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Article

Genesis of Fe-Ti-(V) Oxide-Rich Rocks by Open-System Evolution of Mafic Alkaline Magmas: The Case of the Ponte Nova Massif, SE Brazil

by
Amanda Andrade de Souza
1,2,*,
Rogério Guitarrari Azzone
1,*,
Luanna Chmyz
1,
Lina Maria Cetina Tarazona
1,2,
Fábio Ramos Dias de Andrade
1,
José Vinicius Martins
1,
Excelso Ruberti
1 and
Celso de Barros Gomes
1
1
Department of Mineralogy and Geotectonics, Institute of Geosciences (IGc), University of São Paulo (USP), São Paulo 05508-080, Brazil
2
Postgraduate Program for Geosciences (Mineralogy and Petrology), Institute of Geosciences (IGc), University of São Paulo (USP), São Paulo 05508-080, Brazil
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(7), 724; https://doi.org/10.3390/min14070724
Submission received: 5 June 2024 / Revised: 3 July 2024 / Accepted: 13 July 2024 / Published: 19 July 2024
(This article belongs to the Special Issue Advances in Mantle–Crust Interactions for Petrogenesis and Ore-Forming Processes)
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Abstract

:
The formation of Fe-Ti oxides-rich layers is commonly associated with open-system magma chamber dynamics. These processes are widely discussed due to the economic importance of Fe-Ti-(V) deposits, although an alkaline-system approach to the matter is still scarce. In this study, we use petrography, mineral chemistry, X-ray diffraction and elemental geochemical analyses (whole-rock and Sr isotopes) to discuss the process associated with the formation of Fe-Ti-(V) oxide-rich clinopyroxenite (OCP, 7–15 vol.%) and magnetitite (MTT, 85 vol.%) from the Ponte Nova alkaline mafic–ultramafic massif (PN, K-Ar 87.6 Ma). Ilmenite and Ti-magnetite from both OCP and MTT exhibit higher MgO contents (MgO > 5.0 wt%) than other PN rocks. OCP shows high 87Sr/86Sri ratios, equivalent to crustal-contaminated lithotypes of the PN Central Intrusion, while MTTs are less radiogenic. The oxide supersaturation in silicate mafic magmas is typically associated with the dislocation of the liquid cotectic evolution line, shifting to Fe-Ti-(V) oxide minerals stability field, mainly Ti-magnetite. Different magmatic processes can lead to these changes such as crustal contamination and magma recharge. For the PN massif, the OCP was formed by the assimilation of crustal contaminants in a mush region, near the magma chamber upper walls, which was associated with the evolution of the main pulse. Differently, the MTT would have its origin related to the interaction between magma chamber evolved liquids and more primitive liquids during a new episode of magma recharge. Lastly, post-magmatic events were superimposed on these rocks, generating sulfides.

1. Introduction

Fe-Ti-(V) oxides-rich layers in mafic-ultramafic igneous complexes are widely discussed in the literature (e.g., [1,2]) mainly due to their significant economic importance as ore deposits [3]. These layers are usually hosted in layered intrusions of tholeiitic filiation such as the Bushveld Complex in South Africa [4], the Sept Iles intrusion in Canada [5], the Bjerkreim–Sokndal intrusion in Norway [6] and the Emeishan Large Igneous Province intrusions in SW China [7,8]. For such oxide-rich layers to crystallize from a silicate magma, the crystallization of silicate minerals must temporally halt, allowing a short period of time that only the oxide phase crystallizes [9]. The main petrological models to explain oxide minerals as the sole liquidus phase in the magmatic system include the assimilation of a crustal contaminant [10], magma mixing (e.g., [9,11]) and/or liquid immiscibility [12,13]. Fractional crystallization by in situ and/or the crystal settling of oxides is also invoked to explain the origin of oxide layers in those intrusions [14].
The records of rocks with abnormal concentrations of Fe-Ti oxide minerals are scarce for alkaline systems and usually relate to associated carbonatite rocks. Several carbonatite bodies from tropical regions were initially described as iron ore occurrences due to residual concentrations of magnetite after the extensive weathering of carbonate (e.g., [15]). Primary concentrations of Fe-Ti oxides in alkaline intrusions are mostly found associated with phoscorites, rare plutonic ultramafic rocks formed by magnetite, apatite and forsterite and sometimes forming near monomineralic layers of magnetitite [16,17]. Primary Fe-Ti oxide concentrations directly related to silicate alkaline rocks remain unknown.
The Ponte Nova alkaline mafic–ultramafic massif (PN, Upper Cretaceous; [18]) is part of the extensive Meso-Cenozoic continental alkaline-carbonatite magmatism in the central and southeastern regions of the South American Platform [19]. PN corresponds to a lithologically varied intrusive body with a predominance of cumulatic rocks representative of successive magmatic pulses. In its central–western domain, the massif presents a restricted level particularly enriched in Ti-magnetite, ilmenite and, subordinately, sulfides [18], whose origin is still poorly understood.
Several processes previously mentioned, such as fractional crystallization [18], crustal assimilation [20,21] and magma recharge [22], played an important role in the Ponte Nova evolution. Considering that, this study aims to investigate the magmatic processes that may have contributed to the formation and accumulation of Fe-Ti-(V) oxides in oxide-rich clinopyroxenites (OCPs, 7–15 vol.%) and magnetitites (MTTs, 85 vol.%) in the PN massif. We present new petrographic, mineral chemistry, as well as X-ray diffraction and elemental geochemistry analyses (whole-rock and Sr isotopes) of the studied rocks to be compared with the other PN intrusions and worldwide Fe-Ti-V deposits. This work proposes a model for the origin of OCP and MTT facies looking to encourage and expand discussions related to the origin of Fe-Ti-(V) oxides-rich layers associated with alkaline occurrences, which are considerably less common in the literature, in an open-transcrustal system context. From an economic perspective, OCP and MTT are pointed as potential exploratory targets for Fe, Ti and V [23].

2. Geologic Setting

The Meso-Cenozoic tectono-magmatic reactivation on the South American Platform [24,25] has resulted in intense episodes of magmatic activity, including a restricted, yet notable, manifestation of alkaline and carbonatite magmatism [19,24]. In general, alkaline occurrences in the Brazilian territory are predominantly suites (<100 km2) of evolved lithotypes, usually SiO2-undersaturated to saturated syenites [26]. Mafic occurrences are generally small intrusions (dikes, veins, stocks, plugs and pipes) or cumulates associated with carbonatitic complexes, such as Juquiá [27] and Jacupiranga [28].
Accounting for petrographic and geochronological characteristics [29,30], as well as regional tectonics [19,24], the Brazilian alkaline occurrences are grouped into provinces. The Serra do Mar Alkaline Province (Figure 1a,b; [24,30]) covers most of the coastline of Rio de Janeiro and São Paulo states, extending to the Serra da Mantiqueira in its continental domain. The province is composed of about 20 main alkaline occurrences, mostly plutonic, along with many mafic-ultramafic and felsic dikes, all formed between the Upper Cretaceous and the Lower Paleocene. Most intrusions are syenitic in nature, such as the Passa Quatro [31], Itatiaia [32] and Poços de Caldas [33] occurrences. Syenitic intrusions associated with small mafic-ultramafic cumulate bodies are described in São Sebastião [34] and the Monte de Trigo [35] islands. Exceptionally, the Ponte Nova alkaline mafic-ultramafic massif [18] is the only area with a predominance of mafic–ultramafic rocks in the region.
The PN massif (Figure 1c, K-Ar 87.6 Ma; [18]) is composed of an alkaline gabbroic association representative of successive magmatic pulses in a shallow-level chamber environment [20]. Its parental magma has basanitic composition as well as the mafic dikes that intruded the crystalline basement and the massif itself. The massif is originated from a heterogeneous mantle-derived partial melting, which was enriched by metasomatic events [20].
PN has two exposure areas, with a larger one (5.5 km2) encompassing the massif main intrusions, along with a smaller one (1 km2), constituted by a satellite body to the south of the main occurrence. Following geological and petrographic descriptions [18,20,21,36,37,38], the Central (CI) and Western (WI) intrusions comprise most of the occurrence and are divided into lower and upper sequences. The lower sequences are cumulates composed of clinopyroxenites and melagabbros with variable amounts of olivine and kaersutite-bearing layers. Nepheline monzogabbros exhibiting varied textural features (i.e., massive, banded, equigranular, porphyritic) are present in the upper sequences. The Northern Intrusion (NI), also cumulatic, contains melamonzogabbros with variable amounts of olivine. The Eastern Intrusion (EI) includes more evolved lithotypes, such as inequigranular to porphyritic monzodiorites with variable amounts of nepheline. A small plug (CP) composed of equigranular to porphyritic microgabbros occupies the massif central part. Separated from the rest of the massif by crystalline basement rocks, the Southern Satellite Intrusion (SSI) is formed by porphyritic nepheline-bearing melamonzonites and monzogabbros. Three other subordinate facies in the PN massif correspond to the magmatic breccia facies (Brc), kaersutite-oxide-apatite clinopyroxenites (Koa) and, finally, the oxide-rich clinopyroxenites (OCPs) and magnetitites (MTTs) facies, which are the focus of this study. Covering the massif highest altitude hill (1100 m) are the restricted outcrops of oxide-rich clinopyroxenites and magnetitites (Figure 1d). Both lithotypes are found close to each other, forming isolated blocks with less than 1 m2 and exposed in an area less than 1 km2. Primarily characterized by [18], the OCP facies was described as cumulates where clinopyroxene is the predominant cumulus phase in addition to subordinate ilmenite. Close to that facies, there is a horizon enriched in Fe-Ti oxides (85% vol.) such as Ti-magnetite (95%) and ilmenite (5%), which is classified as magnetitite [18].

