Neoarchean , Rhyacian and Neoproterozoic units of the Saquinho region , eastern Rio Piranhas-Seridó domain , Borborema Province ( northeastern Brazil ) : implications for the stratigraphic model

*Corresponding author Rogério Cavalcante E-mail address: rogerio.cavalcante@cprm.gov.br


Introduction
The Seridó region, in the Borborema Province, is one of the main metallogenic provinces of Brazil, with important deposits of tungsten, gold, iron, tantalum, niobium, and gems (Beurlen 1995;Biondi 2003;Cavalcante et al. 2015).The mineral activity in this region dates back to more than 70 years ago, being one of the largest tungsten producers in the country, with emphasis on mines in the Currais Novos, Bodó and Lajes regions, in Rio Grande do Norte (Salim 1979(Salim , 1993;;Maranhão et al. 1986;Souza Neto et al. 2008).This mineral abundance has been attributed to several factors including tectonic setting, structural relations, mineralizing systems, intrusive bodies related to the source of mineralizing fluids, and the presence of fertile Archean rocks.Dantas et al. (2014) and Ruiz et al. (2018) demonstrated the contribution of Archean material to basement gneissic rocks and metaultramafic rock lenses in the vicinity of western São Tomé and Bonfim mine (in Lajes, Fig. 1), respectively.The U-Pb zircon dates indicated Paleoarchean ages for these rocks.Costa et al. (2014) suggested that their presence might be associated with the origin of the Fe-Ti-V oxide mineralizations in the region.Thus, it is important to map and improve the knowledge on the distribution of Archean portions in the region since these terrains can be strong indicators of the metal potential.
In the Saquinho region, there are questions regarding the existing iron and carbonate formations as whether they represent Ediacaran supracrustal sequences (Jucurutu Formation of the Seridó Group) or older units, as well as their stratigraphic positioning.
Most of the data presented here are from a stratigraphic borehole in the Saquinho region (FD-SE-002) located 20 km northwest of the municipality of Cruzeta, in the central portion of Rio Grande do Norte.The drilling performed by the CPRM-Geological Survey of Brazil aimed at investigating the lithotypes of the Jucurutu Formation (Pessoa 1986;Medeiros et al. 2012a), the iron ore host of the Saquinho mine, and exploring the possibility of stratigraphic repetition of the Seridó Group/basement.As such, we performed the lithostratigraphic/structural characterization of the Jucurutu Formation in the Saquinho region, analyzed the nature of the contact between the metasediments of the Seridó Group and its basement, evaluated the presence of metamaficmetaultramafic rocks, and finally, presented a stratigraphic model for the studied region, besides suggesting processes responsible for the genesis of the iron ore in Saquinho.

Regional geology
The study site is located in the northeastern Borborema Province as defined by Almeida et al. (1981).In recent years, some authors have proposed a tectonic framework (totally or partially) for the Province, based on the concept of tectonostratigraphic terrain and/or domains (Jardim de Sá et al. 1992;Santos 1996;Jardim de Sá 1994;Van Schmus et al. 1995;Brito Neves et al. 2000;Santos et al. 2000).The area is inserted in the Rio Piranhas-Seridó Domain, bordered by the Portalegre and Picuí-João Câmara Shear Zones to the west and east, respectively.To the north, it is covered by Mesozoic (Potiguar Basin) and Cenozoic sediments, while the southern limit is defined by the Patos Lineament (Fig. 1).
In the study site and adjacencies, Medeiros et al. (2012a) and Cavalcante et al. (2016) mapped units of the Caicó Complex, augen gneisses (Rhyacian) and the Jucurutu Formation (Fig. 2) while more recent studies on the iron units were presented by Campos (2011), Sial et al. (2015) and Dantas et al. (2017).
The augen gneisses are generally granitic, representing a transitional suite with signature between alkaline and potassic calc-alkaline (Medeiros et al. 2012b).Hollanda et al. (2011) and Medeiros et al. (2012b) determined (U-Pb in zircon) Rhyacian ages (2.25 to 2.15 Ga).These are representatives of the Poço da Cruz suite defined by Ferreira (1998) and in part of the G2type granitoids referred to by Jardim de Sá et al. (1981) and Jardim de Sa (1994).
U-Pb ages in detrital zircons of rocks from the Seridó and Jucurutu formations obtained by Van Schmus et al. (2003) and Hollanda et al. (2015), indicated the end of the Neoproterozoic (Ediacaran) as the sedimentation period for these two formations.

