Chloritites of the Tocantins Group, Araguaia fold belt, central-northern Brazil: Vestiges of basaltic magmatism and metallogenetic implications

Journal of South American Earth Sciences 69 (2016) 171e193

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Chloritites of the Tocantins Group, Araguaia fold belt, central-northern Brazil: Vestiges of basaltic magmatism and metallogenetic implications Basile Kotschoubey, Raimundo Netuno Villas*, Benevides Aires
, P. O. Box 8608, 66.075-110, Bel

, Brazil Graduate Program in Geology and Geochemistry, Federal University of Para em, Para

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 July 2015 Received in revised form 29 March 2016 Accepted 6 April 2016 Available online 8 April 2016

~es Velho, Juarina, Morro Grande, Morro do Chloritites from different localities (Arapoema, Couto Magalha Jabuti, Morro do Pau Ferrado, Morro do Salto, Serra do Jacu, Serra do Quatipuru, Serra do Tapa, Serrinha) of the Araguaia fold belt, Tocantins geotectonic province, central-northern Brazil, have been investigated. Based on field work and petrographic, diffractometric, geochemical and mineral chemistry data, these rocks, commonly associated with metacherts and banded iron formations, have been interpreted as products of ocean-floor exhalative-hydrothermal activity on MORB basalts. Distribution patterns of rare earth elements and diagrams of relatively immobile components in the hydrothermal environment highlight not only the genetic link between the chloritites and the basaltic rocks that occur in the region (Serra do Tapa and Morro do Agostinho), but also some peculiar characteristics of the submarine environment. The rock association and anomalous contents of Cu, Zn, Ni, As, and Au are suggestive that the region was favorable to the formation of volcanogenic massive sulfide deposits, what makes it a potential target for mineral exploration programs. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Araguaia fold belt Chloritite Exhalative-hydrothermal alteration Ocean-floor environment Rare earth elements

1. Introduction Rocks composed dominantly of chlorite, associated with metacherts and iron formations, occur in the mid-western portion of the Araguaia belt, where the Tocantins Group is largely exposed. These rocks make up lithological associations that are distinct from the regional, essentially siliciclastic metasedimentary formations (meta-graywackes, meta-arkoses, chlorite-muscovite-albite schists, phyllites, slates), which form also part of that group. Serpentinized or talcified ultramafic lenticular bodies commonly are present in these associations. Originally termed chlorite-schists, these chlorite-rich rocks have been named chloritites since the early 1990s (Kotschoubey and Hieronymus, 1990) and assigned to the Pequizeiro Formation, a subdivision of the Tocantins Group, and considered to be of plutono-volcano-sedimentary nature (Abreu, 1978). In the ~es region, they were interPequizeiro-Araguacema-Couto Magalha preted as product of intense magnesian metasomatism experienced by pelitic rocks at the contact with intrusive ultramafic

* Corresponding author. E-mail address: [email protected] (R.N. Villas). http://dx.doi.org/10.1016/j.jsames.2016.04.001 0895-9811/© 2016 Elsevier Ltd. All rights reserved.

bodies (Gorayeb, 1981, 1989). Preliminary investigations in the ~es Velho areas (Tocantins state) led Serrinha and Couto Magalha Kotschoubey and Hieronymus (1990) and Aires and Kotschoubey (1994) to suggest a genetic relationship between chloritites, metacherts and iron formations, and to conclude that the chloritites were most likely products of submarine hydrothermal-exhalative alteration of mafic volcanic rocks. The occurrence of serpentinized ultramafic rocks and pillow basalts that represent dismembered ophiolite sequences is well documented in the westernmost part of the Araguaia belt ~o and Nilson, 2001a; Kotschoubey (Kotschoubey et al., 1996; Paixa ~o et al., 2008; Miyagawa and Gorayeb, and Hieronymus, 2005; Paixa 2013). In the central-east zones of this belt, in the Tocantins Group domain, ultramafic bodies have been also identified (talc-schists and to a lesser extent serpentinites), but basalts are lacking. Further east, in the Estrondo Group domain, occur amphibolites subordinately in relation to the regional metasedimentary rocks, though systematically associated with iron formations. Instead of basalts, chloritites are widespread in the central-east zones of the Araguaia belt commonly intercalated with ultramafic and chemical sedimentary rocks. In this paper, the nature and origin of the Araguaia chloritites

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and the associated iron and siliceous formations are investigated. The unusual nature of the chloritites led to the hypothesis they may have a direct relationship with the mafic submarine volcanism, which is recorded in the western part of the belt. This hypothesis gained further credence considering the fact that the chloritites are commonly associated with ultramafites, and that cherts and iron formations occur in the same geological setting, discounting the less intense tectonics in the western part. It is also intended to assess the potential for the formation of mineral deposits in a region where exhalative activity seems to have been significant. 2. A brief overview on chloritites Chloritites generally refer to chlorite-rich rocks that result from intense hydrothermal alteration of both mafic and felsic volcanites in exhalative submarine environments and represent the innermost part of the alteration zone that underlies massive to semimassive sulfides, particularly the volcanogenic type (VMS). Toward the innermost part of the alteration conduit, there is a gradual replacement of the seafloor volcanic rocks by chlorite. The most advanced stage of chloritization is characterized almost exclusively by chlorite, accompanied or not by subordinate quartz (Large, 1975). Usually, this scenario also comprises cherts, containing or not Fe or Mn oxides, in addition to distal deposits that consist of iron and/or manganese formations and silicate exhalites (Large, 1975; Schermerhorn, 1978; McQueen, 1989; Billaud and Leblanc, 1990). Intense chloritization of seafloor volcanites has been described in many VMS deposits (Iwao, 1970; Schermerhorn, 1978; Bernard et al., 1988; Liaghat and MacLean, 1995; Zang and Fyfe,
nchez-Espan ~ a et al., 2000; Ruiz et al., 2002; Nimis et al., 1995; Sa 2004, 2008; Yoshitake et al., 2009; among others). Although pure chloritites are rarely described, experimental studies show that, in submarine environment under high seawater/ rock ratio and high temperature, seafloor basalts tend to evolve into virtually monominerallic rocks (Mottl and Holland, 1978). The association chlorite þ subordinate quartz þ Ti-minerals (titanite, rutile) would be the ultimate expression of submarine hydrothermal alteration, whatever the original volcanic rock (Mottl, 1983). Oceanographic explorations and studies of recent ophiolites have shown that various clay minerals and chlorite, locally in considerable amounts, are the result of the interaction of seafloor volcanic rocks with either seawater or submarine hydrothermal fluids (Humphris and Thompson, 1978; Mottl and Holland, 1978; Schiffman and Staudigel, 1995; Sturz et al., 1998; Lackschewitz et al., 2004). Basalts, gabbros and diabases totally chloritized or transformed into chlorite-quartz-titanite associations, with very subordinate sulfides, epidote and actinolite, have been found in several exhalative fields (Miyashiro et al., 1979; Teagle et al., 1998; Honnorez et al., 1998; Humphris and Tivey, 2000; Teagle and Alt, 2004a,b). Broadly speaking, chloritization would be the dominant, perhaps unique, form of submarine alteration at great depths and temperatures above 300  C (Miyashiro et al., 1979; Bettison and Schiffman, 1988; Erzinger, 1989). Strong enrichment in smectite, corrensite or chlorite occurs also in siliciclastic sedimentary formations e mainly in its clay fraction e that have been affected by submarine hydrothermal alteration (Buatier et al., 1995, 2001; Lackschewitz et al., 2000a,b). German et al. (1995) and Pierret et al. (2000), among others, emphasized the importance of metasomatic reactions between hydrothermal sediments and seawater. Furthermore, according to Buatier et al. (1995, 2001), Lackschewitz et al. (2000a,b) and Nimis et al. (2004), chlorite may be directly precipitated together with magnesian or iron-magnesian clay minerals, talc, carbonate, quartz and sulfides at discharge areas, as a result of the reaction between hydrothermal fluids and seawater. It is, in this case, a typical

exhalative deposit (Schermerhorn, 1978; McLeod and Stanton, 1984; McQueen, 1989; Leblanc and Billaud, 1990; Haïmeur, 1998) The common association of chlorite with talc and ironmagnesian clay minerals in environments marked by submarine exhalative activity suggests, in certain situations, a similar origin. According to McLeod and Stanton (1984), in these environments chlorite may precipitate in its definitive form or be formed by transformation of pre-existing magnesian minerals such as sepiolite and magnesian montmorillonite, among others. In ultramafic rocks, the formation of chlorite requires lithotypes with original Al-bearing minerals as clinopyroxene, feldspar, spinel or garnet. Therefore, in depleted mantle rocks (harzburgites and dunites), whose Al2O3 content is low, chloritization is not normally observed or is insignificant; instead, serpentinization or talcification are commonly observed. Only the alteration of non-depleted ultramafites (sub-continental or deep-derived mid-oceanic ridge lherzolites) can lead to the formation of chlorite and, even so, in amounts that are normally lower than those of serpentine and/or talc, which are the characteristic alteration minerals of these rocks. On the other hand, chloritites normally comprise together with actinolitites, biotitites or phlogopitites the border zone (black wall) of alpine-type ultramafic bodies, at the contact with Al-rich rocks, such as granites, gneisses, mica schists and pelitic sedimentary rocks. They represent, in this case, late tectonic-induced metasomatic products. 3. Materials and methods Over two-hundred outcrop samples of chloritites and ultramafites were collected in the study area for analytical work. Following petrographic studies, a number of samples were analysed (powder method) with a PANalytical diffratometer (X0 Pert Pro MPD model/PW 3040/60) housed in the Geosciences Institute of the Federal University of Par
a (IG-UFPA) and equipped with a goniometer PW3050/60 and a ceramic X-ray tube of Cu anode (Ka1 ¼ 1,540598 Å), model PW3373/00. Data were treated and interpreted with aid of the software X'PertHighScore Plus, 2003 version. The petrographic work was complemented by scanning electron microscopy (SEM/EDS) in the laboratories of the Emilio Goeldi Museum (MPEG) and IG e UFPA using a Zeiss equipment (LEO-1430 model) and carbon metalized polished thin sections. The operational conditions of the analyses were as follows: accelerating voltage of 20 kV; electron beam current of 90 mA; working distance of 15 mm; spot size of 350e400 m; and counting time of 30 s. Chemical analyses for major, rare earth and selected trace elements of fifty-three samples were performed in laboratories of IG
m, Brazil), Lakefield-Geosol (Belo Horizonte, Brazil) and UFPA (Bele Acme (Vancouver, Canada), using X-ray fluorescence (fusion disk) and Inductively Coupled Plasma Emission/Mass Spectroscopy (ICPES and ICP-MS) techniques. Loss on ignition was determined by calcination at 1000  C. Forty samples represent chloritites (Ara~es Velho, Juarina, Morro Grande, Morro do poema, Couto Magalha Jabuti, Morro do Pau Ferrado, Morro do Salto, Serra do Jacu, Serra do Quatipuru, Serra do Tapa and Serrinha), five chromitites (Morro Grande), three serpentinites (Arapoema, Morro Grande and Serra do Quatipuru), three basalts (Morro do Agostinho), one talc-schist (Serra do Tapa) and one serpentinized olivine gabbro (Serra do Quatipuru). A complete description of the analytical methods is available in the Acmelab and Geosol home pages (www.acme.com and www.geosol.com.br, respectively). Despite the use of the three different laboratory facilities, the results turned out to be consistent, except for the very low Pb contents in the WL sample set. Electron microprobe analyses on carbon metalized polished thin sections were carried out in laboratories of the Pierre and Marie

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Curie University (Paris VI), France, using a Cameca SX 50 spectrometer. Both synthetic and natural mineral standards were used during the runs that were done at accelerating voltage of 15 kV and varying electron beam current.

4. Geological setting In the Neoproterozoic Araguaia fold belt (Fig. 1) dominate rocks of the Baixo Araguaia Supergroup, which is composed of the Estrondo (base) and Tocantins (top) groups (Abreu, 1978). The Estrondo Group occupies the eastern portion of the belt and is divided into the lower Morro do Campo Formation (mainly quartzites interlayered with subordinate meta-conglomerates, amphibolites, iron formations, minor mica schists, garnet
Formation (biotite staurolite schists) and the upper Xambioa schists, staurolite-biotite schists, granadiferous schists, minor amphibolites). The western portion of the belt consists essentially of rocks that belong to the Tocantins Group, a thick metasedimentary pile of dominantly siliciclastic nature, affected by intense folding, faulting and low-angle thrust. As a rule, the Tocantins Group rocks show vergence from W to NW and increasing metamorphic grade from ~es and Pequizeiro Formations west to east. The Couto Magalha represent the western and eastern portions of that group, respec~es Formation comprises turbidites, artively. The Couto Magalha koses, graywackes, unmetamorphosed to weakly metamorphosed limestones, as well as carbonaceous slates and phyllites. The Pequizeiro Formation consists of phyllites, feldspathic schists, Carich schists, carbonaceous shales, meta-limestones and metaarkoses, marking the transition from low to high greenschist facies. Metacherts and banded iron formations are interspersed

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with siliciclastic metasedimentary rocks of both formations. In the Serra do Tapa and, further south, near the Morro do Agostinho in the neighborhood of Araguacema, outcrop MORB pillow basalts and basaltic breccias (Souza and Moreton, 1995; Kotschoubey et al., ~o and Nilson, 2001a,b; Paix~ 1996; Paixa ao and Nilson, 2002; ~o et al., 2008; Miyagawa and Kotschoubey et al., 2005; Paixa Gorayeb, 2013). The Tocantins Group rocks host tabular, submeridian ultramafic bodies along low angle faults and shear zones (Gorayeb, 1981, 1989). These bodies are composed of dunites and harzburgites, which, despite the intense serpentinization or talcification, commonly exhibit their original texture. Locally, thin dikes and sills of pyroxenite and olivine gabbro, partially to completely altered, crosscut these ultramafites. In Serra do Quatipuru and Morro Grande, these ultramafites host podiform chromite ~o and Nilson, deposits (Kotschoubey and Hieronymus, 1996; Paixa ~o et al., 2008; Paixa ~o and 2002; Kotschoubey et al., 2005; Paixa Gorayeb, 2014). Most likely, the belt started evolving with the opening of the proto-oceanic Araguaia basin in Neoproterozoic times. In the course of this initial stage of oceanization, lithospheric mantlederived peridotites were emplaced, being followed by the eruption of basaltic magma and the subsequent formation of silica-rich ferruginous rocks at great depths. The deposition of the Baixo Araguaia Supergroup rocks came next and it was followed, in the eastern part of the basin, by the intrusion of gabbroic bodies of the Xambica Intrusive Suite (Gorayeb et al., 2004). Later on, the tectonic inversion of the basin provided not only the upward displacement of the ophiolite assemblage along thrust surfaces, but also its intermingling with the supracrustal rocks (Alvarenga et al., 2000; Kotschoubey et al., 2005; Miyagawa and Gorayeb, 2013). Concomitantly the rocks were metamorphosed with intensity that

Fig. 1. Geological map of the Araguaia fold belt (on the right) and location map of the localities studied, most of them occurring in the southeast quadrant (on the left). Modified from Alvarenga et al. (2000).