3. Analytical Methods

Seven polished sections (6 from OCP, 1 from MTT) were selected for electron microprobe analyses (EPMAs) of the main cumulus phases (clinopyroxene, Ti-magnetite and ilmenite). The analyses were performed by wavelength-dispersive spectroscopy (WDS) using JEOL JXA-8530F equipment at the GeoAnalitica (GeoAnalitica, Phoenix, AZ, USA) facilities of the Institute of Geosciences, University of São Paulo, São Paulo, Brazil. Backscattered electron images via energy-dispersive X-ray spectroscopy (EDS) were also obtained. Operating conditions were 15 kV for accelerating potential, 20 ηA for probe current and 5 μm for beam diameter. Peak counting times ranged from 10 to 40 s for Fe-Ti oxides and 10 to 60 s for clinopyroxene. Standards included silicate minerals and synthetic oxides with data correction using the PROZA program [39]. Structural formulae were calculated in accordance with [40] and [41]. Fe3+/Fe2+ ratios for clinopyroxene were calculated by stoichiometry following the method of [42], while Fe3+/Fe2+ ratios for Fe-Ti oxides were calculated by stoichiometry, as described by [43]. Standard details and other analytical parameters, as well as the obtained data, are presented in the Supplementary Materials S1.
Four thin sections (3 from OCP, 1 from MTT) were selected for in situ minor and trace elements analyses in Ti-magnetite and ilmenite. The measurements were carried out by laser ablation in a CETAC LSX-213 G2+ laser coupled (Teledyne CETAC Technologies, Omaha, NE, USA) to a Thermo Scientific iCap Q-ICP-MS instrument (Thermo Fisher, Waltham, MA, USA) at the Chemistry and ICP Laboratory of the GeoAnalitica. Analytical time was 70 s with background acquisition of 30 s (gas blank) followed by 40 s of sample data acquisition. Operating conditions included 50 to 100 μm for a beam diameter through a 15 Hz repetition rate and energy fluence of 7.11 J/cm2. Element contents were calibrated utilizing the following international glass standards: BCR-2G for quantification and drift control, and BIR-1G for quality control (see Supplementary Materials S2). Data integration and the determination of concentrations, detection limits and analytical errors were conducted using the software Glitter Version 2008 [44]. The mean of FeO(T) contents from the electron microprobe analyses was employed as internal standard with a standard deviation of 1.10% for Ti-magnetite (MTT) and 0.79% and 1.59% for ilmenite in the MTT and OCP facies, respectively.
Whole-rock major, minor and trace elements (see Supplementary Materials S3) were analyzed in fused glass discs and pressed powder pellets using a Panalytical Axios Max Advanced X-ray fluorescence spectrometer (XRF) (Malvern Panalytical, Malvern, Worcestershire, UK) at the GeoAnalitica core facility with standard methods described by [45]. Whole-rock trace elements, including rare earth elements, were also obtained by inductively coupled plasma mass spectrometry (ICP-MS), utilizing a Thermo Scientific iCap Q-ICP-MS instrument (Thermo Fisher, Waltham, MA, USA) at the same laboratory. Analytical procedures followed the routines described by [46]. Whole-rock Sr isotope analyses were carried out at the Centro de Pesquisas Geocronológicas (CPGeo) of the Institute of Geosciences with equipment and procedures as described in [47] and [48].
X-ray diffraction analyses (powder method) were conducted on 3 samples (2 from MTT, 1 from OCP) using a Panalytical Empyrean diffractometer (Malvern Panalytical, Malvern, Worcestershire, UK) at the Laboratório de Caracterização Tecnológica (LCT) of the Department of Mining and Petroleum Engineering, Polytechnic School of the University of São Paulo. The instrumental parameters included CoKα radiation, 40 mA, 45 kV, an angular range from 5° to 80° 2-theta, an angular step of 0.02° 2-theta and 300 s per step. Data interpretation and quantitative phase analysis (Rietveld method) were performed using the software tools Match! Version 5.0.2 (Crystal Impact), Profex [49] and High Score Plus Version 4.0 (Panalytical) (Malvern Panalytical, Malvern, Worcestershire, UK). Crystallographic information files from the Crystallography Open Database [50] were utilized for this analysis.

4. Results

4.1. OCP and MTT Petrography

The OCP facies corresponds to coarse-grained ultramafic cumulates constituted predominantly by clinopyroxene and ilmenite (Figure 2a,c) in addition to sulfides, apatite, biotite and, locally, kaersutite (Table 1). Clinopyroxene is the dominant phase in the cumulus assemblage (85–90%), forming micro to macrocrystals oriented grains up to 1 cm, euhedral to subhedral, often zoned (diopside cores and Ti-augite rims). Ilmenite is secondary in the cumulus assemblage, occurring as rounded, small inclusions in clinopyroxene or homogeneous aggregates of polygonal habit and typical triple-junctions (Figure 2g) within the clinopyroxene framework. Ilmenite also constitutes the main intercumulus assemblage as homogeneous, subhedral to anhedral crystals (Figure 2b,c), occasionally replaced by hematite (Figure 2h). Sulfides are represented by pyrite chalcopyrite, mostly found with ilmenite or in veins (Figure 2e,f), but they can be present as euhedral microcrystals dispersed in the intercumulus association.
Hydrothermal and weathering alteration processes are present at the edges and along fractures of clinopyroxene grains, but they are also evident in some crystals of Fe-Ti oxides. Due to these alterations, ilmenite crystals frequently exhibit corroded edges (Figure 2h). Ilmenite in acicular shapes (Figure 2e,f) are specifically attributed to previous ilmenite exsolution lamellae of Ti-magnetite pseudomorphs, the latter replaced by sulfides. Apatite occurs as an accessory phase, as acicular inclusions in clinopyroxene or matrix-disseminated granular microcrystals (Figure 2b). Exceptionally, a restricted apatite-rich domain was recognized with euhedral apatite associated with intercumulus kaersutite and ilmenite (Figure 2d). Kaersutite is only identified in the apatite-rich domain. Biotite is generally secondary and found along clinopyroxene fractures, in veins or enclave rims. OCP presents mafic microgranular enclaves of gabbroic composition, similar to the ones found at the CI upper sequence, as well as enclaves of OCP’s of similar composition but of fine grain texture.
The MTT facies is particularly characterized by the predominance of equigranular Ti-magnetite (about 80–85 vol.%) defining a cumulatic texture (Figure 3a,b). Ti-magnetite crystals are homogeneous, euhedral and strongly fractured, with well-defined contacts and fine oxi-exsolutions (Figure 3c,d). Overlaying the oxi-exsolutions, submillimetric ilmenite lamellae are arranged orthogonally to each other (Figure 3c,e). Substitution textures, as indicated by the formation of ulvöspinel in Ti-magnetite rims, are common (Figure 3d). Clinopyroxene is subordinate, disseminated through the matrix or present as euhedral to subhedral micro and macrocrystals. Clinopyroxene grains are also often zoned (diopside cores and Ti-augite rims; Figure 3f) and occasionally host Ti-magnetite inclusions (Figure 3f). Although less abundant, ilmenite occurs interstitially to Ti-magnetite and clinopyroxene crystals (Figure 3e,g,h).

