The chromitites of the Neoproterozoic Bou Azzer ophiolite (central Anti-Atlas, Morocco) revisited

The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpre- tation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 – 0.74) and high-Cr (Cr# = 0.79 – 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS 2 ) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “ exotic ” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and dia- spore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle- derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.

The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 -0.74) and high-Cr (Cr# = 0.79 -0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS 2 ) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but "exotic" zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantlederived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.

Introduction
The presence of chromitites is one of the characteristics shared by most ophiolites worldwide (Leblanc and Nicolas, 1992 and references therein). These bodies of rocks made almost exclusively by chromite are usually hosted in peridotites that represent the upper mantle section of the ophiolite (e.g., González-Jiménez et al., 2014a;Arai and Miura, 2016). Recent observation of unusual ("exotic") mineral assemblages (Table 1) typical of continental crust rocks (e.g., Yamamoto et al., 2013;Robinson et al., 2015;González-Jiménez et al., 2015aProenza et al., 2018) or formed under super-reduced (SuR) conditions either at ultra-high pressure (UHP) or low-pressure (e.g., Bai et al., 1993;Yang et al., 2014Yang et al., , 2015Griffin et al., 2016;Pujol-Solà et al., 2018, 2020aXiong et al., 2019;Farré-de-Pablo et al., 2019a) have been profusely used for the formulation, testing, and establishment of models on the lithosphere-asthenosphere system dynamics (Arai, 2013;Yang et al., 2014Yang et al., , 2015McGowan et al., 2015;Robinson et al., 2015;Griffin et al., 2016). Far-reaching geodynamic models derived from these findings propose that the ophiolitic mantle -or at least portions of it-experiences UHP conditions prevailing in the Mantle Transition Zone (410-660 km depth) or deeper down to the mantle-core boundary before it returns to the shallow lithospheric oceanic mantle (e.g., Arai, 2013;McGowan et al., 2015;Griffin et al., 2016). However, new works have shown that some SuR/UHP minerals (e.g., moissanite, nano-to-micronsized diamond, native Si) found in ophiolitic chromitite orebodies form, instead, under super-reduced conditions prevailing in the shallow mantle during ocean-floor metamorphic processes of serpentinization of ultramafic/mafic rocks at low pressure and low temperature , 2020aFarré-de-Pablo et al., 2019a, 2019b. Besides, the identification of continental-derived zircons in some ophiolitic chromitites and associated gabbros has been used as a tool to track the recycling and transport of continental material through the mantle wedge above subduction zones (Rojas-Agramonte et al., 2016;González-Jiménez et al., 2017a;Proenza et al., 2018).
Most of the recent advances in these issues come from the study of the well-exposed Phanerozoic examples (e.g., González-Jiménez et al., 2014a;Yang et al., 2015;Robinson et al., 2015;Griffin et al., 2016;Proenza et al., 2018). However, there is still great uncertainty and limited information regarding the less-abundant Archaean and Proterozoic chromitite-bearing ophiolites. To date, the oldest known chromitite-bearing ophiolites are located in China (the 2.5 Ga old Zunhua and Dongwanzi ophiolites; Kusky et al., 2004aKusky et al., , 2004b and in Brazil (the 2.1 Ga old Santa Luz ophiolite; Oliveira et al., 2007). Examples of relatively well-studied Proterozoic chromitite-bearing ophiolites include those from Outokumpu-Jormua in Finland (1.95 Ga; Peltonen et al., 2008) and from the Arabian-Nubian shield in Saudi Arabia and Egypt (0.87 Ga -Pallister et al., 1988;and 0.75 Ga -Ali et al., 2010, respectively). The study of preserved Precambrian ophiolitic chromitites is significant to understand the chemical evolution of the oceanic mantle (e.g., Arai and Ahmed, 2018).
