1. Introduction

The Colombian Andes, located in the northwest of South America, are divided into three mountain ranges separated by valleys, namely from east to west: the Eastern Cordillera, the Magdalena River Valley, the Central Cordillera, the Cauca River Valley, and the Western Cordillera (Figure 1A ). The western flank of the Central Cordillera, the Cauca River Valley, the Western Cordillera, and the Baudó Range make up the geographic region called “Western Colombia” (Restrepo and Toussaint, 1973;Bourgoiset al., 1987;Moreno-Sánchez and Pardo-Trujillo, 2003). Due to its high potential for precious metals, mainly gold, silver and Platinum-Group Elements (PGE), this region has been largely explored since pre-hispanic and colonial times.

Figure 1
(A) Distribution map of peridotite bodies and mafic-ultramafic rock associations in Colombia (modified fromCorrea-Martínez, 2007;Gómezet al., 2015). (B) Geological map of the Medellín region (modified fromGarcía-Cascoet al., 2020 and references therein). The location of the studied samples is shown in the map.

Western Colombia is characterized by the occurrence of several ultramafic bodies (Figure 1A, some of them interpreted as: (1) fragments of an ophiolite mantle sequence (Restrepo and Toussaint, 1973;Álvarez, 1987;Bourgoiset al., 1987;Correa-Martínez, 2007) and (2) mantle portions of the Caribbean-Colombian oceanic plateau (Nivia, 1987;Kerret al., 1996;Serrano, 2009). The main ultramafic body of the Central Cordillera of Colombia is historically known as the Medellin Dunite (e.g.,Restrepo and Toussaint, 1984;Álvarez, 1987). It is located in the western flank of the Central Cordillera, northeast of Medellín (Department of Antioquia;Figure 1A) and is formed by peridotites metamorphosed at amphibolite facies conditions (ca. 600 ºC, <6 kbar;Restrepo, 2008;García-Cascoet al., 2020 and references therein). This ultramafic body has been interpreted as the mantle section of the so-called Aburrá Ophiolite (Correa-Martínez, 2007). Using a bulk-rock major element geochemical approach,García-Cascoet al. (2020) inferred that this ultramafic body is mainly harzburgitic, and that the term Medellin Metaharzburgitic Unit (MMU) is more appropriate than the historical “Medellin Dunite” term. The MMU hosts the best-known chromian spinel mineralization of Colombia, including the large deposit of Patio Bonito, located in the southern area of the ultramafic body (Figure 1B). This ore deposit contains refractory grade chromian spinel (Al-rich;Proenzaet al., 2004a;Correa-Martínez, 2007).

The origin of the MMU and its associated Cr-PGE mineralization has been a matter of debate for various decades (e.g.,Restrepo and Toussaint, 1973,1984;Restrepo, 1986,2008;Álvarez, 1987;Correa-Martínez and Nilson, 2003;Proenzaet al., 2004a;Pereiraet al., 2006;Correa-Martínez, 2007;García-Cascoet al., 2020 and references therein).Proenzaet al. (2004a) andCorrea-Martínez (2007) suggested that the MMU represents a fragment of oceanic lithospheric mantle formed in a supra-subduction environment. Pereiraet al. (2006) documented a high PGE content in two samples of metadunite (up to 1.2 ppm), mainly of Pt, Pd, and Rh, suggesting a primary magmatic origin of the PGE content, chromian spinel and pentlandite. Accessory chromian spinel in ultramafic rocks provides valuable information about the petrogenesis of the host rock. Nevertheless, to date, there are no detailed studies of the textural and compositional characteristics of the accessory chromian spinel of the MMU, as well as its alteration.

This paper focuses on the study of the chromitite bodies hosted in the MMU. We have studied mineralogical, petrological and geochemical characteristics of chromitites and associated metaperidotites from Las Palmas, Patio Bonito and San Pedro, which cover the southern and northern sections of the ultramafic body (Figures 1B and2).

2. Geological setting

2.1. Mafic/Ultramafic rock associations in The Colombian Andes

The central part of the Colombian Andes (Central Cordillera) comprises a metamorphic basement overlain by Mesozoic and Cenozoic sedimentary successions intruded by plutons of different ages, ranging from the Triassic to the Neogene (Feiningeret al., 1972;Vinascoet al., 2006). In the western part of the Central Cordillera, there is an important tectonic boundary between Cretaceous oceanic crust to the west and the Paleozoic metamorphic continental basement to the east, called the Romeral Shear Zone (Figure 1A;Caseet al., 1971;McCourtet al., 1984;Nivia, 1996;Vinasco, 2019). This complex shear zone consists of three main faults (from E to W): San Jerónimo, Silvia-Pijao and Cauca-Almaguer (Maya and González, 1995). The Cauca-Almaguer fault is considered to be the tectonic boundary between the Cretaceous rocks of oceanic affinity and the Paleozoic basement of the Central Cordillera (Nivia, 1996,2001;Moreno-Sánchez and Pardo-Trujillo, 2003;Lópezet al., 2009). A series of mafic-ultramafic bodies of different ages lie on both sides of the Cauca-Almaguer fault (Figure 1A) (Restrepo and Toussaint, 1973;Bourgoiset al., 1987;Nivia, 1993;Moreno-Sánchez and Pardo-Trujillo, 2003).

