The Ni-Cu-PGE mineralized Brejo Seco mafic-ultramafic layered intrusion, Riacho do Pontal Orogen: Onset of Tonian (ca. 900 Ma) continental rifting in Northeast Brazil

Accepted Manuscript The Ni-Cu-PGE mineralized Brejo Seco mafic-ultramafic layered intrusion, Riacho do Pontal Orogen: Onset of Tonian (ca. 900 Ma) continental rifting in Northeast Brazil Silas Santos Salgado, Cesar Fonseca Ferreira Filho, Fabrício de Andrade Caxito, Alexandre Uhlein, Elton Luiz Dantas, Ross Stevenson PII:

S0895-9811(16)30085-2

DOI:

10.1016/j.jsames.2016.06.001

Reference:

SAMES 1568

To appear in:

Journal of South American Earth Sciences

Received Date: 1 November 2015 Revised Date:

30 May 2016

Accepted Date: 1 June 2016

Please cite this article as: Salgado, S.S., Ferreira Filho, C.F., de Andrade Caxito, F., Uhlein, A., Dantas, E.L., Stevenson, R., The Ni-Cu-PGE mineralized Brejo Seco mafic-ultramafic layered intrusion, Riacho do Pontal Orogen: Onset of Tonian (ca. 900 Ma) continental rifting in Northeast Brazil, Journal of South American Earth Sciences (2016), doi: 10.1016/j.jsames.2016.06.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The Ni-Cu-PGE mineralized Brejo Seco mafic-ultramafic layered intrusion, Riacho do

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Pontal Orogen: onset of Tonian (ca. 900 Ma) continental rifting in Northeast Brazil.

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Silas Santos Salgado1, Cesar Fonseca Ferreira Filho2, Fabrício de Andrade Caxito1, Alexandre

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Uhlein1, Elton Luiz Dantas2, Ross Stevenson3

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1 – Centro de Pesquisas Manoel Teixeira da Costa, Instituto de Geociências, Universidade

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Federal de Minas Gerais, Campus Pampulha, Av. Antônio Carlos 6627, CEP 31270-901, Belo

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Horizonte, MG, Brazil.

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2 – Instituto de Geociências, Universidade de Brasília, Campus Universitário, Asa Norte, CEP

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70910-900, Brasília, DF, Brazil.

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3 – GEOTOP, Université du Québec à Montréal, P.O. Box 8888, Station Centre Ville,

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Montreal, Quebec H3C 3P8, Canada.

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Abstract

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The Brejo Seco mafic-ultramafic Complex (BSC) occurs at the extreme northwest of the Riacho do Pontal Orogen Internal Zone, in the northern margin of the São Francisco

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Craton in Northeast Brazil. The stratigraphy of this medium size (3.5 km wide and 9 km long)

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layered intrusion consists of four main zones, from bottom to top: Lower Mafic Zone (LMZ;

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mainly troctolite), Ultramafic Zone (UZ; mainly dunite and minor troctolite); Transitional

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Mafic Zone (TMZ; mainly troctolite) and an Upper Mafic Zone (UMZ; gabbro and minor

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anorthosite, troctolite, and ilmenite magnetitite). Ni-Cu-PGE mineralization occurs at the

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contact of the UZ with the TMZ, consisting of an up to 50 meters thick stratabound zone of

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disseminated magmatic sulfides. An Mg-tholeiitic affinity to the parental magma is indicated

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by the geochemical fractionation pattern, by the magmatic crystallization sequence and by the

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elevated Fo content in olivine. A Sm-Nd isochron yielded an age of 903 ± 20 Ma, interpreted

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ACCEPTED MANUSCRIPT as the age of crystallization, with initial εNd = 0.8. Evidence of interaction of the BSC

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parental magma with sialic crust is given by the Rare Earth and trace element patterns, and by

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slightly negative and overall low values of εNd(900 Ma) in between -0.2 and +3.3. Contrary to

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early interpretations that it might constitute an ophiolite complex, based mainly on the

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geochemistry of the host rocks (Morro Branco metavolcanosedimentary complex), here we

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interpret the BSC as a typical layered mafic-ultramafic intrusion in continental crust, related

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to an extensional regime. The BSC is chrono-correlated to mafic dyke swarms, anorogenic

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granites and thick bimodal volcanics of similar age and tectonic setting in the São Francisco

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Craton and surrounding areas. Intrusion of the BSC was followed by continued lithospheric

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thinning, which led to the development of the Paulistana Complex continental rift volcanics

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around 888 Ma and ultimately to plate separation and the generation of new oceanic crust

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(Monte Orebe Complex) around 820-650 Ma ago. Thus, the BSC provides a benchmark for

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the onset of Tonian continental rifting in this area, and is an important marker for the

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processes of Rodinia breakup and dispersion recorded in South America.

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Rodinia break-up, São Francisco craton, Riacho do Pontal Orogen.

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Keywords: Mafic-ultramafic complex, Ni-Cu-PGE mineralization, Tonian rifting,

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1. INTRODUCTION

Active continental rifting is conditioned by asthenospheric thermal circulation, which

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leads to lithospheric extension and ultimately the breakup and dispersion of continents. In

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ancient crustal fragments, the records of this continental rifting stage can be preserved as A-

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type granitoids, mafic dyke swarms, and related rocks. In the Brazilian Shield, for example,

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records of an extensional event which occurred around 930-875 Ma are widespread and

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connected to the demise of Rodinia fragments, which in turn, were later re-grouped to form 2

ACCEPTED MANUSCRIPT the West Gondwana supercontinent during the Brasiliano Orogeny at 630-530 Ma ago (e.g.,

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Cordani et al., 2003; Tupinambá et al., 2007; Silva et al., 2008; Li et al., 2008; Fuck et al.,

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2008; Brito Neves et al., 2014). Ancient mafic-ultramafic intrusions are very important in this

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context, as they record the transfer of high volumes of mantle material to the continental

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crust, and the possible site of ancient mantle plume activity (e.g., Ernst et al., 2013).

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Furthermore, layered mafic-ultramafic igneous complexes are notable for hosting magmatic

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Ni-Cu±PGE deposits, being the most important Ni and Cu sources around the world

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(Naldrett, 2004; Arndt et al., 2005). However, in the Brazilian Shield, large mafic-ultramafic

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intrusions of this age are not common, and not yet well described.

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In this paper, we present the first systematic mapping, stratigraphic, petrographic,

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mineral and whole-rock geochemistry (including Sm-Nd isotope and Platinum Group Element

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analyses) studies on rocks of the Brejo Seco Complex (BSC), a ca. 3.5 km thick layered

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mafic-ultramafic intrusion in the Riacho do Pontal Orogen, southern Borborema Province,

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bordering the northern São Francisco-Congo Craton margin (Brito Neves, 1975; Gava et al.,

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1983; Marimon, 1990; Caxito, 2013; Caxito and Uhlein, 2013; Caxito et al., 2014a; 2014b;

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Salgado, 2014; Salgado et al., 2014). Although primarily interpreted as part of an ophiolite

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complex by Marimon (1990), we here suggest that the BSC represents in fact a layered mafic-

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ultramafic intrusion in a continental rift setting. The amassed data suggest that the BSC was

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intruded around 900 Ma ago, and involved the mixing of large volumes of mantle-derived

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magma with variable amounts of older continental crust. This provides a benchmark for

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Rodinia breakup and dispersion in the northern portion of the Brazilian shield – a region

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which is strategically located between the three major cratonic landmasses of West

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Gondwana, namely the São Francisco-Congo, West Africa-São Luiz and Amazonian cratons

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(Caxito et al., 2014b).

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2. GEOLOGIC SETTING The São Francisco Craton in east central Brazil is a geotectonic unit composed of an

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Archean to Paleoproterozoic basement covered by Proterozoic and Phanerozoic sedimentary

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rocks (Fig. 1; Almeida et al., 1981; Alkmim, 2004). It is delimited by the marginal fold belts

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of the extensive Brasiliano/Pan-African orogenic system, generated during the amalgamation

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of West Gondwana at ca. 630-530 Ma ago (e.g., Cordani et al., 2003; Almeida et al., 1981;

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Trompette, 1994; Brito Neves et al., 2014).