4.2. Major Elements

4.2.1. Clinopyroxene

Clinopyroxene cores from both OCP and MTT (Figure 4) present two distinct Mg#cpx [Mg/(Mg + FeT), molecular proportions] populations that are divided into relict and cognate cores. Relict cores have a higher Mg#cpx (0.84–0.88) and higher Cr2O3 contents (0.22–0.82 wt%) but lower TiO2 (0.88–1.20 wt%), Na2O (0.38–0.48 wt%) and Al2O3 (3.24–4.20 wt%) contents compared to intermediates and rims. The second core population has lower Mg#cpx (0.76–0.83) and Cr2O3 contents (0–0.04 wt%) but higher TiO2 (1.55–3.15 wt%), Na2O (0.4–0.6 wt%) and Al2O3 (4.32–7.35 wt%) contents. Intermediate and rim domains have lower Mg#cpx (0.76–0.83) and are equivalent to the second core population. Clinopyroxene crystals are positively correlated with Cr2O3 and negatively correlated with TiO2, Al2O3 and Na2O.

4.2.2. Fe-Ti Oxides

Fe-Ti oxides from OCP and MTT analyses, as with all PN intrusions, are consistent with the respective ulvöspinel–magnetite and ilmenite–hematite solid solution lines (Figure 5). Magnetite forms continuous solid solutions with ulvöspinel, originating Ti-magnetite. Ilmenites from MTT are close to the pure FeTiO3 end-member while other populations from PN, including the OCP ilmenites, are more enriched in the R3+ component (Fe3+, Cr, V, Al), mainly due to its Fe3+ content.
Ilmenite and Ti-magnetite Mg# [Mg/(Mg + Fe2+), molecular proportions] compositions in violin-type diagrams (Figure 6) allow a better understanding of the OCP and MTT data distribution when compared to other PN intrusions. It is notable that Fe-Ti oxide crystals from OCP and MTT stand out from the other rocks of the massif due to their higher Mg# (crystals from both are Mg-rich). Ti-magnetite contents vary between Mg#ti-mgt = 0.01–0.16 and Mg#ti-mgt = 0.02–0.1 in CI and other intrusions, respectively. In the magnetitites, the range is Mg#ti-mgt = 0.18–0.23. Although some data have equivalent values to those observed in the rest of PN, most of the compositions have higher values. Ilmenite values vary between Mg#ilm = 0.10–0.26 and Mg#ilm = 0.03–0.26 in CI and other intrusions, respectively. As noted for Ti-magnetite, some of the data are consistent with PN, but most of them are higher (Mg#ilm = 0.37).

Ti-Magnetite

MTT’s Ti-magnetites have TiO2 (Figure 7a) and Cr2O3 (Figure 7c) contents within the observed range for the other lithologies of PN. Most of the results for Ti-magnetite indicate a substitution of Fe3+ by Si. The data show a positive correlation for FeO (Figure 8a) and a negative correlation for Fe2O3 (Figure 8b) in both EPMA and LA-ICP-MS analyses. They present particularly high contents of V (2523–3542 ppm), Mn (2267–3610 ppm) and Mg (11,581–40,590 ppm) when compared to Ti-magnetites from Fe-Ti-V deposits (Figure 9). Ni (36–65 ppm), Co (106–208 ppm), Cu (4–125 ppm), Cr (89–179 ppm), Nb (3–7 ppm) and Zr (24–46 ppm) contents are low, even though Zr and Nb contents are higher than most of the compiled data (Figure 9b).

Ilmenite

MTT ilmenites have higher TiO2 contents (Figure 7b) than those observed in PN, while OCP ilmenites exhibit chemically coherent behavior with that observed in PN. OCP ilmenites (Figure 7d) have higher Cr2O3 contents than those observed in both MTT and the rest of the massif but, in general, all PN ilmenites are Cr-poor and close to lower detection limits. OCP ilmenites with lower Mg#ilm and compositionally closer to PN are associated with a few rims or small crystals. Most of the OCP ilmenite presents higher contents of Mn (7534–9549 ppm), V (3575–4995 ppm), Zn (141–306 ppm) and Cr (339–794 ppm), while the MTT ilmenite presents higher Co (7–15 ppm), although in low concentration. Both facies have equivalent contents of Cu (14–32 ppm) and Zr (90–584 ppm) (Figure 10). Despite being derived from different textural features (i.e., enclaves, apatite-rich domain, matrix and inclusions in clinopyroxene), OCP ilmenites are within the same compositional range.

4.2.3. Temperature and Oxygen Fugacity Conditions

Temperature (solidus, equilibrium) and oxygen fugacity conditions were estimated using magnetite-ilmenite pairs through the WinMIgob program [52] with calculations according to [53] and [54], respectively. Obtained pairs are considered to be in chemical equilibrium conditions as they passed a Mg/Mn partitioning test between magnetite and ilmenite (Figure 11a; [55]). The test assesses the quality of analytical data and determines whether Fe-Ti oxide pairs had their composition altered in post-magmatic processes. A total of 22 pairs from MTT, 13 pairs from CI and 10 pairs from other PN intrusions were obtained. The results show that temperatures from MTT pairs are between 769 and 886 °C and, therefore, equivalent to the CI distribution peak and range distribution within other PN intrusions (Figure 11b). Oxygen fugacity conditions (fO2), plotted in a buffer generator [56], range from fO2 = −16 to −13, indicating a formation ambient controlled by the FMQ (fayalite–quartz–magnetite) buffer for magnetitites as well as most of the magnetite–ilmenite pairs from PN (Figure 11c). The results show a linear tendency for the MTT facies (ΔFMQ = −0.4 to 0.5) within PN’s range (Figure 11d).

4.3. Whole-Rock Composition

4.3.1. Elemental Geochemistry

The magmatic evolution trend of PN intrusions is established through R1-R2 indices (Figure 12a; [57]) with the most primitive lithotypes at the top and the most evolved ones at the bottom of the line. OCP is aligned with the massif evolution trend but subtly displaced to the right and close to the SiO2-saturation line. MTT, on the other hand, deviates from the geochemical behavior of the massif. Whole-rock behavior for OCP also indicates intermediate compositions between the average of PN clinopyroxene and Fe-Ti oxides mineral chemistry analyses, within the PN magmatic evolution trend, while MTT again diverges from the trend (Figure 12b).
Whole-rock results (Figure 13) highlight the TiO2-rich nature of OCP (TiO2: 7.81 wt%) and, especially, MTT (TiO2: 15.36 and 17.87 wt%), as well as some CI hydrated lithotypes. Regarding trace element behavior, OCPs are enriched in transition elements, such as Co (220 ppm), Cu (391 ppm) and V (713 ppm), and they are depleted in Cr, Ni, Zr, Zn and Nb. MTT is enriched in Co (145–192 ppm), Cu (103–277 ppm) and Zn (398–492 ppm), besides presenting the highest values of V (1756–1834 ppm) in the massif. As OCPs, they are depleted in Zr (57.9–88.6 ppm) and Nb (5.36–7.16 ppm).
The MgO trend line (Figure 13) for these facies shows a compatible behavior for Cr and Ni. OCP exhibits higher MgO (8.62 wt%) compared to MTT (4.62 and 6.64, wt%) as well as a higher Cr value. Although the MgO contents differ in the two MTT samples, the Cr concentration remains relatively similar. For Ni, there is an enrichment in OCP and samples with lower MgO in MTT facies.

