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OToole, Fortaleza de Minas

Minas Gerais, Brazil

Main commodities: Ni Cu PGE PGM
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The O'Toole or Fortaleza de Minas sulphide nickel deposit is located 4 km to the South of the small Fortaleza de Minas village, in the SW of Minas Gerais State, Brazil, some 10 km to the south east of the Morro do Niquel lateritic nickel deposit.

The deposit lies in an Archaean crustal nucleus of granite-greenstone terrain within the core of a broad nappe structure of Proterozoic meta-sediments, the Passos Nappe. This block was once part of the Archaean Paramirim Craton, which was subsequently reworked into the Araxá mobile belt during the Proterozic. The Fortaleza de Minas Ni–Fe sulphide deposit occurs in the rocks interpreted to be the basal part of the 200 x 50 km Morro do Ferro greenstone belt. The upper portion of this sequence is a thick pile of chemical-detrital sediments. The crystalisation of the host lava flows to the Ni deposit have been isotopically dated at ~2.86 Ga. The area was regionally metamorphosed during the 2.16 to 2.0 Ga Paleoproterozoic Transamazonian Orogeny producing high grade amphibolite facies in the deposit area. The region was subsequently subjected by intense transcurrent shearing during the 900 to 520 Ma Brasiliano Cycle with reactivation of major regional faults associated with retrograde metamorphism.

The nickel deposit is located in the uppermost of four cyclic flow units of serpentinite-clino pyroxenite-gabbro separated by banded iron formations. These flow units comprise the 'intermediate sequence' which lies between a lower and an upper sequence, composed ofkomatiitic basalt flows and subordinately komatiite flows, and graphitic meta-cherts and tuffs (referred to as the black shales). The upper and lower sequences are the dominant units in the area, while the intermediate sequence represents a thick highly fractionated flow, where the individual cycles represent individual flow units separated by thin banded iron formation beds. The volcanic environment of the Fortaleza de Minas deposit is interpreted to be a ponded facies in a distal shallow lava lake.

The Fortaleza de Minas Ni-Cu-Co-PGE sulphide mineralisation is located within a sinistral shear zone at the base of a mineralised olivine-cumulate in contact with a footwall BIF. Typically the hanging wall is represented by a massive serpentinite or a talc–carbonate schist, both of which are metamorphic products after an olivine-cumulate basal zone of a fractionated komatiitic flow. While the footwall is predominantly composed of banded iron formation, it can be locally represented by talc carbonate schist, pyroxene-cumulates or sepentinite.

The Fortaleza de Minas orebody is tabular and comprises five interconnected shoots, defined by the plus 4 m thickness contour, distributed over a strike length of 1600 m, an average width of 5 m and down dip extent of 500 m.  

There are three major types of ore within the deposit, from top to base, namely  i).  disseminated,  ii). matrix, and  iii). breccia ores.   In addition there are remobilised ore types,  iv). sheared  v). BIF hosted and  vi). late hydrothermal massive sulphide.

The disseminated and matrix ores are considered to be of primary magmatic origin, which although variably deformed, still displaypreserved magmatic characteristics with superimposed cataclastic textures. The breccia ore is completely recrystalised with indications of polyphase evolution from a ductile to a brittle regime, interpreted to have been derived from an originally magmatic massive sulphide with a matrix which was partially reworked in intensely sheared zones.

The disseminated ore was the first to be developed in the trough zones towards the base of the differentiated flow. It is hosted by serpentinites, is defined by a 0.60% Ni natural cut-off, and has an average 15 to 25 wt% total sulphides comprising 65% pyrrhotite, 30% pentlandite and 5% chalcopyrite, accompanied by around 15% magnetite. The average grade is approximately 1% Ni, which at depth decreases to 0.8% Ni. The sulphides are fine grained and are generally well disseminated through the host serpentinite. Typical grades are 0.95% Ni, 0.25% Cu, 260 ppm Co, 2.70% S, 0.13 g/t Pt, 0.24 g/t Pd, 0.08 g/t Au. The disseminated ore grades over a short distance downwards into matrix ore towards the base, while on the hangingwall transition, the sulphides start to locally concentrate into clouds or small nuclei, but rapidly grade into sparse disseminations.

