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Article

HRTEM Study of Desulfurization of Pt- and Pd-Rich Sulfides from New Caledonia Ophiolite

by
Néstor Cano
1,*,
José M. González-Jiménez
2,
Fernando Gervilla
3 and
Thomas N. Kerestedjian
4
1
Instituto de Geología, Universidad Nacional Autónoma de México (UNAM), Ciudad Universitaria, Coyoacán, 04510 Mexico City, Mexico
2
Instituto Andaluz de Ciencias de la Tierra (IACT-CSIC), Avda. de las Palmeras 4, Armilla, 18100 Granada, Spain
3
Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Avda, Fuentenueva s/n, 18002 Granada, Spain
4
Geological Institute, Bulgarian Academy of Sciences, 24 Georgi Bonchev Str., 1113 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(1), 66; https://doi.org/10.3390/min15010066
Submission received: 30 November 2024 / Revised: 5 January 2025 / Accepted: 8 January 2025 / Published: 12 January 2025
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
Oxygen-bearing platinum group minerals (O-bearing PGMs) are intergrown with base metal sulfides (BMS, e.g., pentlandite–[NiFe]9S8) within fractures in chromite grains from chromitite bodies on Ouen Island, New Caledonia. These PGMs are hosted in chlorite and serpentine, which formed during serpentinization of olivine and pyroxene. The O-bearing PGM grains are polygonal, show microfracturing (indicating volume loss), and contain Pt-Pd-rich sulfide remnants, suggesting pseudomorphic replacement of primary (magmatic) sulfides. They display chemical zonation, with Pt(-Pd-Ni-Fe) relict sulfide cores replaced by Pt-Fe-Ni oxidized alloy mantles and Pt-Cu-Fe(-Pd) alloy rims (tulameenite), indicating desulfurization. The core and mantle show a nanoporous structure, interpreted as the result of coupled dissolution–reprecipitation reactions between magmatic sulfides and low fO2–fS2 serpentinite-related fluids, probably formed during olivine transformation to serpentine + magnetite (early stages of serpentinization). This fluid infiltrated magmatic sulfides (PGE-rich and BMS), degrading them to secondary products and releasing S and metals that were accommodated in the mantle and rim of O-bearing PGMs. Upon olivine exhaustion, an increase in fO2 might have stabilized Pt-Fe-O compounds (likely Pt0/Pt-Fe + Fe oxyhydroxides) alongside Ni-Fe alloys. Our results show that post-magmatic desulfurization of primary sulfides produces complex nano-scale intergrowths, mainly driven by changes in the fluid’s physicochemical properties during serpentinization.

1. Introduction

Magmatic Pt-Pd-rich sulfides (e.g., cooperite–PtS, malanite–Cu[Ir,Pt]S4) can undergo in situ desulfurization when exposed to late-magmatic or metamorphic-related hydrothermal fluids ([1] and references therein). The desulfurization rection of Pt- and Pd-rich sulfides usually produces complex assemblages of Pt-Fe(-Cu) alloys, e.g., isoferroplatinum (Pt3Fe) ± tetraferroplatinum (PtFe) ± tulameenite Pt2CuFe [2,3]. Some authors have suggested that these secondary Pt-Fe(-Cu) alloys remain stable across various fO2 conditions, even in the supergene environment, as evidenced by the presence of allogenic Pt-Fe(-Cu-Pd) alloys in placers derived from magmatic ore bodies [4,5,6]. Nevertheless, contrasting views suggest a different transformation sequence in highly oxidizing environments (e.g., weathering), where Pt-Pd-rich sulfides are supposedly transformed into Pt-Pd-rich oxyhydroxides (via oxidative desulfurization), and Pt-Fe alloys are considered an intermediate product [7,8,9,10,11]. However, up to date, only palladinite (PdO)—documented by [12] as an ochreous coating on palladian gold at Itabira (Brazil)—has been approved as a valid Pt-group element (PGE) oxide by the International Mineralogical Association (IMA), although it is ascribed as “questionable” because its identification was conducted by conventional mineralogical methods.
More recent studies that applied X-ray microbeam absorption on O-bearing Pt-Fe grains from the Pirogues deposit (New Caledonia ophiolite; Figure 1b) have revealed that these “O-bearing” PGMs (Pt-group minerals), in fact, consist of fine intergrowths of Pt0 or isoferroplatinum and ferrihydrite (Fe10O14[OH]2) [13]. In agreement, later studies based on X-ray computed tomography and X-ray diffraction (XRD) [14] and micro-Raman spectroscopy and synchrotron tts-μXRD (synchrotron through-the-substrate micro-XRD) [15] have suggested that Ru-Os-Ir(-Fe-Ni) oxides/hydroxides consist of fine intergrowths of PGMs and Fe-rich oxyhydroxides. Nano-scale observations by [16] provided definitive evidence for this debate, showing that previously considered Ru-Os-Ir-Fe-Ni oxides from the Monte Bueno mine (Cuba) were, in reality, nano-sized intergrowths of Ru-Os-Ir alloys and Fe-Ni-Mn oxide/hydroxide. This latter work showed that there could be a full sequence of transformation of Ru-Os-Ir sulfides to secondary Ru-Os-Ir alloys and Fe-Ni-Mn oxide/hydroxide and that this sequence could be tracked down to the atomic scale. However, it is not clear how these transformations operate in Pt(-Fe-Ni-Pd-Cu) sulfides and whether Pt-Fe-Ni alloys represent relict phases, intermediate steps, or final products of desulfurization.
This paper addresses the desulfurization process of Pt(-Fe-Ni-Pd-Cu) sulfides based on the atomic-scale analysis of supposed PGE oxides (referred to as O-bearing PGMs hereafter). For this purpose, we have focused on those O-bearing PGMs reported by [3] within fractures or altered zones of chromite crystals in PGE-rich chromitites from the New Caledonia ophiolite (Figure 1). These PGM grains display comparable features to those “PGE oxides” described by [7] and later reinterpreted by [13], i.e., a beige-brownish hue with noticeable pleochroism, strong anisotropy, rugged granular texture, and fracturing. We conducted in situ analyses on PGM grains directly in the parent rock, rather than in heavy mineral concentrates from laterite deposits—as was the case in previous studies on the New Caledonia ophiolite [7,16]. This allowed us to capture crystallographic variations in the entire sequence of desulfurization of Pt-Pd(-Ni-Fe-Cu) sulfides. These results shed new light on the true nature of Pt-Pd oxides and their formation mechanisms during the serpentinization of ophiolitic complexes.