4.3.2. Sr Isotope Geochemistry

Geochemistry data of Sr isotopes were utilized to determine the 87Sr/86Sri ratio from OCP and MTT (Figure 14). 87Sr/86Sri ratios in different lithotypes of PN massif show a variation predominantly between 0.7043 and 0.7064. The highest values correspond to monzonites (87Sr/86Sri > 0.7060; [36]) to the west–northwest and to south of the main exposure area. Lithotypes with the lowest 87Sr/86Sr values correspond to the Central Plug (CP) facies, one of the last magmatic activities of PN, while the lithotypes with highest 87Sr/86Sri ratios are more contaminated by the assimilation of crustal material from the host rocks (87Sr/86Sri = 0.7150–0.7233; [20]). OCP presents 87Sr/86Sri = 0.7049, similar to ratios found in CI’s upper sequence or hydrated facies of the lower sequence of the same intrusion. MTT exhibits a 87Sr/86Sri ratio (0.7047) between OCP and the CP facies.

4.4. X-ray Diffraction

X-ray data are consistent with chemical and petrographical results. In the MTT samples (Table 2), Ti-magnetite is the predominant phase, which is accompanied by minor quantities of ilmenite, spinel and varying amounts of Ti-bearing augite. In addition, anatase and goethite were also identified. The OCP (Figure 15) sample exhibits a more complex mineralogical composition, dominated by Ti-augite (67 wt%), alongside small proportions of diopside, ilmenite and pyrite. The presence of melantherite (FeSO4·7H2O) in significant amounts and traces of rhomboclase ((H5O2)Fe(SO4)2·2H2O), both as alteration products of pyrite (Figure 2e–g), is noteworthy. Rietveld simulations used phases with chemical compositions similar to those observed in the samples. The refinement process accounted for sample height displacement, cell parameters, peak broadening, preferential orientation and thermal parameters. The substitution of Si in magnetite, up to 2.5 wt%, did not yield discernible shifts in its diffraction peaks.

5. Discussion

The processes responsible for the concentration of oxides in igneous rocks remain widely discussed in the literature (e.g., [8,59,60,61,62,63]) and involve a wide variety of geological, petrographic and compositional aspects. Several processes (both magmatic and hydrothermal) may be responsible for their formation and, although many mechanisms have been proposed, no single mechanism can explain all the different types of layering [14]. Some processes operate during the magma emplacement in the magma chamber, when the system is still predominantly liquid (e.g., [9,11,64]). Other processes act in response to magma convection patterns and/or to mechanical processes (e.g., [4]) or are responses of the magmatic system to intensive parameters (i.e., temperature, pressure, oxygen fugacity) variations (e.g., [62,63]). There are also processes occurring in the final stages of crystallization and during cooling (e.g., [65]). Recent studies indicate that high-temperature hydrothermal systems are able to precipitate high-Ti magnetite with ilmenite lamellae textures [51]. Many layers may have been formed from combinations of different processes (e.g., [66]).
Magnetite can be formed under diverse conditions, either crystallizing from silicate and sulfide melts at high temperatures or precipitating from hydrothermal fluids at lower temperatures [67]. Even though the Fe3+-Si substitution observed in the MTT magnetite seems to be found in both magmatic and hydrothermal deposits (e.g., IOA deposits; [51]), MTT chemical signatures are characteristically magmatic (Figure 9a) with trace elements contents coherent with Fe-Ti-V magmatic deposits data (Figure 9b). The observed enrichment in Zr and Nb, when compared to other Fe-Ti-V deposits, may be associated with the alkaline filiation of the PN complex, once magnetites from alkaline mafic–ultramafic intrusions are particularly enriched in these elements [68].
Petrographic and textural evidence indicate different magmatic crystallization stages for the Fe-Ti oxides of the studied PN lithotypes. The concentration of oxides in OCP occurs dispersedly between the clinopyroxene grainswith the initial establishment of the cumulate fabric by clinopyroxene cores at an early stage, prior to the supersaturation, which is then followed by the continuous crystallization of clinopyroxene and ilmenite in the later stages. MTT facies do not display the same petrographic features recognized in OCP, besides the fact that the Fe-Ti oxides are evidently higher in volume. As magnetite corresponds to the main cumulus phase of MTT, the supersaturation in Fe-Ti oxides in this facies has occurred in a predominantly liquid context. Thus, it is suggested that the formation and accumulation of Ti-magnetite and ilmenite present in OCP and MTT rocks are associated with a particular process other than crystal-liquid fractionation and, yet, that both lithologies were not originated from the same process.

5.1. Crustal Contamination

Oxide supersaturation can occur in association with crustal assimilation processes, in which the interaction between the magmatic system and crustal felsic rocks forces the liquid out of its cotectic crystallization line toward the stability field of an oxide phase, consequently causing its unique crystallization [9,11]. PN host rocks are mainly metasyenogranites, metamonzogranites and biotite gneisses from the Serra da Água Limpa Batolith (645–630 Ma; [69]), with the latter being found especially in the northwestern contacts with PN [20]. The assimilation of crustal components, associated with the crystal-liquid fractionation of different magmatic pulses, has an important role in the evolution of the PN massif [20,22]. In this context, processes of crustal contamination could occur as a consequence of the interaction between the walls of the magma chamber and the adjacent host rock during magma ascent, as well as in the upper portions of the chamber, where the emplacement of magmatic material generates partial melting of the overlying supracrustal materials [20,22]. Direct evidence of such processes are registered in the various crustal xenoliths found in the most contaminated facies from PN.
Petrographic features such as the orientation and compaction of clinopyroxene grains (Figure 2a–c) and the presence of enclaves, as well as the stratigraphic level of OCP (Figure 1d), at the top of CI, suggest that the formation of this lithotype took place in the upper mush zones, near the upper magma chamber walls, where the assimilation of crustal contaminants from the partial melting of the crust would be favored. Evidence of crustal contamination are mainly found in the OCP facies, whose Sr isotopic ratio is equivalent to those found in considerably contaminated rocks from the upper sequence of CI and kaersutite-rich facies from PN (Figure 14). Sr isotopic signatures could also be affected by processes such as hydrothermal activity and mantle heterogeneities [70]. However, typically magmatic signatures observed in Ti-magnetite (Figure 9) suggest that the first scenario would not be plausible. Also, there are several limitations to discuss mantle source when dealing with a fossil magmatic chamber that present strong geological, petrographic and geochemical evidence of assimilation processes (e.g., [20,22]) OCP is also close to the SiO2-saturation plane in the R1-R2 diagram, with whole-rock compositions close to highly contaminated rocks (Figure 12a). Additionally, the presence of acicular apatite is commonly related to assimilation events [71].

5.2. Magma Recharge

Mixing between partially differentiated and primitive liquids during episodes of magma chamber replenishment [9] is a viable model that explains Ti-magnetite supersaturation in MTT. Successive magmatic pulses of crystal-laden basanites characterize the geometry and evolution of PN intrusions (e.g., [21,36,38]). The first studies on the OCP and MTT facies [36] proposed that both lithotypes could be associated with a new magmatic influx mixed with residual fractionated magma from a PN chamber. The magma recharge hypothesis is suggested by the geometry of these facies in the PN geological map (Figure 1c) where the rounded shape and arrangement of the body suggest a new pulse intruding the Central Intrusion. MTT has significantly less radiogenic Sr isotopic ratios than those from OCP and associated CI rocks, with values closer to those from the Central Plug facies (CP; Figure 14), which is located near the studied rocks and one of the last records of magmatic recharge of the massif. In this sense, the MTT presents MgO content slightly higher than that from CP, but lower from those from most PN rocks (Figure 13), which may be a further suggestion of hybridization of a new pulse when interacting with more evolved liquids. The arrival of a new pulse in a residual and evolved environment also explains the lower amount of silicate phases in MTT rocks. Similarly to the effect of crustal contamination on a primitive liquid, the interaction between the evolved magmatic system and new primitive batch of magma forces the liquid out of its cotectic crystallization line toward the stability field of an oxide phase (mainly Ti-magnetite), consequently causing its unique crystallization [9,11]. The authors of [22] also indicate a process of magmatic recharge, after the contamination process in the PN massif, through microanalytical isotopic approaches based on in situ analyses of alkali-feldspar crystals and contrasting compositions of crystal cores, intermediates and rims. Thus, for MTT facies, the interaction of a more evolved liquid resident in the magma chamber with a new, more primitive pulse could generate the expected Ti-magnetite supersaturation (e.g., [65]).