The transition from disseminated to matrix ore is defined by a natural 2.00% Ni cut-off. However, due to the amount of shearing within the deposit area, the contacts between the two ore types are predominantly tectonic, marked by thin fine grained talcose mylonitic zones, while locally, tectonic wedges or slabs of disseminated and matrix ores are interleaved. The average sulphide content is 50 wt% comprising 65% pyrrhotite, 30% pentlandite and 5% chalcopyrite, accompanied by around 16 wt% magnetite, in similar proportions to the disseminated ore, and has an average grade of around 3.00 to 3.50% Ni. The matrix ore has a distinctive net-textured fabric, with rounded serpentine-magnetite gangue, interpreted as metamorphosed cumulate olivine crystals, which are surrounded by the sulphide network. This fabric is interpreted to be a primary magmatic texture, although it usually displays an overprinting cataclastic texture. The grain size ranges from a fine to coarse, with in places, an alternation of finer and coarser ores interpreted to reflect primary flow layering structures. The ore grade is typically 3.40% Ni, 0.63% Cu, 724 ppm Co,13.2% S, 0.53 g/t Pt, 0.74 g/t Pd and 0.14 g/t Au.

The massive sulphide breccia ore is the most widespread ore type, occurring throughout the entire deposit, due to its high plasticity. It is commonly well recrystalised but can also be intensely sheared in a transcurrent ductile-brittle regime, and is typical found at the contact between the hanging wall serpentinite and footwall BIF, usually extending beyond the primary embayment limits. It exhibits a gradational decay in both grade and quantity of sulphide from the centre to the lateral periphyry of the deposit. Where sheared it contains rounded and elongated fragments of wall rocks and occasionally segregation of pentlandite in layers following the foliation plane. While the breccia texture is regarded to be of tectonic origin, some of the BIF fragments may have been derived from thermo-mechanical erosion of the footwall. There is a strong correlation between the composition of the breccia fragments and the adjoining wall rocks, while there is evidence of tectonic fragmentation of adjacent wall rocks. The breccia ore has been subdivided into BR1 and BR2 sub-types reflecting the varying remobilisation process under a transcurrent shear regime. The BR1 sub-type has higher Ni grades and occurs in the central embayment structure, commonly below the matrix and disseminated ores. It has a clean recrystallised groundmass with rounded, slightly flattened and angular clasts of both serpentinite and BIF and minor fragments of talc schist, varying in size from millimetric to decimetric to metric in sizes. The BR2 sub-type is characterised by a talcose sulphide matrix with a lower average Ni grades than BR1, and strongly deformed fragments within the foliated BR2 matrix. The fragments are dominantly talc schist and BIF, with the talc fragments being strongly flattened, well oriented and smaller than those of BR1. BR2 typically occurs at the lateral margins of the embayment zone where the ore shows a consistent reduction in both thickness and grades. At the limits of the economic central shoots, BR2 gradually merges into the sheared ore where the cataclastic texture typical of BR2 ore remains unchanged, although the total amount of sulphide drops considerably into the sheared low grade talcose ore. The breccia ore typical contains 4.14% Ni, 0.56% Cu, 870 ppm Co, 16.0% S, 0.42 g/t Pt, 0.69 g/t Pd and 0.14 g/t Au, averaging 65 wt% suphide in BR1 ancomprising 65% pyrrhotite, 25% pentlandite, 10% chalcopyrite and 10 to 12% magnetite, similar to that of the disseminated and matrix ores.