2. Geological Background

Oxygen-bearing PGMs analyzed in this study were identified in a chromitite body (#3 in [3]) cropping out NE of Baie Tranquille of the Island of Ouen, at the southern part of New Caledonia (coordinates 166°49′40.8″ E, 22°25′15.6″ S; Figure 1c). Located in the southwestern Pacific Ocean, east of Australia and northwest of New Zealand (Figure 1a), the New Caledonia mainland records a tectonic evolution related to the development of marginal basins and island arcs since the Late Cretaceous and the collision of microcontinental fragments and terranes [17,18]. During the Late Eocene (~52–48 Ma), arc–microcontinent collision led to the emplacement of the Cretaceous (100–77 Ma) oceanic lithosphere that constitutes the New Caledonia ophiolite [19,20,21]. Most of the ophiolite exposures represent upper mantle material [22,23], while the crustal section is limited to cumulate mafic rocks of the lower crust [19,24,25,26]. Ophiolites cover ~41% of New Caledonia’s surface area and are largely exposed in the southern massif (or Grand Massif du Sud [19]). This massif is made up of tectonite-structured mantle harzburgite, locally capped by dunite, wehrlite, rare pyroxenite, and layered gabbro [19,24,27]. The massif hosts abundant podiform chromitite bodies within mantle peridotite (e.g., Anna-Madeleine mine), and smaller chromite seams in the overlying dunite (e.g., Pirogues deposit, Figure 1b [26,28,29]).
The southernmost expression of the southern massif (and the New Caledonia ophiolite) occurs on Ouen Island (Figure 1b). The central–western part of the island hosts a unit of strongly serpentinized mantle harzburgites with a tectonite structure (Figure 1c). The base of this unit is represented by a tectonic mélange that comprises altered gabbros within a serpentinite-rich matrix, with frequent intrusions and dikes of plagiogranite. The central–eastern part (around Baie Tranquille) hosts mantle harzburgites that transition upward into interlayered dunite–wehrlite, which further grades into wehrlite with gabbro intervals and minor chromitite bodies, which are studied herein. According to [3], these chromitite bodies are hosted by layered plagioclase-bearing wehrlite and occur as small pods, irregular bodies, or schlieren up to 1 m long and less than 0.5 m thick (Figure 2a,b). These authors indicate that the chromitites contain numerous small (<20 µm across) particles of PGMs, including Pt-Fe-Cu alloys (isoferroplatinum and tulameenite), Pt(-Rh-Pd) sulfides (cooperite and malanite), Pt “oxides”, Ru-Os-Ir sulfides (laurite–RuS2), pentlandite ([NiFe]9S8), and native Ru, Pd, Pt, and Os. These minerals occur as solid inclusions within unaltered chromite or primary silicates (olivine or pyroxene), or within fractures filled by chlorite cutting across altered chromite. In particular, Pt “oxides” are restricted to fractures or alteration rims of ferrian chromite (coating cores of chromite). They occur as single grains around partially desulfurized malanite or cooperite in association with pentlandite, pyrrhotite, and chalcopyrite [3].