The age of the ophiolite is still controversial; SHRIMP U-Pb zircon dating yielded 697 ± 6 Ma in a metagabbro located in the Bougmane complex (El Hadi et al., 2010) and LA-ICP-MS U-Pb zircon dating yielded 759 ± 2 Ma in a layered metagabbro from the crustal section at Ait Ahmane . These differences suggest that the younger age obtained by El Hadi et al. (2010) may correspond to the volcanic arc stage (Triantafyllou et al., 2018Hodel et al., 2020) or may be indicative of a long-lived construction of the oceanic lithosphere. The ophiolitic sequence is intruded by two groups of arc-related granitoids: 1) syn-orogenic granitoids emplaced during arc-continent collision (632-659 Ma;El Hadi et al., 2010;Inglis et al., 2005); 2) Late-orogenic granitoids represented by the Bleida granodiorite dated at 579 Ma (Inglis et al., 2004). Low-to medium-grade metamorphic assemblages overprint the Bou Azzer ophiolite (Leblanc, 1981;Saquaque et al., 1989;Hefferan et al., 2000;El Hadi et al., 2010).

Studied samples and analytical techniques
We studied representative samples from three chromitite mining sites in the Bou Azzer ophiolite: Inguijem, Filon 60 and Ait Ahmane (Figs. 1 and 2), including massive chromitites from individual pods (n = 4), semi-massive chromitites (n = 2), banded chromitites (n = 1) and the host dunites. Polished thin sections were studied by optical and electron microscopy using a Quanta 200 FEI XTE 325/D8395 scanning electron microscope (SEM) equipped with an INCA Energy 250 EDS microanalysis system and a JEOL JSM-7100 field-emission SEM at the Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB). Operating conditions were 20 kV accelerating voltage. Micro-Raman spectra were obtained with a HORIBA JobinYvon LabRam HR 800 dispersive spectrometer equipped with an Olympus BXFM optical microscope at the CCiTUB. Non polarized Raman spectra were obtained in confocal geometry by applying a 532 nm laser, using a 100x objective (beam size around 2 µm), with 3-5 measurement repetitions for 10-15 s each. The Si band at ~520 cm − 1 was used for calibration. The obtained spectra were processed using the LabSpec® software (JobinYvon; Villeneuved'Ascq, France).
Quantitative electron microprobe analyses (EMPA) on chromite and platinum group minerals (PGM) were also conducted at the CCiTUB using a JEOL JXA-8230 operating in wavelength-dispersive spectroscopy (WDS) mode. Analytical conditions are described in detail in Appendix 1. X-ray (XR) images of chromite (512 × 512 pixels, 0.52 × 0.52 µm pixel size and 266.24 × 266.24 µm total area) and platinum group minerals (three images of 300 × 300 pixels: 0.22 µm pixel size and 66 × 66 µm total area, 0.08 µm pixel size and 24 × 24 µm total area and 0.1 µm pixel size and 30 × 30 µm total area) were obtained with the same a JEOL JXA-8230 electron microprobe operated at 20 kV and 20nA in spot mode with focused electron beam and counting time of 20 to 60 ms/ pixel. The images were treated with DWImager software (Torres-Roldán & Garcia Casco, unpublished; see Garcia-Casco, 2007). Minor and trace elements in chromite were analyzed using a Resolution M-50 Excimer laser coupled to a ThermoICap Qc inductively coupled mass spectrometer at the Laboratorio de Estudios Isotópicos (LEI) from the Centro de Geociencias, UNAM (Mexico). The analytical conditions and procedure are described in Appendix 1.
A representative sample (m = 4.2 kg) of massive chromitite from the Inguijem deposit was crushed, sieved and processed using hydroseparation (HS Lab, Universitat de Barcelona; http://www.hslab-barce lona.com/) to obtain mineral concentrates. The resulting concentrates were mounted as polished monolayers on resin blocks (SimpliMet 1000) in order to identify mineral phases. Whole-rock platinum-group elements (PGE) analyses were performed on 8 chromitite and 5 dunite  N. Pujol-Solà et al. Ore Geology Reviews 134 (2021) 104166 samples at Genalysis Ltd (Perth, Western Australia) after nickel sulfide fire assay collection, following the method described by Chan and Finch (2001). The analytical procedure is described in detail in Appendix 1.