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increased from west to east (anchi-metamorphism to amphibolite facies). Syn-to late-collisional granitoids intruded the Araguaia belt rocks spanning from 680 to 545 Ma, all related to the meta~o and Kotschoubey, 1996; morphism of the Estrondo Group (Lamara Poinsignon, 2009; Poinsignon et al., 2010). The ultramafic bodies and pillow basalts correspond to remains of a poorly evolved oceanic crust that was dismembered ~o and Nilson, 2002), during (Kotschoubey et al., 2005) or not (Paixa the closure of the basin. In turn, the structuring of the Araguaia belt may have taken place by the end of the Brasiliano orogenic cycle (Kotschoubey et al., 1996; Kotschoubey and Hieronymus, 1996; Alvarenga et al., 2000; Paix~ ao and Nilson, 2001a, b; Kotschoubey ~o et al., 2008). et al., 2005; Paixa 5. The chloritites in the Araguaia belt 5.1. Field relations In the investigated region, the rocks are in general strongly deformed, having responded heterogeneously to the shear stress to which they were submitted. Rheologically more competent, most silica-rich ferruginous rocks preserved their thickness, despite the intense breaking and dismemberment experienced during the late stages of the Brasiliano cycle when they formed hm-to km-long, m to dam-thick lens-shaped bodies or slices within the Tocantins Group. Thicker and longer bodies are only found in Serra do Tapa, Morro do Agostinho and Serra do Quatipuru where these rocks are little deformed and practically non-metamorphosed (in a few cases they even exhibit biogenic microstructures). In turn, the much less competent chloritites show a more random distribution, since their layers commonly taper along the strike disappearing and then reappearing further up. Commonly several slices of the silica-rich ferruginous rocks are stacked up and separated by m to damthick layers of chloritite, the whole pile displaying similar structural features, particularly the same foliation. These “sandwiches” are well represented in Pau Ferrado (Fig. 2A), Morro Grande (Fig. 2B), Serrinha (Fig. 2C), Couto Magalh~ aes Velho and Juarina. The silica-rich ferruginous rocks are more resistant to weathering and erosion, and sustain, elongated hills that rise tens up to 130 m above the surrounding flattened surface. These hills extend from 100 m to several km and present, except in a few localities, sub-meridian orientation. Outside those relatively restricted areas that stand out in the intensively denuded regional topography, good outcrops are rare, what hinders the assessment of the lateral extent of the chloritites. In fact, chloritites might have a larger occurrence, but they seem to be hidden under the cover of soil, regolith and fragments of altered rock and quartz. Available geological maps (Gorayeb, 1981, 1989; CPRM, 1994; Alvarenga et al., 2000) suggest that those ribbon-like hills denote the presence of adjacent low angle, Eto SE-dipping thrust faults. In this geomorphological-structural framework, the chloritites and associated siliceous rocks would belong to one or more stratigraphic units that constitute the basal portion of the Tocantins Group and are exposed primarily in upthrusting zones. The systematic association of silica-rich ferruginous rocks with chloritites or amphibolites in the Araguaia belt, clearly recognized even in the Estrondo Group (Sousa and Kotschoubey, 2005), is highly indicative that the former are direct products of sedimentary processes. Despite deformation, banding is generally remarkable. Some specimen exhibit an alternation of light and dark cryptocrystalline micro-bands, while others are more massive with colors that vary from whitish grey to black. Many of them reveal microfolds and develop a distinct foliation. These silica-rich ferruginous rocks have been interpreted as meta-exhalites (metacherts and iron formations) and are distinct from the late, locally thick, massive

milky quartz veins that commonly crosscut the metacherts and line off faults and other discontinuities (Kotschoubey et al., in preparation). The abundance of chloritites in relation to the meta-exhalites is very variable. In some localities, these rocks occur in comparable proportions, but metacherts are dominant in others. As a rule, the iron formations are subordinate to the metacherts. Meta-limestone was not found in the region, but in western Serrinha area where it occurs as a thick magnesian mica- and sulfide-bearing lens sandwiched in the meta-exhalitedchloritite pile. The contacts between chloritites and other lithotypes are usually sharp. The nature of the interleaving of these rocks is not always clear due to deformation d mylonitization and transposition d that affected them and to the scarcity of good exposures. This interleaving could correspond to either a normal rock sequence of different compositions or an imbrication of rock slices caused by the dismemberment of the original layers, in response to the intense tangential tectonics that accounted for the structuring of the Araguaia belt. It is very likely that both situations exist. In spite of good exposures of ultramafites, pillow basalts, siliceous rocks and discrete iron formations, only a few chloritites were recognized in the Serra do Tapa and Serra do Quatipuru, where the tectonic regime was milder. In the Morro do Agostinho no chloritites were observed. On the other hand, abundant chloritites associated with meta-exhalites (commonly including iron formations) and, to a lesser extent, ultramafites (often talcified) ~o do Araguaia region, particularly in were identified in the Conceiça Serrinha, Couto Magalh~ aes Velho and Juarina. In the southernmost localities, closer to Araguacema, metacherts are dominant, but some expressive chloritites occur in Pau Ferrado, Salto, Jabuti, Morro Grande and Morro do Jacu. At last, it is worth reassuring that, from the tectonic standpoint, two different domains can be distinguished in the investigated region: one characterized by practically intact or little deformed siliceous formations and altered basalts preserving many primary features; the other by intensively deformed chloritites, metacherts and iron formations. 5.2. Petrography The chloritites are, in general, very fine grained-rocks that present incipient to strong foliation and dark green to olive green, locally grey, colors (Fig. 3AeD). They can also be massive or isotropic and coarse-grained. Veins of quartz or chlorite commonly occur conformably or crosscut the foliation planes. Sub-millimeter crystals of magnetite, titano-magnetite, rutile and ilmenite are usually scattered in the fine matrix. These rocks are composed almost exclusively of chlorite (Figs. 4A and 5A), although it is common some quartz and subordinate talc. In the Morro Grande chloritite, acicular tremoliteactinolite crystals occur immersed in the fine chlorite-rich matrix. Locally, this amphibole is very abundant or even dominant, as seen in Juarina where tremolitites and chlorite-tremolitites are associated with chloritites and iron formations. Similar rocks, enriched in tremolite-actinolite and associated with chloritites, and serpentinites, occur further east about 25 km west of Colinas do Tocantins town. The main accessory minerals are magnetite, titanomagnetite, ilmenite, rutile and apatite. Monazite, zircon and, to a lesser extent, xenotime were identified by SEM-EDS. Very locally, tourmaline is also present in both chloritites and tremolitites. Chlorite represents in general more than 90% of the rock volume, forming fine oriented flakes or lamellae. Less commonly, it develops aggregates (up to 0.5 mm) of randomly oriented lamellae. Quartz comes next in abundance. It displays fine anhedral grains, isolated or forming aggregates, dispersed regularly or randomly in

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Fig. 2. Schematic geological sections across the Pau Ferrado (A), Morro Grande (B) and Serrinha (C) areas of the Araguaia belt showing the contact relationships among the various rock units defined by thrust surfaces.

the chlorite-rich matrix (Fig. 4A and C), not exceeding 15 vol. %, except for samples from Quatipuru (>20 vol. %). Quartz occurs also in veinlets that transect locally the rock matrix (Fig. 4B). Talc shows similar mode of occurrence, but the aggregates tend to be fibroradial. Often it is a trace mineral, although in a very few samples it amounts to approximately 10 vol. %. Locally, at Serrinha, this phyllosilicate makes up a substantial part of the rock (~40 vol. %) in the form of aggregates interdigitated with the chlorite mass. Tremolite-actinolite is not common in the chloritites. When present (<5 vol. %), it exhibits acicular crystals in sheaf-like arrangement with conspicuous to very irregular, almost random, orientation (Fig. 4D). Some chloritite varieties show higher proportions of amphibole (up to 50 vol. %), thus being described as tremoliteactinolite chloritites. Normally magnetite, ilmenite, titanomagnetite and rutile constitute together 5% of the rock volume, exceptionally 10%, and are present as micro-porphyroblasts scattered in the chlorite-rich matrix (Fig. 4E). Subhedral to euhedral magnetite crystals appear partially to completely replaced by martite. Among the titaniferous minerals rutile is certainly the most abundant. It occurs in aggregates of fine acicular to prismatic crystals displaying the typical yellow internal reflections. These clustered tiny prismatic crystals are generally roughly oriented according to the host rock foliation. Locally, rutile replaces ilmenite crystals, still preserving their skeletal structures. Titano-magnetite

is rarer and occurs in prismatic crystals. Ilmenite is not common, being a residual mineral largely altered to rutile and Fe oxide. It occurs as fine euhedral blades that normally display skeletal features. Leucoxene is normally associated with rutile in a number of chloritites, in which it forms diffuse clouds around ilmenite remnants. Titanite is seldom observed and grows as fine anhedral crystals. Apatite is relatively common (<3 vol. %) and abundant in some samples (up to 5 vol. %). It occurs as fine rounded or elongated grains dispersed in the rock or as small clusters (Fig. 4F). More rarely, apatite develops fine prismatic crystals. Monazite, zircon and xenotime are very common trace minerals that occur as irregular to rounded grains finely dispersed in the chloritic mass. Monazite is locally abundant. This phosphate locally displays skeletal degradation features and alteration to a poorly-crystallized or amorphous product. Zircon is also quite common in the chloritites. Xenotime is rare. Tourmaline occurs as fine euhedral prismatic crystals diffusely clustered or as mm-thick beds or lenses with poorly defined edges. An unidentified mineral containing Th and U locally occurs in very fine-grained aggregates. At Serrinha, native gold was detected in a chloritite sample forming aggregates of very fine particles dispersed in the chlorite-rich matrix (Kotschoubey and Hieronymus, 1990). Within the chromitites hosted by serpentinite lenses, chloritites occur locally with a quite distinct composition, as observed in the

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Fig. 3. Photographs of chloritite hand samples from different localitites of the Araguaia belt, showing their monotonous aspect, except for the color variation (from dark to light green). A, B and D are finely foliated chloritites, whereas C is a less foliated tremolite-chloritite.

Morro Grande area. Their mineralogical assemblage includes residual Cr-spinel, fuchsite and occasionally Cr-dravite (Fig. 5B), in addition to chlorite. These chloritites are derived from ultramafic rocks and, hence, are not directly related to the chloritites focused in this study. However, being an important evidence of submarine hydrothermalism, the composition and genesis of these rocks are discussed below. 5.3. Mineral chemistry The chemical composition of chlorites from four chloritite ~es Velho samples from Serrinha (SER-4 and SER-5), Couto Magalha (BREJ2/2) and Juarina (FLCl) localities are shown in Table 1. In general, the data point out a homogeneous composition for samples SER4, SER5 and FLCl. Sample SER4/2, in relation to these three samples, is compositionally quite distinct in that it presents higher SiO2 (28.85e43.01 wt. %), Al2O3 (16.12e25.32 wt. %) and NiO (0.41e2.46 wt. %), and much lower FeO (13.77e18.71 wt. %) and MgO (11.09e16.44 wt. %) contents. In turn, sample BREJ2/2 records the lowest values for FeO (12.56 wt. %) and the highest values for MgO (21.74e24.32 wt. %) and Cr2O3 (0.10e0.66 wt. %). Its NiO values are comparable to those of sample SER4/2. The expressive Cr and Ni concentrations are suggestive that chlorite formed from hydrothermal fluids that interacted with ultramafic rocks, particularly in the Couto Magalh~ aes Velho area. The Fe/(Fe þ Mg) ratios range from 0.23 to 0.46, corresponding to clinochlore. Its high content of nickel (up to 2.84 wt. % NiO) is probably due to the isomorphous substitution of Mg by this element. The chromium content is usually less than 200 ppm Cr, while CaO, Na2O, K2O, TiO2 and P2O5 are normally below the

detection limits of the technique used. On the Ni vs Mg# diagram (Fig. 6A) most data of the Serrinha and Juarina chlorites cluster in a restricted field. A few points fall outside this field and show that higher Ni contents are not ~es Velho dependent on Mg#. In turn, the data of the Couto Magalha chlorite occupy a narrow vertical band, showing also that Ni increase may occur regardless of Mg#. Similar picture is observed on the Cr vs Mg# diagram (Fig. 6B), although all points representing both the Serrinha and Juarina chlorites cluster very close to the Mg# axis. On the Cr vs Ni diagram (Fig. 6C) the data of the Serrinha and Juarina chlorites are scattered within a short range slightly ~es Velho above the Ni axis, whereas those of the Couto Magalha chlorite are randomly distributed, suggesting that these metals might have been provided by more than one source rock. Talc reveals high total Fe content (Table 1) and NiO concentration of approximately 1 wt. %. Most punctual analyses did not detect chromium; when detected, the values did not exceed 500 ppm. The chemical data for magnetite, Ti-magnetite, ilmenite, rutile and apatite are in Table 2. Magnetite reveals rather high Cr contents (up to 1.5 wt. % Cr2O3), lower Ni contents (up to 0.3% wt. NiO) and notable concentrations of ZnO (up to 860 ppm). Titano-magnetite shows average TiO2 content of approximately 7.5% and local enrichment in Cr. Ilmenite is slightly manganesian (~1.0%e1.3% wt. MnO) and at the crystal edges reveals strong Fe depletion and enrichment in Ti, Mn, and Ni. Commonly, ilmenite is altered to slightly ferriferous rutile (between 0.4 and 0.8% wt. FeO). At Juarina, rutile records FeO contents of 3e3.5 wt. % (Table 2). Noteworthy are the anomalous contents of ZnO (0.7 wt. %) found in a few apatite crystals (Table 2).

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Fig. 4. Photomicrographs of chloritites and associated rock from the Araguaia fold belt. A) Sparse crystals of quartz (Qz), titanomagnetite (Ti-Mag) and rutile (Rt) immersed in a weakly foliated chlorite-rich matrix (Juarina locality). B) Quartz veinlet hosted by a fine-grained matrix mostly composed of quartz (Qz) and chlorite (Chl). Note a magnetite (Mag) crystal on the upper left corner of the photo. A microfault filled by titanomagnetite (Ti-Mag) transects both the veinlet and the matrix (Morro Grande locality). C) Deformed quartz (Qz) aggregate interspersed in a chlorite-rich matrix containing sparse magnetite (Mag) crystals (Serrinha locality). D) Moderately oriented columnar to acicular microporphyroblasts of tremolie-actinolite (Tr-Act) in a sheaf-like arrangement commonly found in tremolitites-actinolitites associated with chloritites (Morro Grande locality). E) Abundant fine crystals of rutile (Rt) and titanomagnetite (Ti-Mag), some oriented conformably to the rock foliation planes (Serrinha locality). F) Aggregate of sub-rounded apatite (Ap) crystals associated with magnetite (Mag) (Serrinha locality). Non-polarized light, except in D.

Chemical data of monazite and xenotime are not tabulated. The composition of monazite varies substantially, with a predominance of either Ce (up 30.8 wt. %) or La (up 37.5 wt. %). In all cases, Nd is an important constituent (11.9e15.9 wt. %). Its alteration product revealed notable contents of Al and Ca, and is probably a REE-rich Al-phosphate of the crandallite family. Xenotime shows high Y and heavy REE contents. 5.4. Bulk chemical composition Forty chloritite samples from the investigated localities were analysed for major, trace and rare earth elements (20 samples), and the results are in Tables 3 and 4. The silica content lies in general between 23% and 35 wt. %, but it can reach up to 64.2 wt. % (sample QUAT-12). The higher values of SiO2 refer to quartz-rich chloritites.

The alumina content is typically in the range of 13%e21 wt. %; a few values are below 13 wt. %, being characteristic of those samples particularly rich in quartz. Although the total Fe2O3 content varies from 5 to 37 wt. %, values above 25 wt. % and below 12 wt. % are uncommon. These extreme values correspond, in general, to samples very poor and very rich in MgO, respectively. As a rule, Fe2þ predominates strongly over Fe3þ. The concentrations of MgO are between 7% and 32 wt. %, the lower values referring to either Fe- or quartz-rich samples. The samples richer in MgO often contain subordinate talc. Most TiO2 contents are in the 0.5e1.5 wt. % range, yet some record 5 wt. %. When detected, Ca, Na and K concentrations are low, although in a few samples their values are expressive, especially with regard to the first two elements. P2O5 contents rarely exceed 1 wt. %. The highest concentrations correspond, in general, to relatively high Ca contents, indicating that apatite is

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Fig. 5. Diffractograms of two chloritite samples from Morro do Jabuti locality. WL2-04 (A) is representative of the Araguaia chloritites, being essentially composed of chlorite (Chl). WL2-03A is uncommon, the main peaks corresponding to chlorite (Chl), Cr-spinel (Spl) and fuchsite (M).