5.3. OCP and MTT Petrological Model

Previous works proposed a complex petrological model for the PN massif, highlighting the significant role of open-system processes [20,21,38]. Repeated influxes of antecryst-laden basanite magmas, possibly derived from the same magmatic reservoir, deposited their suspended crystals on the floor of an upper-crust magma chamber. In the initial stages of crystallization, crustal contamination processes occur due to the assimilation of xenoliths and partial melts from the surrounding host rock. Each PN intrusion is thus representative of relatively primitive magmas that had assimilated different degrees of partial melts of heterogeneous host rocks. The results of this study suggest that the accumulation of Fe-Ti oxides in the OCP facies occurred in a mush region and, therefore, is associated with the main pulse (Central Intrusion) evolution.
Clinopyroxene compositions from OCP are coherent with the complex plumbing system model proposed for PN massif by [38]. Clinopyroxene cores with higher Mg# resemble those described as antecrysts by [38], which were interpreted as formed in a deeper reservoir that fed the Ponte Nova massif. The more evolved cores determined in OCP, as well as the intermediates and rims with equivalent Mg#, crystallized in equilibrium with the magmatic system in lower pressure crystallization conditions.
Clinopyroxene grains are completely associated with CI evolution as well as the ilmenite inclusions in clinopyroxene (Figure 10) and other ilmenite crystals with the same chemical composition as PN (Figure 6b and Figure 7b). The origin of a compositionally distinct Fe-Ti oxide population would be enabled by the assimilation of crustal contaminants in the mush region, generating a local disequilibrium and, thus, promoting the supersaturation. From the results obtained for the MTT facies, in turn, we propose that the Fe-Ti oxides (i.e., Ti-magnetite and ilmenite) from the MTT facies have its origin associated with the interaction of PN residual liquids and a more primitive one, characterizing a new pulse in PN magmatic evolution.
The proposed unified model (Figure 16), therefore, consists of the following stages: (1) establishment of the cumulate framework in the mush zone of PN’s upper walls, with consequent heat loss to its host rocks and eventual assimilation of partially melted crustal components; (2) Fe-Ti oxide supersaturation triggered by crustal assimilation, forming OCP; (3) subsequent to OCP, in the final stages of crystallization, a new episode of magma recharge forming MTT; (4) the eventual erosion of the uppermost sequences of PN, exposing the lithologies approached in this study.
Regarding imprints of post-magmatic processes on the studied rocks, the absolute absence of magnetite in OCP is unusual (e.g., [8,60,61,62,63,66]). The minerals composing the interstitial assembly consist exclusively of the ilmenite–sulfide pair or just ilmenite. Ilmenite crystals, in turn, exhibit various degrees of corrosion (Figure 3e–h), suggesting that post-magmatic reactions may have affected the rock. Ti-magnetite is unstable in the presence of sulfur-rich fluids and reducing conditions of oxygen fugacity [72,73]. Equilibrium processes within the oxide structure tend to remove Ti contents from magnetite to ilmenite according to the following equilibrium:
6Fe2TiO4 (in magnetite) + O2 = 6FeTiO3 (in ilmenite) + 2Fe3O4 (magnetite)
From the reaction, typical ilmenite trellis lamellae would be formed. With continued cooling (approximately 300 °C; [74]), the interaction with hydrothermal S-rich fluids would cause the ilmenite to be incipiently corroded and unstable magnetite to be completely replaced by sulfides (mainly pyrite and chalcopyrite). The elongated ilmenite crystals, often acicular as oriented orthogonally, would therefore represent relict features. The greater presence of sulfides evidently associated with veins and fractured domains in the cumulate framework, i.e., regions with the possibility of fluid percolation, supports this hypothesis. The absence of magnetite hampers to estimate temperature and fO2 conditions for the OCP facies. However, studies estimate that the conditions in which oxi-exsolutions would occur start at 700 °C and QFM ~ 0.5 [73], which are values consistent with those obtained for MTT and within the estimated variation range of PN massif (Figure 11).

6. Conclusions

Based on petrography, mineral chemistry, whole-rock geochemistry and radiogenic isotopic ratios, we conclude that the Fe-Ti-oxide-rich facies of the PN massif are formed in an open-system environment. However, OCP and MTT originated in different stages of the chamber evolution, and they are representative of distinct open-system processes. OCPs are formed as a consequence of the assimilation of crustal contaminants in the upper levels of the PN chamber, which is a process also registered in marginal facies of the massif. For MTT, we propose that magma recharge would be a more appropriate model to justify the observed Ti-magnetite and ilmenite supersaturation. This mechanism is consistent with our preferred model for the entire massif, which considers repeated influxes of antecryst-laden basanite magmas in a shallow magma chamber.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14070724/s1. Supplementary Materials S1—Major element compositions of clinopyroxene and Fe-Ti oxides from the Ponte Nova massif. Table S1. Analytical parameters and calibration standards used for WDS analyses of clinopyroxene, Ti-magnetite and ilmenite. Table S2. Major element compositions in mass% of clinopyroxene from the OCP and MTT facies. Structural formula calculated on the basis of 6 oxygens. Table S3. Major element compositions in mass% of Ti-magnetite from the Ponte Nova massif. Structural formula calculated on the basis of 32 oxygens. Table S4. Major element compositions in mass% of ilmenite from the Ponte Nova massif. Structural formula calculated on the basis of 6 oxygens. Table S5. Temperature and oxygen fugacity estimations obtained in magnetite-ilmenite pairs utilizing the WINMIgob software (version 1.0.7) [52]. Supplementary Materials S2—LA-ICP-MS analyses of Fe-Ti oxides from OCP and MTT facies. Table S6. Trace element compositions for BCR-2G (USGS) calibration (primary) standard used for routines of trace elements analyses. Recommended values are from [75]. Table S7. Trace element compositions for BIR-1G quality-control monitor (reference materials) used for routines of trace element analyses. Recommended values are from [75]. Table S8. Trace element compositions of Ti-magnetite from the MTT facies. Table S9. Trace element composition compositions of ilmenite from OCP and MTT facies. Supplementary Materials S3—Table S10. Elemental geochemical analyses (whole-rock and Sr isotopes) from the OCP and MTT facies.

Author Contributions

Conceptualization, R.G.A. and L.C.; investigation, A.A.d.S., R.G.A., L.C. and L.M.C.T.; data curation, A.A.d.S., R.G.A., L.C., L.M.C.T., F.R.D.d.A., J.V.M., E.R. and C.d.B.G.; writing—original draft preparation, A.A.d.S., R.G.A., L.C., L.M.C.T. and F.R.D.d.A.; writing—review and editing, A.A.d.S., R.G.A., L.C., L.M.C.T., F.R.D.d.A., J.V.M., E.R. and C.d.B.G.; visualization, A.A.d.S. and R.G.A.; project administration, R.G.A.; funding acquisition, R.G.A. and E.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grants from Brazilian Agencies: FAPESP (Procs. 2019/22084-8; 2023/11675-0) and Brazilian National Research Council CNPq for research and productivity grants (404020/2021-6, 310055/2021-0).

Data Availability Statement

All data are included within the article.