The mafic-ultramafic bodies located to the west of the Cauca-Almaguer fault are associated with volcano-sedimentary rocks of oceanic affinity and are mainly considered to be fragments of a Cretaceous oceanic plateau (Nivia, 1987,1996;Kerret al., 1996;Serrano, 2009).Nivia (1993) grouped these bodies into the Western Cretaceous Lithospheric Province. On the other hand, the majority of the mafic-ultramafic bodies and complexes located to the east of the Cauca-Almaguer fault are interpreted as of ophiolitic origin (Restrepo and Toussaint, 1973;Álvarez, 1987;Bourgoiset al., 1987;Correa-Martínez and Martens, 2000).

The Aburrá Ophiolite (Correa-Martínez, 2007;Figure 1B) is one of the mafic-ultramafic units that crops out to the east of the Cauca-Almaguer fault. It lies to the west of the low-grade metamorphic rocks of the Cajamarca Complex (Maya and González, 1995) that represents the paleo-continental margin. The Aburrá Ophiolite comprises ultramafic rocks that form the Medellin Metaharzburgitic Unit (MMU) and several mafic units, which include the El Picacho Metagabbros and the Espadera-Chupadero Amphibolites (Correa-Martínez and Martens, 2000;Correa-Martínez and Nilson, 2003;Correa-Martínezet al., 2004,2005;Correa-Martínez, 2007;Restrepo, 2008). The contacts between the different units of the Aburrá Ophiolite are tectonic and the MMU overthrusts the Espadera-Chupadero amphibolites (Figure 1B) (Restrepo and Toussaint, 1973;Rodríguezet al., 2016).

2.2. Medellin metaharzburgitic unit (MMU)

The MMU covers approximately 71 km² and represents the main ultramafic body of ophiolitic affinity in the Colombian Central Cordillera (Figure 1B), which is in fault contact with Jurassic Sajonia mylontic gneisses and Cretaceous amphibolites (Figure 1B;Restrepo, 2008;Rodríguezet al., 2016). This unit has been of special interest for chromium mining and exploration since it is a host for several small bodies of chromitite that were exploited over various decades (e.g.,Botero-Restrepo, 1945;Hall,et al., 1970;Álvarez, 1987).Restrepo and Toussaint (1973) interpreted the ultramafic rocks of the MMU as part of an obducted ophiolite.Correa-Martínez and Nilson (2003) considered this unit to have formed in a subduction zone environment, and suggested that the origin of the MMU can be related to a Triassic back-arc basin. However, the age and the geodynamic setting of the ophiolite formation still remains a subject of debate (seeRodríguezet al., 2016;Spikings and Paul, 2019;García-Cascoet al., 2020 and references therein).Correa-Martínez (2007) obtained an age of 216.6 ± 0.36 Ma (Late Triassic) in zircons from a plagiogranite dike intruding the El Picacho metagabbros, which was interpreted as the minimum age for the formation of the ophiolite oceanic crust. In addition, the age of emplacement of the Aburrá ophiolite onto the continent is still unknown, but a minimum pre-Cretaceous age is constrained by the intrusion of several apophysis of the Cretaceous Antioquia Batholith (Figure 1B) (Feininger and Botero, 1982;Correa-Martínez, 2007;Rodríguezet al., 2016).

The MMU comprises mainly metaharzburgites and minor metadunites, containing tremolite, talc, chlorite, fine-grained recrystallized olivine, serpentine group minerals, magnetite, and chromian spinel, and to a lesser extent, carbonates, anthophyllite and relicts of magmatic olivine, chromian spinel and orthopyroxene (e.g.,Álvarez, 1987;Restrepo and Toussaint, 1984;Correa-Martínez, 2007;Restrepo, 2008), indicating medium-grade metamorphic conditions (ca. 600 ºC).García-Cascoet al. (2020) analyzed two possible geodynamic settings in order to explain the metamorphism of the MMU and associated metabasites: i) ocean-floor metamorphism (e.g., Correa-Martínez, 2007), and ii) intra- back-arc subduction-initiation metamorphism, which supposes a new tectonic scenario for the MMU (García-Cascoet al., 2020).

2.3. The chromitite bodies

Chromitite bodies occur mainly as centimetric to metric pods, but also as dikes, lenses, and disseminated schlieren (Álvarez, 1987), showing massive to disseminated textures. Chromitites usually show sharp contacts with the enclosing strongly serpentinized dunite (metadunite) and are concordant to subconcordant with the foliation of the host ultramafic rock (Correa-Martínez, 2007). The ore bodies are small, containing a maximum tonnage estimate of ~20000 tons of ore. They were exploited during the 1970’s-1980’s by metallurgical and glass industries (Correa-Martínez, 2007). The Patio Bonito deposit was the largest in the area, with 30 m length and up to 7 m width (Álvarez, 1987). Artisanal mining activity was reactivated in the 2000’s with the opening of small mines and quarries in the northern and southern bodies (Correa-Martínez, 2007).