more abundant. The chemical composition of most samples clearly reflects their high chlorite content and is close to, with a few exceptions, the own composition of this mineral. It also reflects the variable contents of apatite and titaniferous and REE minerals. The REE contents vary over a large interval. Some samples afford SREE that exceed 600 ppm, with a maximum of 1315 ppm in the sample QUAT-12 (Table 4). REE values normalized to chondrite show moderate to strong enrichment of LREE relative to HREE and similar distribution patterns (Fig. 7A, B and D), except for those of the Salto, Morro Grande and Pau Ferrado samples which tend to be sub-horizontal (Fig. 7C). Moderate negative Eu and Ce anomalies are common, the latter more pronounced and in some cases quite prominent. Among the essentially immobile elements, only Zr and Y reach contents of several hundred ppm; all others show in general low concentrations. With regard to other trace elements, Y reveals a reasonable positive correlation with SREE. Nickel contents range from 165 ppm to about 1.7 wt. % (Brej3b), being common the values higher than 1 wt. %. Chromium is less abundant and only in some cases its content exceeds 1000 ppm, although locally reaches values much higher (almost 5000 ppm in the sample Brej3b). The zinc contents, generally in the order of a few hundred ppm, reach the maximum of 750 ppm also in sample Brej3b. The manganese contents typically vary from several hundred ppm to approximately 1500 ppm. Sample WL26-01, however, presents a content of almost 2800 ppm. The contents of Co, Cu and Pb range from a few ppm to several ten ppm. Exceptionally, Cu and Co exceed 300 ppm. Locally, there is a significant Cu enrichment, as observed in three chloritite samples (WL26-01, WL26-02 and WL27-01) from Morro Pau Ferrado, which reveal Cu concentrations between 900 and slightly above 2800 ppm. In this same locality, oxidized Cu ore fragments were found consisting mainly of malachite and goethite with important amounts of Mo (65 ppm), As (94 ppm), Ag (11 ppm), Au (40 ppb), Hg (4.4 ppm) and Se (>100 ppm). Vanadium content is normally relatively high, reaching slightly more than 400 ppm. 6. The origin of the chloritites 6.1. Geological setting of the chloritites and evidences for submarine hydrothermalism The basal part of the Tocantins Group, exposed in thrust fronts in the mid-west portion of the Araguaia belt, is composed of rock types other than the siliciclastic metasedimentary formations that

make up the remaining of this stratigraphic unit. In addition to chloritites, the nature of the associated rocks e metacherts and iron formations e suggests that significant hydrothermal activity occurred during the opening of the proto-ocean basin, which was the precursor of that belt. Mantle ultramafite slices e essentially harzburgites and dunites e, derived from the basin floor, are completely serpentinized or talcified (Kotschoubey et al., 1996, 2005). These ultramafic rocks contain locally high contents of Cr and especially of Ni (>1 wt. %), the latter occupying octahedral sites of silicate lattices. The dunitic matrix of the Morro Grande and Serra do Quatipuru chromitites is also wholly serpentinized. Chromite is normally enriched in Al, Mg and Fe, and shows no evidence of alteration, except for a local and discrete chloritization at the contact with the serpentine-rich matrix. More intensely altered chromitites can be locally found at Morro Grande, where these rocks have been transformed into spinel-bearing chloritites, with skeletal crystals of Cr-spinel immersed in a chlorite-rich matrix. This matrix was most likely generated by the recombination of Al, Mg, Fe and Cr released as chromite was partially altered or destroyed. Si and Mg would have come from the matrix original silicates, i.e. olivine, orthopyroxene, serpentine (Kotschoubey and Hieronymus, 1996). Commonly, fuchsite occurs along micro-fractures that transect both the chlorite-rich matrix and the residual Cr-spinel crystals, while euhedral Cr-dravite crystals are dispersed in the rock, indicating that K, B and Na were also incorporated into those ultramafites during the alteration process, what is confirmed by chemical analyses (Table 5). The entry of these elements suggests interaction with seawater at high temperatures and high water/rock ratios. On the other hand, serpentinized chromitites and chloritized chromitites, bearing or not mica and tourmaline, differ somewhat as far as the trace-element contents are concerned. The concentrations of Zn (up to 8600 ppm) and Mn (up to 2.1 wt. %) are higher in serpentinized chromitites than in chloritized chromitites. Ni and V contents are also high and variable, but no particular difference distinguishes one chromitite type from the other. The REE totals (SREE) are always very low and similar to those of other ultramafites of the region, although the chloritized chromitites tend to be locally more enriched in these elements. More importantly, it is the fact that the REE distribution pattern in serpentinized or chloritized chromitites is distinct from those of serpentinites from the same area, and clearly shows that there was a close interaction with seawater. Indeed, the REE profiles display a fairly pronounced Ce anomaly and a well-defined Eu anomaly, either positive or negative (Fig. 7),

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Table 1 ~es Velho (BREJ) and Juarina (FLCl) localities and talc from Juarina (FLTl) locality Chemical composition (punctual analyses) of chlorite (wt. %) from Serrinha (SER), Couto Magalha (last nine columns). na ¼ not analysed; nd ¼ not detected. Samples/Oxides

SER4-1

SER4-2

SER4-3

SER4-4

SiO2 28.21 28.42 28.71 28.82 TiO2 0.02 0.03 0.04 0.03 Al2O3 17.74 17.97 18.27 17.83 20.48 20.23 20.39 20.01 FeOt MgO 19.93 19.97 19.73 20.32 MnO 0.06 0.03 0.13 0.10 CaO nd 0.04 0.01 nd Na2O nd nd nd nd K2O 0.02 0.01 0.01 0.03 NiO 0.37 0.28 0.31 0.27 ZnO 0.02 0.04 0.02 0.04 P2O5 0.04 0.05 nd nd Cr2O3 0.02 0.01 0.01 0.01 Total 86.90 87.08 87.63 87.43 Number of cations on the basis of 28 oxygen atoms Si 5.845 5.860 5.881 5.909 Ti 0.003 0.004 0.006 0.004 iv Al 2.155 2.140 2.119 2.091 vi Al 2.177 2.119 2.293 2.218 Fe 3.549 3.488 3.493 3.431 Mg 6.157 6.140 6.027 6.212 Mn 0.010 0.005 0.022 0.017 Ca 0.000 0.006 0.002 0.000 Na 0.000 0.000 0.000 0.000 K 0.005 0.002 0.002 0.008 Ni 0.061 0.046 0.051 0.045 Zn 0.003 0.006 0.003 0.006 P 0.001 0.008 0.000 0.000 Cr 0.001 0.001 0.001 0.001 Mg# 0.634 0.638 0.633 0.644 Samples/Oxides

SER5-5

SiO2 28.27 TiO2 0.06 Al2O3 18.14 FeOt 19.73 MgO 19.31 MnO 0.05 CaO 0.06 Na2O nd K2O 0.18 NiO 0.25 ZnO 0.05 P2O5 0.03 Cr2O3 0.03 Total 86.16 Number of cations on the Si 5.881 Ti 0.009 Aliv 2.119 Alvi 2.329 Fe 3.432 Mg 5.989 Mn 0.009 Ca 0.013 Na 0.000 K 0.048 Ni 0.042 Zn 0.008 P 0.005 Cr 0.005 Mg# 0.636

SER5-6

SER5-7

27.56 28.02 0.02 0.05 18.37 18.48 19.98 19.82 19.71 19.52 0.06 0.07 0.07 0.03 nd nd 0.08 0.08 0.22 0.29 nd 0.04 0.06 0.10 0.05 nd 86.18 86.50 basis of 28 oxygen 5.751 5.805 0.003 0.008 2.249 2.195 2.270 2.318 3.487 3.434 6.133 6.030 0.006 0.012 0.016 0.006 0.000 0.000 0.021 0.021 0.037 0.048 0.000 0.006 0.006 0.017 0.008 0.000 0.637 0.637

SER5-8 28.11 0.01 17.94 20.12 19.45 0.12 0.03 nd 0.16 0.30 0.06 0.06 0.03 86.39 atoms 5.857 0.001 2.143 2.263 3.506 6.042 0.021 0.007 0.000 0.042 0.05 0.,009 0.002 0.005 0.633

SER4-5

SER4-6

SER4/2-1

SER4/2-2

SER4/2-3

SER4/2-4

SER5-1

SER5-2

SER5-3

SER5-4

27.76 0.06 19.31 22.13 18.81 0.14 nd na nd 0.31 na nd nd 88.52

28.00 0.20 18.04 20.42 19.04 0.15 0.05 na 0.03 0.19 na 0.03 nd 86.15

32.56 0.03 25.32 16.84 11.09 0.04 0.03 na na 0.41 nd na 0.03 86.36

28.85 0.12 21.35 18.71 16.44 0.11 0.04 na na 0.88 nd na 0.03 86.53

43.01 nd 16.12 13.77 11.21 0.08 0.17 na na 2.01 nd na 0.02 86.39

31.45 0.07 22.09 17.02 13.98 0.08 0.06 na na 2.46 nd na nd 87.21

27.97 0.08 17.99 22.48 18.53 0.06 0.01 nd nd 0.30 0.05 nd 0.01 87.57

27.38 0.06 18.69 22.35 18.12 0.09 nd nd 0.01 0.28 nd 0.02 nd 87.00

27.69 0.02 18.38 21.95 18.52 0.04 nd nd nd 0.27 0.03 nd nd 86.90

27.88 0.06 17.76 22.26 18.85 0.01 0.02 nd 0.01 0.26 0.02 0.04 0.01 87.18

5.687 0.009 2.313 2.350 3.703 5.746 0.024 0.000 0.000 0.000 0.051 0.000 0.000 0.000 0.608

5.847 0.031 2.153 2.288 3.567 5.929 0.026 0.011 0.000 0.008 0.032 0.000 0.005 0.000 0.624

6.470 0.004 1.530 4.400 2.798 3.286 0.007 0.006 0.000 0.000 0.065 0.000 0.000 0.005 0.540

5.910 0.018 2.090 3.065 3.205 5.022 0.019 0.009 0.000 0.000 0.145 0.000 0.000 0.005 0.610

8.324 0.000 0.000 3.677 2.229 3.325 0.013 0.035 0.000 0.000 0.313 0.000 0.000 0.003 0.592

6.322 0.010 1.678 3.556 2.861 4.190 0.014 0.013 0.000 0.000 0.398 0.000 0.000 0.000 0.594

5.814 0.012 2.186 2.221 3.908 5.743 0.010 0.002 0.000 0.000 0.050 0.008 0.000 0.002 0.595

5.727 0.009 2.273 2.335 3.910 5.610 0.001 0.000 0.000 0.003 0.047 0.000 0.003 0.000 0.589

5.788 0.003 2.212 2.304 3.831 5.763 0.0007 0.000 0.000 0.000 0.045 0.005 0.000 0.000 0.600

5.813 0.009 2.187 2.278 3.882 5.861 0.002 0.004 0.000 0.003 0.043 0.003 0.007 0.002 0.601

SER5-9

SER5-10

SER5-11

SER5-12

BREJ2/2-1

BREJ2/2-2

BREJ2/2-3

BREJ2/2-4

BREJ2/2-5

BREJ2/2-6

28.10 0.07 17.95 20.23 19.35 0.03 0.02 nd 0.10 0.29 0.02 0.11 0.01 86.36

27.70 0.03 17.85 20.28 20.28 0.02 0.04 na nd 0.18 0.03 0.03 nd 86.44

28.64 0.03 17.85 19.79 19.79 0.03 0.06 na 0.12 0.29 0.04 0.15 0.04 86.83

28.56 0.02 18.17 19.83 19.15 0.01 0.08 na 0.26 0.40 0.04 0.13 0.01 86.66

30.15 0.09 19.01 12.68 23.97 0.15 0.08 na na 1.32 nd na 0.10 87.55

31.20 0.04 18.38 12.57 21.74 0.15 0.12 na na 2.84 nd na 0.28 87.32

30.22 0.07 18.23 12.56 22.41 9.18 0.16 na na 2.38 nd na 0.24 86.45

29.26 0.04 18.28 14.51 23.38 0.19 nd na na 0.79 nd na 0.46 86.91

29.22 0.01 18.40 13.79 24.32 0.15 0.03 na na 0.65 nd na 0.66 87.23

29.58 0.06 18.56 14.02 24.10 0.16 0.02 na na 0.69 nd na 0.29 87.48

5.855 0.011 2.145 2.263 3.525 6.011 0.005 0.004 0.000 0.026 0.048 0.003 0.019 0.002 0.630

5.767 0.005 2.233 2.155 3.531 6.295 0.003 0.009 0.000 0.000 0.030 0.004 0.005 0.000 0.641

5.905 0.005 2.095 2.293 3.412 6.084 0.005 0.013 0.000 0.031 0.048 0.006 0.026 0.006 0.641

5.788 0.003 2.212 2.286 3.483 5.997 0.002 0.018 0.000 0.070 0.067 0.006 0.023 0.002 0.632

5.956 0.013 2.044 2.382 2.095 7.060 0.025 0.017 0.000 0.000 0.210 0.000 0.000 0.015 0.771

6.205 0.006 1.795 2.513 2.091 6.446 0.025 0.025 0.000 0.000 0.454 0.000 0.000 0.044 0.755

6.075 0.010 1.925 2.395 2.111 6.717 0.031 0.034 0.000 0.000 0.385 0.000 0.000 0.038 0.761

5.882 0.006 2.118 2.213 2.439 7.008 0.032 0.000 0.000 0.000 0.128 0.000 0.000 0.073 0.742

5.835 0.001 2.165 2.166 2.303 7.241 0.025 0.006 0.000 0.000 0.104 0.000 0.000 0.104 0.759

5.882 0.009 2.118 2.132 2.332 7.146 0.027 0.004 0.000 0.000 0.110 0.000 0.000 0.045 0.754

Samples/Oxides

FLCl-1

FLCl-2

FLTl-1

FLTl-2

FLTl-3

FLTl-4

FLTl-5

FLTl-6

FLTl-7

FLTl-8

FLTl-9

SiO2 TiO2 Al2O3 FeOt MgO MnO CaO Na2O

28.27 0.06 18.14 19.73 19.31 0.05 0.06 nd

27.56 0.02 18.37 19.98 19.71 0.06 0.07 nd

59.76 nd 0.17 7.13 25.60 nd nd na

59.38 nd 0.15 7.07 25.05 nd nd na

59.96 0.04 0.14 7.18 24.94 0.01 0.01 na

60.30 nd 0.22 7.37 24.94 0.01 0.05 na

60.59 nd 0.08 7.28 25.41 0.02 0.01 na

59.89 nd 0.18 6.70 24.89 nd 0.04 na

60.17 0.04 0.37 6.94 25.55 0.01 nd na

59.67 nd 0.21 6.63 25.33 0.02 0.01 na

59.77 0.03 0.10 7.11 25.15 0.04 0.01 na

(continued on next page)

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Table 1 (continued ) Samples/Oxides

FLCl-1

FLCl-2

FLTl-1

FLTl-2

0.18 0.08 nd nd K2O NiO 0.25 0.22 0.77 1.09 ZnO 0.05 nd 0.01 0.05 P2O5 0.03 0.06 nd 0.02 Cr2O3 0.03 0.05 nd nd Total 86.16 86.18 93.44 92.81 Number of cations on the basis of 28 (chlorite) and 22 (talc) oxygen atoms Si 5.678 5.777 7.986 8.004 Ti 0.000 0.007 0.000 0.000 Aliv 2.322 2.223 0.014 0.000 Alvi 2.423 2.285 0.013 0.024 Fe 3.203 2.545 0.797 0.797 Mg 6.206 6.793 5.101 5.035 Mn 0.043 0.023 0.000 0.000 Ca 0.013 0.030 0.000 0.000 Na 0.000 0.000 0.000 0.000 K 0.005 0.000 0.000 0.000 Ni 0.034 0.080 0.083 0.118 Zn 0.000 0.009 0.001 0.005 P 0.000 0.012 0.000 0.002 Cr 0.024 0.107 0.000 0.000 Mg# 0.659 0.727 0.865 0.863

FLTl-3

FLTl-4

FLTl-5

FLTl-6

FLTl-7

FLTl-8

FLTl-9

0.01 0.94 0.05 nd nd 93.28

0.01 1.00 nd nd 0.05 93.95

0.01 0.91 0.01 0.01 nd 94.33

nd 0.99 0.06 nd nd 98.75

0.01 1.07 0.04 nd 0.02 94.22

0.01 0.97 nd nd nd 92.85

nd 0.99 0.03 nd nd 93.23

8.032 0.004 0.000 0.022 0.804 4.982 0.001 0.001 0.000 0.002 0.101 0.005 0.000 0.000 0.861

8.028 0.000 0.000 0.034 0.820 4.951 0.001 0.007 0.000 0.002 0.107 0.000 0.000 0.005 0.858

8.026 0.000 0.000 0.012 0.806 5.019 0.002 0.001 0.000 0.001 0.097 0.001 0.001 0.000 0.862

8.053 0.000 0.000 0.028 0.753 4.990 0.000 0.006 0.000 0.000 0.107 0.006 0.000 0.000 0.869

7.980 0.004 0.020 0.038 0.770 5.053 0.001 0.000 0.000 0.002 0.114 0.004 0.000 0.002 0.868

8.013 0.000 0.000 0.033 0.774 5.072 0.001 0.001 0.000 0.002 0.105 0.000 0.000 0.000 0.868

8.017 0.003 0.000 0.016 0.798 5.030 0.004 0.001 0.000 0.000 0.107 0.003 0.000 0.000 0.863

Fig. 6. eMg# vs Ni (A), Mg# vs Cr (B) and Cr vs Ni (C) diagrams for chlorite from chloritites sampled at Serrinha, Juarina and Couto Magalh~aes Velho areas showing no correlation of Cr with Ni or of Mg# with either Cr or Ni.

showing not only the involvement of seawater but also the tendency to equilibrium with the marine environment under the influence of high temperature solutions. The metasedimentary rocks spatially associated with chloritites reveal also strong evidence of having been generated or altered by hydrothermal solutions. In fact, metacherts and iron formations commonly display anomalous contents of Cr (up to 5000 ppm) and

Ni (up to 2000 ppm). In a few samples Co, Cu and As are somewhat high, the former reaching almost 300 ppm. Moreover, a more detailed analysis of the REE distribution and the relationships among major elements allowed interpreting those rocks as metaexahalites (Kotschoubey, unpublished data). On the other hand, chemical evidences indicate that the protoliths of the siliciclastic metasedimentary rocks were also locally

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Table 2 Chemical composition (wt. %) of magnetite, Ti-magnetite, ilmenite, rutile and apatite present in the Araguaia Belt chloritites.