Acknowledgments

CAPES is acknowledged for the post-graduate scholarship for A.A.S. and L.M.C.T. L.C. Thanks FAPESP for the postdoctoral fellowship (process 23/12323-0). We thank the analytical support of the technicians of the laboratories of GeoAnalítica-USP and LCT-USP facilities.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Geological setting of alkaline intrusions of Cretaceous magmatism in the southeastern part of the South American Platform showing the distribution of major alkaline occurrences of Alto Paranaíba, Serra do Mar and Ponta Grossa Arch alkaline provinces (modified after [20]). (b) Detail of (a), with the location of the main occurrences. Legend: I—Sediments or sedimentary rocks (Cenozoic); II—Alkaline complexes (Meso-Cenozoic); III—Sedimentary rocks of the Paraná Basin (Ordovician-Silurian); IV—Crystalline Basement, Brasília and Ribeira orogenic belts (Precambrian). (c) Simplified geological map of the Ponte Nova alkaline mafic–ultramafic massif (modified after [20]). (d) Schematic model of the lithostratigraphy of the studied rocks, after [36].
Figure 1. (a) Geological setting of alkaline intrusions of Cretaceous magmatism in the southeastern part of the South American Platform showing the distribution of major alkaline occurrences of Alto Paranaíba, Serra do Mar and Ponta Grossa Arch alkaline provinces (modified after [20]). (b) Detail of (a), with the location of the main occurrences. Legend: I—Sediments or sedimentary rocks (Cenozoic); II—Alkaline complexes (Meso-Cenozoic); III—Sedimentary rocks of the Paraná Basin (Ordovician-Silurian); IV—Crystalline Basement, Brasília and Ribeira orogenic belts (Precambrian). (c) Simplified geological map of the Ponte Nova alkaline mafic–ultramafic massif (modified after [20]). (d) Schematic model of the lithostratigraphy of the studied rocks, after [36].
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Figure 2. Petrographic aspects of the OCP facies. (a) OCP scanned thin section. Transmitted and plane-polarized light: (b) Photomicrograph of OCP cumulatic texture and mineral assemblage constituted of clinopyroxene, ilmenite and apatite (granular and acicular) disseminated through the matrix. (c) Orientation and compaction structures. (d) Apatite-rich domain with subordinate kaersutite, ilmenite and biotite. Reflected and plane-polarized light: (e,f) Anhedral and acicular ilmenite associated with cumulus clinopyroxene and weathered sulfides along fractured zones. Backscattered electron image (BSE): (g) Ilmenite (in triple-junction contacts) and sulfides; and (h) Ilmenite corroded rims and substitution by hematite along fractures. Abbreviations: bt—biotite, cpx—clinopyroxene, cpy—chalcopyrite, hem—hematite, ilm—ilmenite, py—pyrite, sul—weathered sulfide, opq—opaque minerals.
Figure 2. Petrographic aspects of the OCP facies. (a) OCP scanned thin section. Transmitted and plane-polarized light: (b) Photomicrograph of OCP cumulatic texture and mineral assemblage constituted of clinopyroxene, ilmenite and apatite (granular and acicular) disseminated through the matrix. (c) Orientation and compaction structures. (d) Apatite-rich domain with subordinate kaersutite, ilmenite and biotite. Reflected and plane-polarized light: (e,f) Anhedral and acicular ilmenite associated with cumulus clinopyroxene and weathered sulfides along fractured zones. Backscattered electron image (BSE): (g) Ilmenite (in triple-junction contacts) and sulfides; and (h) Ilmenite corroded rims and substitution by hematite along fractures. Abbreviations: bt—biotite, cpx—clinopyroxene, cpy—chalcopyrite, hem—hematite, ilm—ilmenite, py—pyrite, sul—weathered sulfide, opq—opaque minerals.
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Figure 3. Petrographic aspects of the MTT facies. (a) MTT scanned thin section. Transmitted light: (b) Photomicrograph of MTT cumulatic texture. Reflected and plane-polarized light: (c) Euhedral magnetite crystals with submillimetric ilmenite lamellae and fine oxi-exsolutions. (d) Substitution textures to spinel predominantly at crystal rims. Backscattered electron image (BSE): (e) Cumulus Ti-magnetite, clinopyroxene and ilmenite. (f) Zoned clinopyroxene macrocrystal. Reflected and plane-polarized light: (g) Clinopyroxene grains with magnetite inclusions, cumulus magnetite and ilmenite (reflected and plane-polarized light). (h) Intercumulus ilmenite, cumulus magnetite and clinopyroxene (reflected and plane-polarized light). Abbreviations: cpx—clinopyroxene, ilm—ilmenite, Ti-mgt—Ti-magnetite, ulv—ulvöspinel.
Figure 3. Petrographic aspects of the MTT facies. (a) MTT scanned thin section. Transmitted light: (b) Photomicrograph of MTT cumulatic texture. Reflected and plane-polarized light: (c) Euhedral magnetite crystals with submillimetric ilmenite lamellae and fine oxi-exsolutions. (d) Substitution textures to spinel predominantly at crystal rims. Backscattered electron image (BSE): (e) Cumulus Ti-magnetite, clinopyroxene and ilmenite. (f) Zoned clinopyroxene macrocrystal. Reflected and plane-polarized light: (g) Clinopyroxene grains with magnetite inclusions, cumulus magnetite and ilmenite (reflected and plane-polarized light). (h) Intercumulus ilmenite, cumulus magnetite and clinopyroxene (reflected and plane-polarized light). Abbreviations: cpx—clinopyroxene, ilm—ilmenite, Ti-mgt—Ti-magnetite, ulv—ulvöspinel.
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Figure 4. OCP and MTT clinopyroxene compositions compared to compiled clinopyroxene data (after [38]) of CI (green line) and other PN intrusions (gray line). (a) Mg#cpx vs. Cr2O3 (b) Mg#cpx vs. TiO2 (c) Mg#cpx vs. Na2O (d) Mg#cpx vs. Al2O3. Pink dotted lines refer to core–intermediate–rim composition of one selected crystal from OCP facies. Orange dotted lines refer to core–rim composition of one selected crystal from MTT facies.
Figure 4. OCP and MTT clinopyroxene compositions compared to compiled clinopyroxene data (after [38]) of CI (green line) and other PN intrusions (gray line). (a) Mg#cpx vs. Cr2O3 (b) Mg#cpx vs. TiO2 (c) Mg#cpx vs. Na2O (d) Mg#cpx vs. Al2O3. Pink dotted lines refer to core–intermediate–rim composition of one selected crystal from OCP facies. Orange dotted lines refer to core–rim composition of one selected crystal from MTT facies.
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Figure 5. Ti-R2-R3 ternary diagram of Fe-Ti oxides of PN with the magnetite–ulvöspinel and ilmenite–hematite solid solution lines, in atoms per formula unit (apfu).
Figure 5. Ti-R2-R3 ternary diagram of Fe-Ti oxides of PN with the magnetite–ulvöspinel and ilmenite–hematite solid solution lines, in atoms per formula unit (apfu).
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Figure 6. Mg# index of Ti-magnetite and ilmenite from OCP, MTT, CI and other PN intrusions. The grey field represents analyses with equivalent Mg# to other PN intrusions. (a) Mg# in Ti-magnetite. (b) Mg# in ilmenite. Abbreviations: OCP—oxide-rich clinopyroxenites, MTT—magnetitites, CI—Central Intrusion.