3. Studied samples and analytical techniques

A total of 19 representative samples from Las Palmas, San Pedro and Patio Bonito outcrops (Figure 2) were selected to be studied on polished thin sections using optical microscopy (transmitted and reflected light). These include metaharzburgites and metadunites from Las Palmas, San Pedro and Patio Bonito also chromitites at San Pedro and Patio Bonito. The textural study of the chromian spinel alteration and the mineralogical characterization of the fine grain phases was carried out with Scanning Electron Microscopy with Energy-Dispersive X-Ray Spectroscopy (SEM-EDS) using the ESEM Quanta 200 FEI, XTE 325/D8395 at the Serveis Científics i Tecnològics (CCiTUB), Universitat de Barcelona. For the mineral chemistry analyses, the JEOL JXA-8230 electron microprobe was used at the same institution. Operating conditions were set to an acceleration voltage of 20 kV and a beam current of 20 nA. The elements were acquired using the analyzing crystals: LiF for Fe, Mn, and Ni; TAP for Mg and Al; PET for Cr, V, and Ti. The standards used were chromian spinel (Cr, Al, Fe), periclase (Mg), rhodonite (Mn), nickel oxide (Ni), rutile (Ti), metallic vanadium (V), albite (Na), wollastonite (Ca), and orthoclase (Si, K).

Figure 2
Field photographs of San Pedro, Patio Bonito, and Las Palmas localities. (A) Metaperidotite outcrop in the San Pedro locality. (B) Metaperidotite outcrop crosscut by a mafic dike or sill in the Patio Bonito locality. (C) and (D) Metaharzburgite outcrops in the Las Palmas locality. (E) Chromitite body in the San Pedro locality. (F) Massive chromitite hand-specimen from the San Pedro locality. (G) General view of the Patio Bonito Mine. (H) Zoom on the massive chromitite in the Patio Bonito locality.

Whole rock analyses of peridotite samples were carried out in the Centro de Instrumentación Científica (CIC), Universidad de Granada. Major elements and Zr were analyzed using a Philips Magix Pro (PW-2440) X Ray Fluorescence (XRF). Trace elements abundances (except Zr) were obtained by ICP Mass Spectrometry (ICP-MS). Details of the analytic protocols have been described byLázaroet al. (2014). Results are shown inTable 1. Platinum-Group Elements analyses were performed by ICP-MS method at Genalysis Laboratory Services Pty. Ltd. in Maddington (Australia). The detection limits were 1 ppb for Rh and 2 ppb for Os, Ir, Ru, Pt, and Pd. The details of the analytical procedure can be found inGervillaet al. (2005) and the results are shown inTable 2.

Table 1

Whole rock analyses of the Medellin Metaharzburgitic Unit (MMU) metaperidotites.

b.d.l. = below detection limit.

Table 2

Whole rock platinum group elements (PGE) analyses of the MMU chromitites (ppb).

b.d.l. = below detection limit.

4. Results

4.1. Petrography

4.1.1. Ultramafic rocks

The metaperidotite samples (Figure 2) correspond to metaharzburgites (>40% Ol and >5% Opx) and metadunites (>90% Ol), both partially serpentinized (Figures 3A to3D). The preserved primary textures are typical of mantle tectonites, predominantly porphyroclastic/granoblastic textures, and minerals show undulose extinction and kink-bands. The ultramafic rocks contain relicts of mantle-derived olivine, chromian spinel, and minor orthopyroxene. However, much of the mineral assemblage is secondary and includes secondary fine-grained olivine, tremolite, chlorite, talc, serpentine, secondary chromian spinel, and minor carbonates and anthophyllite (García-Cascoet al., 2020 and references therein).

Figure 3
Microphotographs of metaharzburgites, metadunites and chromitites from the MMU. (A) Metaharzburgite with tremolite (Tr) and relict olivine (Ol1) replaced by serpentine and chlorite (Chl). Transmitted light and crossed nicols. (B) Metaharzburgite with relict olivine (Ol1), secondary olivine (Ol2), chlorite (Chl) and late carbonate veins. Transmitted light and crossed nicols. (C) Metadunite showing magmatic olivine (Ol1) altered to serpentine (Srp) and iddingsite. Talc (Tlc) and accessory spinel (Spl) are also observed. Transmitted light and crossed nicols. (D) Metadunite with olivine (Ol1) partially altered to serpentine. Chromian spinel (Spl) is surrounded by a decussate chlorite (Chl) corona. Transmitted light and parallel nicols. (E) Chromitite with coarse chromian spinel (Spl) crystals in a chlorite matrix (Chl). Transmitted light and parallel nicols. (F) Same as E in transmitted light and crossed nicols. (G) Chromitite showing strongly altered and fractured chromian spinel (Spl) crystals surrounded by chlorite (Chl). Reflected light. (H) Chromitite showing alteration rims in chromian spinel (Spl) crystals in chromitite. Reflected light.

Metaharzburgites: Metaharzburgites show pseudomorphic mesh and bastite textures, where mantle olivine is replaced by serpentine and iddingsite along rims and fractures, and pyroxene by serpentine and tremolite (Figure 3A and3B). There are two generations of olivine: primary olivine (Ol1), which forms 100-500 μm porphyroclasts, and secondary recrystallized olivine (Ol2) that forms smaller (< 20 μm) rounded grains (Figure 3B). Primary olivine is variably deformed and oriented, showing undulose extinction and kink-bands, and is also found as relict inclusions in pyroxene pseudomorphs. Non-pseudomorphic textures, characterized by intergrowths of antigorite, tremolite and secondary olivine are also observed (Figure 3A). Talc and chlorite replace mainly serpentine.

Chromian spinel (600-800 μm) is a common accessory phase in the MMU metaperidotites. Three types of chromian spinel can be distinguished petrographically, and can be classified according to the textural classification proposed byGervillaet al. (2012): type I partially altered chromian spinel (Figures 4A and4B), type II porous chromian spinel, which is completely altered and contains abundant chlorite within the porosity (Figures 4C and4D), and type III homogeneous chromian spinel (Figures 4E and4F). Both types I and II chromian spinel in the MMU are surrounded by a decussate chlorite corona.