Sampling and analytical procedures
The samples were obtained from the Saquinho region outcrops and from the FD-SE-002 drilling hole (Fig. 2) performed in the homonymous iron mine.The drill-core samples were oriented every 30 meters (approximately), allowing to obtain the structural parameters, and correlating them with the field data, logging and determining the geological profile of the borehole, following the methodology described by Medeiros et al. (2017) for a stratigraphic hole in the Currais Novos region (Fig. 1).
Petrographic analyses were performed at several borehole intervals to characterize the lithologies, while the whole rock C and O isotopic compositions were determined in the carbonate layer with a more homogeneous mineralogical constitution (purest marbles).The U-Pb zircon determinations were conducted in three samples (granitic augen gneiss, monzogranitic microaugen gneiss, and metagabbronorite), collected in drill-core, to define the chronology of the geological events.
Isotopic data (δ 13 C and δ 18 O) were obtained at the Laboratory of Stable Isotopes of the Universidade of Brasília (UnB), where the isotopic composition of carbon and oxygen was determined by continuous flow Isotope Ratio Mass Spectrometry (IRMS) and gaseous source with a DELTA V PLUS (Thermo®) magnetic sector and double entry.For the analyses, the Thermo® GasBench II accessory was used to insert 300μg of sample into clean glass bottles closed with rubber septum caps, which were stored in a block at a controlled temperature of 72ºC.A gas chromatography needle is then used to perform the so-called flushfill process for exchanging the flask atmospheric air by injecting a continuous helium stream for 5 min to render the reaction medium inert.A metering pump is used to insert five drops of 100% H 3 PO 4 into each flask, allowing the CO 2 extraction reaction to occur for one hour.After CO 2 extraction, a second chromatographic needle collects the gas, sending it to a chromatographic column for separation using helium as a carrier; the gas is then injected into the source of ions for the isotopic measurements.The δ 13 C and δ 18 O values are expressed as per thousand (‰) based on the Vienna Pee Dee Belemnit (V-PDB) and Vienna Standard Mean Ocean Water (V-SMOW) standards, respectively.The analytical errors are 0.05 and 0.10‰ for δ 13 C and δ 18 O, respectively.
For the U-Pb zircon study, the crystals were separated after crushing, sieving (˂500μ) and heavy minerals concentration via panning in the Grinding Laboratory of the Department of Geology of the Universidade Federal do Rio Grande do Norte (DG/UFRN).Subsequently, a Frantz Magnetic Separator was used to separate the non-magnetic minerals, of which the zircon fractions were separated manually using a binocular loupe to select clear minerals with no visible inclusions.The crystals were mounted using double-sided tape, embedded in epoxy resin (cold), worn out for exposing the inside of the grains and polished with 0.25 μm diamond paste, followed by ultrasound bath in dilute nitric acid (3%), Nanopure® water, and finally in acetone to extract any residual moisture.The analyses were conducted in the Laboratory of Geochronology of the UnB, following the procedure described by Buhn et al. (2009).
The isotopic determinations were performed on the LAM-MC-ICP-MS Neptune (Thermo-Finnigan) spectrometer coupled to the Nd-YAG (λ = 213nm) Laser Ablation System (New Wave Research, USA).The ablation occurred in 25-40 μm spots, at 10 Hz frequency and 0.19 to 1.02 J/cm 2 fluence.The pulverized material was carried by He (~0.40 L/min) and Ar (~0.90 L/min) flow.The GJ-1 international standard was used for correcting equipment drift and the fractionation between the U and Pb isotopes.Analyses were also performed in the international standard zircon 91500 to verify accuracy.The data were acquired in 40 cycles of 1 second while the collection procedure followed the reading sequence: 1 blank, 1 standard, 4 samples, 1 blank and 1 standard.The intensities of the 202 Hg, 204 (Pb+Hg), 206 Pb, 207 Pb, 208 Pb and 238 U masses were determined at each reading.The raw data, including blank corrections, equipment drift and common lead, were reduced using the tools of the Chronus software in Excel (Oliveira 2015).The ages were calculated and the Concordia diagrams were plotted using ISOPLOT 3.0 (Ludwig 2003).