3. Materials and Methods

We prepared five polished thin sections from the studied chromitite body (Figure 1c) and performed petrographic observations at the Instituto Andaluz de Ciencias de la Tierra (Granada, Spain) in order to choose the best O-bearing PGM grains for further scanning electron (SEM) and transmission electron microscopic (TEM) analyses. SEM studies were conducted using a field emission Nova NanoSEM 450 (Thermo Fisher-FEI; Hillsboro, OR, USA) at the Instituto Universitario de Investigación en Microscopía Electrónica y Materiales, University of Cádiz (Spain). The SEM images were obtained using backscattered electron detectors, and compositional maps using energy-dispersive spectrometer (EDS) silicon drift detectors. The accelerating voltage and beam current were adjusted for each case to ensure high-resolution imaging and sufficient counts for EDS measurements.
Once the optimal area for thin foil extraction was selected, a focused-ion beam (FIB) SEM was employed to prepare one thin foil of a concentrically zoned O-bearing PGM (Figure S1) following the methodology explained in [30,31]. FIB-SEM was performed using a Dual Beam Thermo Fisher-FEI (Hillsboro, OR, USA), model Helios 650 at the Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragón, University of Zaragoza, Spain. The region of interest was protected with a ~300 nm C layer and a ~1 μm Si layer prior to milling, polishing, and foil extraction. The bulk material on both sides of the lamella was removed using Ga ion milling at 30 kV and 2.5 nA, followed by polishing with 30 kV at 0.23 nA. Final polishing was achieved using a 5 kV beam at 68 pA to monitor electron transparency, with a final step of 5 kV at 10 pA to minimize amorphization. The thin foil, approximately 100 nm thick, was then lifted out and transferred to a TEM Cu grid using an OmniProbe nanomanipulator (Thermo Fisher-FEI; Dallas, TX, USA) with a tungsten tip.
High-resolution (HR) TEM analyses of the thin foil were conducted using a probe-corrected Titan (Thermo Fisher-FEI; Eindhoven, The Netherlands) TEM at the LMA (Zaragoza, Spain), equipped with an XFEG field emission gun (Thermo Fisher-FEI; Hillsboro, OR, USA). The TEM has a spherical aberration Cs-corrector in the condenser system for probe correction. The areas of interest within the thin foil were imaged using (1) high-angle annular dark field (HAADF) to produce high-contrast Z images via scanning transmission electron microscopy (STEM) and (2) high magnification (HM-) TEM images to observe the nanostructure of O-bearing PGMs and to identify mineral species. The Titan was operated at 300 kV, while HM-TEM images were being captured using a Gatan CCD Camera. Mineral compositions were determined through EDS analyses with an Ultim Max detector, and data processing was completed using the AZtecTEM software package (Oxford Instruments; Abingdon, England).

4. Micron- to Atomic-Scale Characterization of the Pt-Pd O-Bearing PGMs

Platinum-Pd O-bearing PGMs occur as small (<20 µm) subrounded and polygonal grains, usually isolated within fractures that cut across chromite (Figure 2c–f). Under reflected light, they show a beige-brownish gray color, pleochroism, rugged aspect, microfracturing, and strong anisotropy (Figure 2d,f). These features agree with those previously reported in PGE “oxides” by some authors [3,7,11,32]. In most cases, Pt-Pd O-bearing PGMs are spatially associated with pentlandite replaced by Ni-rich Fe oxyhydroxides and various Pt-Rh-Ru-Ir sulfides (Figure 2c–f, Figure 3a–e and Figure S2a,b). In back-scattered electron images, Pt-Pd O-bearing PGMs exhibit a characteristic internal microfracturing and chemical and textural heterogeneity, as evidenced by mottled and plain (dark and bright) areas (Figure 3a–i and Figure S2c–f). SEM-EDS analyses reveal that mottled areas are enriched in Cu-Pd and depleted in Fe-Ni-S relative to plain areas (Figure 3i and Figure 4a). Plain areas are dark and bright (Figure 3d,e), the former being enriched in Fe-Ni-S relative to the latter (Figures S2f and S3). Remnants of Pt-Pd-bearing sulfides are preserved in some grains of Pt-Pd O-bearing PGMs (Figure 3f; S map in Figure 4a) and may be concentrically replaced at the micron scale (Figure 4a,b). We chose one of those concentrically zoned grains for thin foil extraction and further HRTEM inspection (Figure 4b).
The thin foil confirms the concentrical zonation observed in BSE images, which is defined by a core-to-rim sequence of Pt(-Pd-Ni-Fe) sulfide → Pt-Fe-Ni oxidized alloy → Pt-Cu-Fe(-Pd) alloy (Figure 4b, Figure 5, Figure 6a, and Figure S4). HAADF images show that the contacts between each zone are very irregular (Figure 6a–f and Figure S4), suggesting coupled dissolution–reprecipitation reactions. The Pt(-Pd-Ni-Fe) sulfide core displays a rugged nanoporous structure consisting of nano-sized needles of S-poor Pt(-Pd-Ni-Fe) compounds elongated along a preferential direction (Figure 6a–c, and Figure S4a). It is compositionally equivalent to the plain dark areas seen in the BSE images (e.g., Figure 3d,e). A similar nanostructure is observed in the Pt-Fe-Ni oxidized alloy mantle that coats the relict S-poor Pt(-Pd-Ni-Fe) core (Figure 6a–e), although the mantle is enriched in O and depleted in S-Ni relative to the core (Figure 6c and Figure S4a–c). Such a mantle is compositionally equivalent to the plain bright areas from the BSE images (e.g., Figure 3d,e). Unfortunately, the foil was not thin enough in its central region to conduct HM-TEM observations on the contact between the Pt(-Pd-Ni-Fe) sulfide core and Pt-Fe-Ni oxidized alloy mantle. Despite that, STEM-EDS single-spot spectra and mapping show that the mantle is chemically heterogeneous, as it comprises a mixture of nano-sized needles (<30 nm), probably including tetrataenite (?; FeNi) and antitaenite (?; Fe3Ni) within a matrix of Pt-Fe-O compounds (Figure 6f and Figure S4c; see STEM-EDS spectra in Figure S5). After examining the Fe-Ni oxidized alloy mantle in electro-transparent areas of the thin foil using HM-TEM, we could not find measurable d-spacings in the needles of Fe-Ni alloys (Figure 7a,b).
The outermost rim of the grain is a Pt-Cu-Fe(-Pd) alloy that shows a mottled texture in HAADF images (Figure 6d–f and Figure S4b–d), which is also observed in the BSE images (Figure 3b,i). Compared to the Pt-Fe-Ni oxidized alloy mantle, the alloy rim is enriched in Pd and Cu and depleted in O, Fe, and Ni (Figure 5 and Figure S4b–d). A careful HM-TEM inspection revealed that the alloy rim produces embayments in the oxidized alloy mantle (Figure 7a) and yields measurable d-spacings of ~1.82 and ~1.90 Å (Figure 7c–e) that are close to the ideal 1.79 and 1.94 Å of (002) and (110) of tulameenite [33].

