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Santa Rita Nickel

Bahia, Brazil

Main commodities: Ni Cu Co PGE PGM
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The Santa Rita nickel sulphide deposit is hosted by the Fazenda Mirabela Intrusion in southeastern Bahia, Brazil, ~140 km south of Salvador. Mining commenced in November, 2009.

Four separate crustal segments have been defined within the São Francisco craton in Bahia: (i) the oldest Gavião block in the west and southwest is composed of granitic, granodioritic and migmatitic continental crust, including remnants of 3.4 Ga tonalite-trondhjemite-granodiorite terranes and associated Archaean greenstone belts; (ii) the Archaean Jequié block, to the south and southwest, comprises granulitic migmatites with inclusions of supracrustal rocks, intruded by charnockite plutons; (iii) the Archaean Serrinha block, to the northeast, is composed of orthogneisses and migmatites, which form the basement for Palaeoproterozoic greenstone belts; and (iv) the youngest Archaean to Palaeoproterozoic Itabuna-Salvador-Curaça belt, which extends from southeast Bahia along the Atlantic coast to Salvador, then northward into northwest Bahia. This latter belt was formed by collision between the Serrinha block and the combined Gavião-Jequié blocks during the Palaeoproterozoic Transamazonian orogeny at ~2.15 to 2.05 Ga and is mainly composed of a low K calcalkaline plutonic suite with intercalated metasediments and ocean floor and/or back-arc basin gabbro and basalt. Crustal thickening associated with this collision, resulted in regional metamorphism at ~2.10 to 2.05 Ga.

The Fazenda Mirabela and Fazenda Palestina mafic-ultramafic intrusions were emplaced into the cores of antiformal structures, adjacent to the western margin of the Itabuna-Salvador-Curaça belt, within previously deformed charnockite, enderbite, gneisses, meta-norite and banded iron formation. Intrusion accompanied the terminal Palaeoproterozoic transpression in the southern part of the belt at ~2.08 to 2.06 Ga, following peak granulite facies metamorphism, and was localised in antiformal cores in response to westward thrusting.

The Fazenda Mirabela and Fazenda Palestina intrusions were intruded into a supracrustal succession and orthogneisses, typically deformed charnockites, which have equilibrated at granulite facies. The metamorphosed supracrustal succession is represented by quartzo-feldspathic gneisses, bands of fine-grained meta-mafic volcanics, meta-gabbronoritic sills, silicate-oxide facies banded iron formations, khondalites (quartz-feldspar-garnet-sillimanite gneisses) and rare serpentinites, derived from intercalated metasediments and ocean floor and/or back-arc basin gabbros and basalts. The metasediment of this sequence are sulphide-poor.

Three phases of deformation are recognised in the country rocks, producing: (i) recumbent folds; (ii) isoclinal folds coaxial with the previous structures, and together resulting in east-west shortening; and (iii) open folds with vertical axial planes orthogonal to the first two phases, resulting in dome- and basin-type fold interference patterns and north-south compression.

Both the Fazenda Mirabela and Fazenda Palestina intrusions are undeformed, and show no evidence of the penetrative deformation that has affected the surrounding rock. They display sub-solidus re-equilibration temperatures around 850°C based on pyroxene thermometry, and have very well preserved igneous textures.

The Fazenda Mirabela intrusion is a layered body of mafic-ultramafic cumulate rocks with an areal extent of 4.2 x 2.5 km, covering area of ~7 km2, occurring as a broadly funnel- or boat-shaped layered intrusion with a lower (western) ultramafic zone, varying from dunite to harzburgite, capped by pyroxenite, and an upper (eastern) mafic zone of gabbronorite to augite norite. The intrusive rocks have been dated at 2.2 Ga (Sm-Nd), although this is inconsistent with the observation that it post-dates the ~2.1 Ga peak granulite metamorphism in the country rocks. It was apparently intruded a pressure shadow developed during NNE-SSW sinistral transpression to the south of the Sao Pedro Gabbronorite-BIF massif. The extent of the ultramafic series is obvious in outcrop and magnetic response, and is marked by a topographic high.

The Fazenda Mirabela intrusion is a typical ultramafic cumulate sequence overlying a basal reversal, comprising bronzitite (~130 m thick) overlying contact gabbronorite (~90 m thick). The ultramafic series has a core of fine-grained dunite (~600 m thick), successively overlain by harzburgite (50 m thick) to olivine orthopyroxenite (25 m thick) and orthopyroxenite (50 m thick) units, with a cap a few metres thick of websterite. The peridotites become gradually richer in pyroxene and poorer in olivine outward from the core of the dunite. Olivine-bearing cumulates are almost completely serpentinised at surface, with the preservation of relic texture, although the intensity of serpentinisation diminishes with depth below the current erosion level, persisting to maximum depths of about 500 m. Pyroxenes are entirely fresh, aside from amphibolitisation along faults and major fractures.