SiO2 TiO2 Al2O3 FeO MgO MnO ZnO NiO AuO CaO K2O P2O5 Cr2O3 Total

Magnetita

Ti-magnetita

Ilmenita

Juarina

Juarina

Serrinha

0.006 0.06 nd 89.94 0.01 0.04 0.09 0.15 0.02 nd nd 0.03 0.63 90.97

0.004 1.55 nd 89.38 0.002 0.02 0.05 0.14 0.0 0.006 nd 0.014 1.55 91.23

nd nd nd 90.48 0.013 0.04 0.04 0.21 0.04 nd nd 0.018 nd 90.83

0.015 0.025 nd 89.69 nd nd 0.02 0.19 0.03 nd 0.001 0.018 0.032 90.02

0.16 nd 0.002 89.31 0.02 nd nd 0.26 0.02 nd 0.006 0.03 0.076 89.93

0.036 0.023 0.002 88.43 0.007 nd nd 0.29 0.013 0.02 0.05 nd 0.55 89.42

0.009 0.012 nd 89.81 0.002 0.035 nd 0.30 0.027 nd nd 0.014 0.905 91.11

nd nd nd 89.00 0.005 0.026 nd 0.22 0.0 nd nd 0.014 0.913 90.18

0.237 nd 0.085 86.11 0.051 nd nd 0.17 0.056 0.069 0.034 nd 0.631 87.47

0.098 6.93 0.032 79.41 0.025 0.012 nd 0.03 0.031 0.02 0.005 nd 0.817 87.44

Rutilo

2.90 86.0 1.77 3.57 2.32 0.07 nd nd nd 0.095 0.018 0.025 0.145 96.92

0.04 6.90 0.004 84.41 0.005 0.014 0.009 nd 0.04 nd nd nd 0.02 91.45

0.045 55.38 nd 44.28 0.035 0.966 nd nd nd nd 0.052 nd 0.022 100.78

nd 52.60 nd 46.84 0.025 1.203 0.031 nd 0.085 nd 0.006 0.018 nd 100.80

0.009 52.19 nd 46.64 0.028 1.313 0.032 0.024 0.004 nd nd 0.005 nd 100.25

Apatita

Juarina SiO2 TiO2 Al2O3 FeO MgO MnO ZnO NiO AuO CaO K2O P2O5 Cr2O3 Total

nd 8.07 0.025 83.60 0.018 nd 0.065 0.033 0.053 nd 0.001 nd 0.01 91.88

Serrinha 2.38 87.73 1.76 2.90 2.09 0.22 nd nd nd 0.053 0.007 0.004 0.069 97.22

0.041 99.54 nd 0.444 0.008 0.006 0.017 nd 0.07 0.003 0.008 nd 0.001 100.14

Serrinha 0.073 98.92 0.009 0.545 nd nd nd 0.015 0.056 0.035 nd nd 0.016 99.67

0.049 99.61 nd 0.565 nd nd 0.005 nd nd nd 0.006 0.016 nd 100.25

0.024 99.84 0.006 0.675 0.012 0.026 nd 0.055 0.015 0.013 nd nd 0.039 100.70

0.011 99.70 0.008 0.805 0.012 nd 0.01 0.009 0.071 0.008 0.004 nd 0.001 100.64

0.073 0.025 nd 0.12 0.041 0.081 0.666 nd 0.607 55.87 nd 41.80 nd 99.28

0.073 nd nd 0.12 0.017 0.053 0.712 0.041 0.535 55.59 nd 41.60 nd 98.74

0.011 0.007 nd 0.329 0.003 0.026 0.667 nd 0.652 55.11 nd 43.05 0.018 99.87

0.071 nd nd 0.143 0.018 0.019 0.716 0.047 0.604 55.98 nd 41.96 0.006 99.56

nd ¼ not detected.

affected by submarine hydrothermalism. This is particularly true for the well-preserved low grade banded feldspathic schists, metagreywakes and meta-arkoses from Serrinha, Juarina and Serra do Quatipuru. Thus, although most analytical data point out only to a discrete enrichment of metallic elements in these rocks (yet not exceeding 1200 ppm Mn, 450 ppm Cr, 300 ppm Ni, 400 ppm Zn, and 100 ppm Cu) two meta-greywacke samples from the northern portion of the Serra do Quatipuru present quite anomalous concentrations of certain trace elements. Sample SL- 2/3A reveals remarkable values for Ni (8646 ppm), Zn (2704 ppm) and Y (2500 ppm), and sample SL-2/3B a high Ni content (2273 ppm). Worth to note also, the SREE in SL-2/3A is ten to twenty-fold (3666 ppm) that of all other siliciclastic metasedimentary rock samples, whose SREE varies from 137 to 367 ppm. Normalized to the NASC, the REE contents of the metasedimentary samples show similarly to the chloritites variable distribution patterns on regional scale, although they tend to be more uniform within each area. Thus, in the Serra do Quatipuru, the pattern is usually subhorizontal with occasional negative Ce anomaly that is particularly significant in sample SL-2/3A. In the Serra do Tapa, the profile displays a relatively pronounced enrichment in LREE, also with occasional negative Ce anomaly. In the Serrinha-Juarina area, the pattern varies from crudely horizontal to LREE-enriched accompanying a change in Ce anomaly from positive to negative. Finally, in Morro Grande, the pattern is horizontal with very strong negative Ce anomaly. This variation in the distribution of REE seems to reflect the varying degree of hydrothermal alteration imposed on the siliciclastic sedimentary deposits. Therefore, based on these data, it can be assumed that even heavily affected by exhalative activity, the siliciclastic sedimentary formations of the Tocantins Group have undergone few mineralogical modifications, making it

unlikely that chloritites and quartz chloritites are derived from these formations. It is worth noting that pyrite, as well as traces of chalcopyrite, arsenopyrite, pyrrhotite and pentlandite, besides phlogopitic mica, have been identified in the meta-limestones intercalated with chloritites at Serrinha area. Though discrete, the presence of these sulfides suggests a metal influx by hydrothermal solutions. In addition, thin Mn-rich beds interlayered with metacherts from the Serrinha, Morro do Jabuti and Morro do Salto areas reveal high Ni, Cu, and Zn contents. All these evidences, sometimes modest as they may be, support the occurrence of hydrothermal activity at seafloor during the basin opening. The infiltration and circulation of aqueous solutions into the proto-ocean floor led to a mobilization of metals, mainly Ni and Cr, and subordinately Zn and Cu, which were fixed in sedimentary formations deposited in the basin. The chloritites, closely associated with the aforementioned rocks, fit into this scenario. Evidences for hydrothermal alteration of the basaltic rocks are also plenty. The pillowed basalts and basaltic breccias of Serra do Tapa, in particular, are of T-type MORB (Fig. 8), exhibit practically isochemical propylitic-type alteration and consist mainly of actinolite, albite, chlorite, epidote, titanite, quartz and residual ilmenite. Augite is the only primary silicate still preserved, though partly (Kotschoubey et al., 2005). Commonly, these rocks still reveal their original textures without significant differences in chemical composition between the inner parts and their more vitreous edges (Kotschoubey et al., 1996, 2005). Their REE distribution pattern shows enrichment in LREE and discrete negative Eu anomaly. In turn, the pillowed basalts of Morro do Agostinho are N-type MORB (Fig. 8), exhibit the typical depletion in LREE and significant negative Eu anomaly (Fig. 9). They contain appreciable amounts of devitrified volcanic material, the cores of the pillows being affected

182 Table 3 Chemical composition of chloritite samples from the Araguaia fold belt (Serrinha, Juarina and Couto Magalh~ aes Velho localities) in terms of major components (wt. %) and rare earth and selected trace elements (ppm). na ¼ not analysed. Serrinha

Couto M. Velho

MM3/1

MM4/1

SR1/1

SR2/1

SR3/1

SR4/3

BK1/2

SER2/2

SER2/4

SER2/6

BK2/7

BK2/9

BK4

BK4/2

BK4/3

BK5/2

BK5/3

F.Luiz3

Erotil2

Brej2/2

Brej3b

32.27 1.30 17.30 20.42 4.64 14.20 17.36 0.02 0.35 0.03 0.03 9.38 98.46 17.95 21.19 22.68 5.30 1.33 4.46 3.90 0.75 1.87 1.44 0.18 281.05 300 504 2087 92 24 55 242 3 na na nd nd nd 35 155

37.86 1.33 13.27 26.02 5.30 18.64 10.48 0.05 0.40 0.03 0.10 9.52 99.06 na na na na na na na na na na na

33.50 0.91 16.95 19.00 4.26 13.26 17.52 0.02 0.60 0.02 0.12 9.50 98.14 na na na na na na na na na na na

31.37 0.80 14.80 22.87 8.02 13.36 16.89 0.20 1.66 0.38 0.20 9.02 98.19 na na na na na na na na na na na

30.68 0.77 15.33 19.63 2.50 15.41 19.64 0.90 0.59 0.03 0.25 9.96 97.78 na na na na na na na na na na na

31.80 0.68 14.67 22.95 3.83 17.20 17.64 0.02 0.55 0.04 0.12 9.89 98.36 na na na na na na na na na na na

26.36 1.50 17.28 24.32 2.98 19.20 18.13 1.42 0.05 0.04 0.84 9.29 99.23 na na na na na na na na na na na

32.45 0.87 21.01 24.77 17.22 6.79 10.22 0.07 0.03 0.19 0.37 8.29 98.27 na na na na na na na na na na na

51.88 0.99 12.06 12.49 1.72 9.69 14.70 0.02 0.23 0.07 0.15 7.08 99.67 na na na na na na na na na na na

35.12 3.02 16.33 25.77 6.44 17.39 9.48 0.71 0.44 0.08 1.53 7.30 99.78 na na na na na na na na na na na

34.38 1.02 19.50 16.41 2.47 12.54 17.24 0.03 0.91 0.56 0.18 8.99 99.22 na na na na na na na na na na na

24.64 3.07 19.25 21.09 2.56 16.67 18.39 0.80 1.99 0.22 0.91 9.42 99.78 na na na na na na na na na na na

23.70 2.69 19.76 22.29 2.81 17.53 18.62 0.47 0.88 0.07 0.39 9.54 98.41 na na na na na na na na na na na

29.68 0.27 17.94 17.30 8.11 8.27 19.98 0.13 0.77 0.05 0.14 10.63 96.89 na na na na na na na na na na na

29.25 0.38 17.87 15.69 7.18 7.66 17.89 0.18 0.03 0.02 0.17 12.73 94.21 na na na na na na na na na na na

378 411 2087 123 5 45 146 3 na na nd nd 34 85 43

234 419 2500 140 7 51 132 3 na na 25 nd 17 768 149

1344 119 3522 589 9 53 96 4 na na 55 6 42 534 67

23.00 1.40 16.60 31.46 10.42 18.93 16.60 0.06 0.01 0.01 0.31 9.24 98.,69 329.1 43.39 254.2 38.16 6.96 27.07 19.08 3.50 7.92 3.89 0.53 733.80 610 100 1100 55 190 8 284 nd 62 767 17. 7 18 47 27

26.55 5.00 16.23 24.18 2.18 19.80 16.14 2.27 0.08 0.03 0.84 8.21 99.53 na na na na na na na na na na na

400 93 2500 67 11 51 267 3 na na 57 nd 191 395 79

42.55 0.80 15.05 22.88 6.81 14.46 9.28 0.06 0.04 0.04 0.25 7.49 98.44 33.83 60.74 48.55 9.33 2.13 7.21 4.56 0.74 1.10 0.43 0.06 168.68 240 95 645 140 31 8 131 na 73 167 28 6 14 30 16

29.25 0.87 19.67 14.87 2.50 11.13 22.79 0.06 0.08 0.02 0.25 11.12 98.98 na na na na na na na na na na na

360 274 1109 92 41 38 151 3 na na 21 nd nd 28 155

33.16 1.00 17.24 21.25 2.51 16.86 17.13 0.07 0.03 0.02 0.26 9.13 99.29 351.90 46.35 290.20 38.44 7.07 26.57 18.08 3.28 7.18 3.96 0.49 793.52 810 1220 2990 145 122 4 148 na 38 96 20 6 33 127 38

36.40 0.93 17.14 17.12 2.69 12.98 18.12 0.04 0.03 0.03 0.14 9.71 99.66 na na na na na na na na na na na

355 119 407 84 34 69 230 3 na na 28 10 7 29 146

23.00 2.20 17.00 36.98 14.64 20.10 9.20 0.05 0.56 0.03 0.21 9.71 98.94 178.00 164.70 161.10 33.56 5.65 40.66 41.06 8.77 25.63 10.98 1.50 671.61 533 1839 10,300 193 34 67 448 3 na na 67 nd 23 801 795

390 670 1210 112 37 4 285 nd 34 na 14 nd nd 27 116

925 1350 3420 112 20 6 409 1 69 na 271 nd nd 34 247

715 135 1085 65 105 7 179 na na na na na na na na

310 280 14,370 63 27 11 397 na na na 27 10 6 29 232

230 84 827 63 5 nd 125 na na na 13 nd nd 47 84

1160 21 1773 36 8 nd 357 na na na na na na na na

960 158 1130 67 9 nd 198 na na na na na na na na

650 248 1500 218 193 nd 422 na na na na na na na na

900. 262 3500. 329 12 na 206 na na na 19 nd 13 106 280

1100 820 10,178 191 11 nd 573 na na na na na na na na

865 4865 17,075 153 23 15 750 na na na na na na na na

B. Kotschoubey et al. / Journal of South American Earth Sciences 69 (2016) 171e193

SiO2 TiO2 Al2O3 Fe2O3T Fe2O3 FeO MgO CaO Na2O K2O P2O5 LOI Total La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu SREE Mn Cr Ni Co Cu Pb Zn Ag Pt Ba Nb Rb Sr Y Zr

Juarina

MM1/2

Table 4 Chemical composition of chloritite samples from the Araguaia fold belt (Serra do Tapa, Arapoema, Quatipuru, Morro Grande, Morro do Jabuti, Pau Ferrado, Salto and Jacu) in terms of major components (wt. %) and rare earth and selected trace elements (ppm), except for Pt (ppb). S. do Tapa

Quatip.

Morro Grande

Morro do Jabuti

Pau Ferrado

Salto

Jacu

SAP-05

SAP-04

ARAP-8

QUAT-12

MGE-3

MGE-11

MGE-12

WL8-01

WL11-01

WL2-04

WL2-05

WL3-04

WL3-06

WL26-01

WL26-02

WL27-01

WL22-01

JAC-1

34.30 0.89 15.80 7.4 3.30 3.70 29.90 0.12 0.02 nd 0.14 11.12 99.69 62.51 114.30 na 70.67 14.81 2.45 11.01 na 9.25 1.84 4.89 na 4.46 0.50 296.69 1340 155 233 22 4 12 67 na 168 76 na 20 na 7 15 60 199 na na

42.30 0.60 11.7 5.5 2.2 3.0 29.8 0.05 0.01 nd nd 9.72 99.68 na na na na na na na na na na na na na na

36.4 0.85 16.4 11.3 3.4 7.10 25.2 0.08 0.08 nd 0.36 9.27 99.94 23.31 15.18 na 18.82 3.75 0.96 4.77 na 6.50 1.38 4.11 na 3.50 0.47 82.75 774 224 601 59 4 14 65 na 137 na na 30 na 7 35 54 257 na na

64.20 0.87 9.8 11.5 6.4 4.6 7.1 0.05 0.01 0.20 0.19 5.16 99.08 383.9 26.84 na 451.35 86.02 22.31 130.4 na 103.1 20.47 54.39 na 31.56 4.32 1314.66 1626 268 4408 26 256 15 200 na 198 641 na 32 na 15 6 1049 261 na na

31.80 0.27 14.9 11.2 7.3 3.5 28.8 0.44 0.01 0.02 nd 11.18 98.62 0.83 0.95 na 0.54 0.10 0.02 0.09 na 0.10 0.02 0.05 na 0.04 0.01 2.75 1161 1710 4087 24 26 69 15 na 263 nd na 8 na 6 nd nd 13 na na

29.30 1.1 16.5 16.4 6.5 8.9 24.1 nd 0.02 0.03 nd 10.64 98.09 na na na na na na na na na na na na na na

30.54 0.43 16.57 7.26 e e 32.64 0.02 0.02 nd 0.01 12.20 99.69 nd 0.5 0.7 nd nd nd 0.12 0.01 0.07 nd 0.10 na 0.15 0.02 1.67 387.5 321.5 1110.3 45 26.6 0.7 16 0.9 189 nd 9.5 1.8 na 0.6 0.9 0.7 9.6 na 29

27.54 2.20 18.22 16.96 e e 23.35 0.26 nd nd 0.22 10.9 99.65 37.4 70.7 7.85 33.1 7.9 2.92 9.65 1.94 11.75 2.46 7.21 1.12 6.20 0.80 201.0 697.5 314.6 309.4 124 91.7 0.3 71 9.5 412 6.2 18.8 23.8 1.7 nd 6.4 68.2 127.1 3.8 46

32.32 0.12 16.72 5.41 e e 32.11 nd nd nd nd 12.8 99.48 na na na na na na na na na na. 0.05 na na na