5. Discussion

5.1. Origin of the Nano-Scale Structure of O-Bearing PGMs

Our petrographic and FE-SEM observations on O-bearing PGMs (Figure 2c–f, Figure 3a–i and Figure 4a) are consistent with previous microstructural descriptions of similar grains from chromitites on Ouen Island [3]. The BSE images reveal that most grains exhibit polygonal outlines with a complex chemical zonation, consisting of irregular cores of Pt(-Pd-Ni-Fe) sulfides surrounded by mantles of O-bearing Pt-Fe-Ni(-S) compounds and outermost rims of Pt-Fe-Cu(-Pd) alloys (Figure 3a–c, Figure 4a,b and Figure S3). In some O-bearing PGM grains, there are small (~1.5 µm in size) and irregular remnants of the Pt-Pd-rich sulfide precursor (Figure 3f–i). These features suggest that O-bearing PGMs represent the pseudomorphic replacement of preexistent Pt-Pd(-Fe-Ni-Cu) minerals. Indeed, ref. [3] reported O-bearing PGM grains with relict cores of cooperite (PtS) and malanite (CuPt2S4) (see their Figure 7g) that show a remarkable substitution of Pt by other PGEs (up to 14.6 wt% Rh, 5.1 wt% Ir, 3.7 wt% Pd) and Cu by Ni + Fe (5.2 wt% Ni + Fe). Therefore, similar cooperite and malanite crystals are considered potential precursors for the O-bearing PGMs studied herein.
Oxygen-bearing PGMs usually occur intergrown with Ni-Fe sulfides variably degraded to Ni-rich oxyhydroxides (Figure 2d–f, Figure 3a–d, Figures S2g and S3). EDS mapping (using FE-SEM and STEM) of the O-bearing PGM grain examined by HRTEM reveals that Cu and Pd are mostly concentrated in the outermost rim (Figure 4a, Figure 5, and Figure S4b–d). Such paragenetic association and chemical zonation likely reflect pristine features of the original magmatic grain, which might have consisted of cooperite (or malanite) intergrown with PGE-Cu-bearing Ni-Fe sulfides (e.g., pentlandite, pyrrhotite, and/or chalcopyrite; base metal sulfides–BMS). In agreement, we found euhedral cooperite crystals intergrown with BMS, both encapsulated in unaltered chromite and clinopyroxene (Figure S6; Figure 5d in [3]. Considering that O-bearing PGMs occur with secondary minerals (e.g., chlorite and serpentine) along fractures (Figure 2c–f), we postulate that magmatic sulfides were exposed to hydrothermal fluids during the post-magmatic alteration of the chromitite [1,34,35], thus leading to their transformation into O-bearing PGMs. Further, widespread microfracturing of O-bearing PGM grains (Figure 3), suggestive of volume reduction, along with their detectable O contents (Figure 5 and Figure S2c–f), suggests that the post-magmatic alteration of Pt-Pd-rich sulfides might have followed a (oxidative?) desulfurization path [16,35,36,37].
STEM-EDS, HRTEM, and HAADF observations revealed a nanoporous structure in the relict S-poor Pt(-Pd-Ni-Fe) sulfide core and its wrapping Pt-Fe-Ni oxidized alloy mantle (Figure 6a–d). Such a nanoporous structure consists of nanometric needles of metallic phases, although with core-to-mantle chemical differences (Figure 6c and Figure S4a–c). The core is made up of Pt(-Pd-Ni-Fe)-S compounds and is mantled by Ni-Fe alloys embedded in an O-bearing Pt-Fe matrix (Figure 6f, Figure S4c and Figure 7a). Nevertheless, the nanotextural similarity between the core and mantle—which is certainly not resolvable by conventional FE-SEM BSE imagining or EDS elemental mapping (Figure 3)—suggests a common link during the post-magmatic alteration processes. Based on these nanotextural and compositional features, we postulate that the crystalline matrix of a Pd-rich cooperite (PGE sulfide in Figure 