Figure 6. Mg# index of Ti-magnetite and ilmenite from OCP, MTT, CI and other PN intrusions. The grey field represents analyses with equivalent Mg# to other PN intrusions. (a) Mg# in Ti-magnetite. (b) Mg# in ilmenite. Abbreviations: OCP—oxide-rich clinopyroxenites, MTT—magnetitites, CI—Central Intrusion.
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Figure 7. Mg# Fe-Ti oxides compositions vs.TiO2 and Cr2O3 from OCP, MTT, CI and other PN intrusions. (a) Mg# vs. TiO2 for Ti-magnetite populations. (b) Mg# vs. TiO2 for ilmenite populations. (c) Mg# vs. Cr2O3 for Ti-magnetite populations. (d) Mg# vs. Cr2O3 for ilmenite populations.
Figure 7. Mg# Fe-Ti oxides compositions vs.TiO2 and Cr2O3 from OCP, MTT, CI and other PN intrusions. (a) Mg# vs. TiO2 for Ti-magnetite populations. (b) Mg# vs. TiO2 for ilmenite populations. (c) Mg# vs. Cr2O3 for Ti-magnetite populations. (d) Mg# vs. Cr2O3 for ilmenite populations.
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Figure 8. (a) SiO2 vs. FeO (wt%) analyses of Ti-magnetite populations from MTT facies. (b) SiO2 vs. Fe2O3 (wt%) analyses of Ti-magnetite populations from MTT facies. Results obtained in this study via laser ablation ICP-MS and electron microprobe (EPMA).
Figure 8. (a) SiO2 vs. FeO (wt%) analyses of Ti-magnetite populations from MTT facies. (b) SiO2 vs. Fe2O3 (wt%) analyses of Ti-magnetite populations from MTT facies. Results obtained in this study via laser ablation ICP-MS and electron microprobe (EPMA).
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Figure 9. (a) Discriminant diagram of log(Al) + log(Ti) + log(V) vs. log(Mn)/[log(Co) + log(Mg)] for Ti-magnetite from MTT facies and compiled data of Fe-Ti-V deposits, adapted from [51]. (b) Trace elements (ppm) diagrams for Ti-magnetite from MTT facies. Ti-magnetite compiled data from worldwide Fe-Ti-V deposits are presented for comparison [51].
Figure 9. (a) Discriminant diagram of log(Al) + log(Ti) + log(V) vs. log(Mn)/[log(Co) + log(Mg)] for Ti-magnetite from MTT facies and compiled data of Fe-Ti-V deposits, adapted from [51]. (b) Trace elements (ppm) diagrams for Ti-magnetite from MTT facies. Ti-magnetite compiled data from worldwide Fe-Ti-V deposits are presented for comparison [51].
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Figure 10. TiO2 (wt%) vs. Mg, Mn, V, Co, Cu, Zr, Zn and Cr (ppm) contents of ilmenites from the OCP and MTT facies.
Figure 10. TiO2 (wt%) vs. Mg, Mn, V, Co, Cu, Zr, Zn and Cr (ppm) contents of ilmenites from the OCP and MTT facies.
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Figure 11. (a) Mg/Mn equilibrium test after [55] in magnetite–ilmenite pairs from PN. 2σ represents the standard deviation of the equilibrium line (in orange). (b) Kernel density estimation (KDE) and box plot diagrams for the obtained temperatures. Temperature estimation after [53]. (c) Temperature vs. oxygen fugacity of magnetite-ilmenite pairs from PN massif. (d) Temperature vs. ΔFMQ of magnetite-ilmenite pairs from PN massif. Oxygen fugacity estimation after [54]. Buffers were obtained in [56]. Abbreviations: MH—magnetite-hematite, NNO—nickel-nickel-oxide; FMQ—fayalite-magnetite-quartz; WM—wüstite-magnetite.
Figure 11. (a) Mg/Mn equilibrium test after [55] in magnetite–ilmenite pairs from PN. 2σ represents the standard deviation of the equilibrium line (in orange). (b) Kernel density estimation (KDE) and box plot diagrams for the obtained temperatures. Temperature estimation after [53]. (c) Temperature vs. oxygen fugacity of magnetite-ilmenite pairs from PN massif. (d) Temperature vs. ΔFMQ of magnetite-ilmenite pairs from PN massif. Oxygen fugacity estimation after [54]. Buffers were obtained in [56]. Abbreviations: MH—magnetite-hematite, NNO—nickel-nickel-oxide; FMQ—fayalite-magnetite-quartz; WM—wüstite-magnetite.
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Figure 12. (a) R1-R2 diagram [57] for rocks from the PN massif and host rocks. Average mineral poles were compiled from [36] and are representative of the main assemblage found in PN (modified after [20]). (b) Al-Si-(Ti + Fe + Mg + Mn) ternary diagram for rocks from the PN massif and host rocks in molar proportions. Mineral chemistry data (in gray) for clinopyroxene and Fe-Ti oxides are from this work. Nepheline, feldspar and olivine compositions in the ternary are from their ideal formula. Abbreviations: cpx—clinopyroxene, Ti-mgt—Ti-magnetite, ol—olivine, ilm—ilmenite, nph—nepheline, fsp—alkali-feldspar, pl—plagioclase, ap—apatite, bt—biotite, krs—kaersutite.
Figure 12. (a) R1-R2 diagram [57] for rocks from the PN massif and host rocks. Average mineral poles were compiled from [36] and are representative of the main assemblage found in PN (modified after [20]). (b) Al-Si-(Ti + Fe + Mg + Mn) ternary diagram for rocks from the PN massif and host rocks in molar proportions. Mineral chemistry data (in gray) for clinopyroxene and Fe-Ti oxides are from this work. Nepheline, feldspar and olivine compositions in the ternary are from their ideal formula. Abbreviations: cpx—clinopyroxene, Ti-mgt—Ti-magnetite, ol—olivine, ilm—ilmenite, nph—nepheline, fsp—alkali-feldspar, pl—plagioclase, ap—apatite, bt—biotite, krs—kaersutite.
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Figure 13. Bivariant diagrams of TiO2 (wt%) vs. V, Zn, Co and Cu (ppm) from the PN massif rocks, and MgO (wt%) vs. Cr and Ni (ppm) from the PN massif rocks.
Figure 13. Bivariant diagrams of TiO2 (wt%) vs. V, Zn, Co and Cu (ppm) from the PN massif rocks, and MgO (wt%) vs. Cr and Ni (ppm) from the PN massif rocks.
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Figure 14. (a) 87Sr/86Sri of PN massif intrusions. Sr isotopes obtained in whole-rock, apatite and plagioclase analyses. (b) 87Sr/86Sri vs. Sr (ppm) from the PN massif rocks and host rocks. Compiled data from [20]. Abbreviations: AFC—assimilation + fractional crystallization.
Figure 14. (a) 87Sr/86Sri of PN massif intrusions. Sr isotopes obtained in whole-rock, apatite and plagioclase analyses. (b) 87Sr/86Sri vs. Sr (ppm) from the PN massif rocks and host rocks. Compiled data from [20]. Abbreviations: AFC—assimilation + fractional crystallization.
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Figure 15. XRD diffractogram of the OCP facies. Abbreviations: rmb—rhomboclase, mlnt—melantherite, aug—augite, ilm—ilmenite.
Figure 15. XRD diffractogram of the OCP facies. Abbreviations: rmb—rhomboclase, mlnt—melantherite, aug—augite, ilm—ilmenite.
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Figure 16. Schematic model for the proposed origin to OCP and MTT facies. (a) Assimilation of partially melted crustal components into the Central Intrusion upper walls, leading to the displacement of the crystallization cotecticinto the oxide phase and formation of OCP facies. (b) In the final stages of crystallization, and after the formation of OCP facies, a new episode of magma recharge in the PN massif. (c) Displacement of crystallization cotectic into the oxide phase due to the interaction of primitive and evolved liquids during a magma recharge episode, forming the MTT facies. (d) Eventual erosion of the uppermost sequences of PN, exposing the lithologies approached in this study.