Petrography and mineralogy of chromitites
The chromitites from Bou Azzer consist of chromite in an altered silicate matrix, variations in the relative proportions between chromite and the matrix result in the different observed textures (Fig. 3). The Inguijem orebody only contains massive chromitites, whereas Filon 60 is formed by massive chromitites and semi-massive chromitites. Banded chromitites were only observed in Ait Ahmane, where the chromite-rich layers show either massive or semi-massive textures; hence, these samples have been grouped and are described with the two main types.
Massive chromitites ( A large variety of globular, elongated to lamellae-like inclusions and negative crystals, with sizes ranging from few micrometers to 60 µm with average sizes around 10 µm, have been observed in chromite crystals both from massive and semi-massive chromitites. These contain mainly serpentinized olivine, sulfides (in situ pyrite and sphalerite; galena and arsenopyrite in mineral concentrates) and platinum group minerals (PGM) in chromite; and andradite, millerite and a Pb-Ni-S phase in the altered silicate matrix. Appendix 2 contains the list of accessory minerals identified both in situ and in mineral concentrates. Furthermore, PGM also occur intergrown with alteration minerals such as chlorite (Fig. 5f).

Unusual mineral assemblages
Less common mineral phases found as inclusions within chromite grains consist of submicrometric clinopyroxene lamellae (up to 22 µm long and <1 µm wide) variably distributed following the (1 1 1) crystallographic planes of the unaltered chromite cores (Fig. 6a-c). Raman analyses (Fig. 6c) show spectra compatible with diopside after subtracting the chromite signal, with peaks at 328, 394, 563, 669, 1012 and 1045 cm − 1 . Diaspore was found associated with hematite and clinochlore in inclusions within chromite ( Fig. 6d-e). Diaspore and hematite have been identified by Raman spectroscopy (Fig. 6f) with characteristic peaks at 153, 329 and 448 cm − 1 , and 226, 293, 410, 500, and 662 cm − 1 , respectively. This type of inclusions, with sizes between <1 µm and 15 µm, are arranged along trails that can be followed across different grains and are distributed heterogeneously (Fig. 6d, g-h), with clinochlore present in most of them (Fig. 6h). The same types of inclusions were identified in chromitites from the three studied deposits, Inguijem, Filon 60 and Ait Ahmane.
Altered chromite rims ( Fig. 8; Appendices 3 and 4) show higher Cr and Fe 2+ but lower Al and Mg, than unaltered chromite cores, generally progressing towards Cr-rich chromite. Altered chromite, derived from both intermediate-Cr and high-Cr chromite, reach Cr 2 O 3 contents of 70 The matrix is made of silicates, mostly chlorite and serpentine. Abbreviation: Chrchromite. N. Pujol-Solà et al. Ore Geology Reviews 134 (2021) 104166 wt%. The Fe 3+ # [Fe 3+ /(Fe 2+ +Fe 3+ ) atomic ratio] in the altered porous chromite rims (Appendix 3) remains similar to that of unaltered chromite (Fig. 8), except in semi-massive chromitite samples in which ferrian chromite [Fe 2+ (Fe 3+ ,Cr) 2 O 4 ] developed. Two clear alteration trends can be observed (Fig. 8): first an increase in Cr followed by an increase in Fe 3+ . The analyzed hematite is relatively enriched in Cr 2 O 3 (up to 7 wt %; Appendix 3) and represents the final product of alteration.