8a,b) was degraded in an early stage of alteration to the nanoporous structure, consisting of S-poor Pt(-Pd-Ni-Fe) nano-sized needles (Figure 8c,d). This probably resulted from volume reduction due to S loss, and hence the frequent microfracturing. The relict core is enriched in Pt, Pd, Ni, and Fe (Figure 4a, Figure 5, and Figure S4a), suggesting that these metals were lattice-bounded in the magmatic cooperite/malanite [3] and remained immobile during the early alteration. As the alteration progressed, the interconnected network of nanopores facilitated the circulation of fluids, thus contributing to the subsequent dissolution of S-poor Pt(-Pd-Ni-Fe) nano-sized needles. This resulted in a decoupling between Ni (and Fe) and Pt (and Pd) via the formation of Ni-Fe alloys + Pt-Fe-O compounds, which constituted a newly formed oxidized alloy mantle (Figure 8e,f). Indeed, our HRTEM data show that such a mantle comprises irregular, nano-sized intergrowths of Ni-Fe alloys (seemingly tetrataenite and antitaenite; Figure S5b,c) and Pt-Fe-O compounds (Figure 6f, Figure S4c and Figure 7a). Quite admittedly, the thickness of the foil (~100 nm) near its central area (core and mantle) did not allow us to obtain resolvable HRTEM images in order to assess the true nature of the Pt-Fe-O matrix that hosts Ni-Fe alloys. Nevertheless, these phases could be fine intergrowths of Pt0 or isoferroplatinum with ferrihydrite, such as those reported by [13] based on X-ray micro-beam absorption spectra on micron-sized Pt-rich “oxides” from the nearby Pirogues mineralization (Figure 1).
The sequence of alteration explained above implies that Pt-Fe-rich alloys are not necessarily intermediate phases that must dissolve to produce intergrowths of PGE alloys and Fe oxyhydroxide, as proposed by Hattori and coworkers. Rather, our nanotextural and compositional evidence suggests that direct alteration of magmatic Pt-Pd-rich sulfides—via the formation of S-depleted Pt-Pd-rich nanodomains—is also feasible. This scenario must have involved coupled dissolution–reprecipitation reactions that disproportionated Fe and Ni in the original sulfide (or the hosting silicate matrix; see next section) to produce Ni-Fe alloys coexisting with a Pt-Fe-O matrix. Further, this interpretation agrees with recent works that have documented the direct transformation (i.e., desulfurization) of other magmatic sulfides (e.g., laurite [RuS2]-erlichmanite [OsS2]) to fine intergrowths of metallic PGE alloys (i.e., Os-Ir-Ru alloys) and Fe(-Mn) oxides/hydroxides [14,15,16,38,39].
The oxidized alloy mantle of O-bearing PGMs is rimmed by a Pt-Pd-Cu-Fe alloy (i.e., tulameenite), which shows a mottled texture that is observable at the micron- and nano-scale (Figure 3b,i, Figure 5 and Figure 6d). The contact between the oxidized alloy mantle and tulameenite rim is sinuous (embayed), and relicts of needle-like nano-compounds are found as “islands” within tulameenite (Figure 6d,e and Figure S4b). These textural features suggest that tulameenite grew at the expense of the oxidized alloy mantle, during which time Cu and Pd were remobilized by fluids, which originally entered the porous nanostructure but ultimately diffused toward the margins (Figure 8e,f). Moreover, the close association between O-bearing PGMs (former cooperite) and Cu-bearing Ni-Fe sulfides (Figure 2c–f and Figure 3a–c) suggests that Cu and Pd could have also been remobilized from the accompanying BMS. Thus, the tulameenite rim may represent a former boundary between the magmatic Pt-Pd-rich sulfide and BMS.