Surface geology of the Saquinho area
The geology of the Saquinho mine area (Fig. 3) consists of Rhyacian biotite augen gneisses, superimposed by schists, paragneisses, banded iron formations and marbles of the Jucurutu Formation.Cover/soil and colluvial deposits correspond to younger units.
The biotite augen gneisses with amphibole occur in the southern and eastern regions (Fig. 3), displaying granitic composition, pinkish to grayish color, and coarse grain (Fig. 4A).The porphyroclastic texture is given by potassic feldspar crystals up to 5 cm long, which define a mineral stretching lineation with a low angle plunge (Fig. 4A).
Quartz-chlorite-biotite schists, locally with garnet, outcrop in the central and eastern portions of the area (Fig. 3).These are gray to yellowish rocks, with grano-lepidoblastic texture, and a prominent foliation marked by phyllosilicates.
The banded iron formations occurring in the central and eastern areas (Fig. 3) are characterized by an ocher-reddish soil.The lithotypes alternate laminations rich in quartz and magnetite+hematite, magnetite+hematite+amphibole, magnetite+hematite+sulfide, and more rarely magnetite +carbonate.
The marbles that crop out in the western and mid-eastern areas (Fig. 3) are granoblastic with medium grain, consisting essentially of whitish, and more rarely gray, calcite forming alternating white and grayish levels (Fig. 4C).
The amphibole-biotite schists with garnet are restricted to a single outcropping body in the eastern region (Fig. 3), and show green to greenish-gray colors.

Stratigraphic compartments
The lithologies intercepted by the drilling hole (FD-SE-002) can be grouped into three large compartments (Fig. 5), from top to bottom, as metasedimentary rocks (Jucurutu Formation), granitic augen gneisses (Rhyacian), and a succession of
In this compartment, there is a group of metamaficmetaultramafic rocks of greenish coloration, medium-to coarse-grained, composed essentially of amphiboles/ pyroxenes that constitute the amphibolite and gabbroic varieties (Figs.8C and 8F).

Structural features
Steeply-dipping NE-SW trending transcurrent shear zones, associated with the D3-phase, were observed at the outcrop scale and regionally northeast of the studied region, as well as generally open folds with a sub-vertical axial surface and a gently-dipping NE-NW axis.
The brittle deformation (faults and/or fractures), late-or post-D3, is commonly evidenced in both the lithotypes of the Jucurutu Formation (Fig. 9) and the Rhyacian augen gneisses.Faults presenting normal displacement were observed the pit of the Saquinho mine (Fig. 9A), and in the drill-core, being the latter expressed by the displacement of carbonate and/or quartz veins in amphibole-granite-biotite paragneiss/schist (Fig. 9B) and in quartz veins in banded iron formations (Fig. 9C).
The fault/fracture direction oscillates between 0º and 40ºAz predominantly, with steep dips to east-southeast in the western portion of the Saquinho ore deposit, and westnorthwest in the eastern part, which are consistent with the data reported by Pessoa (1986).These features, along with the associated kinematic markers, indicate an architecture similar to a system of normal faults.

C and O isotopes
The C and O isotopic compositions were determined in the marbles as to correlate them with those of the Jucurutu Formation already studied by Nascimento et al. (2004Nascimento et al. ( , 2007)), Campos (2011), Figueiredo (2012) and Sial et al. (2015).The δ 13 C and δ 18 O values were determined in fifteen marble samples collected between 47 and 66 meters deep, from the Saquinho borehole (Table 1).

SAMPLE DEPTH (m)
δ 13 C‰ (V-PDB) Figure 10 -Isotopic (C and O) chemostratigraphy of the marbles of the Saquinho stratigraphic hole, using data from Table 1.
All the analyzed samples correspond to light gray, calcitic granoblastic marbles, with δ 13 C values between 2.4 and 10.4‰, and δ 18 O between -9.7 and -5.3‰ (Table 1 and Fig. 10) The δ 13 C and δ 18 O values available in the literature for the studied region (Nascimento et al. 2007;Campos 2011;Figueiredo 2012;Sial et al. 2015) show that δ 13 C values for the Jucurutu Formation marbles are generally positive and tend to be constant when the original composition of the rock is preserved (Fig. 11).Whereas the marbles of the Seridó Formation present positive (São Mamede) and negative values (SW of Cruzeiro da Maniçoba-Currais Novos).
The  13 C and  18 O values obtained in this work and in the literature suggest that the Saquinho marbles can be correlated with those of the Jucurutu Formation (Fig. 11) and, consequently, with the deposition in a marineglacial environment, corroborating the models proposed by Nascimento et al. (2007) and Sial et al. (2015) for this unit.