Minor and trace element compositions of unaltered chromite cores ( Fig. 9; Appendix 5) shows low Sc (1-5 ppm) and Ga (18-42 ppm) contents, moderate concentrations of Ti (209-370 ppm) and Co (192-347 ppm), and higher Zn (407-1028 ppm), Ni (565-1425 ppm), V (662-1161 ppm), and Mn (1020-5128 ppm) contents. The Ti and Co contents ( Fig. 9) are within the same range for both intermediate-Cr and high-Cr chromite; however, it is remarkable that high-Cr chromite shows lower contents of Ga, V and Ni and higher contents of Mn and Sc compared to intermediate-Cr chromite. Similar relationships were already observed in ophiolitic chromitites showing different compositions in terms of Cr# from Cuba and Dominican Republic by Proenza et al. (2011). The altered chromite rims (Appendix 5) exhibit enrichment (at ppm levels) of Ni, V, Zn, and Mn, which is typically observed in chromitites elsewhere (e.g., Colás et al., 2019 and references therein).

Bulk-rock contents of PGE
Bulk-rock PGE contents in chromitites are between 116 and 313 ppb (Appendix 6), being the IPGE (Ir-Os-Ru: 94-265 ppb) more abundant than PPGE (Pd-Pt-Rh: 11-48 ppb). Chondrite-normalized patterns Ore Geology Reviews 134 (2021) 104166 ( Fig. 10a) show a positive slope to flat line from Os to Ru and a negative slope from Ru to Pd. In addition, four samples show a negative anomaly in Pt (Fig. 10a). These results compare well with previous data from the Bou Azzer chromitites (El Ghorfi et al., 2007;Ahmed et al., 2009). PGE contents in the host dunites are low (ΣPGE = 3-30 ppb; Appendix 6), especially compared with previous results by Ahmed et al. (2009), who reported PGE values in the Bou Azzer host-rocks of chromitites ranging from 22 to 60 ppb. In general, the Bou Azzer dunites hosting the chromitites show a positive slope from Os to Ru and a negative slope from Ru to Pd (Fig. 10b), but they do not show preferential enrichment in IPGE nor PPGE.

Platinum group minerals (PGM) composition
Four grains of alloys from the Bou Azzer chromitites were analyzed by electron microprobe (Appendix 7) and classify mostly as native Os with relevant concentration of Ir (Fig. 11a)  Ore Geology Reviews 134 (2021) 104166 significant zoning. Electron microprobe analyses revealed that some laurite grains are partially altered, showing Ru concentrations up to 70.21 wt%, Os between 1.61 and 4.93 wt%, Ir between 1.06 and 2.82 wt %, up to 1.42 wt% Rh, up to 0.50 wt% Pt, and up to 0.34 wt% Pd (Appendix 7). One grain of altered laurite shows an inverse correlation between Ru and S contents, indicating the transformation of the sulfide into an alloy (X-ray maps of Appendix 8b). A third type of PGM is observed surrounding the alloys (Fig. 5e-f; Appendix 8c), sometimes intergrown with silicates. This phase was too heterogeneous to obtain a well-defined composition, but the analyses confirm the presence of Ir-As-S, probably corresponding to secondary irarsite.

Discussion
In this section, petrological, mineralogical and geochemical data are discussed in order to explore the effects of alteration on element mobility, to determine the parental melts for chromitites, to examine the origin and significance of PGE/PGM and unusual mineral phases within the chromitites, to compare with other Precambrian chromites, and to ultimately provide a genetic model for the formation of the Bou Azzer chromitites.