5.2. Stability Conditions for Pt-Pd Sulfide Transformation to Secondary Oxide–Alloy Intergrowths

Desulfurized, O-bearing PGMs in association with base metal sulfides (BMS) are located within fractures that cut across chromite replaced by ferrian chromite rims (Figure 2c–f; [3]). These fractures are infilled with hydrated phases, such as serpentine and chlorite (Figure 2c,e), thus suggesting an origin linked to hydrothermal fluids derived from serpentinization. The serpentinization-driven desulfurization of PGMs and BMS requires low fS2, which is commonly attributed to the low fO2 conditions (e.g., below ΔFMQ –4) that are typical of the initial stages of serpentinization [38,39,40,41,42].
At Ouen, the initial stages of the serpentinization of chromitites (and their host peridotites) involved the hydration of olivine, which generated lizardite ± magnetite while releasing significant amounts of H2 and/or CH4 [3,43]. The release of such reducing agents potentially drove down both fO2 and fS2 by forming water and H2S [38,39,44,45], which might have promoted the mobilization of S out from primary sulfides (Figure 8c,d). Our micro- to nano-scale textural and compositional evidence from O-bearing PGMs supports a mobilization of S from cooperite (or malanite; Figure 3f,g), following the sequence of desulfurization: PGE-Fe-Ni sulfide → S-poor Pt(-Pd-Ni-Fe) sulfides → Ni-Fe alloys + Pt-Fe-O → tulameenite (see discussion above; Figure 8). These PGMs show a nanoporous structure that is preserved in the relict core and its surrounding mantle (Figure 6a–e and Figure S4), which resembles porous structures that result from coupled dissolution–reprecipitation reactions between a solid phase and a circulating fluid [46]. In our case study, this structure is interpreted to have resulted from the degradation of magmatic cooperite or malanite (the solid phase) due to their interaction with the reducing, H2-rich fluids (the circulating fluid) derived from the ongoing serpentinization of olivine and pyroxene (Figure 8a–d). Further, the infiltration of reducing fluids could have also promoted the mobilization of base metals (Fe, Ni and Cu) from BMS or silicates [47], thus contributing to the formation of Ni-Fe alloys (e.g., tetrataenite and antitaenite; Figure S5b,c and Figure 8d).
Hydrogen and O isotopic data from the New Caledonia ophiolite indicate that peridotites started to hydrate at 250–350 °C by fluids released from the dehydrating altered oceanic crust [43]. Although the effect of pressure is not fully understood, at these temperatures of serpentinization, the PtS/Pt0 equilibrium curve predicts a congruent decomposition of PtS to Pt0 at around –16.5 log fS2 [48]. This suggests that serpentinite-related fluids must have had lower log fS2 to promote the breakdown of PtS (i.e., cooperite) to the Pt-rich alloys present in the mantles and rims of O-bearing PGMs. At these conditions, quantitative estimates of the relative stabilities within the system Fe-Ni-O-PGE-S ± Co ± H ± Si ± Mg [3,40,47,49,50,51,52] indicate the coexistence of Fe-Ni alloys (awaruite [Ni3Fe] and taenites [Ni,Fe]) in equilibrium with other alloys (e.g., PGE alloys) or native metals (e.g., Pt0). These thermodynamic constraints agree with the occurrence of tetrataenite and antitaenite (Fe-Ni alloys) in the Pt-Fe-Ni oxidized alloy mantles (Figure 6f and Figure S4c) and tulameenite (Pt-bearing alloy) in the outermost rims of O-bearing PGMs (Figure 5, Figure 6f and Figure S4d). However, the stability fields for taenites are not well established at these conditions, even though tetrataenite—an equiatomic and highly ordered Fe-Ni alloy—has been documented in several localities of serpentinized oceanic peridotites [53,54,55,56]. In fact, it is still discussed whether these Ni-Fe alloys are stable or metastable during serpentinization (see discussion in [54]), although they have been synthesized via H reduction of nanometric NiFe2O4 [57]. In any case, the discussion on the natural stability of these Ni-Fe alloys goes beyond the scope of our paper.
Previous works have shown that alloys formed early during the serpentinization process may be eventually destroyed (to form oxides or newly stable sulfides) due to an increase in fO2 and fS2 when olivine is fully exhausted [38,40,47,50]—H2 production declines when olivine is consumed [58]. Nickel-Fe alloys, and probably Pt-Fe alloys and Pt0, coexist with Fe oxyhydroxides in the studied O-bearing PGMs (Figure 6f, Figure 7a, and Figure S4c), thus suggesting that circulating fluids attained fO2 conditions that were high enough to stabilize oxyhydroxides but still preserved alloys. The spatial association between Ni-Fe, Pt-Fe, or metallic Pt and oxides containing Fe3+ (e.g., magnetite, ferrihydrite, ferrian chromite) has been documented in serpentinized rocks from several localities (e.g., Iberia abyssal plain, Loma Baya–Mexico, Samail ophiolite–Oman [16,49,59,60,61]. In the studied chromitite body, chromite cores are replaced by ferrian chromite rims along fractures infilled with chlorite + serpentine + O-bearing PGMs [3]. Thermodynamic modeling [58,62,63,64,65] suggests that chlorite + Fe3+-chromite may form in equilibrium at less than 350 °C and −35 log fO2 (Figure 8e,f). Consequently, the co-crystallization of chlorite + ferrian chromite marks and increase in fO2, thus leading to the stabilization of oxidized Fe3+-bearing species alongside Ni-Fe alloys (e.g., Figure 6f and Figure S4c).

6. Conclusions

Oxygen-bearing PGMs from chromitites on Ouen Island (New Caledonia) formed alongside serpentine, chlorite, and ferrian chromite during the serpentinization. Starting from magmatic Pt-Pd-rich sulfides (with Ni and Fe in solid solution), we observed their progressive transformation into (1) nanoporous cores of S-depleted Pt(-Pd-Ni-Fe) needle-like compounds, (2) nanoporous mantles of needle-like Pt-Fe-O + Ni-Fe alloys, and (3) mottle-textured rims of Pt-Cu-Fe(-Pd) alloys. These features suggest a sequence of desulfurization resulting from the infiltration of serpentinite-related reducing fluids into magmatic PGE-rich sulfides, in which sequence alloys are not necessarily intermediate phases. The fluids also replaced neighboring base metal sulfides, thus remobilizing Cu and Pd that eventually concentrated in the alloy rims. A subsequent shift to more oxidizing conditions stabilized Pt-Fe-O compounds (likely Pt0 or Pt-Fe + Fe oxyhydroxides) alongside Ni-Fe alloys. Thus, the products of post-magmatic desulfurization of primary sulfides are highly influenced by shifts in the physicochemical properties of fluids during serpentinization.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min15010066/s1. Figure S1: Thin foil preparation using focused-ion beam scanning electron microscopy; Figure S2: Energy-dispersive X-ray spectroscopy (EDS) results for the studied minerals; Figure S3: EDS elemental mapping of Figure 3d or Figure S2f; Figure S4: TEM-EDS elemental mapping of different segments of the thin-foil; Figure S5: STEM-EDS spectra of the Pt-Fe-Ni oxidized alloy with needle-like texture (site indicated in Figure 6f); Figure S6: BSE images and corresponding EDS spectra of unaltered magmatic sulfide grains, consisting of cooperite, pentlandite, and pyrrhotite-pyrite.