The sheared ore occurs outside of the trough zones in which the disseminated and matrix ores are dominant, and is widespread through the peripheral parts of the deposit, extending for hundreds of metres outwards from the primary central embayment zone as persistent, continuous centimetric sheared talcose zone. It is located at the base of the hanging wall serpentinite (or talc schist), although it is sometimes found in contact with pyroxenites when the serpentinite is pinched out by shearing. It may occur alone, or intermixed with the BR2 ore, or in association with the BIF hosted ore, sometimes tectonically interleaved. The sheared ore is characterised by a flaser mylonitic fabric, and has lower grades, comparable to the disseminated ore, although it is gradational with the sheared BR2 breccia ore from which was tectonically remobilised. The ore averages 22 wt% sulphide comprising 65% pyrrhotite, 25% pentlandite and 10% chalcopyrite with 14 wt% magnetite. The gangue is typically talc with lesser carbonate, chlorite and magnetite. The sulphides occur as interconnected, anastomosing veinlets with typical triangular cuspade junctions.

The BIF hosted ore occurs in the footwall contact zone between the breccia and sheared ore types, and the footwall BIF, and comprises a remobilised ore type formed during polyphase transcurrent shearing. Ni sulphides occur as massive or breccia sulphide veins, veinlets, stringers, tension gashes fillings or as disseminated banded sulphides, cutting a barren silicate facies BIF to produce economic grades. The footwall BIF is commonly absent, rare or thin in the trough zones, interpreted to reflect thermal erosion of the thick footwall BIF unit. In the North of the central embayment there is a gradation from BR1 breccia ore to the BIF hosted ore. It averages 28 wt% sulphides, with 65% pyrrhotite, 25% pentlandite and 10% chalcopyrite, with 15% magnetite and typically has grades of around 1.00% Ni, 0.35% Cu, 239 ppm Co, 4.4% S, 0.11 g/t Pt, 0.20 g/t Pd, 0.08 g/t Au.

The high grade hydrothermal massive sulphide ore is characterised by coarse pentlandite crystals and occurs in steep oblique hanging wall faults and fractures and has a post-tectonic fabric, lacking metamorphic foliation, although it can be locally sheared. It occurs as typical fault plane or fracture infills in the immediate hanging wall of the main ore zone, with a clear relationship to the breccia ore, from which it seems to be derived. Away into the hanging wall it gradually thins and pinches out a few metres to a few tens of metres from the main ore zone and can be split into smaller veinlets along vertical to horizontal fractures to form a stockwork array. In places it has a zoned contact with the wall rocks with pentlandite rich phases bordered by a massive chalcopyrite zone and a massive magnetite contact halo, and has associated talc, carbonate and chalcopyrite hydrothermal breccias. The hydrothermal massive sulphide ore has very high (8 to 14%) nickel grades. It can exceed 90 wt% sulphides with an abnormal concentration of PGM of up to 5 g/t Pt and 40 g/t Pd, locally with decimetric, poikiloblastic pentlandite crystals. It has a greenschist facies paragenesis, with unusual associated minor element such as Ag, Mo, Bi. Typical grades are: 9.10% Ni, 0.71% Cu, 1650 ppm Co, 28.3% S, 0.35 g/t Pt and 4.44 g/t Pd.

The weathering profile typically comprises a 30 m thick, partially outcroping gossan overlying a 20 m transition zone, characterised by a violarite/bravoite zone which passes downward into primary sulphide ore.

Pre-mining reserves were: >7 Mt @ 2.6% Ni, 0.4% Cu, 1.3 g/t PGE, 0.05% Co.

This deposit was initially developed by RTZ Mineracão Ltda, but is now operated by Votorantim Metais Ltda.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1989.    
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:
Brenner T L, Teixeira N A, Oliveira J A L, Franke N D, Thompson J F H  1990 - The OToole Nickel deposit, Morro do Ferro Greenstone Belt, Brazil: in    Econ. Geol.   v85 pp 904-920
Marchetto C M L  1990 - Platinum-group minerals in the OToole (Ni-Cu-Co) deposit, Brazil: in    Econ. Geol.   v85 pp 921-927


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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