Effects of metamorphism and alteration on mobility of major and minor elements
As the Bou Azzer ophiolite is variably affected by post-magmatic processes, including metamorphism, serpentinization and carbonation (e.g., Gahlan et al., 2006;Fanlo et al., 2015;Hodel et al., 2017), interpretations based on the composition of chromite in terms of primary magmatic processes require an assessment of the potential effects of alteration on element mobility. The intermediate-Cr chromite cores are partially mantled by wide rims of porous altered chromite (Fig. 4b, e-f; Appendix 4), which is texturally very similar to the "porous chromite" reported in chromitites affected by greenschist to amphibolite-facies metamorphism elsewhere (e.g., Gervilla et al., 2012;González-Jiménez et al., 2015a, 2015bColás et al., 2016Colás et al., , 2019Hernández-González et al., 2020). The "porous chromite" in the Bou Azzer chromitites is enriched in Cr and Fe 2+ and depleted in Al and Mg when compared to the unaltered chromite cores ( Fig. 8; Appendix 3). On the other hand, alteration is restricted to thin homogeneous ferrian chromite rims in high-Cr chromitite (Fig. 4c, h). Semi-massive chromitites show more evolved alteration than massive chromitites due to stronger interaction (i.e. lower chromite/silicate ratio) with the fluids altering the host peridotites (e.g., Proenza et al., 2004;González-Jiménez et al., 2015a;Gervilla et al., 2012;Colás et al., 2019).
The textural and chemical evidences suggest two different stages of alteration (Fig. 8), similar to the alteration reported in many other ophiolitic chromitites (Proenza et al., 2004;Gervilla et al., 2012;Colás et al., 2014Colás et al., , 2019Colás et al., , 2020González-Jiménez et al., 2015a, 2015bHernández-González et al., 2020): first, Cr and Fe 2+ increase under water-saturated reduced conditions accompanied by a volume reduction of ~43%  in order to form the porous chromite (Fig. 8a); and afterwards, oxidizing conditions prompt the formation of ferrian chromite (Fig. 8b). A latter stage of alteration involved the circulation of Fe 3+ -rich fluids that formed Cr-magnetite, as reported in the Bou Azzer serpentinites by Hodel et al. (2017). This magnetite veins were later oxidized forming hematite, explaining the Cr-rich composition of the studied Bou Azzer hematite (Appendix 3).
Regarding the composition of unaltered chromite cores, both intermediate-Cr and high-Cr chromite overlap the field typical for ophiolitic chromitites (Fig. 7a). The Ti content (Fig. 9a) (Dick and Bullen, 1984). (c) Cr# vs. TiO 2 (wt.%). Data sources for chromian spinel of different tectonic settings are Irvine (1967), Leblanc and Nicolas (1992), Arai (1992), Bonavia et al. (1993) (Fig. 9), suggesting that the primary chromite composition is preserved and that high-Cr chromitites are not the alteration product after intermediate-Cr chromitites, allowing petrogenetic interpretations related to the formation of the deposits (see below). Altered chromite rims exhibits the typical Ni V, Zn, and Mn enrichment at ppm levels (Appendix 5) related to the formation of Cr and Fe 2+ -rich porous chromite (Colás et al., 2019 and references therein). MORB-normalized trace element patterns of unaltered intermediate-Cr and high-Cr chromite are very similar to those of chromitites from the oceanic supra-subduction zone (SSZ) mantle, in particular to fore-arc chromitites (Fig. 12), suggesting that the minor and trace elements contents in the unaltered chromite cores represent the magmatic fingerprint of a fore-arc environment.