U-Pb zircon dating
Drill-cores of the intermediate (granitic augen gneiss) and lower (monzonitic microaugen gneiss and meta-gabbronorite of the Saquinho Complex) compartments were collected from the Saquinho borehole to obtain their crystallization age, considered as representative of the basement of the supracrustal sequence of the Jucurutu Formation (Fig. 5).

Augen gneiss
The amphibole-bearing biotite augen gneiss with granitic composition was sampled between 121.0 and 122.0 meter deep (Fig. 5).The zircon grains (Fig. 12) are generally elongated prismatic, about 150-200 μm long (length/width = 2.6 to 3.1), usually with growth zonation (concentric) along the rims.The scanning electron microscope (SEM) show a constant presence of numerous xenomorphic inclusions and fractures.The crystals are not clear, displaying corroded terminations frequently and, sometimes, evident corrosion gulfs, whereas crystals with rims of metamorphic overgrowth were not observed.
The analyses shown in Table 2 and Figure 13A are generally discordant while only 3 of the 25 zircons have     discordance <5%.The 206 Pb/ 204 Pb ratio also varied greatly, suggesting possible continuous losses of Pb and apparent ages of 700 to 2000 Ma.These varied analytical results obtained for the same rock possibly resulted from the previously mentioned frequent fractures observed in these non-clear and corroded zircons and/or the effect of a metamorphic event (Ediacaran?).
This Rhyacian age (2210 ± 13 Ma) is considered the best approximation to the crystallization age of the augen gneiss protolith of the Saquinho hole, in agreement with the U-Pb zircon age (2171 ± 20 Ma) obtained for similar lithotypes in the Genezaré/Caicó region by Medeiros et al. (2012b), and other augen gneiss bodies in the Rio Piranhas-Seridó Domain by Hollanda et al. (2011).Souza et al. (2016) also obtained Rhyacian ages for orthogneisses from the Caicó region.
The Ediacaran age (ca.609 Ma) of the discordant lower intercept (Fig. 13A) could possibly reflect the action of the Brasiliano Orogenesis.However, the texture analysis of the zircons showed no evidence of neoformed zircons or overgrowth textures and/or rims of metamorphic recrystallization.Even so, Pb loss due to orogeny may be the cause for that age.

Microaugen gneiss
A sample of amphibole-biotite microaugen gneiss (AR-62C), of monzogranitic composition and grayish color, was collected between 210.3 and 211.3 meters deep (Fig. 5).The zircons are predominantly prismatic, sometimes bipyramidal, with well-preserved faces, although some crystals have corroded rims.They are well developed, between 350 and 500 μm long whereas the small ones are 150-200 μm long (length/width = 3.1 to 3.9).Although there are few fractured crystals with, sometimes, filled fractures and ellipsoidal quartz inclusions, most zircon crystals are clear, with simple, homogeneous structures and some discrete growth zoning.Rarely, the crystals exhibited a thin rim that could eventually be attributed to metamorphic growth.The analysis points were generally positioned in clear locations in the crystal center, although the crystals were always homogeneous.Fig. 14 shows photos illustrating the types of crystals analyzed.
The analytical results in Table 3 do not show significant differences in the isotopic ratios and apparent ages between major and minor zircons.All points are concordant or less than 2% discordant, and yielded a concordant age of 2512 ± 3.2 Ma (Fig. 15).The zircon textures and the Th/U ratio between 0.13 and 0.65 are compatible with magmatic rocks (Beluosova et al. 2002;Hoskin and Schaltegger 2003), allowing to interpret this age as the crystallization time of the protolith of this microaugen gneiss, indicating the end of Neoarchean as the crystallization age of this unit.