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The Riacho do Pontal Orogen (Fig. 1 and Fig. 2; Brito Neves, 1975; Brito Neves et al.,

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2000; Caxito, 2013) borders the northern São Francisco Craton margin and extends for 250

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km in an E-W trend. It represents a complete plate tectonics cycle developed in the

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Neoproterozoic (Caxito, 2013; Caxito et al., 2014b) and consists of a south-verging

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(cratonwards) imbricated nappe system. The whole nappe system is intruded by distinct

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generations of Neoproterozoic (Brasiliano) granitoid suites (syn-collisional Rajada Suite, syn-

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to late-collisional Serra da Esperança Suite, and late- to post-collisional Serra da Aldeia /

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Caboclo Suite), dated at around 630-550 Ma by Rb-Sr and U-Pb determinations (Angelim et

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al., 1988; Gomes and Vasconcelos, 1991; Angelim and Kosin, 2001; Jardim de Sá et al.,

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1992; Caxito, 2013). The Riacho do Pontal Orogen can be subdivided, from South to North,

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into three main tectonic domains or zones (Fig. 2; Oliveira, 1998, 2008; Caxito, 2013):

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External Zone – This is the external thrust-and-fold belt sensu strictu, composed of the

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supracrustal rocks of the Casa Nova Group. Those were deposited in a shallow, platformal sea

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of the northern São Francisco Craton passive margin (lowermost Barra Bonita Formation

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quartzites and marbles; Souza et al., 1979; Caxito, 2013) and in a syn-orogenic basin around

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630 Ma ago (uppermost Mandacaru Formation metagraywackes; Figueirôa and Silva Filho,

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1990; Santos and Silva Filho, 1990; Caxito, 2013).

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ACCEPTED MANUSCRIPT Central Zone – Constituted of metabasalts and deep-sea metasedimentary rocks of the Monte

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Orebe Complex. The position of this unit in the axial zone of the orogen, along with

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geophysical evidence which suggests the presence of a suture zone in between the External

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and Internal zones, the T-MORB geochemistry and depleted mantle Nd isotope characteristics

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of the metabasalts (εNd(t) around +4.4) led Caxito et al. (2014b) to interpret these rocks as

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remnants of a Neoproterozoic oceanic crust. Caxito et al. (2014b) presented a Sm-Nd

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isochron of 819 ± 120 Ma for the metabasalts of the Monte Orebe Complex; later, Brito

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Neves et al. (2015) recovered ca. 650-850 Ma zircon grains from metavolcanosedimentary

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rocks interleaved within the metabasalts.

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Internal Zone – The northernmost zone corresponding to the metamorphic/anatectic orogenic

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core. It is constituted of volcano-sedimentary complexes (Paulistana, Santa Filomena and

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Morro Branco complexes; Santos and Caldasso, 1978; Gomes and Vasconcelos, 1991;

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Angelim and Kosin, 2001; Caxito, 2013; Caxito and Uhlein, 2013). Most importantly, the

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augen-gneisses of the Afeição Suite are widespread in the internal zone, but absent in the

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other zones. These rocks were crystallized around 1000-960 Ma (U-Pb ages; Angelim, 1988;

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Jardim de Sá et al., 1988, 1992; Van Schmus et al., 1995; Caxito et al., 2014a), and thus are

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interpreted as part of the Cariris Velhos orogenic belt of the central Borborema Province

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preserved in this portion of the RPO (Brito Neves et al., 1995; Santos et al., 2010; Caxito et

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al., 2014a).

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The Morro Branco Complex (Caxito, 2013; Caxito and Uhlein, 2013) represents the

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host rocks into which the BSC is intruded. It consists of a low-grade (greenschist facies)

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metavolcanosedimentary sequence, outcropping for 60 km in the NW-SE direction (Fig. 1).

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As it is intruded by Afeição Suite granites dated at around 1000 Ma (Caxito et al., 2014a), it is

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considered as a sedimentary basin possibly related to the Cariris Velhos Orogeny. The Morro

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Branco Complex is composed of a supracrustal association of quartz-mica schist, phyllite,

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ACCEPTED MANUSCRIPT quartzite, metarhythmite and metachert, associated with metabasalts, metarhyodacite,

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metadacite and metarhyolite.

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Marimon (1990) described the mafic-ultramafic rocks of the Brejo Seco Complex, consisting

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mainly of dunite, troctolite and gabbro, as a fragment of a subduction-related ophiolitic

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complex. In this regard, it is useful to clarify that Marimon (1990) considered both the mafic-

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ultramafic intrusive complex and the metavolcanosedimentary sequence as part of the same

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cogenetic sequence. However, recent works (Caxito, 2013; Caxito and Uhlein, 2013; Salgado

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et al., 2014) propose to separate the volcanosedimentary portion (Morro Branco Complex)

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from the mafic-ultramafic layered intrusion (BSC).

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3. Materials and Methods

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Field work was conducted in the BSC, in order to constrain its stratigraphy and general

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geology. Drillholes were provided by Vale S.A.; the combination of drillcore and field data

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provided access to unweathered samples throughout the entire stratigraphy of the layered

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complex. Representative samples were then collected both in the field and from drillhole

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cores for petrographic, lithogeochemical, geochronological and isotopic analyses.

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For the lithogeochemical and Sm-Nd analysis, care was taken to select fresh samples free

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from weathering. Only the homogeneous parts of the samples were used (i.e., to avoid veins).

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In preparation for the geochemical and Nd isotope analysis, samples were crushed in a press

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with low-Cr plates and then a fraction of the resulting fragments was powdered in a ceramic

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shatterbox.

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Major, trace and rare earth elements analyses were conducted at the ACME Analytical

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Laboratories Ltd., Vancouver, Canada. Element contents were analyzed via ICP-MS after 6

ACCEPTED MANUSCRIPT fusion with lithium metaborate / tetraborate and digestion with diluted nitric acid. Analytical

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errors are within 5% for oxides and 10-15% for trace elements. Base and precious metal

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concentrations were determined by digestion in Aqua Regia followed by ICP-MS analysis.

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The Loss On Ignition (LOI) was determined by the weighing difference after ignition at 1000

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°C.

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The Sm-Nd analyses were conducted both at the Laboratório de Geocronologia,

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Universidade de Brasília, Brazil, and at the GEOTOP-UQàM Research Center, at Montréal,

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Canada. Samples were dissolved in a HF-HNO3 mixture in high-pressure Teflon vessels.

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A 150Nd-149Sm tracer was added to determine Nd and Sm concentrations. The REE were then

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purified by cation exchange chromatography, and Sm and Nd were subsequently separated

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following the procedure of Gioia and Pimentel (2000). Sm and Nd analyses were done using a

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double filament assembly in a Thermoscientific Triton Plus mass spectrometer operating in

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static mode in both laboratories. The Sm and Nd concentrations and the 147Sm/144Nd ratios

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have an accuracy of 0.5% that corresponds to an average error on the initial εNd value of ±

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0.5 epsilon units, based on repeated measurements of standards JNdi, BCR-1 and BHVO on

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both machines.

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Mineral analyses were performed on polished thin sections using a 5-spectrometer JEOL

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JXA-8230 SuperProbe at the Electron Microprobe Laboratory of the University of Brasília

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(Brazil). The wavelength dispersive (WDS) analyses were performed at an accelerating

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voltage of 15 kV and a beam current of 10 nA. Both synthetic and natural mineral standards

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were used for the analyses and the same standards and procedure were retained throughout the

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analytical work. Count times on peak and on background were 10s and 5s respectively, except

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for nickel (60s and 30s, respectively). Using these analytical conditions, detection limits for

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major elements and nickel are about 0.01 wt. %.

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4. The Brejo Seco Complex The mafic-ultramafic Brejo Seco Complex (BSC) crops out in the western portion of

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the Riacho do Pontal Orogen (Fig. 3). This complex is thrust by the Morro Branco Complex

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metavolcanosedimentary rocks northwards, and thrust over a syn-collisional Brasiliano

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granite of the Rajada Suite southwards. To the west and east, it is covered by sedimentary

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rocks of the Parnaíba basin and Cenozoic deposits, respectively.

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The BSC is a typical medium-size mafic-ultramafic layered intrusion. Its actual

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exposure level reveals an E-W trending 3.5 km wide and 9 km long intrusion, but gravimetric

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and magnetometric data suggest that it extends westwards below the Parnaíba basin (Oliveira,

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2008). Geological sections (e.g., Fig. 3) indicate that igneous layers have steep dip (an

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average of 70°) to the north and thickness of approximately 3.2 km.

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4.1. Stratigraphy

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The BSC is divided into four main zones: Lower Mafic Zone (LMZ), Ultramafic Zone (UZ),

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Transitional Mafic Zone (TMZ) and Upper Mafic Zone (UMZ). The UMZ is in turn

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subdivided into UMZa and UMZb units (Fig. 3 and 4). The new stratigraphy is based on

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detailed geology and geological sections supported by drilling. The facing criteria applied to

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the nomenclature of these zones consider that the magmatic stratigraphy of the BSC is

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inverted. An overturned layered sequence is supported by the fractionation from primitive

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rocks in the UZ toward progressively more fractionated rocks in the TMZ and UMZ, as

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described in the following sections.