As expected, we have identified two different parental melts for the two types of studied chromitites (Fig. 13). Intermediate-Cr chromitites yield average 12.87 wt% of Al 2 O 3 , 0.16 wt% of TiO 2 and 0.90 FeO/MgO ratio, which is similar to MORB melts in terms of Al 2 O 3 content ( Fig. 13a; e.g., Gale et al., 2013) but not in terms of TiO 2 ( Fig. 13b; typical MORB 1.68 wt% TiO 2 ; Gale et al., 2013). The low Ti content in MORB-akin melts, together with low Ti/V and Yb/V ratios, is characteristic of fore-arc basalts (FAB; Reagan et al., 2010). According to Shervais et al. (2019), FAB involve a more depleted source than NMORB source mantle, usually recording up to 23% melting. On the other hand, high-Cr chromitites yield average 9.69 wt% of Al 2 O 3 , 0.16 wt% of TiO 2 and 1.08 FeO/MgO ratio, corresponding to typical boninitic parental melt compositions (Fig. 13c; e.g., Hickey and Frey, 1982;Dick and Bullen, 1984;Kelemen et al., 2004). The PGE contents in the intermediate-Cr and high-Cr chromitites are similar (Fig. 10a) despite having formed after two different parental melts. This feature could either indicate that these melts originated from the same source (e.g., Zaccarini et al., 2011;González-Jiménez et al., 2015a) or could be the result of higher degree of partial melting (than for typical MORB) in the depleted mantle that originated the FAB melts that formed the intermediate-Cr chromitites (e.g., Shervais et al., 2019).

PGE signature and PGM formation
The distribution of PGE in the upper mantle is mainly controlled by  Evans and Frost (1975) and Suita and Strieder (1996).  the accessory base metal sulfides and PGM disseminated within the mantle peridotites (O'Driscoll and González-Jiménez, 2016 and references therein). During partial melting of mantle peridotites, PGE fractionates into two groups: the most refractory IPGE (Ir-Os-Ru) remain trapped within sulfides or PGM in the mantle residue, while the more mobile PPGE (Pd-Pt-Rh) are transferred to the silicate melt (Bockrath et al., 2004a). However, if moderate to high degrees of mantle partial melting (20-25%) are achieved, such as in suprasubduction zones (Barnes et al., 1985;Prichard et al., 1996;O'Hara et al., 2001), it is possible to extract the IPGE by dissolution of IPGE-bearing sulfides and PGM of the mantle residue, thus forming chromitites enriched in IPGE relative to PPGE (Bockrath et al., 2004a;Prichard et al., 2008;González-Jiménez et al., 2014b).
In the Bou Azzer chromitites, there is a clear enrichment in IPGE relative to PPGE ( Fig. 10a; El Ghorfi et al., 2007;Ahmed et al., 2009), similar to what is observed in the Oman chromitites ( Fig. 10a; Ahmed and Arai, 2002) and in other high-Cr (Cr#>0.6) chromitites elsewhere (González-Jiménez et al., 2014a, 2014b. This enrichment is reflected in the mineralogy, characterized by Ru-Os-Ir bearing minerals, such as laurite and Os-Ir alloys (Fig. 5). The origin of chromite-hosted PGM may be linked to chromite crystallization, since it preferentially incorporates trivalent Cr and Fe from the magma, producing a local decrease of fO 2 that reduces the solubility of the dissolved IPGE in the magma and triggers the crystallization of nano-nuggets of IPGE at the chromite rims (Mungall, 2005;Gervilla et al., 2005;Finnigan et al., 2008;González-Jiménez et al., 2014b). According to O'Driscoll and González-Jiménez (2016), chromite and IPGE nuggets crystallize most likely from S-undersaturated melts with low fS 2 in supra-subduction environments. Experimental works (Brenan and Andrews, 2001;Andrews and Brenan, 2002;Bockrath et al., 2004b) demonstrated that Os-free laurite precipitates in equilibrium with Os-Ir alloys at the T-fS 2 -fO 2 -P conditions predominating during chromite formation in the upper mantle. Therefore, the Bou Azzer Os-poor laurite grains (Os content between 1 and 6 at.%) would have crystallized contemporaneously with the Os-Ir alloys, as indicated by petrographic observations. Metamorphism, serpentinization and hydrothermal alteration in the Bou Azzer ophiolitic sequence triggered the destabilization and modification of the primary PGM assemblage. One of the first modifications is the segregation of IPGE nanoparticles from primary laurite grains ( Fig. 5b; Appendix 8b) due to the decrease in fS 2 associated with serpentinization that favors the formation of PGE alloys instead of sulfides (Garuti and Zaccarini, 1997;Jiménez-Franco et al., 2020 and references therein). Another modification of the primary PGM assemblage is the formation of secondary Ir-As-S phases, which have only been found as spongy envelopes around Os-Ir alloys ( Fig. 5e; Appendix 8c) or coexisting with chlorite (Fig. 5f). The formation of these secondary phases requires an increase in the a As of the system (e.g., Thalhammer et al., 1990;Malitch et al., 2001;González-Jiménez et al., 2010, which could be caused by post-serpentinization Cl-rich hydrothermal fluids that circulated in the Bou Azzer area between 380 and 240 Ma Hodel et al., 2017).

