorphosed supracrustal sequence, now represented by banded gneisses, graphitic gneiss, banded iron formation, calc-silicates and alumina rich gneiss, and biotite-hornblende-bearing quartz-feldspar gneiss, with minor amphibolites and quartzites (Sá et al., 1982; Hasui et al., 1982; Sá and Reinhardt, 1984; Silva, 1985).
These supracrustal rocks were subjected to three principal phases of deformation with associated granitic intrusion, metamorphism and migmatisation:
• D1 is characterised by tight isoclinal folding, dated at 2328 Ma (Silva, 1985), which produced the gneissic banding, and was accompanied by tonalitic orthogneiss intrusion (G1), migmatisation and amphibolite facies metamorphism. F1 folds are generally of ~10 cm-scale, rootless and intrafolial relative to SI.
• D2 is characterised by tight isoclinal folding with east-west axes, followed by the intrusion of a second generation of tonalites (G2), mafic dykes and eventually granulite facies metamorphism (at 5.5 to 6.5 kbar and 750 to 720°C; Ackerman et al., 1987). Individual F2 folds (and S2) are not readily distinguished, except in more felsic lithotypes, where F2 folds with east-west trending, sub-vertical to sub-horizontal axes, interference folding of F1 folds, and an east-west trending S2 mineral foliation of biotite and orthopyroxene are evident.
• D3 resulted in tight, cm- up to km-scale, non-cylindrical folds with north-south axial planes that dip at 70 to 75°W and plunge either northerly or southerly. S3 is a penetrative mineral foliation across the entire terrane, defined by biotite, hornblende and strongly flattened quartz and feldspars. Non-mica minerals define a penetrative L3 mineral stretching lineation parallel to the axial planes. D3 was accompanied by granitoid sheets (G3) and amphibolite to granulite facies metamorphism (Sá et al., 1982; Silva, 1985) and may represent the Transamazonian cycle (2050 Ma; Silva, 1985). The G3 granitoids occur as a swarm of north-south trending dykes of reddish pink alkaline granites that are tens of centimetres to tens of metres in thickness, persist over strike lengths of kilometres, and are emplaced parallel to and commonly displaying S3 foliation. They cross-cut F3 hinge-lines, and overprint all older rocks, including the mineralised orthopyroxenites that host the ore deposits.
Northward from ~20 km to the south of Caraiba, nearly three hundred mafic-ultramafic bodies intrude the Curaçá gneiss terrane. Those that are Cu-bearing are magnetite orthopyroxenite (hypersthenites) and/or diorites (locally called norites), whilst the vast majority, which are Cu-poor or barren, are gabbros, gabbronorites and anorthosites (Lindenmayer, 1981).
The relationships between the different mafic to ultramafic bodies within the terrane is not certain. They may represent individual intrusions localised in the crests of small anticlinal folds, or possibly form part of a single, tectonically disrupted, thrust sheet controlled intrusion, based on the similarity in composition between the orthopyroxenites from several localities distributed over an 80 km of strike (Maier and Barnes, 1996).
The mafic-ultramafic bodies are apparently broadly conformable with the D1 tonalitic and granitic orthogneisses dated at 2328 to 2050 Ma (U/Pb, Silva, 1985), and are interpreted to have been intensely folded by all of the three main phases of deformation described below, and hence older than D1 (Silva, 1985; D'el Rey Silva et al., 1994). However, Oliveira and Tamey (1995) dated the orthopyroxenites at 1890±60 Ma (Sm/Nd) and noted the occurrence of apophyses of orthopyroxenite and massive sulphide ore cutting foliation and lithological boundaries within the country rock. Maier and Barnes (1999) suggest this may be explained by differences in competency during folding and remobilisation of sulphides during high temperature deformation.
The geology of the Curaçá gneiss terrane, as described above, represent three main rock groupings, namely, i). the metamorphosed supracrustal sequence, ii). the mafic-ultramafic intrusive suite, and iii). the migmatitic gneisses and syn-tectonic granitoid intrusions (Gl and G2) of grey tonalites and granodiorites (Jardim de Sá et al., 1982). Apart from G2, which are far more numerous than Gl around Caraiba, most of the rocks above are overprinted by an amphibolite-facies regional metamorphic banding (S1) that commonly encloses sub-parallel, 1 to 10 mm, up to 1 m thick, sheet-like layers of quartz and feldspar or simply feldspar that are highly deformed and suggest syn-D1 migmatisation (D'el Rey Silva et al., 1999). â€‹
Deposit Geology, Structure and Mineralisation
At the surface, the mineralised orthopyroxenite has an irregular amoeboid shape, with a strike length of ~1750 m, distributed over an area with north-south dimensions of ~1200 m, and width of ~500 m. It comprises a central, contorted, 500 m long, east-west oriented section where the mineralised layers dip at ~70°to the N, with two north-south arms one curving north from the western extremity of the central zone. This arm splits into two layers that dip at ~80° generally to the W. These layers stretch continuously northwards, although the easternmost plunges to the north and is only continuous below the surface. The second north-south arm occurs as two layers that form a narrow corridor juxtaposed to the south of the inner parts of the central zone (D'el Rey Silva et al., 1999).