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Thus, the base of the magmatic chamber corresponds to the structural top of the complex (Fig. 3 and Fig. 4).

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ACCEPTED MANUSCRIPT Lower Mafic Zone (LMZ) – Constitutes the base of the complex at the northern border of

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the intrusion. The LMZ is approximately 250 meters thick and forms a poorly outcropping

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narrow zone consisting of leucotroctolite (Pl+Ol±Chr cumulates; mineral abbreviations

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according to Whitney and Evans, 2010). This zone is extensively covered by lateritic crusts,

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and thus its delimitation is mainly based on borehole and geophysical data. The contact with

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the Morro Branco Complex is characterized by mylonitic greenschists, which thrust

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southwards over the Brejo Seco Complex. The upper contact with the Ultramafic Zone its

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marked by the transition of Pl+Ol±Chr cumulates to Ol+Chr cumulates.

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Ultramafic Zone (UZ) – The ultramafic zone underiles the Bacamarte plateau (Fig. 5a) and

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is the protolith for a lateritic nickel deposit. The UZ is approximately 1500 meters thick, and

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consists mainly of serpentinized dunite (Ol+Chr cumulate) and minor interlayered

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leucotroctolite (Pl+Ol±Chr cumulate). These intercalations are up to tens of meters thick and

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occur throughout the UZ. Interlayered leucotroctolite becomes progressively thicker and more

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frequent toward the upper stratigraphic portions of this zone, grading thus to the Transitional

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Mafic Zone. Dunite located in the upper stratigraphic portion of the UZ is also characterized

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by larger amounts of interstitial plagioclase, eventually showing a diffuse layering defined by

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variable modal proportion of interstitial plagioclase.

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Transitional Mafic Zone (TMS) – The transitional mafic zone is located between the UZ

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and the Upper Mafic Zone. The TMZ is up to 400 meters thick in the wider western portion

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where abundant outcrops of troctolites occur. It is characterized by Pl+Ol cumulates, being

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constituted mainly by leucotroctolite (Pl+Ol±Chr cumulate) and minor troctolite and olivine

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gabbro (Pl+Ol±Cpx cumulates; Fig. 5b). The gradational disappearance of olivine cumulates

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marks its contact with the Upper Mafic Zone.

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Upper Mafic Zone (UMZ) – The upper mafic zone is about 1000 meters thick and the best

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exposed portion of the complex. To the south and west, it is thrust southwards upon the

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Rajada Suite granite, which becomes mylonitic near the contact. The UMZ is subdivided into UMZa and UMZb, which are distinguished by the

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presence of ilmenite and magnetite in the latter. UMZa is composed essentially of gabbro

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(Pl+Cpx cumulate; Fig. 5c), and subordinately of olivine gabbro (Pl+Cpx+Ol cumulates) and

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troctolite (Pl+Ol±Chr cumulate). Discontinuous layer of adcumulate dunite, eventually

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bearing troctolite xenoliths, crops out in the lower portion of the UMZa. This layer of dunite

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is followed by troctolite and olivine gabbro for about 150 m southwards, and then gabbro

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reappears as the main lithotype.

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The UMZb consists of Ilm-Mag gabbro (Pl+Cpx+Ilm+Mag±Ap cumulate; Fig. 5d),

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with thin interlayered (< 5 m) Ilm magnetitite (Mgt+Ilm cumulate) and anorthosite

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(Pl+Cpx+Apt cumulate). Massive Ilm magnetitite occurs as few meters thick elongated zones

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of blocks and boulders following the EW layering of the complex, as well as rare outcrops of

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interlayered Ilm magnetitite and gabbroic rocks.

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4.2.Petrography

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The BSC rocks generally preserve textures that are typical of cumulates (Wager and

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Wadsworth, 1960), but primary igneous minerals are heterogeneously affected by retrograde

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hydration. Because the original igneous texture is largely preserved in these rocks, such that

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original cumulate minerals can be identified, they are described using igneous terminology.

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The following description of lithotypes is focused on the primary cumulate rocks.

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ACCEPTED MANUSCRIPT Dunite – a medium-grained olivine (> 90 vol. %) and chromite (< 7 vol %) adcumulate (Fig.

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5e). Intercumulus minerals consist of variable proportions of plagioclase, clinopyroxene,

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orthopyroxene and rare phlogopite. Olivine crystals are extensively replaced by serpentine

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minerals and magnetite, while intercumulus minerals are rarely preserved in dunite. Chromite

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occurs as fine-grained euhedral to subhedral crystals partially replaced by ferrichromite and/or

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magnetite. Irregular pods or layers of different sizes (from few centimeters to several meters

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thick) with larger amounts of intercumulus minerals occur within dunite. They form coarser-

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grained meso- to orthocumulate textured peridotites or troctolite. The latter is particularly

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common in the upper portion of the UZ and is characterized by extensive replacement of

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interstitial plagioclase by chlorite (Fig. 5f).

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Leucotroctolite and Troctolite – Leucotroctolite an troctolite occur as medium- to coarse-

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grained olivine (10-40 vol. %) and plagioclase (40-70 vol. %) adcumulate and mesocumulate

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rocks. These rocks commonly have prominent igneous layering or lamination defined by

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alternating leucotroctolite and troctolite. Laminated rocks show tabular plagioclase aligned

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parallel to the layering. Intercumulus minerals consist of variable proportions of

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clinopyroxene and rare orthopyroxene and phlogopite (Fig. 5g). Clinopyroxene appears as a

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cumulus mineral in olivine gabbro associated with troctolite and leucotroctolite (Fig. 5h).

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Chromite (< 2 vol. %) is a frequent cumulus mineral in leucotroctolite and troctolite, usually

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as tiny euhedral crystals, but is absent in olivine gabbro. Serpentinization of olivine and

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saussuritization of plagioclase are highly variable in gabbroic rocks, and pristine rocks occur

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closely associated with pervasively altered ones.

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Gabbroic Rocks – Gabbroic rocks consist of gabbro (Pl+Cpx cumulates), Ilm-Mag gabbro

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(Pl+Cpx+Ilm+Mag±Apt cumulates) and olivine gabbro (Pl+Cpx+Ol cumulates). These

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medium-grained rocks are massive or have prominent igneous layering or lamination defined

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by alternating leucogabbro and gabbro. Laminated rocks usually have tabular plagioclase

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ACCEPTED MANUSCRIPT aligned parallel to the layering. The modal composition of the cumulus minerals is difficult to

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precise due to extensive transformation of clinopyroxene and plagioclase. Saussuritization of

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plagioclase and alteration of clinopyroxene to tremolite or actinolite are extensive, such that

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these minerals are rarely preserved in the UMZ. Cumulus magnetite and ilmenite characterize

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the gabbroic rocks of the UMZb, while apatite occurs as an additional cumulus mineral in the

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most fractionated rocks (Pl+Cpx+Ilm+Mag+Apt cumulate). Ilmenite (< 5 vol. %) and

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magnetite (< 5 vol. %) occur as fine- to medium-grained euhedral to subhedral crystals in Ilm-

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Mag gabbro, while apatite (< 1 vol. %) occurs as fine-grained euhedral crystals.

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Ilmenite Magnetitite – Ilmenite magnetitite occurs as medium- to coarse-grained magnetite

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(~ 60 vol. %) and ilmenite (~ 40 vol. %) adcumulate rocks. Interstitial minerals are secondary

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silicates (mainly chlorite). Exsolution lamellae and globules of ilmenite in magnetite are

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common.

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Anorthosite – Anorthosite occurs as partially to extensively weathered coarse-grained

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plagioclase cumulate with minor pyroxene and apatite. Due to weathering a detailed

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petrographic description of this rock type is compromised.