28.76 1.32 19.09 14.60 e e 24.12 0.01 nd nd 0.08 11.6 99.58 45.9 6.5 6.06 20.9 4.5 1.80 8.20 1.86 11.54 2.47 7.60 1.07 6.01 0.86 125.27 1085 707.5 429.4 88.2 1.4 0.1 72.0 4.8 294 30.4 14.9 9.5 0.7 nd 4.4 99.1 71.0 2 37

31.12 5.10 13.95 14.77 e e 19.61 2.23 n.d nd 2.08 10.6 99.66 49.8 77.0 21.69 111.1 24.7 8.12 23.99 3.35 16.95 2.98 7.26 0.91 5.15 0.63 353.63 465 218.9 1401.4 55.1 316.1 0.5 51 2.4 337 75.1 15.3 21.8 1.5 nd 77.8 86.6 61.9 1.7 29

46.61 1.31 12.89 19.94 e e 11.51 0.01 nd nd 0.03 7.1 99.40 19.9 13.0 3.15 12.7 2.4 0.61 2.35 0.37 1.83 0.30 0.87 0.14 0.97 0.18 48.77 697.5 855 2034.6 154.5 3.3 0.1 69 54.3 325 20.2 10.7 11.7 0.9 nd 2.8 9.5 66.5 1.9 31

27.83 3.32 18.55 15.28 e e 23.79 0.25 0.02 nd 0.24 10.5 99.78 19.3 45.5 5.74 25.7 6.3 2.06 7.54 1.42 8.64 1.77 5.05 0.83 4.59 0.67 135.11 232.5 75.2 168.3 50.2 1399 0.2 33 4.1 432 13.4 20.8 20.6 1.5 0.8 13.2 52.4 185.4 4.8 56

26.87 1.07 20.43 19.50 e e 20.69 nd nd nd 0.06 10.7 99.32 12.8 22.8 4.17 19.2 5.0 2.32 5.68 1.15 6.05 1.06 3.15 0.40 2.53 0.37 86.68 2790 567.7 415.7 97.3 1035.8 0.1 386 5.1 206 17.2 16.9 9.3 0.6 nd 1.9 34.2 71.6 1.9 37

27.90 1.41 19.17 19.60 e e 20.70 0.01 nd nd nd 10.3 99.09 4.6 13.9 2.25 11.5 3.4 1.19 4.74 1.02 5.53 1.12 3.38 0.48 3.36 0.44 56.91 1472.5 608.8 1404 307 2831.9 0.2 397 4.0 179 3.4 14.4 4.0 0.2 nd nd 33.4 85.1 2.1 40

28.22 3.60 17.96 17.02 e e 21.53 0.01 nd nd 0.06 10.9 99.30 36.7 56.4 9.19 39.5 8.0 2.60 9.02 1.82 10.48 2.00 6.17 0.86 5.51 0.83 180.08 465 1285.9 2057.9 116.8 894 0.7 80 7.7 408 43.6 17.2 47.1 2.7 0.7 5.4 57.7 253.4 6.6 75

25.18 0.42 18.60 29.21 e e 13.46 0.01 nd nd nd 10.5 97.38 3.2 1.0 0.58 2.0 0.4 0.11 0.57 0.11 1.00 0.17 0.59 0.09 0.52 0.10 10.44 465 906.3 9699.9 351.8 34 0.1 205 11.4 375 8.8 16.1 0.6 nd nd 0.5 6.6 23.1 0.6 63

27.80 0.97 19.9 12.2 e e 27.3 0.06 0.02 0.03 0.07 11.11 99.46 76.79 163.3 na 74.51 12.50 1.28 7.43 na 6.44 1.22 2.92 na 2.14 0.27 348.8 1548 567 165 58 5 37 213 na 226 65 na 21 na nd 15 34 167 na na

1216 113 284 22 4 10 46 na 81 na na 17 na 10 8 20 163 na na

1470 2052 10,218 157 19 46 204 na 163 17 na 13 na 6 nd 7 44 na na

232.5 868.7 1642 14.5 0.7 0.1 15.0 4.8 133 0.9 9.2 na na na na 0.3 2.0 na 25

B. Kotschoubey et al. / Journal of South American Earth Sciences 69 (2016) 171e193

SiO2 TiO2 Al2O3 Fe2O3T Fe2O3 FeO MgO CaO Na2O K2O P2O5 LOI Total La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu SREE Mn Cr Ni Co Cu Pb Zn W V Ba Ga Nb Ta Rb Sr Y Zr Hf Sc

Arap.

Arap. ¼ Arapoema; Quatip. ¼ Quatipuru na ¼ not analysed; nd ¼ not detected. 183

184

B. Kotschoubey et al. / Journal of South American Earth Sciences 69 (2016) 171e193

Fig. 7. REE distribution patterns of chloritite samples in relation to the chondrite of Nakamura (1974), except for Ho (Haskin et al., 1968). A. Serrinha; B. Jabuti; C. Salto (WL22-01), Pau Ferrado (WL26-01, WL26 and WL27-02-01) and Grande (MGE-3 and WL8-01); D. Arapoema (ARAP-8), Quatipuru (QUAT-12), Jacu (JAC-1) and Tapa (SAP-05).

by moderate hydrothermal alteration and composed of albite, clinopyroxene and hematite, besides subordinate quartz and chlorite. Epidote is mainly present in the anastomosing irregular veins and veinlets that crosscut randomly the basaltic mass. The edges of the pillows, however, are intensely chloritized and contain some residual clinopyroxene and mica, in addition to a small amount of pumpellyite. In comparison with the cores of the pillows, these edges show strong depletion of SiO2 (from 52 to 36 wt. %), nearly complete Na leaching, Ca preservation, significant increase in Mg content and fixing of remarkable amount of K, previously absent. The enrichment in K and Mg at the edges of the pillows would indicate that the alteration occurred at low temperature, but sufficiently high to allow the incorporation of seawater Mg into the rock (Mottl, 1983). 6.2. General considerations about possible chloritite protoliths No primary textures or minerals have been preserved in the chloritites, except traces of titanite, ilmenite and titano-magnetite, the latter two being apparently residual minerals. Likewise, the distribution of chloritites is not indicative of its origin, because these very ductile rocks record intense deformation caused by transposition, dismemberment and displacement, as well as thinning or, conversely, thickening of the original layers. Therefore, only the mineralogical and chemical compositions should function as guides to attempt identifying their protoliths. Their unusual mineralogical composition and high Al, Ti, Ni, Zn and REE contents make the chloritites distinct from all well-defined

metasedimentary or igneous formations that occur in the Tocantins Group. On the other hand, the close association of the chloritites with metacherts and iron formations, interpreted as metaexhalites, suggests that the chloritites are also products of submarine hydrothermal activity. However, their high concentrations of Al and Ti, and the low contents of Si, besides the extremely low amounts of alkalis, certainly do not correspond to the actual composition of the protoliths, reflecting rather the changes e enrichment or depletion of some elements - that the original rocks underwent during the alteration processes and, consequently, the intensity of such processes. In sedimentary environment, Al and Ti are elements indicative of terrigenous contribution and thereby occur in particularly low concentrations in deposits of essentially chemical origin whatever their nature is. Silicate chemical deposits directly precipitated from ocean water are essentially composed of magnesium and/or iron silicates (talc, saponite-type smectite and magnesian chlorite, among others) that do not contain or contain only traces of Al and Ti (Buatier et al., 1995, 2001). Chloritites could hardly have such an origin. Regarding the magmatic rocks, Al and Ti contents are relatively high in both mafic and felsic varieties, but very low in ultramafic rocks, particularly in depleted harzburgites and dunites from the upper mantle underlying mid-ocean ridge zones. On the other hand, Al and Ti behave as essentially immobile elements in the hydrothermal environment. It is therefore unlikely that hydrothermal alteration changes ultramafic rocks into either chloritites or chemically close products that might have preceded them.

B. Kotschoubey et al. / Journal of South American Earth Sciences 69 (2016) 171e193

185

Table 5 Chemical composition of chromitites, serpentinites and associated altered rocks from the Araguaia fold belt. Oxides in wt. %; trace and rare earth elements in ppm. Chromitites

SiO2 TiO2 Al2O3 Fe2O3 FeO Fe2O3t MnO MgO CaO Na2O K2O Cr2O3 B2O3 P2O5 LOI Total Ni Co Cu Pb Zn V Zr Y Rb Sr Ba Nb La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu SREE

T-SCH

OL-GB

MGE-1

MGE-2

WL36-01

MG-3

WL11-02

ARAP-4

Serpentinites QT-29

MG-6/2

STP-12

SL-1/5

25.10 0.07 18.40 e e 11.89 0.33 9.10 0.08 0.13 2.30 26.50 2.50 nd 3.06 99.46 1022 83 6 31 166 689 23 4 46 144 109 nd 1.40 2.32 1.95 0.56 0.30 0.55 0.86 0.19 0.51 0.48 0.08 9.20

25.10 0.11 16.00 e e 14.09 0.25 12.40 Nd 0.06 3.50 22.70 0.46 Nd 5.11 99.78 1415 88 6 43 204 520 14 11 69 18 242 8 7.46 3.43 13.30 3.40 0.85 3.28 2.46 0.47 1.12 0.71 0.10 36.58

26.09 0.06 19.18 e e 8.15 0.25 11.57 0.05 0.80 2.45 26.61 na 0.01 4.50 99.72 1031 85.5 7.2 0.5 1.0 564 1.3 2.4 67 153.7 291.2 nd 1.40 0.90 1.00 0.20 nd 0.28 0.36 0.07 0.26 0.29 0.05 4.81

14.8 0.09 10.90 e e 22.40 2.10 18.60 0.11 0.03 0.02 25.30 na nd 5.34 99.69 1492 200 22 54 5014 412 8 nd 7 nd nd nd 1.03 2.01 0.81 0.17 0.26 0.14 0.13 0.03 0.09 0.08 0.02 4.77

13.59 0.08 16.68 e e 15.91 1.11 16.22 0.01 0.01 0.04 29.66 na 0.01 6.10 99.42 508 67.9 2.9 2.4 73 368 26.2 5.5 1.2 1.0 48.2 1.9 0.6 0.6 1.2 0.5 0.09 0.67 1.12 0.24 0.68 0.62 0.10 6.42

42.00 0.06 1.30 5.40 1.70 e 0.13 36.80 0.05 nd nd 0.81 na nd 11.81 100.06 2114 60 13 17 43 38 na na na na na na 1.51 3.30 1.17 0.24 0.05 0.25 0.22 0.04 0.10 0.10 0.03 7.01

39.60 nd 0.87 6.70 0.56 e 0.05 38.00 nd nd nd 0.65 na nd 12.87 99.30 4958 57 6 12 26 37 23 nd 9 nd nd 5 0.72 1.31 0.58 0.10 0.03 0.10 0.11 0.02 0.07 0.08 0.02 3.14

43.00 0.08 0.70 3.70 0.56 e nd 39.30 nd 0.01 0.02 0.74 na nd 11.80 99.91 2201 114 8 31 71 18 6 nd 9 6 nd nd 0.66 1.14 0.37 0.08 0.03 0.09 0.10 0.02 0.05 0.04 0.01 2.59

55.70 nd 0.43 10.00 0.56 e 0.01 27.50 nd 0.01 nd 1.45 na nd 4.48 100.14 2176 53 3 50 22 30 24 nd 8 5 na 6 0.85 3.99 1.50 0.36 0.09 0.37 0.41 0.09 0.29 0.25 0.04 8.24

33.70 0.24 11.6 4.3 0.28 e 0.18 35.10 0.06 0.02 0.03 0.22 na nd 13.58 99.31 3536 74 7 31 30 216 10 Nd 8 Nd 18 Nd 0.68 2.00 0.80 0.23 0.05 0.25 0.29 0.06 0.16 0.18 0.03 4.73

nd ¼ not detected na ¼ not analysed LOI ¼ loss on ignition T-SCH ¼ talc-schist OL-GB ¼ serpentinized olivine gabbro. ARAP ¼ Arapoema; QT, SL ¼ Serra do Quatipuru; MG, MGE, WL ¼ Morro Grande; STP ¼ Serra do Tapa.

It follows from the above considerations that other alternatives are to be sought for the protoliths of the chloritites. In this regard, clay mineral-rich siliciclastic sedimentary and/or volcanic rocks seem to be the most appropriate candidates.

6.2.1. Hypothesis of a siliciclastic sedimentary origin The abundance of chlorite, the locally remarkable presence of tremolite-actinolite and the absence of quartz or its occurrence only in subordinate amounts in most samples do not support a siliciclastic sedimentary origin for the chloritites. In fact, although terrigenous sedimentary deposits that have been subjected to submarine hydrothermalism can exhibit significant chemical and mineralogical changes, only the clay fraction seems to have actually been affected by the process, while the coarser fractions composed of quartz, feldspar, mica and mafic minerals are not or are only slightly modified (Lackschewitz et al., 2000a). Thus, in the investigated region, no decisive mineralogical factor is sufficiently reliable to establish a relationship between the chloritites and the regional metasedimentary rocks derived from siliciclastic deposits, even those most closely associated with meta-exhalites (metacherts and iron formations). Actually, these metasedimentary rocks are made up of abundant quartz, mica and, in some lithotypes, feldspars as well. Chlorite should be the most symptomatic mineral

of hydrothermal alteration. However, it is only locally present and found in rather significant amounts exclusively in the feldspathic chlorite-mica schists from the Serrinha-Juarina area. Unlike the other rock types that do not exhibit any unusual structure, these schists show a banding that may suggest a pyroclastic origin (volcanic ash?). This banding is fairly regular and characterized by thin bands of quartz and albite alternating with bands of almost colorless chlorite and low temperature pale green biotite. Despite the greenish color of these schists, the amount of chlorite is incomparably much lower than that observed in the chloritites and quartzchloritites. The accessory minerals are mainly magnetite, rutile, apatite, xenotime, tourmaline and rare titanite, none of them being actually diagnostic of any hydrothermal process. Hence, it remains to be demonstrated that the protoliths of these schists have undergone any mineralogical change caused by hydrothermal fluids and, if they have, it seems that the change was just incipient. Therefore, the scarcity of chlorite, the systematic predominance of quartz and mica, and the presence of feldspar in certain rocks, as well as the lack of any formation that might be intermediate between the common metasedimentary rocks and the chloritites/ quartz chloritites discount siliciclastic sedimentary rocks as the protoliths of the chloritites. Therefore, based on the available chemical data, it can be assumed that, despite being intensively

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Fig. 8. Nb  Zr diagram in which most chloritite and basalt samples fall in the T-type MORB field (Le Roex et al., 1983). Chloritites from Serrinha, Juarina and Couto Mag~es Velho (open circles); Chloritites from Jabuti, Salto Grande and Pau Ferrado alha (filled circles); Chloritites from Serra do Quatipuru (vertical semi-circles); Chloritites from Serra do Tapa (filled squares); basalts from Morro do Agostinho (diamonds); and basalts from Serra do Tapa (horizontal semi-circles).

of quartz chloritites and chloritites (Mottl, 1983; Mottl and Holland, 1978) very similar to the rocks studied here. In fact, in the western portion of the region of study, several pillowed basalts are exposed at east, west and south of Serra do Tapa, and, further south, in the proximity of Morro do Agostinho, near Araguacema (Souza and ~o and Nilson, Moreton, 1995; Kotschoubey et al., 1996; Paixa ~o and Nilson, 2002; Kotschoubey et al., 2005). Most 2001a,b; Paixa likely, these volcanites are actually more abundant along the western margin of the Araguaia belt, although hidden under the thick weathering cover, and likewise may exist in other areas of the Tocantins Group domain, albeit compositionally modified. The hypothesis of derivation of the chloritites from basalt was tested based on lithogeochemical data plotted on compositional diagrams usually applied to mafic volcanics. Due to the deep changes experienced by the original rocks, the use of diagrams based on mobile major elements was considered inadequate except for those proposed by Reinhardt (1987) to discriminate products of hydrothermal alteration derived from volcanics and magnesian pelitic sediments. Worthwhile noting, the cordierite-anthophyllite rocks, which are the basis for the discriminant diagrams proposed by this author, may be products of high grade metamorphism of volcanic rocks, particularly mafic volcanites, that have been previously altered by hydrothermal solutions in submarine exhalative environment (Vallance, 1967; Schermerhorn, 1978; ElliotMeadows, 1999). As cordierite-anthophyllite rocks can be derived directly from chloritites under practically isochemical conditions, the chloritite composition can be used without major restrictions.