Figure 4
Back-scattered electron images (BSE) of the accessory chromian spinel in MMU. (A) and (B) Type I chromian spinel (Spl) (Al-rich) in metaperidotite. Chlorite (Chl) is present as inclusions within the chromian spinel that follow the (111) crystallographic planes and as a corona surrounding the spinel. (C) and (D) Type II chromian spinel (Cr-Fe2+-rich, and Al-Mg-depleted) with chlorite inclusions and a chlorite halo surrounding the spinel. (E) and (F) Type III chromian spinel (Fe3+-rich) intergrown with tremolite (Tr).

Metadunites: Metadunites are less abundant than metaharzburgites in the MMU, and have been observed in Las Palmas and San Pedro localities (Figure 1B). In San Pedro, metadunites occur as envelopes around chromitite bodies. Metadunites show pseudomorphic mesh textures, where serpentine replaces primary olivine porphyroclasts along grain boundaries and fractures (Figure 3C). Primary mantle olivine forms crystals ~100 μm in size and secondary olivine forms smaller (~10 to 20 μm) crystals. Tremolite and talc are found as overgrowths in serpentine and filling thin veinlets (Figure 3C).

Accessory chromian spinel (0.6 - 1 mm) shows unaltered cores surrounded by rims of porous chromian spinel with chlorite following the altered chromian spinel (111) crystallographic planes (Figure 4A and4B). This partially altered chromian spinel corresponds to type I spinel according to the classification ofGervillaet al. (2012). A decussate chlorite corona surrounds the type II chromian spinel (Figure 3D). Chlorite crystals reach up to 500 μm in size and neither primary pyroxene nor pseudomorphs after pyroxene are observed.

4.1.2. Chromitites

Chromitite samples from San Pedro and Patio Bonito localities (Figures 3E to3H) exhibit massive (>80% vol. chromian spinel) and semi-massive (60-80% vol. chromian spinel) textures. Massive chromitites consists of large (0.5-0.8 cm) chromian spinel crystal aggregates, which have unaltered cores surrounded by thin alteration rims of ferrian chromian spinel with abundant chlorite inclusions (Figure 3G), also showing typical pull-apart textures. Semi-massive chromitites are made up of smaller chromian spinel grains (up to 0.2 cm) that are strongly fractured and altered along rims to ferrian chromian spinel (Figure 3H), locally forming brecciated textures. Chlorite is the predominant intergranular mineral accompanied by minor serpentine, rutile, ilmenite, and titanite. Mineral inclusions observed within chromian spinel include olivine, chlorite, serpentine, ilmenite, rutile, and amphibole. In both chromitite bodies, chromian spinel alteration rims are highly porous and the voids are filled by chlorite and minor sulfides. The voids within the alteration rims in the less altered chromian spinel crystals are rounded (Figure 3H), whereas the ones in the more altered crystals have coarse irregular or acicular-shaped voids (Figures 4A to4D).

4.2. Whole rock geochemistry

4.2.1. Metaperidotites

The LOI values of the MMU metaperidotites range from 8.17 to 11.29 wt% (Table 1). The Al₂O₃ contents (0.84 - 0.96 wt%) are rather low (except for sample LP-7), SiO₂ ranges between 31.18 and 40.60 wt%, Fe₂O₃ between 7.58 and 10.80 wt%, MgO between 37.20 to 41.20 wt%, and CaO and TiO₂ contents are low (0.07 - 0.74 wt% and 0.02 - 0.30 wt% respectively) (Table 1). One metadunite from Las Palmas (sample LP-7) shows the lowest contents of SiO₂ and CaO (31.18 wt% and 0.07 wt% respectively), and the highest contents of TiO₂ (0.30 wt%), Al₂O₃ (5.27 wt%), and Fe₂O₃ (10.80 wt%) (Table 1).

Chondrite-normalized rare earth element (REE) patterns (Figure 5A) are almost flat or U-shaped. The metaharzburgite sample from San Pedro (SP-1) shows a positive Ho anomaly and a negative Er anomaly. One metaharzburgite sample from Las Palmas (LP-4) is clearly depleted in REEs when compared to the other samples. The metadunite (LP-7), which is already particular in terms of major elements, shows a noticeable negative Eu anomaly, not observed in the other samples.

In the multielemental diagram normalized to the primitive mantle (Figure 5B) metaperidotites are typically depleted in large-ion lithophile elements (LILE) and slightly enriched in high field strength elements (HFSE). All samples show positive Th and Pb anomalies.

Figure 5
Whole rock geochemistry of the Las Palmas (LP-1, LP-4 and LP-7) and San Pedro (SP-1) metaperidotites. (A) Chondrite-normalized REE patterns. (B) Multi-elemental diagram normalized to the primitive mantle. The fields for abyssal mantle peridotites (Godardet al., 2008) and hydrated mantle wedge serpentinites (Tso Morari, Himalaya,Deschampset al., 2010; Mariana Forearc,Savovet al., 2005) are projected for comparison. Normalizing values were taken fromMcDonough and Sun (1995).