In detail the ore deposit represents a tight, asymmetric, F2 synform-antiform pair, with steep south dipping, east-west trending axial planes, that have been refolded by an F3 fold with an upright, north-south trending axial plane, and shallowly south plunging axis. The east-west central section of the exposed deposit represents the steep, contorted southern limb of the tight F2 synform and adjacent antiform. The northern arm is the western limb of the F3 synform folding the gently dipping northern limb of the F2 synform, although the eastern F3 limb is absent where the orebody has lensed out below the surface. The southern arms of the deposit are the F3 folded dislocated, gently south dipping southern limb of the F2 antiform. This composite structure resembles a north-south trending conical mushroom lying on its side, sliced from the crest to the base of the stem by the current surface. The deepest limit of the mineralised orthopyroxenite is at the intersection of the F2 and F3 synforms in the central zone, where it reaches depths of ~1600 m (D'el Rey Silva et al., 1999).
The Cu sulphide mineralisation is almost exclusively hosted by orthopyroxenites, although elevated Cu sulphide levels also occur in small diorites (to ~0.4% Cu) and glimmerites (to ~1.9% Cu), as well as some calc-silicate gneisses and felsic breccias directly adjacent to the orthopyroxenites. Other lithologies are generally barren. Gabbronorites not directly associated with the pyroxenites and those in the hanging-wall sequence of the Caraiba orthopyroxenite sill, may contain up to 3 percent sulphides as pyrite and pyrrhotite but are generally Cu poor (Maier and Barnes, 1999).
The mineralised orthopyroxenite is closely associated with a set of Cu-barren gabbros, gabbronorites and anorthosites, but is also found in contact with barren supracrustal rocks (mostly gneisses) to the west, and with a sequence of well-banded mafic gneisses to the east. The contacts with the barren mafic intrusives and mafic gneisses is generally gradational, marked by increasing amounts of layer-parallel feldspathic bands. The contact with the gneisses on both sides is generally subvertical and commonly marked by zones of intense migmatisation and partially overprinted by late NNW and NNE trending zones of ductile shearing. Chalcopyrite and bomite were remobilised into small shear zone-controlled veins, even within the main orebody and into adjacent rocks.
The average orthopyroxenite contains ~60 to 70 vol.% orthopyroxene, <5 vol.% andesine plagioclase, 5 to 10 vol.% Cu sulphides and oxides each, and 5 vol.% red phlogopite (which may rarely be >50% to form a glimmerite). The diorites (known as norites in local literature) mainly contain andesine plagioclase (>50 vol.%), orthopyroxene (5 to 50 vol.%), quartz (5 to 20 vol.%) and phlogopite (5 to 50 vol.%). Apatite and zircon are common accessories in the orthopyroxenites, diorites and glimmerites (Maier and Barnes, 1999).
According to Maier and Barnes (1999), there is abundant evidence that the orthopyroxenites, diorites and glimmerites have undergone high-grade metamorphism, on the basis of their deformation (particularly the micas), fracturing (well developed in some zircons), undulous extinction (in orthopyroxene, plagioclase, quartz, plagioclase, and mica), granular textures with tripple point junctions, and subgrain walls. D'el Rey Silva et al. (1999) observe that the Cu-bearing rocks have a clear S1 metamorphic banding, generally sub-vertical either near the contact or within the richest parts of the orebody itself, particularly within melanorites that contain <1 to > 10 cm thick layers of (hypersthene) orthopyroxenite interbanded with norites and leuconorites. Both the orebody contact and S1 are folded by 0.1 to 10 m size F3 folds, the hinges of which are invaded by <1 to > 10 cm thick, late folding feldspathic (and quartz) melts, and locally by G3 granites. S2 foliation is rarely seen in the main deposit, although F2 folds with E-W trending, sub-vertical to sub-horizontal axes, F2-F3 interference folds, and an E-W trending S2 biotite and orthopyroxene mineral foliation, are evident in surrounding felsic wall rocks. S3 is penetrative in both the mineralised mafic-ultramafic rocks and wallrock gneisses, which display abundant F3 folds with generally southerly and gently plunging axes. Apart from narrow chlorite-epidote-sericite-carbonate rims around microscopic to macroscopic fractures and veinlets, the orthopyroxenites show little retrograde alteration accompanying this metamorphism.