Fig. 9. REE distribution patterns of basalt samples from Morro do Agostinho and Serra do Tapa in relation to the chondrite of Nakamura (1974). Data for Ho are from Haskin et al. (1968). Same symbols as in Fig. 8.

affected by exhalative activity, the siliciclastic sedimentary formations of the Tocantins Group have at best undergone rather discrete mineralogical changes, making it unlikely that chloritites and quartz chloritites are derived from these formations. 6.2.2. Hypothesis of a basaltic origin In the volcanic rocks and specially in quartz-free mafic volcanites, the changes may be more complete and result, under appropriate temperature and high seawater/rock ratio, in the formation

In the FeOTeAl2O3eMgO and TiO2-FeOT/(FeOT þ MgO) diagrams, most chloritite samples, in particular those from Serrinha, Juarina, ~es Velho and Araguacema localities, occupy the Couto Magalha same fields as do the basalts mapped in west-central Araguaia belt (Figs. 10 and 11). However, some samples, especially from Serra do Tapa, Arapoema and Serra do Jacu, exceptionally rich in magnesium and relatively poor in iron, fall in the field of magnesian pelitic sediments. These samples could also reflect a particularly intense hydrothermal alteration of fine siliciclastic sediments, tuffites or

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tuffs, or alternatively an important chemical contribution to the composition of the original deposit in the form of minerals precipitated directly from seawater. On the V  Ti/1000 diagram, the chloritite samples from the different localities occupy, in general, the field of the oceanic floor basalts (Fig. 12). These same samples, in the Zr/TiO2  Nb/Y, despite some dispersion, fall mainly in the fields of sub-alkaline and alkaline basalts, and subordinately in the fields of basaltic andesites and andesites (Fig. 13). Magnesium-rich and iron-poor samples fall generally close to the field that encompasses the other chloritite samples. Similar distribution is observed in the Zr  TiO2 diagram, in which magnesian chloritites occupy the andesite field, although near the basalt and diabase field where most chloritite samples plot (Fig. 14). According to Liaghat and MacLean (1995), the hydrothermal products that result from the alteration of igneous rocks align normally with their respective precursor samples in the same diagram, reflecting the fact that Ti and Zr are practically immobile in the hydrothermal environment. However, ligands of P and/or F, if present in the hydrothermal fluids, tend to favor Ti and Zr solubility causing them to undergo some transport, especially if the fluids are hot and alkaline, and the water:rock ratios are high (Rubin et al., 1993; Jiang et al., 2005; Louvel, 2014). The chloritite samples from Jabuti, Salto, Pau Ferrado and Morro Grande, except the one unusually enriched in Ti (WL2-05), were plotted on that diagram together with basalt samples from both the Morro do Agostinho and Serra do Tapa (Table 6). The excellent alignment of the 27 samples, combined with the regression coefficient of 0.95, suggests a close genetic relationship between the basalts and chloritites (Fig. 15). Chloritite samples from the northernmost localities (Serrinha, Juarina, Jacu, Serra do Tapa, Arapoema) and from Serra do Quatipuru exhibit a more disordered distribution. The reason for the scattering of these samples could arise from the heterogeneous composition of the original rocks, i.e. magma, or from some mobilization of Zr and/or Ti due to a particular chemistry of the solutions and the intensity of hydrothermal alteration.

187

Fig. 11. Distribution of the chloritite samples in the TiO2-FeOT/(FeOT þ MgO) diagram. Despite a lot of samples falling in the field of basalts (B), many others fall off, especially those from Jabuti, Salto Grande and Pau Ferrado. Field A corresponds to rocks highly enriched in Mg. Based on Reinhardt (1987). Same symbols as in Fig. 8.

Fig. 12. Representation of the chloritite samples in the V  Ti/1000 diagram. Most samples fall within the ocean floor basalt (OFB) field. ARC stands for volcanic arcs. Based on Shervais (1982). Same symbols as in Fig. 8.

Fig. 10. FeOTeAl2O3eMgO diagram in which chloritite and basalt samples are plotted, most of them falling in the field of the altered volcanic rock (A) contoured by the dashed lines. The solid line separates the volcanic rock field from that of the evaporitic clays (B). Based on Reinhardt (1987). Same symbols as in Fig. 8.

The high contents of a few trace elements suggest that the lithologic environment in which chloritites developed was complex. If the Zn contents seem consistent with a relationship with mafic rocks, those of Cr and especially of Ni are more consistent with an ultramafic source. Mantle peridotites are known to occur in the seafloor for quite long time (Aumento and Loubat, 1971;
vel, 2003). The Bonnatti, 1976; Cannat, 1993; Carlson, 2001; Me interaction of seawater with these rocks has been of interest for many researchers (Seyfried and Dibble, 1980; Bonatti et al., 1983; Charlou et al., 1998; Holm and Charlou, 2001; Lowell and Rona, 2002; Alt and Shanks III, 2003). Recently, large mantle peridotite-

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Fig. 13. Chloritite samples represented in the Zr/TiO2  Nb/Y diagram and distributed in the fields of subalkaline and alkaline basalts and, subordinately, in the fields of basaltic andesites and andesites. Based on Winchester and Floyd (1977). Same symbols as in Fig. 8.

Fig. 14. Distribution of the chloritite samples in the Zr  TiO2 diagram, most of which clustering in the fields of basalts and diabases. Based on Floyd and Winchester (1975). Same symbols as in Fig. 8.

exposing areas were discovered in slow and ultra-slow mid-ocean ridges, as well as large hydrothermal systems hosted exclusively in upper mantle ultramafic rocks that were tectonically exhumed
vel, 2003). The hydrothermal fluids (Skelton and Valley, 2000; Me and the alteration products in these systems revealed different characteristics from those hosted by classic oceanic crust of mafic composition (Wetzel and Shock, 2000). Serpentinization is the main type of alteration, during which both olivine and orthopyroxene incorporated water and changed into antigorite, lizardite or chrysotile, along with some brucite and magnetite. Concerning the fluids, they exhibit remarkable variations and may, according to the

tectonic or structural setting, be either very reducing, relatively cold (<100  C) and highly alkaline (Lost City-type) or very hot and relatively acid (Rainbow-type) (McCaig et al., 2007). Generally, these fluids are rich in SiO2, Ca2þ and CH4 (Charlou et al., 2002). Serpentinization would occur, therefore, in accordance with the tectonic setting, at temperature varying from 350  C to well below 100  C (Früh-Green et al., 2004). Comparing with exhalative vents developed on mafic rocks, the major difference would be the presence of ultramafic rocks and columnar buildings consisting essentially of carbonate up to 60 m high, instead of metric-tall chimneys composed primarily of sulfides. The extreme variations of Ni contents (165 ppme1.7 wt. %), although not characteristic of ultramafic rocks, suggest at least pronounced influence of ultramafites. The same applies to Cr, although its contents are well below those of Ni (typically <1000 ppm). It must be emphasized that neither chromite nor Crspinel has been identified in the chloritites. Most authors consider elements such as Ni and Cr little mobile in hydrothermal conditions, but apparently both experienced remarkable mobilization and transfer from ultramafic rocks to the chloritite protoliths. Indeed, the intense chloritization of the plagioclase present in mafic rocks may have been effected by fluids that had previously interacted with ultramafic rocks (McCaig et al., 2005). Nickel enrichment at the edges of ilmenite crystals immersed in the chloritic matrix of chloritites illustrates the mobility of this metal and its capacity, in the presence of aqueous solutions, of replacing other elements in minerals that are known to be refractory to alteration. However, it is unpractical to determine if Ni was extracted from the basin ultramafic floor during the hydrothermal alteration or from that chloritic matrix during the latest metamorphic event. In such conditions, the contents of Ni, Cr and other elements associated with ultramafites (Co, PGE) may have increased at varying degrees in the alteration products of basalts, essentially represented by phyllosilicates, which are particularly prone to fix trace elements. Others metals, namely Zn and V, not characteristic of ultramafic rocks, have been also mobilized by hydrothermal fluids to be then fixed in chloritites or their protoliths. It is worthwhile noting that high contents of Zn (up to almost 8700 ppm) and low, but significant, contents of V (up to 842 ppm) were detected in chromitites from Morro Grande. Most likely, these rocks have been, at least in part, the source of these metals which are concentrated locally both in chloritites and sedimentary formations of the Tocantins Group. The SREE in chloritites (600e1315 ppm) is much higher than in the basalts from both Serra do Tapa (32.4e56.3 ppm) and Morro do Agostinho (45.4e67.2 ppm), and hundreds of times higher than in the ultramafites of the region (1.9e7.3 ppm). The distribution pattern shows, in general, moderate to strong enrichment in LREE relative to HREE (Fig. 7) and is very similar to that of the basalt samples from Serra do Tapa (Fig. 7). Seawater might have played an important role in the non-uniform REE enrichment that was largely controlled by variable fluxes of hydrothermal fluids produced by volcanic-exhalative activity on the ocean floor, allowing the transfer of notable amounts of these elements to the basalts (Humphris et al., 1978; Polat et al., 2003; Teagle and Alt, 2004). The non-uniform REE enrichment of chloritites resulted most likely from variations in space and time of both the hydrothermal fluid composition and the intensity of the hydrothermal fluxes. 7. Metallogenetic considerations Exhalative hydrothermal systems developed in submarine environment present highly favorable conditions for the formation of a number of mineral deposits. These systems are characterized by intense circulation of metal-bearing fluids, thick rock sequences,

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189

Table 6 Chemical composition of basalt samples from Serra do Tapa* and Morro do Agostinho. Major elements (wt. %); trace and rare earth elements (ppm), except Au (ppb). * Kotschoubey et al. (2005). Serra do Tapa

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu SREE Cr Ni Co Cu Pb Zn Zr V Au

STP-07

STP-19

SAP-06

SAP-01

STP-22A

STP-22

STP-20

STP-08

50.60 1.70 13.50 4.70 6.40 0.22 6.20 10.00 3.70 0.06 0.15 1.98 99.21 5.34 15.16 12.24 3.48 1.15 4.79 5.87 1.19 3.31 2.53 0.30 55.36 201 52 34 33 15 62 112 316 150

52.10 1.70 13.80 5.20 6.10 0.15 4.30 10.30 3.30 0.03 0.13 2.16 99.27 na na na na na na na na na na na e 256 78 32 46 15 54 120 247 520

48.30 1.60 13.60 3.70 8.00 0.20 6.40 12.20 2.40 0.75 0.15 1.72 99.02 6.91 18.55 12.69 3.13 1.08 3.89 4.17 0.82 2.19 1.69 0.21 55.33 277 45 33 39 15 48 109 298 1570

51.40 1.90 12.60 6.90 5.70 0.24 5.90 8.90 3.20 0.12 0.19 2.23 99.28 na na na na na na na na na na na e 111 49 37 27 19 77 128 316 nd

48.40 1.30 16.40 3.40 6.40 0.16 5.90 11.50 2.80 0.14 0.11 2.78 99.29 na na na na na na na na na na na e 132 50 27 38 22 51 110 235 7180

54.60 1.60 14.20 5.50 3.00 0.12 3.30 11.50 2.80 0.61 0.13 2.15 99.51 5.23 13.70 11.35 2.98 1.03 4.07 4.35 0.84 2.14 1.34 0.15 47.18 190 46 na 13 19 31 117 273 nd

48.70 1.40 15.60 3.30 6.90 0.15 7.20 8.80 2.60 0.49 0.07 3.96 99.17 4.11 13.31 12.05 3.11 1.03 2.77 3.04 0.59 1.49 1.00 0.13 42.45 300 132 na na na na 67 na 9

51.50 1.40 13.70 6.40 4.30 0.17 5.10 12.30 2.20 0.42 0.11 1.83 99.43 3.38 11.87 10.07 2.62 0.76 2.43 2.66 0.49 1.14 0.67 0.06 36.15 153 47 na na na na 95 na 13

Serra do Tapa

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu SREE Cr Ni Co Cu Pb Zn Zr V Au

Morro do Agostinho

STP-22B

OB14II

STP-16

STP-25

WL5-01A

WL5-01B

WL5-02

48.90 1.20 14.10 3.10 8.10 0.19 7.30 11.00 2.60 0.08 0.09 2.29 98.95 3.27 10.52 8.93 2.23 0.70 2.15 2.26 0.43 1.07 0.75 0.11 32.42 93 54 na na na na 71 na 22

51.00 1.40 14.00 6.70 5.10 0.17 4.80 11.20 2.70 0.03 0.10 2.07 99.27 5.04 15.59 13.37 3.49 0.91 3.54 4.37 0.84 2.12 1.44 0.15 50.86 216 62 na na na na 85 na 17

49.90 1.50 14.90 3.50 6.40 0.15 6.60 9.90 2.60 0.41 0.10 3.21 99.17 4.82 14.23 11.47 2.84 0.88 2.46 2.50 0.47 1.11 0.73 0.10 41.61 233 124 na na na na 85 na 17

49.40 1.80 18.00 2.60 5.80 0.14 5.30 7.50 4.90 0.29 0.15 3.31 99.19 5.35 18.22 16.33 4.24 1.06 3.48 3.69 0.72 1.87 1.22 0.15 56.33 353 140 na na na na 135 na 23

51.78 1.40 14.29 11.93 nd 0.12 4.01 8.65 5.84 <0.04 0.13 1.80 99.97 2.7 8.4 9.5 3.1 1.08 4.38 5.72 1.11 3.17 2.76 0.43 45.37 na 104 na 25 0.3 44 71 na 0.7

36.37 2.50 17.28 14.79 nd 0.22 11.53 8.16 0.71 1.58 0.18 6.50 98.95 2.7 11.3 13.7 5.0 1.61 7.15 8.88 1.70 5.18 4.82 0.70 67.17 na 146 na 18 0.2 123 135 na 0.7

38.04 2.40 19.28 13.06 nd 0.19 10.16 5.83 0.45 3.58 0.13 6.80 99.93 2.8 10.4 11.3 4.2 1.25 5.72 7.49 1.59 5.11 4.44 0.76 58.89 na 91 na 97 0.3 101 126 na 2.5

na ¼ not analysed; nd ¼ not detected.

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Fig. 15. Zr  TiO2 diagram in which twenty-seven chloritite samples show an alignment with a regression coefficient equal to 0.95, suggesting a genetic relationship between them and basalts (based on Liaghat and MacLean 1995).

including volcanic and sedimentary varieties, and thermal anomalies that prompt ore-forming processes on or near sea floor, notably where plate boundaries are active and rifting is underway. Classical examples are volcanogenic massive sulfide (VMS), sedimentary-exhalative (Sedex), and Kupferschiefer type deposits (Flanklin et al., 1981; Rona, 1988, 2005; Jowett, 1986), in addition to iron and manganese formations (Bekker et al., 2010). They represent a significant source of the world's Cu, Zn, Pb, Au, Ag, Fe and Mn ore reserves and production. Despite a great deal of evidence for submarine exhalative activity in the investigated region, no mineralized body has been found associated with the Araguaia chloritites. However, geological surveying and exploration work in the region of Arapoema (Tocantins state) and Conceiç~ ao do Araguaia (Par
a state) have revealed local anomalies of Cu, Zn, Pb, Au, Ag, As, Ba, Fe and Mn in a rock succession which includes exahalites, and epiclastic and pyroclastic rocks interpreted to have been formed above a submarine spreading-center (Osborne, 2001). Likewise, traces of sulfides and particularly anomalous contents of Ni, Cr, Zn, Cu, As, Au and REE were detected in both chloritites and associated metasedimentary rocks in the course of the present research. The erratic distribution and generally low contents of Cu and Zn in the chloritites just suggest that Cu and Zn sulfides may be present somewhere in the studied region. For Ni mineralization, however, the metallogenetic setting seems to have been more favorable. In fact, recent investigations, corroborating former studies, which uphold a hydrothermal origin for some Ni deposits (Lusk, 1976; Robinson and Hutchinson, 1982; Fleet, 1977), contradict the commonly accepted idea that Ni is immobile in the hydrothermal environment and show that at appropriate geological and physico-chemical conditions Ni sulfide deposits may indeed result from hydrothermal activity. Such deposits occur in Australia, Tasmania, Canada, Russia, China and other countries, where Cu, Co, Zn, Au, Ag and other metals are commonly associated with Ni. They exhibit a notable diversity, some being closely related to ophiolites (Nimis et al., 2008; Keays and Jowitt, 2013; Gonzalez-Alvarez et al., 2013; Melekestseva et al., 2013; Capistrant et al., 2015). Since the geological settings of these deposits are similar to that where

chloritites formed, investments aiming at the exploration of Ni sulfide mineralization in the domain of the Tocantins Group could be compensatory. The exploratory efforts should also be extended to Cu and Zn mineralizations. In fact, the submarine hydrothermal activity caused, in addition to a strong alteration of the rocks in some zones of the proto-ocean floor, a significant mobilization of metals from ultramafites and basalts. After their transport, these metals re-precipitated in exhalites, chloritites and occasional siliciclastic deposits, whose mineralogical composition greatly favored the metal fixation. Despite being the hydrothermal alteration quite intense, the sulfide scarcity and the ubiquitous presence of Fe-oxides (magnetite partially replaced by martite) in the chloritites indicate that the aqueous medium was depleted in sulfate and/or the environment was highly oxidized, at least in the studied occurrences. The strong influence of sulfate-poor continental waters, the small thickness of the basaltic flows and, hence, insufficient metal reserve, the scarcity or lack of upward migration of channeled hydrothermal fluids may perhaps explain why mineralization was meager. Nevertheless, locally special conditions favored sulfide accumulation as it was the ~o Martim stratiform mineralization (pyrite, chalcocase of the Sa pyrite, sphalerite and very minor galena) associated with the incipiently metamorphosed siliciclastic sedimentary units of the Couto Magalh~ aes Formation, west of the studied area (Osborne, ~o and Gorayeb, 2014). Unfortu2001; Villas et al., 2007; Paixa nately, the data about this occurrence are indeed incomplete to support a solid genetic model and to attempt any comparison with base metal deposits formed in similar geological setting. 8. Relationships between the supposedly basalt-derived chloritites and the basalts from the western portion of the Araguaia belt Basalts and chloritites occur in the lower part of the Tocantins Group, both outcropping in a number of areas in the western part of the Araguaia belt closely associated with metacherts and subordinate iron formations. Along and close to the western border of the Araguaia belt, however, almost exclusively pillowed basalts occur; chloritites are quite rare and localized. Further east, only welldeveloped chloritites showing no traces of basalt have been found. Thus, two distinct domains can be defined and their different lithological associations reflect most likely the geodynamic evolution of the crust during the development of the proto-ocean basin in Early Neoproterozoic times as indicated by a ~o et al., 2008). After the Sm-Nd isochron age of 757 ± 49 Ma (Paixa early rifting stage, divergent movements set forth the opening of the proto-ocean basin that was accompanied by sub-aqueous basaltic volcanism, deposition of siliceous and iron-siliceous chemical sediments, and abundant siliciclastic sedimentation. Concomitantly, the mantle plume located beneath the basin axial zone caused a progressive rise of harzburgites and dunites from the upper mantle, representing the base of the Moho transition zone ~o et al., 2008). As the extensional movements advanced, (Paixa these rocks were gradually serpentinized during their ascension, were emplaced as protrusions along faults, formed domes and ended up being exhumed at or near the ridges. Thus, the less altered basalts from the Serra do Tapa and Morro do Agostinho would have been early volcanites. These volcanites have been preserved due to the location of the associated ultramafic rocks at so great depths at the onset of the geotectonic process that no sufficiently hot fluids could be generated to cause a more intense alteration of the basaltic flows. As the basin widened and the proto-ocean crust expanded laterally, the early basalts were gradually shifted to the marginal zones, being replaced by later mafic volcanites in the innermost