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4.3. Ni-Cu-PGE Mineralization

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In the contact between the UZ (dunite) and TMZ (troctolite), a zone with disseminated

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sulfides was identified during the exploration program carried out by Vale S.A. between 2005

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and 2006. This stratiform interval with disseminated sulfides is up to 50 meters thick and

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extends for about 1000 meters along strike. The sulfides are mainly hosted in dunite and

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minor interlayered troctolite of the uppermost portion of the UZ. Dominantly, the sulfides are

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disseminated (< 5 vol. %), forming fine-grained aggregates or blebs interstitial to cumulus

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olivine crystals (Fig. 6a). They consist basically of pyrrhotite (>50 vol. %), pentlandite and 12

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chalcopyrite (Fig. 6b, 6c, 6d). Sulfides are commonly partially altered to a fine-grained

295

aggregate of magnetite and/or Fe-hydroxides, as well as partially remobilized along fractures

296

or veins. Normally, this kind of alteration is related to the interaction of sulfides with

297

oxidizing hydrothermal fluids (Vusse and Powel, 1983; Collins et al. 2012). The sulfide zone is enriched in Ni, Cu, Pt, Pd and Au (Fig. 7a) forming a low-grade

299

stratabound mineralization. Very low contents for Ni (up to 3000 ppm), Cu (up to 1500 ppm),

300

Pt+Pd (up to 150 ppb) and Au (up to 52 ppb) in the sulfide zone suggest that metal tenors

301

(i.e., the metal content recalculated in 100% sulfide) should be accordingly low (for

302

comparison see values reported for several Ni-Cu-PGE deposits by Barnes and Lightfoot,

303

2005). Nickel contents in unmineralized dunites adjacent to the sulfide zone range from 1400-

304

1800 ppm, thus suggesting that a significant amount of nickel in the mineralized zone is not

305

associated with sulfides. The plot of Ni vs Cu for rocks of the mineralized zone falls on

306

reasonably well-defined linear trends (r2 = 0.73, Fig. 7b), indicating that nickel contents of

307

dunites in the mineralized zone are upgraded due to Ni-Cu sulfides.

TE D

M AN U

SC

RI PT

298

Platinum Group Elements (PGE) and Au contents were determined for four samples

309

from the sulfide zone. Ir (< 1 ppb), Ru (<5 ppb), Rh (<1 ppb), Pt (<31 ppb), Pd (<17 ppb) and

310

Au (<19 ppb) contents are extremely low (Table 1 of the Supplementary Material). Mantle-

311

normalized profiles for PGE-Au contents are moderately enriched in PPGE (Rh, Pt, Pd) with

312

positive slope (Fig. 7c). This type of profile is similar to those observed in PGE

313

mineralizations in layered intrusions (e.g., Barnes and Lightfoot, 2005).

AC C

314

EP

308

Magmatic Ni-Cu-PGE sulfide mineralization developed as a stratiform/stratabound

315

layer within a layered intrusion is not common, and the Santa Rita deposit in the Mirabela

316

intrusion is the only economic example (Barnes et al. 2011b; Ferreira Filho et al. 2013). This

317

unusual type of stratiform/stratabound Ni-Cu deposits share similarities with several PGE

318

deposits associated with disseminated sulfides (e.g., the Main Sulfide Zone in the Great Dyke;

13

ACCEPTED MANUSCRIPT 319

Wilson, 2001; the PGE mineralization in the Luanga Complex; Ferreira Filho et al., 2007) and

320

provide further evidence for similar processes originating magmatic sulfide deposits with

321

distinctively different metal contents (e.g., Naldrett, 2004; Barnes and Lightfoot, 2005).

322

325

4.4.Geochemistry 4.4.1 Olivine composition

SC

324

RI PT

323

Systematic studies of mineral composition of cumulus minerals of the BSC are limited

327

due to partial to extensive alteration of the igneous assemblage. Detailed sampling and

328

petrographic studies provided just nine samples with igneous Ol crystals appropriate for

329

chemical analyses (see Table 2 of the Supplementary Material for representative analyses).

330

Olivine compositions range from Fo80.7 to Fo89.3 indicating primitive (i.e., high MgO contents)

331

to moderately primitive compositions. The compositional variation of Ol with stratigraphic

332

height in borehole FDS-003 provides the compositional range of olivine in dunite (5 samples)

333

and troctolite (1 sample) in the stratigraphic interval closely associated with the Ni-Cu-PGE

334

mineralization (Fig. 8a). In this interval, olivine compositions indicate a discontinuous

335

fractionation with one sharp compositional reversal (Fig. 8a). Ni contents in olivine range

336

from 740 to 2590 ppm and have poor correlation with Fo content (Fig. 8b). Considering a

337

general trend line through the olivine compositions of samples with the highest Ni content for

338

the identified Fo range (i.e., samples 1, 2, 3 and 5 in Fig. 8b), the composition of olivine from

339

the troctolite sample located stratigraphically above the mineralized sulfide zone (i.e., sample

340

6 in Fig. 8b) is highly depleted in nickel.

AC C

EP

TE D

M AN U

326

341

The compositions of olivine crystals from troctolite samples of the LMZ (collected

342

from borehole FR-007) and TMZ (one sample from borehole FDS-003 and one from outcrop)

14

ACCEPTED MANUSCRIPT are shown in figure 8c. Olivine compositions of two samples of troctolite from the LMZ are

344

primitive and range from Fo87.4 to Fo89.3. Olivine compositions of two samples of troctolite

345

from the TMZ range from primitive (i.e., Fo87.5 to Fo87.9 - sample 6 from borehole FDS-003)

346

to moderately fractionated (i.e., Fo80.0 to Fo81.5 - sample 9 from an outcrop; Fig. 8c). Ni

347

contents of olivine from troctolite samples fit the general Fo-Ni plot delineated for samples

348

from borehole FDS-003, except for the troctolite located immediately above the mineralized

349

sulfide zone (sample 6). Sample 9, located about 150 meters above the contact of the UZ and

350

TMZ, is undepleted in nickel and suggests that new influxes of primitive magma restored the

351

nickel content of the magma chamber within the TMZ.

M AN U

352

353

SC

RI PT

343

4.4.2. Lithogeochemistry Major elements

355

The plot of MgO vs major oxides and selected minor elements provides the main

356

characteristics of cumulate rocks from the BSC (Fig. 9). In the following discussion and

357

diagrams, the composition of major and minor elements will be quoted as weight percent

358

oxide from analyses (see Table 3 of the Supplementary Material) normalized to 100% on an

359

anhydrous basis. The reasoning for recalculation on anhydrous basis is just to limit the

360

differences associated with distinct contents of volatiles (LOI), especially when olivine

361

cumulates (dunite) (~ 10.8-13.4 wt. % LOI) are compared with gabbroic rocks (< 4.8 wt. %

362

LOI).

AC C

EP

TE D

354

363

Distinct SiO2 (39-50 wt.%), Al2O3 (1-28 wt.%), MgO (1-44 wt.%), CaO (< 17 wt.%),

364

Na2O (< 3 wt.%), K2O (< 1 wt.%), TiO2 (< 6 wt.%), Fe2O3 (3-20 wt.%), Ni (13-3965 ppm)

365

and Cr (34-6500 ppm) contents reflect the modal variation of the main cumulus phases for

366

different rock types (Fig. 9). The plot of major element oxides against MgO reflects the 15

ACCEPTED MANUSCRIPT 367

predominance of olivine cumulates (dunite), olivine and plagioclase cumulates (troctolite) and

368

plagioclase-clinopyroxene cumulates (gabbro) in the BSC (Fig. 9). Dunite samples have high modal % of olivine and are characterized by high Mg# (83-

370

88%) and MgO (41-44 wt.%; Fig. 9). Ni contents depend mainly on its concentration in

371

olivine and, in few samples, are upgraded by the presence of pentlandite (e.g., sample FDS-

372

004/1, Fig. 9a). The contents of Cr2O3 (0.35-0.95 wt. %) are consistent with the occurrence of

373

cumulus chromite in dunite (Fig. 7b). Al2O3 contents in dunite (up to 6.56 wt. %) reflect the

374

presence of interstitial plagioclase in several samples.

SC

RI PT

369

Troctolite samples show high Mg# (81-88%) similar to dunite but much lower MgO

376

contents (14-18 wt. %). The negative correlation of MgO with SiO2, Al2O3, CaO and

377

Na2O+K2O (Figs. 9e and 9h) is due to distinct modal % of plagioclase (40-70 vol.%) and

378

olivine (10-40 vol%), consistent with these rocks being Ol + Pl meso- to adcumulate.

M AN U

375

Gabbro and Ilm-Mag gabbro samples are more fractionated as indicated by Mg#

380

between 70-81% and 46-63%, respectively (Table 3 of the Supplementary Material). The

381

fractionation of these rocks are also indicated by higher P2O5 and TiO2 contents, as well as

382

lower Cr and Ni contents (Fig. 9; Table 3 of the Supplementary Material).