Figure 16. Schematic model for the proposed origin to OCP and MTT facies. (a) Assimilation of partially melted crustal components into the Central Intrusion upper walls, leading to the displacement of the crystallization cotecticinto the oxide phase and formation of OCP facies. (b) In the final stages of crystallization, and after the formation of OCP facies, a new episode of magma recharge in the PN massif. (c) Displacement of crystallization cotectic into the oxide phase due to the interaction of primitive and evolved liquids during a magma recharge episode, forming the MTT facies. (d) Eventual erosion of the uppermost sequences of PN, exposing the lithologies approached in this study.
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Table 1. Summary of petrographic features of intrusions of the Ponte Nova massif (after [38]). CI, central intrusion; WI, western intrusion; NI, northern intrusion; EI, eastern intrusion; SSI, southern satellite intrusion; OCP, oxide-rich clinopyroxenite; MTT, magnetitite; Brc, magmatic breccia; LS, lower sequence; US, upper sequence; Krs-rich, kaersutite-rich cumulates; Nph, nepheline-bearing monzonite.
Table 1. Summary of petrographic features of intrusions of the Ponte Nova massif (after [38]). CI, central intrusion; WI, western intrusion; NI, northern intrusion; EI, eastern intrusion; SSI, southern satellite intrusion; OCP, oxide-rich clinopyroxenite; MTT, magnetitite; Brc, magmatic breccia; LS, lower sequence; US, upper sequence; Krs-rich, kaersutite-rich cumulates; Nph, nepheline-bearing monzonite.
IntrusionLithologiesColor
Index		Crystallization Stages
Early-Magmatic
(Primocrysts)		PrincipalLate-MagmaticPost-Magmatic
CILS (regular)(coarse- to medium-grained)
(cumulatic)
Olivine clinopyroxenites,
Olivinel-bearing clinopyroxenites,
Olivine melagabbros,
Olivine-bearing melagabbros		97–70Di, Ti-Aug, Ol, ±Ti-Mgt, ±Ap, ±Pl Ti-Aug, Pl, Krs, Ti-Mgt, Ap, ±Ilm, ±BdyTi-rich Bt,
±Afs, ±Nph		(incipient levels) Anl, Ab, Ms, Cb, Ep/Czo, Chl, Bt, Act, Ttn, Py, ±Po, ±Cpy, ±Sp
LS
(Krs-rich)		(coarse- to medium-grained)
(cumulatic)
Olivine kaersutite clinopyroxenites,
Kaersutite melagabbros		95–80Di, Ti-Aug, Ol, ±Ti-Mgt, ±Ap, ±PlKrs, Ti-Aug, Pl, Ti-rich Bt, Ti-Mgt, Ap, ±Ilm, ±BdyTi-rich Bt, ±Afs, ±Nph
US(coarse- to medium-grained)
(inequigranular to macrocrystic) Nepheline-bearing monzogabbros		62–50Di, Ti-Aug, Pl, ±OlPl, Ti-Aug, Bt, Ti-Mgt, Ti-rich Bt, ApIntergrowth of Afs + Nph + Pl
WILS(coarse- to medium-grained)
(cumulatic)
Olivine melagabbros,
Olivine-bearing melagabbros,
Olivine-bearing clinopyroxenites		99–75Di, Ti-Aug, Ol, ±Ti-Mgt, ±Ap, ±Pl Ti-Aug, Pl, Krs, Ti-rich Bt, Ti-Mgt, Ap, ±Ilm, ±BdyTi-rich Bt, ±Afs, ±Nph
US(coarse- to medium-grained)
(inequigranular to equigranular, banded to massive)
Nepheline-bearing monzogabbros		52–36Pl, Ti-Aug, ±Ti-Mgt,
±Ap, ±Ol		Pl, ti-Aug, Ti-Mgt, Ap, Ti-rich Bt, Afs, Nph, Ba-rich AfsIntergrowth of Afs + Nph + Pl
NI(coarse- to medium-grained)
(cumulatic)
Olivine melamonzogabbros,
Olivine-bearing melamonzogabbros		76–67Di, Ti-Aug, Ol, ±Ti-Mgt,
±Ap, ±Pl		Ti-Aug, Pl, Ti-Mgt, Ap, Afs, Ilm, ±BdyTi-rich Bt,
±Krs,
intergrowth of Afs + Nph		--
EI(medium- to fine-grained)
(seriate to porphyritic)
Nepheline monzodiorites,
Nepheline-bearing monzodiorites		58–51Di, Ol, Pl, ±ApTi-Aug, Pl, Ba-Ti-rich Bt, Ti-Mgt, Ilm, Ap, ±ZrcAfs, Ba-rich Afs, Nph,
±Sdl		(insipient levels) Anl, Ab, Prh, Cb, Ep/Czo, Chl, Bt, Py, ±Cpy, ±Sp
CP(medium-grained)
(porphyritic to equigranular) microgabbros		56–50Ti-Aug, OlTi-Aug, Pl, Ti-Mgt, Bt, ApBt, Afs, Nph--
SSI(porphyritic, medium-grained matrix)
Nepheline-bearing melamonzogabbros		67–60Di, Ti-Aug, OlPl, Ti-Aug, Ti-Mgt, Ilm, ApTi-rich Bt, Afs, Ba-rich Afs, Nph(incipient levels) Cb, Anl, Ab, Ms, Ep/Czo, Chl, Bt, Ttn, Py, Fl
(porphyritic, medium-grained matrix)
Nepheline-bearing melamonzonites		64–62Di, Ti-AugPl, Ti-Aug, Ba-Ti-rich Bt, Afs, Ba-rich Afs, Ti-MgtIlm, Ap ± ZrcNph
coarse- to medium-grained
(inequigranular, porphyritic)
Nepheline-bearing monzonites		46–40Pl, Afs, Ba-rich Afs, Ti-AugBa-Ti-rich Bt, Ap, Prg, Hst, Ti-Mgt, NphNph
OCP MTT(coarse- to medium-grained)
(cumulatic)
Oxide-rich clinopyroxenites		100 Ti-Aug, Di, Ilm (±Ti-Mgt)Ti-Aug, Ilm, (±Ti-Mgt), Ap,Ti-rich Bt, ±Py, Ap, Krs (low-levels) Ti-rich Bt, Hem, Py, ±Cpy, Mlnt, Rmb
(coarse- to medium-grained)
(cumulatic)
Magnetitites		100Ti-Mgt, Ilm, Ti-Aug Ti-Aug, Ti-Mgt,
Ilm 		Ti-Mgt, IlmHem, Ulv, Ant, Gth
Mineral abbreviations: Ab—albite, Anl—analcime, Ant—anatase, Afs—Alkali feldspar, Ap—apatite, Bdy—baddeleyite, Bt—biotite, Cb—carbonates, Cpy—chalcopyrite, Chl—chlorite, Czo—clinozoisite, Di—diopside, Ep—epidote, Fl—fluorite, Gth—goethite, Hem—hematite, Hst—hastingsite, Ilm—ilmenite, Krs—kaersutite, Mlnt—melantherite, Ms—muscovite, Nph—nepheline, Ol—olivine, Pl—plagioclase, Po—pyrrothite, Prg—pargasite, Prh—prehnite, Py—pyrite, Rmb—rhomboclase, Sdl—sodalite, Sp—sphalerite, sulf—sulfides, Ti-Aug—Ti-augite, Ti-Mgt—Ti-magnetite, Ttn—titanite, Ulv—ulvöspinel, Zrc—zirconolite.
Table 2. Mineralogical composition of magnetitite and oxide-rich clinopyroxenite samples determined by XRD_Rieteveld method. GoF (Goodness of Fit) is a measure of the convergence of the calculated model toward the observed data (for details, see [58]).
Table 2. Mineralogical composition of magnetitite and oxide-rich clinopyroxenite samples determined by XRD_Rieteveld method. GoF (Goodness of Fit) is a measure of the convergence of the calculated model toward the observed data (for details, see [58]).
MTTOCP
wt%R118SM82R119
Ti-augite2.025.768.7
Diopside 1.8
Ti-magnetite84.263.6
Ilmenite5.83.35.0
Anatase0.90.5
Spinel5.14.3
Goethite1.92.6
Pyrite 0.7
Melantherite 23.1
Rhomboclase 0.7
GoF3.824.163.94
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de Souza, A.A.; Azzone, R.G.; Chmyz, L.; Tarazona, L.M.C.; de Andrade, F.R.D.; Martins, J.V.; Ruberti, E.; de Barros Gomes, C. Genesis of Fe-Ti-(V) Oxide-Rich Rocks by Open-System Evolution of Mafic Alkaline Magmas: The Case of the Ponte Nova Massif, SE Brazil. Minerals 2024, 14, 724. https://doi.org/10.3390/min14070724

AMA Style

de Souza AA, Azzone RG, Chmyz L, Tarazona LMC, de Andrade FRD, Martins JV, Ruberti E, de Barros Gomes C. Genesis of Fe-Ti-(V) Oxide-Rich Rocks by Open-System Evolution of Mafic Alkaline Magmas: The Case of the Ponte Nova Massif, SE Brazil. Minerals. 2024; 14(7):724. https://doi.org/10.3390/min14070724

Chicago/Turabian Style

de Souza, Amanda Andrade, Rogério Guitarrari Azzone, Luanna Chmyz, Lina Maria Cetina Tarazona, Fábio Ramos Dias de Andrade, José Vinicius Martins, Excelso Ruberti, and Celso de Barros Gomes. 2024. "Genesis of Fe-Ti-(V) Oxide-Rich Rocks by Open-System Evolution of Mafic Alkaline Magmas: The Case of the Ponte Nova Massif, SE Brazil" Minerals 14, no. 7: 724. https://doi.org/10.3390/min14070724

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