4.2.2. Chromitites: PGE geochemistry

Whole rock PGE contents normalized to chondritic values for the massive chromitites associated with the MMU (Figure 6) show a general PGE depletion, around 100 times lower than chondritic values. The San Pedro chromitites have ΣPGE < 41 ppb, Ru being the most abundant element (17 - 18 ppb). The Patio Bonito chromitites have ΣPGE < 37 ppb, Ru also being the most abundant element (7 - 14 ppb), but with a significantly lower content than in San Pedro. The chondrite-normalized PGE patterns (Figure 6) are characterized by comparable Os and Ir values, positive Ru anomalies, and a negative slope from Ru to Pd, which is typical for podiform chromitites (e.g.,Leblanc, 1991). These patterns are similar to those from the Al-rich chromitites from Tehuitzingo, México (Proenzaet al., 2004b).

Figure 6
Chondrite-normalized platinum group element (PGE) patterns for the San Pedro and Patio Bonito chromitites. Normalizing chondritic values are fromNaldrett and Duke (1980). The field for the Tehuitzingo chromitites (México) is fromProenzaet al. (2004b).

4.3. Mineral chemistry

4.3.1. Chromian spinel

Accessory chromian spinel in the metaperidotites has been divided into 3 textural groups. The compositional variations regarding the Fe³⁺, Cr and Al contents are directly related to the textural classification defined petrographically (Figures 7A,7B,8 andTable 3).

Figure 7
Mineral chemistry of accessory chromian spinel in the metaperidotites and in the chromitite bodies of the MMU. (A) TiO2 (wt%) vs. Al2O3 (wt%) diagram for type I chromian spinel. Fields for supra-subduction zone (SSZ) and MORB-type peridotites are fromKamenetskyet al. (2001), and the field for the Puerto Rico Peridotite is fromMarchesiet al. (2011). (B) Cr, Fe3+ and Al compositions for chromian spinel of the MMU. (C) Cr# [Cr/(Cr+Al)] vs. Mg# [Mg/(Mg+Fe)] diagram for chromian spinel of the San Pedro and Patio Bonito chromitites, and primary chromian spinel for the metaperidotites. The podiform and stratiform fields are afterIrvine (1967) andLeblanc and Nicolas (1992) respectively, the Moa Baracoa and Sagua de Tánamo (Cuba) fields are fromProenzaet al. (1999), Tehuitzingo (México) is fromProenzaet al. (2004b), Los Guanacos (Argentina) is from Proenzaet al. (2008), and Cerro Colorado (Venezuela) is fromMendiet al. (2020). (D) Cr# vs. TiO2 (wt%) for the San Pedro and Patio Bonito chromitites, and type I chromian spinel in the metaperidotites. Fields for boninites and MORB are fromArai (1992), references are the same as in (C).

Figure 8
Element distribution maps for accessory chromian spinel in the MMU. (A) Type I chromian spinel from Las Palmas (Sample LP-7). Note the high Al, Fe, and Cr contents in the core that gradually decreases towards the rims. The Al-rich cores are surrounded by a high-V halo. (B) Type III chromian spinel from the Las Palmas metaperidotite (Sample CO-7). The Fe enrichment is related with the Cr depletion, whereas Al is almost absent.

Table 3

Representative electron microprobe analyses of accessory chromian spinel in the metaperidotites.

b.d.l. = below detection limit.

Type I chromian spinel (partially altered) has Al-rich cores (Figures 7A and8A,Table 3) that represent the primary composition. These cores are characterized by Cr# [Cr/(Cr+Al) atomic ratio] ranging from 0.58 to 0.62, Mg# [Mg/(Mg+Al) atomic ratio] from 0.42 to 0.44, Fe³⁺# [Fe³⁺/(Fe³⁺+Cr+Al) atomic ratio] ≤ 0.03, TiO₂≤ 0.36 wt%, MnO from 0.32 to 0.42 wt%, and ZnO from 0.68 to 0.87 wt%. This composition is typical of accessory spinel in mantle rocks at supra-subduction zones (Kamenetskyet al., 2001) (Figure 7A).

Type II chromian spinel (porous) is Al and Mg-depleted and enriched in Cr, Fe²⁺, Ti and Mn, when compared to Type I chromian spinel. The composition is characterized by Cr# that ranges from 0.84 to 0.96, Mg# from 0.13 to 0.27, Fe³⁺# from 0.05 to 0.16, TiO₂from 0.42 to 1.57 wt%, MnO from 0.39 to 0.76 wt%, and ZnO from 0.37 to 0.66 wt% (Figure 7B,Table 3).

Type III chromian spinel (homogenous) is Fe³⁺-rich (Figures 7B and8B,Table 3) and has Cr# that ranges from 0.98 to 1.00, Mg# from 0.09 to 0.22, Fe³# from 0.30 to 0.79, TiO₂from 0.27 to 0.45 wt%, MnO from 0.16 to 0.58 wt%, and ZnO from 0.04 to 0.28 wt%.

Unaltered chromian spinel cores from San Pedro chromitites has Cr# ranging from 0.43 to 0.58, Mg# from 0.54 to 0.68, TiO₂ from 0.02 to 0.5 wt%, Fe³⁺# <0.035, MnO < 0.27 wt%, V₂O₃ < 0.22 wt%, ZnO < 0.17 wt%, and NiO < 0.23 wt% (Figures 7C,7D,Table 4). In contrast, unaltered chromian spinel cores from Patio Bonito chromitites exhibit higher Cr# (0.51 - 0.63) and Mg# (0.67-0.80), similar TiO₂ (0.05 to 0.5 wt%) and NiO (<0.21 wt%) but lower Fe³+# (<0.018 wt%) and slightly higher MnO (0.11-0.49 wt%), V₂O₃ (<0.26 wt%), and ZnO (<0.37 wt%) (Figures 7C,7D andTable 4).