Metagabbronorite
A metagabbronorite sample from the metamafic sequence of the lower compartment (AR-75C) was obtained between 331.15 and 332.05 m deep (Fig. 5).The zircons are generally short prismatic with length to width ratio seldom exceeding 2. In general, the crystals are between 50 and 150 μm long, have almost always corroded and irregular rims, many shaped like gulfs, with broken and jagged frontal endings (Fig. 16).There are inclusions of small circular and, sometimes, aligned minerals whose crystals often exhibit discontinuous, sinuous fractures.However, these crystals do not have complex textures, being homogeneous and limpid with discrete growth zonation, when present.Rarely, the crystals show a probable rim of metamorphic overgrowth, but the analyses were performed in the crystals center to avoid the fractures.
The data in Table 4 show that the great majority of the crystals plot on or very close to the concordia, with <2% discordance, and rare points a little more discordant.The high values of the 206 Pb/ 204 Pb ratio indicate the excellent quality of the obtained values while the Th/U ratios up to 0.6 are compatible with those of igneous zircons (Beluosova et al. 2002;Hoskin and Schaltegger 2003).Together, the points produce an excellent alignment (Fig. 17) defining an age of 2501 ± 2.7 Ma, interpreted as the crystallization age of the rock protolith at the end of Neoarchean.

Geological model
A geological model using the surface mapping data in semi-detail and detail scales, mining pits mapping, Saquinho stratigraphic hole (FD-SE -002) and drill holes provided by Susa Indústria e Comércio de Productos Minerários Ltd is here proposed to understand better the geodynamic events in the region of the Saquinho iron deposit.The vertical geological section (Fig. 18) shows, at the top, a rock sequence of the Jucurutu Formation represented by biotite paragneiss, amphibole-biotite schist, marble, garnet amphibolite and banded iron formation.Below the Jucurutu Formation, a biotite augen gneiss with amphibole was observed, whose contact with the upper strata occurs due to nonconformity, and, in some places, through shear zones.The basal contact between the augen gneiss and the underlying Archean unit was considered intrusive.
These lithological contents and the obtained Neoarchean age, previously unknown for the Rio Piranhas-Seridó Domain, allows suggesting the term Saquinho Complex for this unit.The site of the FD-SE-002 hole at depth of 165 to 440m is taken as the region of its stratotype.In this context, a geological model is proposed for the region of the Saquinho iron mine, from a substrate with age of the end of the Neoarchean.Subsequently, this substrate was intruded by a porphyritic granitoid (protolith of the Rhyacian augen gneiss) representing the intermediate compartment.
A weathering/erosion process followed, with subsidence at least partially favored by the action of normal faults (?) and beginning of sediment deposition of what would become the Saquinho Basin (upper compartment).This compartment is represented by the metasediments of the Jucurutu Formation (Ediacaran), with the banded iron formation developing at the base of the sequence.In the final installation phase, the basin is affected by the Brasiliano orogenesis, when folds and reactivations of the normal faults occurred, determining the final structuring of the basin (Figure 19).
The participation of mafic rocks such as those observed in the lower compartment in the iron mineralization process is commonly reported in the literature (Lindenmayer et al. 2002;Lobato et al., 2005;Rosière et al., 2006;Zuchetti 2007).In the genesis of the iron formations, the iron source would be related to two possibilities: (i) leaching of iron-rich lithologies (e.g., mafic/ultramafic rocks); (ii) introduced by subaquatic hydrothermal discharges into lake environments and ocean basins.For both hypotheses to be plausible, a stratified density system must act, that is, currents need to transport the reduced iron from deeper anoxic waters to a shallow water oxygenated environment where Fe 2+ is oxidized and precipitated as oxides and carbonates.
Currently, several authors (Klein and Beukes 1989;Klein 2005;Pirajno 2009) agree that iron was introduced into the ocean from hydrothermal vents, followed by the deposition of iron formations on the continental shelf and upper slope, in a layer of oxide-anoxic stratified water.
The proposed origin for the iron formations in the Saquinho area would be related to the interaction of hydrothermal fluids with the Neoarchean mafic rocks previously emplaced in the floor of this basin, which acted as leachers, removing (mainly) elements such as iron and magnesium and enriching the paleobasin.In addition, we speculate the presence of vents that would be responsible for the hydrothermal iron discharges in the basins.
The stratigraphic study based on the δ 13 C and δ 18 O isotopic data of the Saquinho marbles show compositions with a broad spectrum signature represented by the δ 13 C (2.4 and 10.4‰) and δ 18 O (-9.7 and -5.3‰) values, which can be interpreted as a carbon source derived from marine carbonate rocks (Nascimento et al. 2004(Nascimento et al. , 2007;;Sial et al. 2015;Campos 2011).