The sulphide assemblage in the orthopyroxenites is almost exclusively of bornite and chalcopyrite, which commonly show symplectitic intergrowth, and on average, occur in approximately equal proportions (Maier and Bames, 1996), although locally relative compositions are very variable. There is also some variation between individual bodies within the district, from bornite to chalcopyrite dominant intrusions (Maier and Barnes, 1999).
Many of the orthopyroxenites, diorites and glimmerites are highly enriched in light REE (most likely contained in apatite and zircon), in contrast to the other mafic-ultramafic rocks elsewhere in the terrane (Maier and Barnes, 1999).
The majority of the orthopyroxenite- (and diorite-) hosted ore is disseminated, occurring as anhedral sulphides interstitial to the silicates. Locally, the disseminated ore grades into massive aggregates, pods, and schlieren of sulphides that may reach a few tens of cm in thickness, and are composed of the same sulphide assemblage as the disseminated ores.
Within the Caraiba deposit, massive sulphides tend to be concentrated in synclinal closures, indicating local syn-kinematic remobilisation. Microscopic and macroscopic veins of sulphides, replacement of oxides by sulphides (Maier and Barnes, 1996), and finely dispersed sulphides in fractures and cleavage planes of silicates indicate local remobilisation of the ore in all orthopyroxenite bodies in the Curaçá gneiss terrane (Maier and Barnes, 1999). However, remobilisation has not generally mineralised the country rocks, except for occasional sulphide veins of up to 20 cm in thickness transgressing the hanging-wall rocks, and local mineralisation of some calc-silicate gneisses.
Oxides, which comprise up to 50% of the mineralised orthopyroxenite, are mainly titanomagnetite and ilmenite, with lesser pure magnetite and rare chromite, occurring as euhedral or anhedral disseminated grains interstitial to silicates, as massive aggregates, and in veins and veinlets where they may or may not be associated with sulphides (both chalcopyrite and bornite). The magnetite contains up to 14 wt.% Cr203, 5.8 wt.% Al203, 2 wt.% V203, 1 wt.% ZnO, and 7 wt.% Ti02 (Maier and Barnes, 1996), and has particularly high Ti, AI and Zn contents, suggesting subsolidus equilibration with orthopyroxene (Maier and Barnes, 1996).
d34S values fall into a mantle range of -1.495 to +0.643‰, although the eNd value of -4.3‰, from the Caraiba dyke (n=3) suggests the involvement of a crustal component in the geologic evolution of the bodies. It is still unclear whether this crustal signature represents a source characteristic or is the result of magmatic contamination, metasomatism, or mechanical hybridization of contrasting lithologies during the intense deformation of the rocks (Maier and Barnes, 1996).
Maier and Barnes (1999) regard the ore as highly atypical of magmatic deposits in that it contains up to 50% titanomagnetite and magnetite, the sulphide assemblage comprises bornite and chalcopyrite (with an average Cu:Ni ratio of 40), and the host orthopyroxenites contain abundant phlogopite and locally, apatite and zircon. In addition, orthopyroxenites distributed over a strike length of ~80 km, contain between 100 and 500 ppb PGEs, although individual samples may contain as much as 2700 ppb. Mantle-normalised noble metal patterns are relatively fractionated, with Pd:Ir ratios of ~70. They therefore conclude, that as hydrothermal Cu sulphide ores typically show much more pronounced noble metal fractionation, caused by the relatively high mobility in low-temperature fluids of Au, Pd and Pt, compared to Os, Jr, Ru and Rh, the Caraiba deposits are likely originally of magmatic origin.
To explain the high Cu:Ni and other unusual features of the Curaçá gneiss terrane ores, Maier and Barnes (1999) suggest that the orthopyroxenites may represent restitites of a sulphide-, magnetite-, phlogopite- and apatite-bearing dioritic protolith which underwent anatexis and effective melt extraction. The sulphides may have been molten but could have remained in the restite due to their relatively high density. Partial dissolution of the sulphide melt by the S-undersaturated silicate melt would cause enrichment of the excess sulphide melt in the highly chalcophile Cu and Se, potentially followed by the crystallisation of bornite and chalcopyrite. This interpretation, they suggest, is supported by the absence of differentiated lithologies that may represent residual liquids and by the fact that the Curaçá gneiss terrane deposits are located in high-grade metamorphic terranes.
Resources and Production
Open pit mining was to a depth of 200 m with a waste:ore ratio of 4.5:1. Below this, ore is being mined underground to a depth of 850m, with plans (in 2013) to extend to a total depth of 1600 m below the surface.
The deposit had pre-mining resources quoted at: 90 Mt @ 1.4% Cu or 135 Mt @ 1.1% Cu or 150 Mt @ 0.8% Cu.
Total production from the open pit and underground mining operations at Caraiba from 1978 up to 30 June, 1998 (D'el Rey Silva et al., 1999):
~60.5 Mt @ 1.6% Cu.