Table 4

Representative electron microprobe chromian spinel analyses from the chromitite bodies in the MMU.

b.d.l. = below detection limit.

4.3.2. Olivine

Olivine is only present in the metaperidotites and its composition ranges from Fo_89.7 to Fo_93.7 (average of Fo_90.5), and NiO from 0.19 to 0.51 wt%, which is within the range of mantle olivine (e.g.,Takahashiet al., 1987). Even though petrographically it was possible to distinguish primary mantle-derived olivine and secondary metamorphic olivine, all the analyzed crystals show similar composition (Table 5).Figure 9A shows a scattering of olivine compositions between the abyssal peridotites (Moghadamet al., 2015) and the supra-subduction peridotite fields (Ishiiet al., 1992).

Table 5

Representative electron microprobe analyses of olivine in the metaperidotites.

b.d.l. = below detection limit.

Figure 9
(A) NiO (wt%) vs. Forsterite content (Fo) in olivine from the metaperidotites of the MMU. Fields for mantle olivine are fromTakahashiet al. (1987), fore-arc peridotite fromIshiiet al. (1992) and abyssal peridotite fromMoghadamet al. (2015). (B) Amphibole classification diagram according toHawthorneet al. (2012), variations ofA(Na + K + 2Ca) a.p.f.u. vs.C(Al + Fe3+ + 2Ti) a.p.f.u. (C) Fe/(Fe + Mg) vs. Si a.p.f.u. classification diagram for chlorite according toHey (1954), for the metaperidotites and chromitites of the MMU. (D) Si, Al, Fe + Mg distribution diagram for serpentine of the MMU metaperidotites.

4.3.3. Tremolite

Amphibole is only present in the metaharzburgites and it is more abundant in samples that contain Type-III chromian spinel. The studied crystals belong to the calcium subgroup (calculations followingLocock, 2014) according to the classification scheme ofHawthorneet al. (2012), and have Mg# [Mg/(Mg+Fe²⁺) atomic ratio] ranging from 0.95 to 0.99 (Table 6). These calcium amphiboles (Ca^B = 1.196 - 2 a.p.f.u; Na^B = 0.001 - 0.19) classify as tremolite (Figure 9B). Analyses yielded very low Ti (<0.02 a.p.f.u.), Mn (<0.06 a.p.f.u.) and K (<0.007 a.p.f.u.) contents, and have variable Si (7.20 - 8.02), Al^TOT (0.016 - 0.131), Mg (3.76 - 4.96), Fe^TOT (0.14 - 0.39), Ca (1.19 - 2.00), and Na (0.001 - 0.19) contents (Table 6). The vacancy in position A is between 0.69 and 1.00 a.p.f.u.

Table 6

Representative electron microprobe analyses of tremolite, chlorite and serpentine in the metaperidotites and chromitites.

4.3.4. Chlorite

Chlorite composition in the metaharzburgites corresponds to clinochlore and high-Si pennantite (Figure 9C) according to the classification byHey (1954). SiO₂ content ranges from 27.91 to 35.21 wt% (up to 6.7 Si a.p.f.u.) and FeO from 1.79 to 3.37 wt% (up to 0.49 Fe²⁺ a.p.f.u.). The Fe/(Fe + Mg) ratio ranges between 0.03 and 0.05, also the Cr₂O₃ content is low (<3.2 wt%) (Table 6).

Chlorite in the chromitites corresponds to clinochlore (Figure 9C), with SiO₂ ranging from 27.07 to 32.61 wt%, FeO from 1.22 to 1.79 wt% and Fe/(Fe + Mg) ratio between 0.02 and 0.03. These values are lower than those from chlorite found in metaperidotites. The Cr₂O₃ content is also low (<2.61 wt%).

4.3.5. Serpentine

Serpentine composition (Figure 9D) ranges from 41.54 to 45.98 wt% of SiO₂, FeO from 2.85 to 4.51 wt%, Al₂O₃ from 0.53 to 2.50 wt%, and Cr₂O₃ from 0.03 to 0.47 wt% (Table 6). The highest SiO₂ values are observed in serpentine from metaperidotites with type III accessory chromian spinel, where chlorite is rare.

5. Discussion

5.1. Tectonic setting of the MMU peridotites

Ophiolitic peridotites are spatially and temporally related with first-order tectonic and magmatic global events, which, together with mantle processes, control the development of different types of oceanic lithosphere (“ophiolites”) in various tectonic settings (Dilek and Flower, 2003;Dilek and Furnes, 2014 and references therein). In general, oceanic ultramafic rocks of ophiolitic affinity can be classified as: (i) subduction zone (supra-subduction zone or volcanic arc) related ophiolites, which might be influenced by the dehydration of the subducting plate, associated metasomatic processes, and repetitive episodes of partial melting of metasomatized peridotites, and (ii) subduction unrelated ophiolites (oceanic ridges, MOR or plumes), which are linked to mantle diapirism, heat advection and melting in the asthenospheric mantle and/or at deeper levels.