Conclusions
The age of 2210 ± 13 Ma obtained for the augen gneiss of syenogranite to alkali feldspar granite, from 107.9 to 165.1 m deep, is similar to those mapped and dated in adjacent regions, indicating a Rhyacian age for this rock.
The ages of 2512 and 2501 Ma (end of Neoarchean) obtained for the rocks of the units placed below the augen gneisses (lower compartment of the stratigraphic hole) indicate a unit distinct from those previously mentioned in the Rio Piranhas-Seridó Domain, which we name here as the Saquinho Complex.
The presence of metamafic-metaultramafic rocks with disseminated sulfide (pyrite, chalcopyrite, and pyrrhotite), even in the small amount observed, allow considering these rocks as a new geological/metallogenic potential for the region.
The marbles in the Saquinho hole occur between 46.7 and 66.3 m deep, presenting petrographic, stratigraphic and isotopic compositions (δ 13 C and δ 18 O) similar to those described in the literature for the Jucurutu Formation, but distinct from those of the Seridó Formation and those considered as of older/Archean ages.The carbon and oxygen isotopic data show values that corroborate the models proposed in the literature, which interpreted as from a source derived from marine carbonate rocks.
The geological model of the Saquinho Basin is divided into three large compartments, with the installation of brittle and/or ductile/brittle structures with successive reactivations that probably acted as conduits for percolation of fluids that generated the ferriferous formations.

Figure 1 -
Figure 1 -Geological framework of northeastern Borborema Province showing the studied area.Modified from Medeiros et al. (2017).

Figure 3 -
Figure 3 -Geological map of the northern portion of the Saquinho iron mine, adapted from the data provided by Susa Indústria e Comércio de Produtos Minerários Ltd.

Figure 4 -
Figure 4 -Field features of the rocks around the Saquinho mine: (A) augen gneiss with stretching lineation given by potassic feldspar crystals; (B) banded iron formation; (C) marble with banding formed by white, light and dark gray levels; (D) biotite (amphibole) paragneiss with alternating whitish and light gray bands.

Figure 5 -
Figure 5 -Cross-section of the drill hole FD-SE-002 performed near the Saquinho iron mine, highlighting the three geological compartments, as well as the sampling sites for the U-Pb geochronological analyses.

Figure 6 -
Figure 6 -Petrography of the upper compartment (Jucurutu Formation) units: (A) marble drill core showing interdigitation of light and dark gray beds; (B) chevron-like folds in drill-core of the banded iron formation; (C) photomicrograph of marble formed by calcite, biotite and muscovite (crossed Nicols); (D) photomicrograph of the banded iron formation evidencing a hinge zone of a tight fold delineated by alternating quartz+phyllosilicates and magnetite (parallel Nicols); (E) magnetite crystal with chalcopyrite at the edges (reflected light).Bt = biotite, Cal = calcite, Ccp = chalcopyrite, Mag = magnetite, Mb = white mica, Qtz = quartz.
Medeiros and Dantas (2015) stated that the rocks of the Saquinho region and neighboring areas were affected by at least three phases of ductile (D1, D2, and D3) and brittle deformations.The first one (D1) is evidenced only in the Paleoproterozoic/Rhyacian (augen gneisses/Caicó Complex) lithotypes to the north, outside the studied area, while the last two (D2 and D3) were also evidenced in Neoproterozoic lithotypes (Jucurutu Formation).

Figure 9 -
Figure 9 -Photos of structural features of the brittle regime in the rocks of the Jucurutu Formation, in the Saquinho mine region (A, B, and C) and respective interpretative drawings (A', B', and C').(A) Subvertical fault (80°/50°Az) affecting paragneiss and iron formation in an open pit operation; (B) and (C) drill-cores showing normal faults with centimeter heave superimposed on paragneiss/schist (B) and iron formation (C).Qtz = quartz; Ca = carbonate (calcite).

Figure 18 -
Figure 18 -Geological section of northern Saquinho iron mine, composed of the FD-SE-002 borehole and others made by Susa Indústria e Comércio de Produtos Minerários Ltd.

Figure 19 -
Figure 19 -Geological model of the Saquinho mine region (without scale), emphasizing the metasediments of the Jucurutu Formation superimposed on a basement composed of Rhyacian and Archean rocks.

Table 1 -
Whole-rock isotopic compositions (C and O)determined in samples of the Saquinho borehole marbles (FD-SE-002).