EP

TE D

379

The only anorthosite sample analysed has the highest Al2O3 and Na2O+K2O contents

384

(Fig. 9f and 9h; Table 3 of the Supplementary Material), consistent with the abundance of

385

cumulus plagioclase (> 90 vol. %).

AC C

383

386

387

4.4.3. Trace elements

388

Mafic-ultramafic rocks of the BSC have relatively low contents of incompatible trace

389

elements (Table 3 of the Supplementary Material), as expected for olivine, clinopyroxene and 16

ACCEPTED MANUSCRIPT plagioclase cumulates. The trace element contents are shown in primitive-mantle normalized

391

spidergrams (Fig.10, normalization values from Sun and McDonough, 1989). The low

392

strength field elements Cs, Rb, Ba and Sr were not utilized due to its mobile behavior during

393

post-magmatic processes (Pearce and Cann, 1973; Rollinson, 1993). The REE values were

394

normalized to the C1 chondrite (Sun and McDonough, 1989) and are shown in multi-element

395

diagrams (Fig.11). Except for the most fractionated gabbroic rocks from the UMZa and

396

UMZb, incompatible trace elements have contents close to or below detection limits of the

397

analytical methods, reflecting in irregular REE profiles and multi-element diagrams (Fig. 10;

398

Fig. 11).

M AN U

SC

RI PT

390

Dunite samples are from the UZ except for sample FRP-115 from the UMZa (Figs.

400

10a and 11a). The distribution of incompatible elements and REE profiles in dunite are

401

heterogeneous but characterized by an enrichment of LREE in several samples, as indicated

402

by (La/Yb)N between 1.1-17.0 and (La/Sm)N in between 1.2-6.5.

TE D

399

Troctolite samples come from TMZ, LMZ and UMZa. Samples FR-007/1 and FR-

404

007/2 from the LMZ (Fig.11b) have distinctively enriched LREE patterns, with high (La/Yb)N

405

(5.6 and 16.7) and (La/Sm)N (3.2 and 5.0) ratios. Samples FDS-003/11, FDS-003/18 and FRP-

406

118 (TMZ) and SS-CBS-061 (UMZa; Fig. 11c) have moderately enriched LREE, as indicated

407

by (La/Yb)N from 3.1 to 5.4 and (La/Sm)N from 2.1 to 3.1. The Eu/Eu* values are positive in

408

all troctolite samples, from 1.8 to 3.8. REE distributions in essentially adcumulate troctolites

409

like those from the BSC are mainly controlled by plagioclase (Charlier et al. 2005). The REE

410

profile of the anorthosite (Fig. 11e) illustrates the distribution of REE expected for cumulus

411

plagioclase in the BSC.

AC C

EP

403

412

The gabbros from UMZa and UMZb (Figs. 10c, 10d, 11d and 11e) are characterized

413

by negative Zr anomalies and predominantly flat REE distribution. (La/Yb)N ratios range from

17

ACCEPTED MANUSCRIPT 0.9 to 2.1 and (La/Sm)N ratios from 0.7 to 1.7 in UMZa gabbros, whereas (La/Yb)N ratios

415

range from 0.9 to 3.1 and (La/Sm)N ratios range from 0.7 to 2.6 in UMZb gabbros. The

416

Eu/Eu* ratios are positive and vary from 1.1 to 1.6 for UMZa and 1.3 to 3.8 for UMZb.

417

Distinctively positive Ti anomalies and higher Ta and Nb values of gabbros from the UMZb

418

(Figs. 10d and 10c) result from abundant ilmenite and magnetite in these rocks (Nielsen and

419

Beard, 2000).

RI PT

414

Distinctively different distribution of REE of gabbroic rocks of the UMZa and UMZb

421

partially result from different modal proportion of plagioclase, responsible for fractionation of

422

LREE, and Ca-clinopyroxene, responsible for the fractionation of HREE (Charlier et al.

423

2005). Possibly, the presence of apatite in UMZb, even in small modal concentrations, causes

424

an increase in ∑REE (Table 3 of the Supplementary Material).

425

4.5. Sm-Nd isotope geochemistry

TE D

426

M AN U

SC

420

Four whole-rock samples from the UMZb, with relatively higher Sm and Nd contents

428

(Table 4 of the Supplementary Material), rendered an isochron suggesting a crystallization

429

age of of 903 ± 20 Ma (2σ) (Fig. 12), with εNd(t) of +0.8 (MSWD = 0.18 and probability of fit

430

of 0.84). Other six samples were analysed (Table 4 of the Supplementary Material) and used

431

in a chemostratigraphy column showing εNd(t) variations throughout the BSC (Fig. 13).

432

Variable values of εNd(900 Ma), in the range of -0.2 and +3.3 (Table 4 of the Supplementary

433

Material), suggest that the parental magma may be originated from a depleted mantle source

434

variably contaminated with continental crust (Fig. 13).

AC C

EP

427

435

436 437

5. Discussions 5.1. Parental magma 18

ACCEPTED MANUSCRIPT The composition of the parental magma of the Brejo Seco Complex cannot be

439

constrained by common approaches used to define their composition in well-exposed and

440

unaltered intrusions (e.g., chilled margin, bulk composition, extrusive equivalents, related

441

dykes, and melt inclusions). However, some of the characteristics of the parental magma are

442

indicated from the crystallization / magmatic fractionation trends, from the Sm-Nd isotope

443

data and from the composition of cumulus olivine.

RI PT

438

The magmatic crystallization sequence (Ol+Chr >> Pl+Ol+Chr >> Pl+Cpx+Ol >>

445

Pl+Cpx+Ilm+Mag+Apt) defined for the BSC parental magma is typically tholeiitic. This

446

crystallization sequence is described in several tholeiitic layered intrusions (e.g., Skaergaard

447

intrusion in Greenland; McBirney, 1989), which usually show enrichment in TiO2 and Fe2O3

448

with fractionation (Fig. 13). A primitive composition (i.e., high MgO content) for the parental

449

magma of the BSC is indicated by the thick pile of dunite in the UZ, which allowed the

450

development of a large nickel laterite deposit. The compositional range of cumulus Ol of the

451

BSC (Fo80-89) supports a primitive composition for their parental magma. The most primitive

452

composition of olivine in the BSC (i.e., Fo89) is comparable with those reported for layered

453

intrusions originated from primitive magmas, as illustrated by the Bushveld Complex (Fo89;

454

Eales and Cawthorn, 1996) and Stillwater Complex (Fo90; McCallum, 1996).

EP

TE D

M AN U

SC

444

The petrogenetic interpretation of lithogeochemical data of mafic-ultramafic cumulate

456

rocks is not a straightforward issue. The distribution of incompatible trace elements in the

457

BSC is largely controlled by cumulus minerals, as illustrated by distinct patterns for

458

associated troctolite, gabbro and anorthosite (Fig. 11). Very low contents for several trace

459

elements hampered the identification of common geochemical features associated with

460

specific petrogenetic processes (e.g., Nb-Ta negative anomalies).

AC C

455

461 462

5.2. A descriptive model for the evolution of the Brejo Seco Complex 19

ACCEPTED MANUSCRIPT 463

Geological and geochemical results of this study indicate that the BSC is similar to other typical layered mafic-ultramafic intrusions around the world. The magmatic stratigraphy

465

of the BSC is interpreted as resulting from successive influxes of parental magma within a

466

continuosly fractionating magma chamber. The evolution of the BSC is briefly described as

467

the result of the following four major stages.

RI PT

464

Stage 1 consists of the initial evolution of the intrusion. Several studies suggest that

469

layered intrusions commonly pass through an initial open-system stage where the inflowing

470

magma becomes increasingly more primitive with time (e.g., Egorova and Latypov, 2012). In

471

particular, they are commonly characterized by a crystallization sequence that follows a

472

reversed trend compared with the one characteristic of the main layered sequence. Although

473

our data are limited to sparce geological and petrographic data, we speculate that the LMZ,

474

which is characterized by a thin zone of olivine + plagioclase cumulates (troctolite), may

475

result from the first batches of magma filling the magma chamber.

TE D

M AN U

SC

468

Stage 2 represents the periodic infilling of parental magma in the chamber and

477

concomitant crystallization that form the UZ. Frequent influx of parental magma sustained

478

the high Mg# values (84-87; Fig. 13) throughout the UZ and the formation of the thick pile of

479

homogeneous dunite (Ol + Chr cumulate).