Author Contributions

Conceptualization, J.M.G.-J., F.G. and N.C.; Methodology, N.C. and J.M.G.-J.; Formal Analysis, N.C., J.M.G.-J., F.G. and T.N.K.; Investigation, N.C. and J.M.G.-J.; Data Curation, N.C. and J.M.G.-J.; Writing—Original Draft Preparation, N.C., J.M.G.-J., F.G. and T.N.K.; Visualization, N.C. and J.M.G.-J.; Funding Acquisition, J.M.G.-J. and T.N.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work benefited from grant NANOMET PID2022-138768OB-I00 funded by MCIN/AEI/10.13039/50110001133 and by “ERDF A way of making Europe” by the “European Union”. Additionally, N.C. received funding from the Geological Society of America (grant 14012-24) to conduct laboratory analyses.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The samples used in this study were donated by Thierry Augé for JMGJ’s PhD project and the authors are indebted for their use. The authors acknowledge the use of instrumentation and the technical advice provided by the National Facility ELECMI ICTS node Laboratorio de Microscopías Avanzadas (LMA), University of Zaragoza–Unizar), and División de Microscopia Electrónica of the Servicios Centrales de Investigación Científica y Tecnológica (University of Cadiz–UCA). Laura Casado, Alfonso Ibarra, Rodrigo Fernández (LMA-Unizar), and Juan González García (UCA) are acknowledged for their assistance during sample preparation and laboratory analyses. Finally, we thank three anonymous reviewers for their constructive comments that helped improve it.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of New Caledonia in the southwestern Pacific (a) and distribution of ophiolite massifs in the island (b). Yellow dots in (b) represent localities and the yellow star the capital city. (c) Geological map of the Ouen Island and location of chromitite bodies (including the one studied here). Adapted from [3].
Figure 1. Location of New Caledonia in the southwestern Pacific (a) and distribution of ophiolite massifs in the island (b). Yellow dots in (b) represent localities and the yellow star the capital city. (c) Geological map of the Ouen Island and location of chromitite bodies (including the one studied here). Adapted from [3].
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Figure 2. Outcrop and photomicrographs of the studied chromitite bodies. (a,b) Outcrop of wehrlite hosting chromitite structures. (c–f) Reflected-light photomicrographs of chromitites containing O-bearing PGMs, pentlandite, Ni-rich Fe oxyhydroxides, and Pt-Rh sulfides within fractures filled with chlorite.
Figure 2. Outcrop and photomicrographs of the studied chromitite bodies. (a,b) Outcrop of wehrlite hosting chromitite structures. (c–f) Reflected-light photomicrographs of chromitites containing O-bearing PGMs, pentlandite, Ni-rich Fe oxyhydroxides, and Pt-Rh sulfides within fractures filled with chlorite.
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Figure 3. Back-scattered electron (BSE) images of O-bearing PGMs and associated minerals. (a–c) O-bearing PGMs surrounded by pentlandite (partly replaced by Ni-rich Fe oxyhydroxides) and Ru(-Ir) sulfides. Notice textural variations (i.e., mottled vs. plain) within single O-bearing PGM grains. (d,e) Ni-rich Fe oxyhydroxides after pentlandite in contact with Pt-Rh sulfides and O-bearing PGM grains, with the latter showing dark and bright areas. The degradation of pentlandite to Ni-rich Fe oxyhydroxides increases from (a) to (e). (f–i) O-bearing PGMs displaying Pt-Pd sulfide remnants (f,g) and concentrical textural and compositional zonation (i). Microfracturing was observed in all O-bearing PGM grains.
Figure 3. Back-scattered electron (BSE) images of O-bearing PGMs and associated minerals. (a–c) O-bearing PGMs surrounded by pentlandite (partly replaced by Ni-rich Fe oxyhydroxides) and Ru(-Ir) sulfides. Notice textural variations (i.e., mottled vs. plain) within single O-bearing PGM grains. (d,e) Ni-rich Fe oxyhydroxides after pentlandite in contact with Pt-Rh sulfides and O-bearing PGM grains, with the latter showing dark and bright areas. The degradation of pentlandite to Ni-rich Fe oxyhydroxides increases from (a) to (e). (f–i) O-bearing PGMs displaying Pt-Pd sulfide remnants (f,g) and concentrical textural and compositional zonation (i). Microfracturing was observed in all O-bearing PGM grains.
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Figure 4. SEM-EDS elemental maps of the O-bearing PGM from Figure 3i (a) showing the location of the thin foil and its textural/compositional zonation (b). The image in (b) presents the mineralogical variations in the O-bearing PGM grain analyzed by TEM.