The MMU protoliths are predominantly harzburgites and dunites with tectonite textures and are similar to abyssal ocean ridge and supra-subduction zone peridotites. The Cr# of the accessory chromian spinel in metaperidotites (0.58 - 0.62, unaltered cores from Type-I chromian spinel) from the MMU overlap those of supra-subduction peridotites from ophiolites (Figure 7A;Pearceet al., 2000;Marchesiet al., 2016 and references therein).

The REE patterns of the studied metaperidotites compare well with those related with supra-subduction zone peridotites (Savovet al., 2005;Marchesiet al., 2006;Deschampset al., 2010) and differ from those of abyssal peridotites (MOR-type) (Godardet al., 2008), characterized by positive LREE to HREE slopes (Figure 5A). The REE patterns (almost flat or U-shaped) are consistent with melt percolation reactions, common in supra-subduction zones due to volatile-rich fluid infiltration produced by slab dehydration (Proenzaet al., 1999;Pearceet al., 2000;Marchesiet al., 2006,2016). Also, primitive mantle-normalized trace element patterns (Figure 5B) of the stBudied MMU metaperidotites are similar to supra-subduction zone peridotites from the Tso Morari fore-arc (Savovet al., 2005).

On the other hand, the MMU hosts the major chromian spinel deposits described for Colombia. Several authors have suggested that podiform chromitites are predominantly formed within supra-subduction zone settings (Pearceet al., 1984;Roberts, 1988;Proenzaet al., 1999;González-Jiménezet al., 2012,2014a and references therein). Consequently, the ultramafic rocks in Medellín probably formed in a supra-subduction zone setting, where ophiolites show typical island arc signatures but have oceanic crust structures clearly formed by expansion processes related to a subduction zone (e.g.,: fore-arc, intra-arc, and back-arc basins). This interpretation agrees with the work byCorrea-Martínez (2007), who considers the Aburrá Ophiolite an oceanic back-arc basin formed during the Triassic and later tectonically emplaced during Jurassic time.

5.2. Origin of the chromian spinel mineralization

The studied chromitite samples from Patio Bonito and San Pedro deposits are Al-rich (Figures 7C and7D) and strongly PGE-depleted (Figure 6). Al-rich chromitites are usually found within the mantle-crust transition zone, or Moho Transition Zone (MTZ), near levels of layered gabbros at the base of the crust in ophiolite complexes (Leblanc and Violette, 1983;Proenzaet al., 1999,González-Jiménezet al., 2014a;Mendiet al., 2020). The primary cores of chromian spinel forming the chromitite bodies associated with the MMU exhibit compositions that plot between the fields defined for chromian spinel from mid-ocean ridge basalts (MORB;Dick and Bullen, 1984) and boninite-like lavas (Figure 7D) (Arai, 1992). These compositions are similar to other Al-rich ophiolitic chromitites in central and southern America and the Caribbean region (Figures 7C and7D) such as Moa-Baracoa, Cuba (Proenzaet al., 1999), Tehuitzingo, México (Proenzaet al., 2004b), Los Guanacos, Argentina (Proenzaet al., 2008), and Cerro Colorado, Venezuela (Mendiet al., 2020).

The composition of the melt in equilibrium with unaltered (magmatic) chromian spinel cores of the Patio Bonito and San Pedro chromitites has been calculated using the following equations (Maurel and Maurel, 1982;Zaccariniet al., 2011):

The calculations give average values of Al₂O_3melt = 15.75 wt%, TiO_2melt = 0.41wt% and (FeO/MgO)_melt = 0.72 wt%, and plot overlapping the field described for tholeiitic magmas (MORB or BABB) in the FeO/MgO (melt) vs. Al₂O₃ (melt) binary diagram (Figure 10). These types of MORB/BABB magmas are characteristic for back-arc basins, a geodynamic environment consistent with the previously proposed geological setting of the MMU by other authors that is based on the study of the host peridotites (Correa-Martínez, 2007;Restrepo, 2008;García-Cascoet al., 2020). The low PGE content in the chromitites is also consistent with the MORB/BABB composition of the parental magmas. PGE content in chromitite is strongly related with the parental magma composition: PGE-rich Cr-rich chromitites usually crystallize from boninitic magmas, whereas PGE-poor Al-rich chromitites (e.g., Colombian chromitites) crystallize from tholeiitic magmas. This is related to the fact that boninitic magmas are S-undersaturated and have higher PGE-contents than tholeiitic magmas, which are S-saturated (Hamlynet al., 1985;Zhouet al., 1998;González-Jiménezet al., 2014b).

Figure 10
Composition diagram [(FeO/MgO)melt vs. (Al2O3)melt (wt%)] of the parental melt in equilibrium with the San Pedro and Patio Bonito chromitites. Melts in equilibrium were calculated using the equations byMaurel and Maurel (1982) andZaccariniet al. (2011). Fields are fromMoghadamet al. (2015).

The mechanism of crystallization of the chromitites within the supra-subduction back-arc mantle may be related with the mingling of MORB/BABB melts with different degrees of fractionation within dunite channels (Arai and Yurimoto, 1995;Melcheret al., 1997;Zhou and Robinson, 1997;Proenzaet al., 1999,2004b; González-Jiménezet al., 2011,2014a). In this model, infiltrating melt reacts with the host harzburgite, generating secondary dunite in equilibrium with a more differentiated MORB/BABB melt, while the necessary Cr for chromitite formation is provided by the dissolution of Cr-rich pyroxene from the host peridotite. Mixing/mingling of melts with different SiO₂ drives supersaturation in Cr in order to crystallize chromian spinel (Arai and Yurimoto, 1995;Melcheret al., 1997), which may be accumulated by bubble flotation in the relatively hydrous melt (Matveev and Ballhaus, 2002), in a self-sustained process to form massive bodies (González-Jiménezet al., 2011,2014a and references therein).