AC C

480

EP

476

Stage 3 represents the formation of the disseminated sulfide Ni-Cu-PGE mineralized

481

zone, at the contact between UZ and TMZ. The compositional variation of Ol in the

482

stratigraphic interval closely associated with the Ni-Cu-PGE mineralization indicates a

483

discontinuous fractionation with one sharp compositional reversal. This feature indicates that

484

the segregation of magmatic sulfides is associated with new influxes of parental magma. Ni

485

contents in olivine range from 740 to 2590 ppm and have poor correlation with Fo content,

486

suggesting that the segregation of the immiscible sulfide liquid led to metal depletion in the

20

ACCEPTED MANUSCRIPT magma chamber. The process leading to sulfide saturation and the segregation of magmatic

488

sulfides in the upper portion of the UZ is likely associated with new influxes of parental

489

magma, possibly enriched in sulfur due to contamination with crustal rocks. The lowest value

490

of εNd(900 Ma) of the BSC is obtained in a sample from the mineralized zone (-0.2; Table 4 of

491

the Supplementary Material), thus supporting the influx of magmas contaminated with older

492

sialic crust.

RI PT

487

Stage 4 represents the processes leading to the development of a thick pile of mafic

494

cumulates of the TMZ and UMZ. Periodic new influxes of magma with reversals in the

495

fractionation trend occur in the lower portion of the mafic cumulates (TMZ and UMZa), but

496

become progressively less significant toward the upper portions of the stratigraphy (UMZb).

497

The upper part of the layered intrusion is characterized by fractionated cumulates (Opx + Pl +

498

Apt + Mag + Ilm cumulate) interlayered with massive magnetite-ilmenite layers.

M AN U

SC

493

TE D

499

5.3. Tectonic setting and implications for Rodinia break-up

501

The new data presented in this study characterize the BSC as a layered igneous

502

complex, similar to layered intrusions emplaced in continental crust. Our results do not

503

support the previous interpretation of the BSC and associated mafic volcanics as an ophiolitic

504

complex (Marimon, 1990). The intracontinental origin of the BSC is supported mainly by: i)

505

highly variable and slightly negative to low positive values of εNd(900 Ma) suggests interaction

506

of the parental magma with sialic crust; ii) the age difference between the BSC (ca. 900 Ma)

507

and the Morro Branco metavolcanics, which are cross-cut by the 1.0 Ga Afeição Suite augen-

508

gneisses (Caxito et al., 2014a), does not support a cogenetic origin for the BSC and host-rock

509

metavolcanics, which were analysed together in Marimon’s (1990) work; iii) the stratigraphic

510

similarity of the BSC with other layered igneous complexes originated in continental rifting

AC C

EP

500

21

ACCEPTED MANUSCRIPT setting (e.g., Skaergaard; Tegner et al., 1998.; Duluth; Miller and Ripley, 1996). Therefore,

512

considering the BSC as a layered complex intrusive into continental crust, this geodynamic

513

setting should be related to specific stages in the evolution of the Riacho do Pontal Orogen

514

and of the São Francisco Craton.

RI PT

511

Regarding to the geotectonic context of the Riacho do Pontal Orogen, the BSC is

516

located in the Internal Zone, and intrudes metavolcanosedimentary rocks related to the Cariris

517

Velhos cycle (1000-960 Ma; Caxito et al., 2014a). But, in the tectonic model presented here,

518

the BSC has no genetic relation to the Cariris Velhos cycle, instead being interpreted as

519

evidence of the initial rifting stage related to the following Brasiliano cycle (900-600 Ma) and

520

related to development of a Neoproterozoic rift system.

M AN U

SC

515

The age of ca. 900 Ma for the intrusion of the BSC corresponds to the ages of other

522

mafic and ultramafic rocks of the Riacho do Pontal Orogen. The Paulistana Complex is a

523

metavolcanosedimentary complex with intercalations of gabbros and metabasalts interpreted

524

as emplaced in a continental rift environment, with an 888 Ma U-Pb age (Caxito, 2013).

525

Those are followed by T-MORB basalts of the Monte Orebe Complex, whose age is not yet

526

well constrained but might fall in between 820 Ma (Sm-Nd isochron; Caxito et al., 2014b)

527

and 650 Ma (U-Pb detrital zircon data; Brito Neves et al., 2015). The latter were interpreted

528

as ophiolitic remnants (Moraes, 1992; Caxito et al., 2014b). Thus, the intrusion of the BSC at

529

ca. 900 Ma ago was followed by lithospheric thinning and development of a continental rift

530

sequence (Paulistana Complex) around 888 Ma ago. Continued thinning of the lithospere

531

eventually led to plate separation and creation of true oceanic crust (Monte Orebe Complex),

532

starting at ca. 820 Ma ago and possibly extending to ca. 650 Ma, when the onset of

533

subduction on the Borborema Province might have caused the inversion of basins and

534

obduction of oceanic crust slices (Caxito et al., 2014b).

AC C

EP

TE D

521

22

ACCEPTED MANUSCRIPT In a global tectonic view, emplacement of the BSC might be correlated with the

536

extensional processes that led to the fragmentation of the Rodinia Supercontinent (ca. 1000-

537

750 Ma; Li et al., 2008), and dismembering of the São Francisco-Congo paleocontinent,

538

which was later amalgamated with other blocks in the Gondwana Supercontinent during the

539

Brasiliano / Pan-African Orogeny (ca. 630-530 Ma; Brito Neves et al., 2000; Tupinambá et

540

al., 2007; Li et al., 2008; Fuck et al., 2008). Both the São Francisco (Brazil) and Congo

541

(Africa; Fig. 14) cratonic portions bear records of this Tonian extension in the form of layered

542

intrusions, anorogenic granites, dykes and sill swarms (Correa-Gomes and Oliveira, 2000;

543

Brito Neves, 2003; Cordani et al., 2003; Tupinambá et al., 2007).

M AN U

SC

RI PT

535

Specifically in the São Francisco Craton, those extensional processes were subdivided

545

by Tupinambá et al. (2007) in three stages: I (1120-1000), II (930-900 Ma) and III (850-815

546

Ma), the later associated to the formation of oceanic crust. The BSC (ca. 900 Ma; Fig.14)

547

might be related to Stage II, and thus correlated to: i) mafic dyke swarms (~906-925 Ma;

548

Machado et al., 1989; Heaman, 1991; Correa-Gomes and Oliveira, 2000) of the São Francisco

549

Craton; ii) anorogenic (A-Type) granites of the Araçuaí Orogen (875 ± 9 Ma; Silva et al.,

550

2008); iii) amphibolite bodies (ca. 959 Ma; Valeriano et al., 2004) of the Brasília Orogen. In

551

the West Congo Belt, which is the African counterpart of the Araçuaí Orogen (Fig.14),

552

Tonian magmatism is represented by the km-thick bimodal volcanism of the Mayumbian and

553

Zadinian groups (~912-920 Ma; Tack et al., 2001) and by the Comba-Sembe-Quesso dyke

554

swarm (ca. 950 Ma; Vicat and Pouclet, 1995).

EP

AC C

555

TE D

544

Two other layered igneous complexes at the São Francisco Craton marginal belts

556

whose genesis was interpreted in a similar manner (i.e., extensional magmatism) are the

557

Ipanema Complex (1104 ± 78 Ma; Angeli et al., 2004), in the Araçuaí Orogen and Canindé

558

Complex (701 ± 8 Ma; Nascimento et al., 2005; Oliveira et al., 2010; Fig.14), in the

559

Sergipano Orogen. The geochronological differences between those three complexes might 23

ACCEPTED MANUSCRIPT 560

indicate the acting of diachronic and distinct instalation processes of rifting associated to the

561

dispersion of Rodinia, which might have evolved to the formation of oceanic crust in the site

562

of the precursor basins to the marginal belts.

564

5.4. Implications for mineral exploration

RI PT

563

Magmatic Ni-Cu± (PGE) deposits form when mafic-ultramafic mantle derived

566

magmas become saturated in sulfur, seggregating and concentrating immiscible sulfide

567

droplets (Naldrett, 2004; Arndt et al., 2005). Intrusion-related Ni–Cu sulfide deposits are

568

typically associated with extensional tectonics in the crust, including rifted continental crust

569

or continental margins, as represented by the occurrences from Noril’sk, in Russia, and

570

Duluth, in USA (Naldrett, 2004). However, this type of mineralization is rarely associated

571

with oceanic rifting, and no significant economic deposit is known in such geological setting.