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portions of the basin. At this stage, the basalts underwent a more intense hydrothermal alteration and became enriched in Ni, Cr and Co, given the proximity of mantle-derived ultramafites, which were then affected by seawater infiltration and, as a consequence, hydrothermalized. The resulting higher thermal gradient, favoring the upward flow of high temperature fluids, may have contributed to the partial or total chloritization of these volcanites. Later on, during the basin inversion and the general shifting of the rock masses from east to west that accounted for the structural organization of the Araguaia belt, significant deformations affected the rocks of the eastern portion, which were then metamorphosed under conditions of the greenschist to amphibolite facies. The rocks of the middle portion of the belt were submitted not only to low/medium greenschist metamorphism, but also to intense deformation and mylonitization. More than the other rock formations, the strongly chloritized and therefore highly plastic basaltic strata experienced extreme deformation, resulting in local thinning or thickening and intense dismemberment. At the same time, the circulation of metamorphic fluids may have contributed to the transformation of these already strongly altered volcanites into chloritites and quartz chloritites after additional leaching of the alkaline and alkaline earth elements still present in the volcanites. Regarding the rocks of the western portion of the belt, they have undergone incipient to low grade metamorphism and were only slightly deformed. During the shifting of the rock masses from east to west, thick piles of the lower sequences, including chloritites and associated formations, as well as slices of mantle rocks, were dragged over or next to the pillowed basalts. As a result, parts of the same litho-stratigraphic unit with distinct degrees of hydrothermal alteration and metamorphism eventually formed a complex imbricated system brought about by vigorous strike-slip movements during the Neoproterozoic Brasiliano tectonic cycle. 9. Conclusions Based on petrographic, diffratometric and geochemical data from samples collected in the western portion of the Araguaia belt and on the geological setting in which these rocks occur, chloritites are interpreted to represent vestiges of mafic volcanism that marked the early, extensional stage of the belt evolution. The volcanic products were probably 1) submarine basaltic flows that, in abrupt contact with seawater, were fractured and fragmented, acquiring high permeability (hyaloclastites?), and/or 2) layers of mafic pyroclastites also showing high porosity/permeability. These rocks, still hot, experienced intense hydrothermal alteration caused by infiltration of seawater under conditions of high water/rock ratios that brought about the formation of chlorite ± quartz replacing the primary mineral assemblage of basalts. There was an almost complete leaching of the alkaline and alkaline earth elements, except for Ca, which remained locally fixed in apatite and/or titanite and/or tremolite-actinolite. Silica was likewise largely leached, what probably allowed the deposition of chemical formations, both siliceous (metacherts) and iron-siliceous (iron formations) closely associated with chloritites. It is also possible that silica has remained in excess in the rock after the formation of the chloritic mass, allowing the generation of secondary quartz in the absence of other mineral able to fix that component. Al, Ti and Fe experienced outstanding relative enrichment, the former two due to the virtual immobility in hydrothermal conditions and the latter easily fixed, mainly in its reduced form, in alteration phyllosilicates. The removal of Mg from seawater was largely responsible for the significant absolute enrichment of this element in the chloritites. Finally, the notable enrichment of Ni and rather high concentrations of Cr and Co in these rocks suggest the removal of these metals from the underlying ultramafites by the hydrothermal fluids

191

and their fixation in the overlying altered volcanites. The profound transformation of basalt into chloritites, which led to the complete destruction of primary igneous textures and minerals, except perhaps of ilmenite, was probably multiphase and caused initially by the action of hydrothermal fluids that resulted from the seawater infiltration in the volcanites at high temperature. However, the transformation continued until the final stages of the evolution of the Araguaia belt, certainly favored by both the intense tangential tectonics, which drastically deformed the rocks, and the concomitant metamorphism of the Tocantins Group at a later stage of the Brasiliano cycle. Indeed, mylonitization and the regional metamorphism were processes capable of causing migration of large amounts of fluids. The intense hydrothermal-exhalative activity, responsible for the generation of cherts, iron formations and chloritites, also produced locally fluids enriched in base metals potentially capable of generating volcanogenic massive sulfide deposits. These deposits have not been yet recorded in the Tocantins Group. However, anomalous concentrations of Zn (up to 750 ppm) and Cu (2832 ppm) in a few chloritite samples would justify working out exploration programs for these metals in the areas here investigated. Not incidentally, stratiform sulfide mineralization was recently described in clastic lithological units of the Couto Mag~es Formation (Tocantins Group) at Sa ~o Martim, near the city of alha ~o (Villas et al., 2007). Whilst dominated by pyrite, minor Redença amounts of chalcopyrite, sphalerite and galena may bear evidence for hydrothermal-exhalative activity in the region. Similarly, the region seems very promising for the search of hydrothermal Ni sulfide mineralization in view of the high Ni concentrations (up to 1.7 wt. %) in the chloritites. Similar Neoproterozoic belts in various parts of the world are endowed with significant mineral wealth. Further research and exploratory work are still needed in the ~1200 km-long Araguaia belt in order to appraise its mineral potential and select areas favorable to discovery of ore deposits. Acknowledgements Thanks are due to the National Council for Scientific and Technological Development (CNPq) for a fellowship granted to BK and to
for the the Geosciences Institute of the Federal University of Para ^ nia Railine laboratory support. The authors are grateful to Anto
Silva, Erika Suellen Santiago and Gustavo Craveiro for much of the artwork. Constructive comments made by Dr. Cesar Ferreira Filho and an anonymous JSAES reviewer are kindly acknowledged. This research was partly funded by the Geosciences Institute of Amazon e Geociam (grant no: INCT-CNPq/MCT/FAPESPA 573733/2008-2). References ~o estrutural do segmento setentrional da Abreu, F.A.M., 1978. Estratigrafia e evoluça Faixa de Dobramentos Paraguai-Araguaia (MSc thesis). Universidade Federal do
, Bele
m, Brazil, p. 90 (in Portuguese, with English abstract). Para Aires, B., Kotschoubey, B., 1994. Contribuiç~ ao ao estudo da origem dos clorititos da
~es, Tocantins. In: SBG, Simpo
sio area de Serrinha, município de Couto Magalha ^ nia, 4, Resumos Expandidos, pp. 247e249 (in de Geologia da Amazo Portuguese). Alt, J.C., Shanks III, W.C.S., 2003. Serpentinization of abyssal peridotites from MARK area: sulfur geochemistry and reaction modelling. Geochimica Cosmochimica Acta 67, 641e653. Alvarenga, C.J.S., Moura, C.A.V., Gorayeb, P.S.S., Abreu, F.A.M., 2000. Paraguay and Araguaia belts. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.), 31st International Geological Congress, Proceedings. Tectonic evolution of South America, Rio de Janeiro, pp. 183e193. Aumento, F., Loubat, H., 1971. The Mid-Atlantic Ridge near 45 N: serpentinized ultramafic intrusions. Can. J. Earth Sci. 8, 631e663. Bekker, A., Slack, J.F., Planavsky, N., Krapez, B., Hoffman, A., Konhauser, K.O., Rouxel, O.J., 2010. Iron formation: the sedimentary product o a complex interplay among mantle, tectonic, oceanic, and biospheric processes. Econ. Geol.

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B. Kotschoubey et al. / Journal of South American Earth Sciences 69 (2016) 171e193

105, 467e508.
s massifs des Bernard, A.J., Maier, O.W., Mellal, A., 1988. Aperçu sur les amas sulfure hercynides Marocaines. Miner. Deposita 23, 104e114. Bettison, L.A., Schiffman, P., 1988. Compositional and structural variations of phyllosilicates from the Point Sal ophiolite, California. Am. Mineral. 73, 62e76. Bonatti, E., 1976. Serpentinite protrusions from the mid-Atlantic ridge. Earth Planet. Sci. Lett. 32, 107e113. Bonatti, E., Simmons, C., Breger, D., Hamlyn, P.R., Lawrence, J., 1983. Ultramafic rock/ seawater interaction in the oceanic crust: Mg-silicate (sepiolite) deposit from the Indian Ocean floor. Earth Planet. Sci. Lett. 62, 229e238. Buatier, M.D., Früh-Green, G.L., Karpoff, A.M., 1995. Mechanisms of Mgphyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley, Juan de Fuca). Contrib. Mineral. Petrol. 122, 134e151. Buatier, M.D., Monnin, Ch., Früh-Green, G.L., Karpoff, A.M., 2001. Fluid-sediment interactions related to hydrothermal circulation in the Eastern Flank of the Juan de Fuca Ridge. Chem. Geol. 175, 343e360. Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at mid-ocean ridges. J. Geophys. Res. 98, 4163e4172. Capistrant, P.L., Hitzman, M.W., Wood, D., Kelly, N.M., Williams, G., Zimba, M., Kuiper, Y., Jack, D., Stein, H., 2015. Geology of the enterprise hydrothermal nickel deposit, north-western province, Zambia. Econ. Geol. 110, 9e38. Carlson, R.L., 2001. The abundance of ultramafic rocks in the Atlantic ocean crust. Geophys. J. Int. 144, 37e48. Charlou, J.-L., Fouquet, Y., Bougault, H., Donval, J.-P., Etoublea, J., Jean-Baptiste, P., Dapoigny, A., Appriou, P., Rona, P.A., 1998. Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15 200 N fracture zone and the Mid-Atlantic Ridge. Geochimica Cosmochimica Acta 62, 2323e2333. Charlou, J.-L., Donval, J.-P., Fouquet, Y., Jean-Baptiste, P., Holm, N., 2002. Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36 14'N, MAR). Chem. Geol. 191, 345e359.
gicos ba
sicos do Brasil (1:250.000) e CPRM, 1994. Programa Levantamentos geolo
gico do Brasil (in Portuguese). Subprojeto Recursos Minerais. Serviço Geolo Elliot-Meadows, S., 1999. Cordierite-anthophyllite-cummingtonite rocks form the lar deposit, Laurie lake, Manitoba. Can. Mineral. 37, 375e380. Erzinger, J., 1989. Chemical alteration of the ocean crust. Geol. Rundsch. 78, 731e740. Flanklin, J.M., Sangster, D.M., Lydon, J.W., 1981. Volcanic-associated massive sulfide deposits. In: Skinner, B.J. (Ed.), Economic Geology 75 th Anniv. Vol., pp. 485e627. Fleet, M.E., 1977. Origin of disseminated copper-nickel sulfide ore at Frood, Sudbury, Ontario. Econ. Geol. 72, 1449e1458. Floyd, P.A., Winchester, J.A., 1975. Magma-type and tectonic setting discrimination using immobile elements. Earth Planet. Sci. Lett. 27, 211e218. Früh-Green, G.L., Boschi, C., Kelley, D.S., Bernasconi, S.M., 2004. Serpentinization of oceanic peridotites and peridotite-hosted hydrothermal systems. European Geosciences Union. Geophys. Res. Abstr. 6, 05507. German, C.R., Barreiro, B.A., Higgs, N.C., Nelsen, T.A., Ludford, E.M., Palme, r M.R., 1995. Seawater metasomatism in hydrothermal sediments (Escanaba Trough, northeast Pacific). Chem. Geol. 119, 175e190. Gonzalez-Alvarez, I., Sweetapple, M., Lindley, I.D., Kiakar, J., 2013. Hydrothermal Ni : Doriri creek, Papua New Guinea. Ore Geol. Rev. 52, 37e57. ~o geolo
gica da regia ~o de Araguacema-Pequizeiro, Gorayeb, P.S.S., 1981. Evoluça
s. MSc. thesis. Universidade Federal do Para
, Bele
m, Brazil, p. 100 (in PorGoia tuguese, with English abstract). ~o de AraGorayeb, P.S.S., 1989. Corpos serpentiníticos da Faixa Araguaia na regia ~o do Araguaia (Goia
s-Para
). Rev. Bras. Geocie ^ncias guacema-Pequizeiro-Conceiça 19, 51e62. Gorayeb, P.S.S., Moura, C.A.V., Calado, W.M., 2004. Suíte Intrusiva Xambica: um
ico pre
-tecto ^ nico no Cintur~ magmatismo toleítico neoproterozo ao Araguaia. In: SBG, Congresso Brasileiro de Geologia, 42, Anais, 35 (in Portuguese).
ochimie et metalloge
nie de l'environnement Haïmeur, J., 1998. Lithostratigraphie, ge
dimentaire de l'amas sulfure de Douar Lahjar (Guemassa, Maroc). volcano-se Bull. d'Institute Sci. Rabat. 21, 15e30. Haskin, L.A., Haskin, M.A., Frey, F.A., Widman, T.R., 1968. Relative and absolute terrestrial abundances of the rare earths. In: Ahrens, L.H. (Ed.), Origin and Distribution of the Elements, Vol. 1 Pergamon, Oxford, pp. 889e911. Holm, N.G., Charlou, J.-L., 2001. Initial indications of abiotic formation of hydrocarbons in the Rainbow ultramafic hydrothermal system, Mid-Atlantic Ridge. Earth Planet. Sci. Lett. 191, 1e8. Honnorez, J.J., Alt, J.C., Humphris, S.E., 1998. Vivisection and autopsy of active and fossil hydrothermal alterations of basalts beneath and within the TAG hydrothermal mound. In: Herzig, P.M., Humphris, S.E., Miller, D.J., Zierenberg, R.A. (Eds.), Proceedings ODP (Ocean Drilling Program), Scientific Results, vol. 158, pp. 231e254. Humphris, S.E., Morrisson, M.A., Thompson, R.N., 1978. Influence of rock crystallization history upon subsequent lanthanide mobility during hydrothermal alteration of basalts. Chem. Geol. 23, 125e137. Humphris, S.E., Thompson, G., 1978. Hydrothermal alteration of oceanic basalts by seawater. Geochimica Cosmochimica Acta 42, 107e125. Humphris, S.E., Tivey, M.K., 2000. A synthesis of geological and geochemical investigations of the TAG hydrothermal field : insights into fluid-flow and mixing processes in a hydrothermal system. In: Dilek, Y., Moores, E., Elthon, D., Nicolas, A. (Eds.), Ophiolites and Oceanic Crust : New Insights from Field Studies and the Ocean Drilling Program, Geological Society of America, Special Paper,