Figure 4. SEM-EDS elemental maps of the O-bearing PGM from Figure 3i (a) showing the location of the thin foil and its textural/compositional zonation (b). The image in (b) presents the mineralogical variations in the O-bearing PGM grain analyzed by TEM.
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Figure 5. STEM-EDS elemental maps of the thin foil extracted from the O-bearing PGM in Figure 4b.
Figure 5. STEM-EDS elemental maps of the thin foil extracted from the O-bearing PGM in Figure 4b.
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Figure 6. TEM inspection of the O-bearing PGM grain from Figure 4b. (a,b,d,e) High-angle annular dark field (HAADF) images of the thin foil and close-ups of the three zones: Pt(-Pd-Ni-Fe) sulfide core, Pt-Fe-Ni oxidized alloy mantle, and Pt-Cu-Fe(-Pd) alloy (tulameenite) rim. (c,f) STEM-EDS elemental maps of the core–mantle (c) and mantle–rim (f) contacts. Red stars in (f) represent EDS spectra in the Supplementary Materials.
Figure 6. TEM inspection of the O-bearing PGM grain from Figure 4b. (a,b,d,e) High-angle annular dark field (HAADF) images of the thin foil and close-ups of the three zones: Pt(-Pd-Ni-Fe) sulfide core, Pt-Fe-Ni oxidized alloy mantle, and Pt-Cu-Fe(-Pd) alloy (tulameenite) rim. (c,f) STEM-EDS elemental maps of the core–mantle (c) and mantle–rim (f) contacts. Red stars in (f) represent EDS spectra in the Supplementary Materials.
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Figure 7. High-magnification TEM images of the thin foil (area indicated in Figure 6e). (a) Contact between the Pt-Fe-Ni oxidized alloy mantle and tulameenite rim showing needle-like crystals of tetrataenite (?) embedded in antitaenite (?) + Pt-Fe-O matrix. Notice that tulameenite produces embayments in the mantle. (b–e) Close-ups of the areas in (a) with measured d-spacings superimposed.
Figure 7. High-magnification TEM images of the thin foil (area indicated in Figure 6e). (a) Contact between the Pt-Fe-Ni oxidized alloy mantle and tulameenite rim showing needle-like crystals of tetrataenite (?) embedded in antitaenite (?) + Pt-Fe-O matrix. Notice that tulameenite produces embayments in the mantle. (b–e) Close-ups of the areas in (a) with measured d-spacings superimposed.
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Figure 8. Schematic process of desulfurization of Pt- and Pd-rich sulfides from chromitite bodies on Ouen Island. (a,c,e) Progressive serpentinization of the olivine + pyroxene matrix surrounding chromite grains with inclusions of PGE sulfides + base metal sulfides (BMS). The degree of serpentinization increases toward the right. (b,d,f) Close-ups of PGE sulfides + BMS inclusions showing their fluid-mediated progressive transformation (as serpentinization progresses) to needle-like S-poor Pt(-Pd-Ni-Fe) sulfides, Pt-Fe-O + Fe-Ni alloys, and, ultimately, Pt-Cu-Fe(-Pd) alloys. The pink dotted line in (f) represents the limit between the core and mantle.
Figure 8. Schematic process of desulfurization of Pt- and Pd-rich sulfides from chromitite bodies on Ouen Island. (a,c,e) Progressive serpentinization of the olivine + pyroxene matrix surrounding chromite grains with inclusions of PGE sulfides + base metal sulfides (BMS). The degree of serpentinization increases toward the right. (b,d,f) Close-ups of PGE sulfides + BMS inclusions showing their fluid-mediated progressive transformation (as serpentinization progresses) to needle-like S-poor Pt(-Pd-Ni-Fe) sulfides, Pt-Fe-O + Fe-Ni alloys, and, ultimately, Pt-Cu-Fe(-Pd) alloys. The pink dotted line in (f) represents the limit between the core and mantle.
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Cano, N.; González-Jiménez, J.M.; Gervilla, F.; Kerestedjian, T.N. HRTEM Study of Desulfurization of Pt- and Pd-Rich Sulfides from New Caledonia Ophiolite. Minerals 2025, 15, 66. https://doi.org/10.3390/min15010066

AMA Style

Cano N, González-Jiménez JM, Gervilla F, Kerestedjian TN. HRTEM Study of Desulfurization of Pt- and Pd-Rich Sulfides from New Caledonia Ophiolite. Minerals. 2025; 15(1):66. https://doi.org/10.3390/min15010066

Chicago/Turabian Style

Cano, Néstor, José M. González-Jiménez, Fernando Gervilla, and Thomas N. Kerestedjian. 2025. "HRTEM Study of Desulfurization of Pt- and Pd-Rich Sulfides from New Caledonia Ophiolite" Minerals 15, no. 1: 66. https://doi.org/10.3390/min15010066

APA Style

Cano, N., González-Jiménez, J. M., Gervilla, F., & Kerestedjian, T. N. (2025). HRTEM Study of Desulfurization of Pt- and Pd-Rich Sulfides from New Caledonia Ophiolite. Minerals, 15(1), 66. https://doi.org/10.3390/min15010066

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