M AN U

SC

565

The tectonic setting for the emplacement of the BSC proposed in the previous

573

discussion, suggesting that it might be correlated to the extensional processes that led to the

574

fragmentation of the Rodinia Supercontinent (ca. 1000-750 Ma; Li et al., 2008), has

575

significant implications for exploration. The proposed setting for the BSC is highly favorable

576

for hosting different type of Ni-Cu-PGE deposits, including Ni-Cu mineralization located at

577

the base of layered intrusions (e.g., Duluth Complex; Ripley, 2014) or in conduits or

578

chonoliths (e.g., Norilsk; Naldrett, 2004; Limoeiro; Mota-e-Silva et al., 2013), as well as reef-

579

type PGE deposits (e.g., Skaergaard Complex; Andersen et al., 1998). An additional positive

580

feature for the potential to host Ni-Cu-PGE magmatic deposits is provided by the primitive

581

composition of the parental magma of the BSC, as indicated by olivine compositions of up to

582

Fo89, as well as the sulfide-rich zone identified in the upper portion of the UZ. Even though

583

the sulfide zone is characterized by very low and sub-economic Ni-Cu-PGE contents, this up

AC C

EP

TE D

572

24

ACCEPTED MANUSCRIPT to 50 meters thick zone of disseminated sulfides indicates that significant amount of sulfur

585

was segregated as immiscible sulfides from the mafic-ultramafic magmas. Our results suggest

586

that several mafic-ultramafic occurrences identified in the Riacho do Pontal Orogen and

587

surrounding areas should be evaluated for Ni-Cu-PGE mineralizations.

RI PT

584

588

589

6. Conclusions

The BSC is a typical layered mafic-ultramafic igneous intrusion, hosted in continental

591

crust, and tectonically inverted. It is subdivided in four main stratigraphic zones: the Lower

592

Mafic Zone (LMZ; troctolite), the Ultramafic Zone (UZ; mainly dunite), the Transitional

593

Mafic Zone (TMZ; mainly troctolite) and the Upper Mafic Zone (UMZ; mainly gabbro, and

594

minor Ilm-Mag gabbro, magnetitite and leucotroctolite). The BSC hosts a stratabound sub-

595

economic Ni-Cu-PGE mineralization located at the contact of the UZ with the TMZ. The

596

BSC originated from a primitive (high-Mg) tholeiitic magma in a dynamic magmatic system,

597

characterized by succesive influxes of parental magma. A crystallization age of around 900

598

Ma is suggested by a Sm-Nd isochron. The contamination of the BSC parental magma with

599

older sialic crust is suggested by trace element patterns and by values of εNd(900 Ma) between -

600

0.2 and +3.3.

M AN U

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The BSC is interpreted as a ca. 900 Ma layered intrusion related to the initial rifting

AC C

601

SC

590

602

stages of Rodinia. The BSC might be correlated to the extensional events which led to the

603

break-up and dispersion of the Rodinia Supercontinent, and thus be related to the mafic dyke

604

swarms, extensive volcanism and anorogenic granite emplacement which took place

605

throughout the São Francisco – Congo paleocontinent and surrounding areas during the

606

Tonian (Tupinambá et al., 2007; Li et al., 2008). Thus, the BSC provides a benchmark for

607

Rodinia breakup in the northern portion of the São Francisco – Congo craton, a region which

25

ACCEPTED MANUSCRIPT 608

would later occupy the heart of the Gondwana Supercontinent. The tectonic setting proposed

609

for the emplacement of the BSC suggests a highly favorable environment for hosting different

610

types of Ni-Cu-PGE deposits.

612

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7. ACKNOWLEDGMENTS

Fundação de Amparo à Pesquisa do Estado de Minas Gerais and Vale S.A. (Brazil)

614

supported this work through grant CRA-RDP-00120-10. The work was greatly facilitated by

615

the Canadian and Québec Government who provided support for multiple short-time research

616

visits by FAC to the GEOTOP Research Center, Montréal, Canada, through the ELAP

617

(Emerging Leaders in the Americas Program) of the Canadian Bureau for International

618

Education and the Merit Scholarship Program of the Ministère de l’Education, du Loisir et du

619

Sports du Québec (MELS). Analytical facilities of the Instituto de Geociências (Universidade

620

de Brasília) provided additional support for this research. The authors acknowledge Walter

621

Riehl and Márcio Erbes from Vale for their support during fieldwork and interpretation of

622

exploration data. AU, ED and CFF are Research Fellows of CNPq, and CFF acknowledges

623

the continuous support through research grants and scholarships for the "Metalogenênese de

624

Depósitos Associados ao Magmatismo Máfico-Ultramáfico" Research Group. The original

625

manuscript was greatly improved after constructive and helpful comments and suggestions by

626

Dr. Christian Tegner, Dr. Stephen J. Barnes and an anonymous reviewer. We thank Regional

627

Editor Dr. Reinhardt A. Fuck for handling the manuscript.

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FIGURE CAPTIONS

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Fig. 1 - Localization of the Riacho do Pontal Orogen considering the São Francisco Craton

922

and the Borborema Province. Craton outline after Caxito (2013), modified from Alkmim

923

(2004).

924 925 926

AC C

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Fig. 2 – Geological map of the Riacho do Pontal Orogen (Simplified from Caxito, 2013).

927 928

Fig.3 - Geological map of Brejo Seco Complex (BSC) area.

929

38

ACCEPTED MANUSCRIPT 930

Fig. 4 - Stratigraphy of the BSC and cumulus minerals.

931

Fig 5 – Field and petrographic aspects of rocks from the BSC. a) View towards north with the

933

Bacamarte hill in the background and the contact between Ultramafic Zone (UZ) and

934

Transitional Mafic Zone (TMZ); b) troctolite with a thin layer of leucotroctolite; c) layered

935

gabbro; d) ilm-mag gabbro; e) adcumulate dunite consisting of cumulus olivine (extensively

936

replaced by serpentine) and chromite (euhedral opaque crystals); f) mesocumulate dunite

937

consisting of cumulus olivine pseudomorphs (serpentine) and intercumulus minerals replaced

938

by a chlorite-rich fine-grained aggregate; g) intercumulus pyroxene in troctolite (Ol + Pl

939

cumulate); h) igneous lamination in olivine gabbro (Ol + Pl + Cpx cumulate).

SC

RI PT

932

940

Fig. 6 - Photomicrographies of sulfides from the mineralized zone. a) Fine-grained aggregate

942

of sulfides interstitial to olivine pseudomorphs (Ol); b) pentlandite (Pn) crystal partially

943

altered along fractures to violarite.; c) pyrrhotite (Po) with flame-type exsolutions of Pn; d)

944

sulfide bleb consisting of Pn, Po and Ccp.

M AN U

941

945

Fig. 7- Geochemical characteristics of the mineralized zone a) Compositional variation of Ni

947

(ppm), Cu (ppm), Au (ppb) and Pt+Pd (ppb) throughout the stratigraphic interval that hosts

948

the mineralized zone. Data from borehole FD003 (unpublished exploration reports from Vale

949

S.A.); b) Plot of Cu vs. Ni for the mineralized interval (same data as referred to in Fig. 7a).

950

Trend-line corresponds to the linear correlation for Cu-Ni. See text for further explanations; c)

951

Primitive Mantle-normalized PGE + Au values. Normalization values after Sun and

952

McDonough (1995).

EP

AC C

953

TE D

946

954

Fig. 8 - a) Compositional variations of olivine in borehole FDS-003; b) Fo-Ni plot for olivine

955

in borehole FDS-003; c) Fo-Ni plot for olivine in troctolite from the LMZ and TMZ. See Fig.

956

2 for location of borehole FDS-003 and Table 2 of the Supplementary Material for

957

representative analyses of olivine.

958

39

ACCEPTED MANUSCRIPT 959

Fig. 9 - Plot of MgO content versus major oxides and selected minor elements for rocks of the

960

BSC. See Table 3 of the Supplementary Material for chemical analyses.

961

troctolite;

-gabbro;

- ilm-mag gabbro and

- Dunite;

-

- anorthosite.

962

Fig. 10 - Primitive Mantle – normalized incompatible element spidergrams for samples of the

964

BSC. Normalization values after Sun and McDonough (1989).

965

RI PT

963

Fig. 11 – Chondrite-normalized REE spidergrams for samples of the BSC. Normalization

967

values for the C1 Chondrite after Sun and McDonough (1989).

968

Fig. 12 - Sm-Nd isochron for samples of the BSC.

M AN U

969 970 971

SC

966

Fig. 13 – Variation of chemical proxies with stratigraphy in the BSC. See text for details.

TE D

972

Fig. 14 - Positioning of the Brejo Seco mafic-ultramafic layered intrusion relatively to other

974

rock assemblages of similar age related to the break-up of the Rodinia Supercontinent in the

975

São Francisco – Congo Craton and adjacent areas (see text for references). São Francisco –

976

Congo Craton outline is adapted from Pedrosa-Soares and Alkmim (2011).