The mafic series lies to the east of the ultramafic series, is ellipsoidal in exposed shape, and occupies two-thirds of the surface area of the intrusion. It is estimated to be at least 1000 m thick, occupies an area of subdued, almost planar, relief and comprises non-magnetic homogeneous coarse-grained gabbro-norites, leuco-gabbronorites and augite norites, locally disrupted by late tectonic overprints. Olivine is absent and plagioclase is the major mineral phase, together with lesser ortho- and clinopyroxene. The mafic series is separated from the country rock at the northern, southern, and eastern margin by a narrow zone of orthocumulate orthopyroxenite, where disseminated magmatic sulphide mineralisation is found at the gabbro-orthopyroxenite contact. This orthopyroxenite is petrographically and compositionally similar to the orthopyroxenite at the top of the ultramafic series and is interpreted to be a marginal zone continuous with that layer but extending up the side-wall of the magma chamber.

The Santa Rita ore zone (and resources) is a continuous stratiform layer containing 0.5 to 5 wt.% disseminated magmatic Fe-Ni-Cu sulphides, confined entirely within a 220 to 290 m thick stratigraphic envelope from the top of the dunite (i.e., the first appearance of cumulus orthopyroxene), upwards to the base of the mafic series (first appearance of plagioclase). It varies from a single 50 to 60 m thick, well-defined layer near the northern margin, to a more complex interval with fluctuating sulphide abundance over an interval of 100 to 180 m toward the south central portion of the intrusion. Grades and thicknesses increase, and the continuity of the mineralisation improves, toward the axis of maximum thickness of the ore zone, the Southern Deeps, a broadly linear "channel" that plunges down-dip along a slightly north of east azimuth, from a point at surface about two-thirds of the way from the northern to the southern margin. Toward the southern margin, south of the Southern Deeps axis, mineralisation tends to occur over a wider zone as multiple layers or fluctuating sulphide abundance, corresponding to a zone of complex intercalation of pyroxenite, apophyses of country-rock gneiss, and dolerite with minor semi-massive sulphide veinlets.

Mineralisation steps gradually upwards through the stratigraphic section from the northern to the southern margin, straddling different parts of the generalised internal stratigraphy of the upper part of the Ultramafic Series. The internal stratigraphy comprises: (i) a Dunite subzone (olivine cumulate with ubiquitous disseminated chromite), overlain by (ii) the Harzburgite subzone, an interval of interlayered and moderately heterogeneous olivine orthopyroxenites and granular to poikilitic harzburgite (olivine-bronzite and bronzite-olivine cumulates) with widespread centimetre to decimetre bands and layers (with sharp contacts) or possible xenoliths of orthopyroxenite; overlain in turn by (iii) a well defined band of olivine orthopyroxenite; then (iv) the Orthopyroxenite subzone, which comprises pure orthopyroxene (mainly bronzite) cumulates; and (v) a thin but persistent layer of websterite defined by the abrupt appearance of cumulus augite.

Sulphides take the form of composite polyminerallic blebs comprising a magmatic sulphide assemblage of abundant pentlandite, chalcopyrite, pyrrhotite and subordinate but ubiquitous pyrite. Pyrite is intergrown with pentlandite, which preferentially occupies the centres of blebs which have a variety of morphologies. The most common form is cuspate, interstitial blebs moulded around characteristically rounded olivine and orthopyroxene grains. Terminations of sulphide blebs show low dihedral angles in some cases but are mostly relatively rounded. Another sulphide morphology occurs as more convex, rounded blebs intergrown with finely granular orthopyroxene, or in some cases with fine granular aggregates of orthopyroxene, clinopyroxene, plagioclase, and phlogopite. Complex "wormy" or "graphic" intergrowths with crystallographically continuous post-cumulus rims of orthopyroxene grains are common, with sulphide assemblages that are typically chalcopyrite rich. Very finely dispersed sulphides with rounded droplet morphologies are frequently found intergrown with orthopyroxene rims around the margins of sulphide blebs.

JORC compliant reserve and resources as at January 2011 (Mirabela Nickel website, 2012) were:
    Proven + probable reserves - 159.3 Mt @ 0.52% Ni, 0.13% Cu, 0.015% Co (based on a weighted average cut-off of 0.25% Ni);
    Open pit measured + indicated + inferred resources - 198 Mt @ 0.52% Ni, 0.13% Cu (based on a weighted average cut-off of 0.25% Ni);
    Underground inferred resources - 198 Mt @ 0.79% Ni, 0.23% Cu (based on a weighted average cut-off of 0.50% Ni).

The open pit mining strip ratio is 5.0:1 and the weighted average metallurgical recovery is 68.7% Ni (Mirabela Nickel website, 2012).

This summary is mostly drawn from, and in places closely paraphrases, Barnes et al., 2011.

The most recent source geological information used to prepare this summary was dated: 2011.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Barnes S J, Osborne G A, Cook D, Barnes L, Maier W D and Godel B,  2011 - The Santa Rita Nickel Sulfide Deposit in the Fazenda Mirabela Intrusion, Bahia, Brazil: Geology, Sulfide Geochemistry, and Genesis : in    Econ. Geol.   v.106 pp. 1083-1110
Knight, R.D., Prichard, H.M. and Ferreira, C.F.,  2017 - Evidence for As Contamination and the Partitioning of Pd into Pentlandite and Co + Platinum Group Elements into Pyrite in the Fazenda Mirabela Intrusion, Brazil: in    Econ. Geol.   v.112, pp. 1889-1912.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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