349, pp. 213e236. Iwao, S., 1970. Clay and silica deposits of volcanic affinity in Japan. In: Watanabe, T. (Ed.), Volcanism and Ore Genesis. University of Tokyo Press, pp. 267e283. Jiang, S.Y., Wang, R.C., Xu, X.S., Zhao, K.D., 2005. Mobility of high field strength elements (HFSE) in magmatic,- metamorphic-, and submarine hydrothermal systems. Phys. Chem. Earth 30, 1020e1029. Jowett, E.C., 1986. Genesis of Kupferschiefer Cu-Ag deposits by convective flow of Rotliegendes brines during Triassic rifting. Econ. Geol. 81, 1823e1837. Keays, R.R., Jowitt, S.M., 2013. The Avebury Ni deposit, Tasmania. A case study of an unconventional nickel deposit. Ore Geol. Rev. 52, 4e17. ~o Couto MagKotschoubey, B., Hieronymus, B., 1990. Clorititos auríferos na regia alh~ aes-Novo Plano (Tocantins): indícios de atividade hidrotermal exalativa na Formaç~ ao Pequizeiro, Grupo Tocantins. In: Congresso Brasileiro de Geologia, 36, p. 147 (in Portuguese). ^nico dos Kotschoubey, B., Hieronymus, B., 1996. Origem e significado geotecto
). In: Conmetassedimentos e meta-ultramafitos da Serra do Quatipuru (Para gresso Brasileiro de Geologia 39, Anais, 6, pp. 22e25 (in Portuguese). Kotschoubey, B., Hieronymus, B., Rodrigues, O.B., Amaral, R.T., 1996. Basaltos e
rea da Serra do Tapa (Pa): prov
serpentinitos da a aveis testemunhos de um complexo ofiolítico pouco evoluído e desmembrado. In: Congresso Brasileiro de Geologia 39, Anais, 6, pp. 25e28 (in Portuguese). Kotschoubey, B., Hieronymus, B., Albuquerque, C.A.R., 2005. Disrupted peridotites and basalts from the Neoproterozoic Araguaia Belt (Northern Brazil): remnants of a poorly evolved oceanic crust? J. S. Am. Earth Sci. 20, 211e230. € nberg, D., Stoffers, P., Horz, K., Lackschewitz, K.S., Singer, A., Botz, R., Garbe-Scho 2000a. Formation and transformation of clay minerals in the hydrothermal deposits of Middle Valley, Juan de Fuca Ridge. ODP Leg 169. Econ. Geol. 95, 361e390. €nberg, D., Stoffers, P., 2000b. Lackschewitz, K.S., Singer, A., Botz, R., Garbe-Scho Mineralogy and geochemistry of clay minerals near a hydrothermal site in the Escanaba trough, Gorda ridge, Northeast Pacific Ocean. In: Zierenberg, R.A., Fouquet, Y., Miller, D.J., Normark, W.R. (Eds.), Proceedings ODP (Ocean Drilling Program), Scientific Results, vol. 169, pp. 1e24. Lackschewitz, K.S., Devey, C.W., Stoffers, P., Botz, R., Eisenhauer, A., Kummetz, M., Schmidt, M., Singer, A., 2004. Mineralogical, geochemical and isotopic characteristics of hydrothermal alteration processes in the active, submarine, felsichosted PACMANUS field, Manus Basin, Papua New Guinea. Geochimica Cosmochimica Acta 68, 4405e4427. Lamar~ ao, C.N., Kotschoubey, B., 1996. Granitoides Santa Luzia: registro do mag~o Araguaia na regi~ matismo granitico brasiliano do Cintura ao de Paraiso do ^ncias 26, 277e288. Tocantins. Rev. Bras. Geocie Large, R.R., 1975. Zonation of hydrothermal minerals at the Juno mine, Tennant creek Goldfield, Central Australia. Econ. Geol. 70, 1387e1413. Leblanc, M., Billaud, P., 1990. Zoned and recurrent deposition of Na-Mg-Fe-Si exhalites and Cu-Fe sulfides along synsedimentary faults (Bleïda, Morocco). Econ. Geol. 85, 1759e1769. Le Roex, A.P., Dick, H.J.B., Erlank, A.J., Reid, A.M., Frey, F.A., Hart, S.R., 1983. Geochemistry, mineralogy and petrogenesis of lavas erupted along the Southwest Indian Ridge between Bouvet triple junction and 11 degrees east. J. Petrol. 24, 267-218. Liaghat, S., MacLean, H., 1995. Lithogeochemistry of altered rocks at the New Insco VMS deposit, Noranda, Quebec. J. Geochem. Explor. 52, 333e350. Louvel, M., 2014. Fluids in the crust: constraints on the mobilization of Zr in magmatic-hydrothermal processes in subduction zones from in situ fluid-melt portioning experiments. Amer. Mineral. 99, 1616e1625. Lowell, R.P., Rona, P.A., 2002. Seafloor hydrothermal systems driven by the serpentinization of peridotite. Geophys. Res. Lett. 29, 26e1-26-4. Lusk, J., 1976. A possible volcanic-exhalative origin for lenticular nickel sulfide deposits of volcanic association with special reference to those in Western Australia. Can. J. Earth Sci. 13, 451e458. McCaig, A.M., Cliff, R.A., Frost, B.R., Escartin, J., MacLeod, C., 2005. Extreme isotopic and chemical alteration in an extensional detachment fault, 15 45'N, midAtlantic ridge: a reaction zone for Rainbow-type vent fluids ? Geophys. Res. Abstr. 7, 09807. McCaig, A.M., Cliff, R.A., Escartin, J., Fallick, A.E., 2007. Oceanic detachment faults focus very large volumes of black smoker fluids. Geology 35, 935e938. McLeod, R.L., Stanton, R.L., 1984. Phyllosilicates and associated minerals in some Paleozoic stratiform sulfide deposits in Southeastern Australia. Econ. Geol. 79, 1e22. McQueen, K.G., 1989. Sediment geochemistry and base metal sulphide mineralisation in the Quidong Area, southern New South Wales, Australia. Miner. Deposita 24, 100e110. Melekestseva, I.Yu., Zaykov, V.V., Nimis, P., Tretyakov, G.A., Tessalina, S.G., 2013. Cu(Ni-Co-Au)-bearing massive sulfide deposits associated with maficultramafic rocks of the Main Ural Fault, South Urals : geological structures, ore texture and mineralogical features, comparison with modern analogs. Ore Geol. Rev. 52, 18e36.
vel, C., 2003. Serpeninization of abyssal peridotites at mid-ocean ridges. Me Comptes Rendus Geosci. 335, 825e852. Miyagawa, L.J.P., Gorayeb, P.S.S., 2013. Basaltos almofadados da suite ofiolítica Morro ^nico na porç~ do Agostinho: registros de fundo ocea ao centro-oeste do Cintur~ ao
rie científica 13, 111e124. Araguaia. Rev. Geol. USP, se Miyashiro, A., Shido, F., Kanehira, K., 1979. Metasomatic chloritization of gabbros in the Mid-Atlantic Ridge near 30oN. Mar. Geol. 33, M47eM52. Mottl, M.J., 1983. Metabasalts, axial hot springs and the structure of hydrothermal

B. Kotschoubey et al. / Journal of South American Earth Sciences 69 (2016) 171e193 systems at mid-ocean ridges. Geol. Soc. Am. Bull. 94, 161e180. Mottl, M.J., Holland, H.D., 1978. Chemical exchange during hydrothermal alteration of basalt by seawater - 1. Experimental results for major and minor components of seawater. Geochimica Cosmochimica Acta 42, 1103e1115. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na, and K in carbonaceous and ordinary chondrites. Geochimica Cosmochimica Acta 38, 757e775. Nimis, P., Tesalina, S.G., Omenetto, P., Tartarotti, P., Lerouge, C., 2004. Phyllosilicate minerals in the hydrothermal mafic-ultramafic-hosted massive-sulfide deposit of Ivanovka (southern Urals): comparison with modern ocean seafloor analogues. Contrib. Mineral. Petrol. 147, 363e383. Nimis, P., Zaykov, V.V., Omenetto, P., Melekestseva, I.Y., Tesalina, S.G., Orgeval, J.J., 2008. Peculiarities of some maficeultramafic- and ultramafic-hosted massive sulfide deposits from the Main Uralian Fault Zone, southern Urals. Ore Geol. Rev. 33, 49e69. Osborne, G.A., 2001. Geotectonics and mineralization in the ophiolite assemblages
and Tocantins states, Central Brazil. In: of the Araguaia marginal basin, Para ^nia, 7, Bele
m, Resumos Expandidos CD- ROM. SBG/Norte, Simp. Geol. Amazo Paix~ ao, M.A.P., Nilson, A.A., 2001a. Basaltos almofadados e harzburgitos do Morro
gica e petrografia do Agostinho (Araguacema, Tocantins): caracterizaç~ ao geolo ^nia de fragmento ofiolítico na Faixa Araguaia. In: SBG/Norte, Simp. Geol. Amazo 7, Resumos Expandidos, CD-ROM (in Portuguese). Paix~ ao, M.A.P., Nilson, A.A., 2001b. Complexo ofiolítico Quatipuru: base de uma ~o do Moho em antiga litosfera oce^ zona de transiça anica da Faixa Araguaia. In: ^nia 7, Resumos Expandidos, CD-ROM (in SBG/Norte, Simp. Geol. Amazo Portuguese). Paix~ ao, M.A.P., Nilson, A.A., 2002. Fragmentos ofiolíticos da Faixa Araguaia:
gica e implicaço ~es tecto ^ nicas. In: Klein, E.L., Vasquez, M.L., caracterizaç~ ao geolo ~es a  Geologia da Amazo ^nia, vol. 3, pp. 85e104 Rosa-Costa, L.T. (Eds.), Contribuiço (in Portuguese). Paix~ ao, M.A.P., Nilson, A.A., Dantas, E.L., 2008. The Neoproterozoic Quatipuru ophiolite and the Araguaia fold belt, central-northern Brazil, compared with correlatives in NW Africa. In: Pankhurst, R.J., Trouw, R.A.J., Brito Neves, B.B., de Wit, M.J. (Eds.), West Gondwana: Pre-Cenozoic Correlations across the South Atlantic Region, Geol. Soc. London, Special Publications, vol. 294, pp. 297e318. ^nese da Faixa Araguaia. In: Silva, M.G., Paix~ ao, M.A.P., Gorayeb, P.S.S., 2014. Metaloge ^nese das Províncias Rocha Neto, M.B., Jost, H., Kuyumjian, R.M. (Eds.), Metaloge ^ nicas Brasileiras. CPRM e Serviço Geolo
gico do Brasil, Brasília, Tecto pp. 467e488. Pierret, M.-C., Clauer, N., Blanc, G., 2000. Influence hydrothermale dans les sedi
tude de quelques e
le
ments traces. ments de fosses de la mer Rouge par
le Oceanol. Acta 23, 783e792. Poinsignon, J.R., 2009. O magmatismo granitico e os seus efeitos na regi~ ao de ~, TO (Dissertaça ~o de Mestrado). Universidade Federal do Xambioa-Araguana
m, Brazil, p. 173 (in Portuguese). Para, Bele ~o do magmaPoinsignon, J.R., Kotschoubey, B., Villas, R.N.N., 2010. Caracterizaça ~o de Xambioa
, TO. In: Contismo granitico e seus efeitos hidrotermais, regia gresso Brasileiro de Geologia, 45, p. 230 (in Portuguese). Polat, A., Hofmann, A.W., Münker, C., Regelous, M., Appel, P.W.U., 2003. Contrasting geochemical pattern in the 3.7-3.8 Ga pillow basalt cores and rims, Issua greenstone belt, Southwest Greenland: implications for post-magmatic alteration processes. Geochimica Cosmochimica Acta 67, 441e457. Reinhardt, J., 1987. Cordierite-anthophillite rocks from north-west Queesland, Australia: metamorphosed magnesian pelites. J. Metamorph. Geol. 5, 451e472. Robinson, D.J., Hutchinson, R.W., 1982. Evidence for a volcanogenic-exhalative origin of a massive nickel sulphide deposit at Redstone, Timmins, Ontario. In: Hutchinson, R.W., Spencer, C.D., Franklin, J.M. (Eds.), Precambrian Sulphide Deposits, H.S. Robinson Memorial Volume, Geological Association of Canada. Special Paper, vol. 25, pp. 211e254. Rona, P.A., 1988. Hydrothermal mineralizations at oceanic ridges. Can. Mineral. 26, 431e465. Rona, P.A., 2005. TAG hydrothermal field: a key to modern and ancient seafloor

193

hydrothermal VMS ore-forming systems. Mineral. Depos. Res. Meet. Glob. Chall. 687e690. Rubin, J., Henry, C.D., Price, J.G., 1993. The mobility of zirconium and other “immobile” elements during hydrothermal alteration. Chem. Geol. 110, 29e47. Ruiz, C., Arribas, A., Arribas Jr., A., 2002. Mineralogy and geochemistry of the Masa Valverde blind massive sulphide deposit, Iberian pyrite belt (Spain). Ore Geol. Rev. 19, 1e22. ~ a, J., Velasco, F., Yusta, I., 2000. Hydrothermal alteration of felsic S
anchez-Espan volcanic rocks associated with massive sulphide deposition in the northern Iberian Pyrite Belt (SW Spain). Appl. Geochem. 15, 1265e1290. Schermerhorn, L.J.G., 1978. Epigenetic magnesium metasomatism or syngenetic €rvi. Trans. Inst. Min. Metall. 87, chloritite metamorphism at Falun and Orija B162eB167. Schiffman, P., Staudigel, H., 1995. The smectite to chlorite transition in a fossil seamount hydrothermal system: the Basement Complex of la Palma, Canary Islands. J. Metamorph. Geol. 13, 487e498. Seyfried, W.E., Dibble, W.E., 1980. Seawater-peridotite interaction at 300  C and 500 bars: implications for the origin of oceanic serpentinites. Geochimica Cosmochimica Acta 44, 309e321. Shervais, J.W., 1982. Ti-V plots and the petrogenesis of modern and ophiolitic lavas. Earth Planet. Sci. Lett. 59, 101e118. Skelton, A.D.L., Valley, J.W., 2000. The relative timing of serpentinisation and mantle exhumation at the ocean-continent transition, Iberia: constraints from oxygen isotopes. Earth Planet. Sci. Lett. 178, 327e338. ~o e ge ^nese das formaço ~es ferríSousa, W.S.P., Kotschoubey, B., 2005. Caracterizaça ~o de Xambio
~ feras e dos xistos grafitosos do Grupo Estrondo na regia a-Araguana e TO. In: Cong. Bras. Geoq. 10 e Simp. Países Mercosul 2, Porto de Galinhas, Pernambuco, Brasil. Resumos Expandidos, CD-ROM (in Portuguese).
. Folha SB 22-Z-B. Estados do Par
Souza, J.O., Moreton, L.C., 1995. Xambioa a e
gicos Ba
sicos do Brasil. CPRM. Tocantins. Programa Levantamentos Geolo Sturz, A., Itoh, M., Smith, S., 1998. Mineralogy and chemical composition of clay minerals, TAG hydrothermal mound. In: Herzig, P.M. (Ed.). In: Humphris, S.E., Miller, D.J., Zierenberg, R.A. (Eds.), Proceedings ODP (Ocean Drilling Program), Scientific Results, vol. 158, pp. 277e284. Teagle, D.A.H., Alt, J.C., 2004a. Hydrothermal alteration of basalts beneath the Bent Hill massive sulfide deposit, Middle Valley, Juan de Fuca. Econ. Geol. 99, 561e584. Teagle, D.A.H., Alt, J.C., Halliday, A.H., 1998. Tracing the chemical evolution of fluids during hydrothermal discharge: constraints from anhydrite recovered in ODP Hole 504B. Earth Planet. Sci. Lett. 155, 167e182. Teagle, D.A.H., Alt, J.C., 2004b. Hydrothermal alteration of basalts beneath the Bent Hill massive sulfide deposit, Middle Valley, Juan de Fuca ridge. Econ. Geol. 99, 561e584. Vallance, T.G., 1967. Mafic rock alteration and isochemical development of some cordierite-anthophyllite rocks. J. Petrol. 8, 84e96. Villas, R.N., Lima, A.D.P., Kotschoubey, B., Neves, M.P., Osborne, G.A., 2007. Contexto
gico e origem da mineralizaça ~o sulfetada estratiforme de Sa ~o Martim, SW geolo ~o Araguaia, Par
^ncias 37, 305e323. do Cintura a. Rev. Bras. Geocie Wetzel, I.R., Shock, E.L., 2000. Distinguishing ultramafic- from basalt-hosted submarine hydrothermal systems by comparing calculated vent fluid compositions. J. Geophys. Res. 105, 8319e8340. Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chem. Geol. 20, 325e343. Yoshitake, N., Arai, S., Ishida, Y., Tamura, A., 2009. Geochemical characteristics of chloritization of mafic crust from northern Oman ophiolite: implications for estimating the chemical budget of hydrothermal alteration of the oceanic lithosphere. Mineral. Petrol. Sci. 104, 156e163. Zang, W., Fyfe, W.S., 1995. Chloritization of the hydrothermally altered bedrock at
Bahia gold deposit, Caraj
the Igarape as. Brazil Miner. Deposita 30, 30e38.