5.3. Chromian spinel alteration

Textural and compositional variations observed in the chromian spinel of the MMU cannot be explained by magmatic/metamorphic processes at mantle temperatures or pressures. Instead, these variations suggest an origin related to the post-mantle metamorphic evolution of the ultramafic bodies. The metamorphic mineral associations in the metaperidotites (chlorite + tremolite + talc + forsterite) indicate temperatures of 550-700 ºC, at medium pressures up to 6 kbar (García-Cascoet al., 2020).

Compositional and textural variations of the studied chromian spinel could be attributed to cooling associated with a metamorphic cycle during the Permian-Triassic period in the Mesozoic paleomargin of South America. The first transformation of chromian spinel, which forms porous chromian spinel (type II), can be explained by chromian spinel dissolution and chlorite precipitation. Chlorite coronas surrounding chromian spinel and the systematic presence of secondary recrystallized olivine, tremolite and talc indicate medium-T metamorphic imprint (amphibolite facies) during cooling (García-Cascoet al., 2020). This transformation is characterized by a decrease in Al and Mg coupled with an increase in Cr and Fe²⁺. At this stage, the residual chromian spinel is enriched in Cr, whereas the Fe³⁺ content stays invariable. The porous chromian spinel (type II) composition (Mg# vs. TiO₂, MnO, ZnO) is typical of chromian spinel metamorphosed at amphibolite facies (Barnes, 2000;Saumur and Hattori, 2013;Coláset al., 2014,2019). The formation of this secondary chromian spinel can be represented with the following reaction proposed byGonzález-Jiménezet al. (2015):

According to these authors, at temperatures between ~ 700 and ~ 400 ºC and in the presence of SiO₂-rich fluids, primary Al-rich chromian spinel reacts with forsterite to produce chlorite and residual porous type II chromian spinel, enriched in Cr and Fe²⁺.

Homogeneous chromian spinel (type III) is Fe³⁺-rich in comparison to the other two chromian spinel types (Figure 7B). The Fe³⁺# values vary from 0.3 to 0.79, and, therefore, this chromian spinel can be classified as ferrian chromian spinel. The formation of ferrian chromian spinel has been interpreted as the reaction product between chromian spinel and secondary magnetite (Barnes, 2000), or between chromian spinel and antigorite (Merliniet al., 2009) during prograde metamorphism. On the other hand, it has also been interpreted as the reaction product between primary chromian spinel and olivine (Gervillaet al., 2012) or between chromian spinel and lizardite (Melliniet al., 2005) during the cooling of an ultramafic body.

The homogenous chromian spinel formation implies the dissolution of already formed Cr-rich spinel, probably at oxidizing conditions through the addition of magnetite to the porous Al-Mg-depleted chromian spinel (type II) during a late hydrothermal process starting at temperatures close to 600 ºC and evolving to temperatures lower than 500 ºC (Gervillaet al., 2012) or 350 ºC (Coláset al., 2019). The composition (Mg# vs. TiO₂, MnO, and ZnO) of homogeneous chromian spinel (type III) from the MMU is also typical for metamorphosed chromian spinel in amphibolite facies (Barnes, 2000;Saumur and Hattori, 2013;Coláset al., 2019) with similarities to chromian spinel metamorphosed at greenschist facies.

6. Conclusions

The petrological and geochemical characteristics of the metaperidotites from the MMU indicate that these represent shallow levels of the suboceanic lithospheric mantle related to a supra-subduction zone setting (back-arc basin/incipient arc scenario). The chromian spinel mineralization associated with the MMU is Al-rich (refractory grade) and is strongly depleted in PGE. The most favorable geodynamic setting for such chromitite formation is a back-arc basin, where the chromian spinel crystallizes from a BABB-type tholeiitic magma. Accessory chromian spinel in the metaperidotites is classified as: (i) partially altered chromian spinel with Al-rich cores, (ii) porous Cr-Fe²⁺-enriched and Al-Mg-depleted chromian spinel, and (iii) homogeneous Fe³⁺-rich chromian spinel. Textural and compositional variations of accessory chromian spinel in the metaperidotites of the MMU give evidence of the superimposed metamorphic processes of the MMU, which has reached amphibolite facies and later retrograded to the greenschist facies conditions.

Acknowledgements

This research has been financially supported by FEDER Funds and the Spanish Projects CGL2012-36263, CGL2015-65824 and PID2019-105625RB-C21. Fieldwork for sample collection was partly supported the Universidad Nacional de Colombia (project no. 35671) and the Convocatoria para la Movilidad Internacional de la Universidad Nacional de Colombia (project no. 6867). We thank Dr. Antonio García-Casco for his support and feedback during the preparation of this paper. Excellent technical support during EPMA sessions by Dr. X. Llovet and during FESEM sessions by Eva Prats at the Serveis Científics i Tecnòlogics (University of Barcelona) is highly appreciated. Two anonymous reviewers and the special volume editors are profoundly acknowledged for their constructive criticism that has helped to greatly improve the quality of the present manuscript.

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Notes

Author notes

*Corresponding author: (N. Pujol-Solà)npujolsola@ub.edu