AC C

EP

973

40

ACCEPTED MANUSCRIPT

Pernambuco Lineament

RPB

PMB

Rio Preto Belt

PAB

SB 48°

RPB- Riacho do Pontal Belt

São Francisco Craton Phanerozoic Cover Proterozoic Cover Cratonic Basement (> 1 Ga)

M AN U

Cities- BH- Belo Horizonte; SV- Salvador

SC

BH

Borborema Province SB- Sergipana Belt PAB- Pernambuco-Alagoas Block PMB- Poço Redondo-Marancó Block

RI PT

SV

Araçuaí Belt

Brasília Belt

12°

AC C

EP

TE D

Fig.1 - Localization of Riacho do Pontal Belt considering São Francisco Craton and Borborema Province.

ACCEPTED MANUSCRIPT

Ü

Pernambuco Shear Zone

Brejo Seco Complex Afrânio

Capitão Gervásio

Rajada

Brasiliano Granitoids Undivided

AC C

EP

Cra co

o inh ra d Sob am D

M AN U ncis

F ra

São

TE D

Casa Nova Group Central Zone Figure 3 Monte Orebe Complex Internal Zone Brejo Seco Complex Paulistana/Santa Filomena Complex- undivided Morro Branco Complex Afeição Suite

t on

External Zone

SC

São Raimundo Nonato

RI PT

Pa rn

aí ba

B

as in

Paulistana

Petrolina Juazeiro 50 Km

ACCEPTED MANUSCRIPT 41°58'30"W

41°57'0"W

Legend

Ü

Phanerozoic Cover eluvio-colluvial cover - ECC

8°27'0"S

¹

PB

¹

58

50

(

( 37

ß

(

A

61

(

(

(

FDS-004

(

LMZ

ECC (

(

(

(

Brejo Seco Complex

FR-007

(

A

Ì

(

¹ ¹ ¹ 75

¹ 65

(

mFDS-003

(

(

70

(

60

(

(

ß 8°30'0"S

350

(

B

¹ 70

10

41°58'30"W

N

(

(

¹ß 40

(

40

EP

40

(

¹ 75

(

(

AC C

¹

(

(

( (

(

Section -AB

UZ

TMZ

UA

(

(

( (

( (

(

¹ « (

0,5

41°57'0"W

UB UB

0,5

Kilometre

S

RG

Kilometre

Transitional Mafic Zone - TMZ Ultramafic Zone - UZ

Lower Mafic Zone- LMZ

Geological and Cartographic Conventions

(

MBC LMZ

Upper Mafic Zone - UMZ

Basement

«

(

UMZa

8°28'30"Stroctolite

40

(

(

gabbro and minor dunite

Morro Branco Complex - MBC

65

UMZb

(

UMZb

dunite and minor troctolite

UMZa

¹

(

( 42

70

(

MBC

(

75

70

TE D

(

¹

70

( (

(

ß

70

85

85

ß

(

¹ « ¹« ¹ « (

¹

60

(

8°28'30"S

(

(

ilmenite-magnetite gabbro

troctolite

TMZ

(

RG

(

45

M AN U

UZ

( (

Neoproterozoic

Rajada Granite - RG

W

m

(

Parnaíba Basin - BP

8°27'0"S

CMB MBC

35

¹

Ü

¹

(

SC

(

RI PT

MBC

(

8°30'0"S

(

Lineament

Road

Creek

Metamorphic Foliation Igneous Layering Compressional Shear Zone Mineral Lineation

Ì

Nickel Lateritic Mine

A

Pilot Plant Drill Hole

A

B Geological Section

Ol Ch r Pl Cp x Ilm Ap +Ma t g

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Morro Branco C.

TE D

UMZb

TMZ

UZ

500 m

AC C

EP

UMZa

LMZ

Rajada Granite

ACCEPTED MANUSCRIPT

a)

b)

Bacamarte Hill

UZ

RI PT

TMZ

d)

SC

c)

e)

M AN U

So

f)

Chr

TE D

Ol

Ol (Srp)

Chr

g)

1.0 mm

h) Pl

Pl

AC C

Cpx

EP

1.0 mm

Ol Cpx

Ol 0.5 mm

0.25 mm

ACCEPTED MANUSCRIPT

b)

a)

Pn

Ccp sulphides

Ol

0.4 mm

c)

RI PT

violarite 0.04 mm

d)

Po

SC

Pn Pn Po

M AN U

Ccp

AC C

EP

TE D

0.04 mm

0.03 mm

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

a

(m)

FDS-003

RI PT

6

TMZ

5

Ni-Cu disseminated sulfide

4 3 2 1

80

M AN U

SC

ZU

85

90

0

1000

Fo (%) Ni (ppm)

4000

FDS-003

olivine in dunite

1 3000

3000

Ni (ppm)

olivine in troctolite

TE D

b

2000

5

3

2000

4

2

EP

1000

AC C

95

c

Ni (ppm)

4000

93

6

91

89

Highly depleted olivine

87

85

83

81

79

77

75

Fo (%) Troctolite

olivine in the LMZ olivine in the TMZ

3000

FR-007 2000

FRP-118

1000

6

95

93

91

89

87

85

83

Fo (%)

81

79

77

75

4000

ACCEPTED MANUSCRIPT

Al2O3 (wt.%)

47

Pl

45 43 41

Ol

10

0 20

40

60

MgO (wt.%)

20

NaO+K2O (wt%)

40

60

MgO (wt.%)

5 4

M AN U

15

5

Ol

6

PL

10

CPX

0

e

Cpx

20

CaO (wt.%)

15

37 25

f

3

Pl

2 1

Ol

0 0

20

40

5 Pl

0 0

20

AC C

d

4 3 2 Cpx

10

20

30

40

MgO (wt.%)

7000

a

Cpx

5000

Cr (ppm)

3000 2500

Ol

1500

50

b

6000

3500

2000

50

5

0

60

FDS-004/1

4000

40

0

40

MgO (wt%)

4500

30

MgO (wt%)

1

EP

CPX

20

Ilm (53 % TiO2)

6

Ol

10

10

7

TE D

15

0

c

Mag (64 % Fe2O3)

20

0

60

MgO (wt.%)

25

Fe2O3 (wt%)

20

5 0

Ni (ppm)

25

39

TiO2 (wt.%)

SiO2 (wt.%)

49

h

Pl

30

RI PT

51

35

g

CPX

SC

53

4000 3000 2000

1000

1000

500 0

0

0

20

40

MgO (wt%)

60

0

10

20

30

MgO (wt%)

40

50

ACCEPTED MANUSCRIPT





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ACCEPTED MANUSCRIPT Isochron

0.5130

0.5128

Initial

0.5122 0.12

Age = 903±20 Ma =0.511512±0.000029 MSWD = 0.18 εNd(T)= +0.8

143Nd/144Nd

0.16

0.20

0.24

SC

0.5124

RI PT

0.5126

EP

TE D

M AN U

147Sm/144Nd

AC C

143Nd/144Nd

n= 4

!



















SC

































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$

TE D

"

EP



AC C

RI PT

ACCEPTED MANUSCRIPT

%

&

























Sergipana belt ACCEPTEDelt MANUSCRIPT

Àfrica

São Francisco-Congo Craton

b al

ho

do

AFRICA

8

nt Po5

ac

Ri

Congo Craton

4

BRAZIL

São Francisco Craton

South American

6

Phanerozoic Covers

2

Tonian Magmatism 1- Amphibolites (~959 Ma)

3

2- Salto da Divida Suite (875±9 Ma)

V

3- Mayumbian and Zadinian Group (~912-920)

7

as Br

L

ília

100 Km

be lt

5- Brejo Seco Complex (~900 Ma)

1

6- Comba-Sembe-Quesso Dykes swarm (~950Ma)

SC

4- Dykes Swarm (~906-925 Ma)

RI PT

C

Araçuaí- Congo Oeste orogen

Layered Intrusions

M AN U

7- Ipanema Complex (1104±78 Ma)

AC C

EP

TE D

8- Canindé Complex (701±8 Ma)

ACCEPTED MANUSCRIPT

EP

TE D

M AN U

SC

RI PT

The BSC is a mafic-ultramafic intrusion in a continental extension setting Crystallization of the BSC occurred circa 900 Ma ago The BSC presents variable interaction of mantle-derived magma with older continental crust Ni-Cu-PGE mineralization detected in the BSC Benchmark for Rodinia’s break up in the northeastern South American platform

AC C

• • • • •