Origin and significance of unusual ("exotic") mineral assemblages
Unusual minerals distinguished in situ in chromite include oriented clinopyroxene lamellae and polyminerallic inclusions of diaspore, hematite and clinochlore. Other minerals identified in the mineral concentrates include moissanite, native Cu and zircon grains with apatite or serpentine inclusions (Fig. 6). Several of these unusual mineral phases have been commonly interpreted as indicators of UHP conditions and sourced from the mantle transition zone in many ophiolitic rocks worldwide (Table 1), but such models have been challenged by recent studies , 2020aFarré-de-Pablo et al., 2019a). The occurrence of similar mixed mineral assemblages of continental crust and super-reduced (SUR) and/or ultra-high pressure (UHP) origin as the ones in Bou Azzer is revised in Table 1.
Zircon grains hosting serpentine inclusions (denoting the former presence of ferro-magnesian minerals, notably olivine; Fig. 6m) recovered from the Bou Azzer chromitites could have crystallized at shallow mantle levels during the formation of the chromitites or during further interaction with silicate melts, as reported in other chromitites (Pujol-Solà et al., 2020b). The presence of apatite inclusions within other recovered zircon grains (Fig. 6l) may however indicate crystallization from a continental crustal magma. These potentially xenocrystic zircon grains may hence be linked to the transference of detrital zircon from subducted sediments to the upper mantle wedge and later encapsulation within chromite Rojas-Agramonte et al., 2016;González-Jiménez et al., 2017a;Proenza et al., 2018;Torró et al., 2018). Subduction of WAC (West African Craton)-derived sediments accumulated on top of the subducting oceanic lithosphere ahead the craton could facilitate the transfer of detrital zircons to the mantle wedge and their incorporation to the chromite-forming magmas.
Overall, we suggest a simple crystallization process for the Bou Azzer chromitites, discarding UHP crystallization (Yang et al., 2014 or recycling of low-pressure chromitites to great depth via mantle convection (Arai, 2013;Griffin et al., 2016). In contrast, we observe that the unusual mineralogy formed during the serpentinization of the chromitites and the host peridotites, from relatively high temperature conditions forming chlorite, moissanite and native Cu, to lower temperature conditions, forming andradite and diaspore.
Precambrian chromitites are mostly typically show massive textures and are centimetric to metric in size, with some exceptions such as the chromitites from the Hoggar ophiolite (Algeria) that reach 250 m in length (Augé et al., 2012) and those from the Tapo Massif (Peru) that reach around a hundred meters in length (Castroviejo et al., 2009;Colás et al., 2017) (Table 2). Regarding Cr#, it is highly variable, from 0.37 (Marlborough chromitites, Australia; Bruce et al., 2000) to 0.91 (Kenticha Hill chromitites, Ethiopia; Bonavia et al., 1993), though most orebodies are high-Cr (Cr# > 0.6; Table 2; Fig. 14a). A clear correlation between the composition of the chromitites and the age of the host ophiolite is not observed (Fig. 14a). However, younger chromitites generally have lower Cr# (Cr# < 0.7) than older chromitites (Fig. 14a), as already observed by Arai and Ahmed (2018). In order to stablish a potential correlation, and since the compositional information is in many cases partial and the post-magmatic chemical variations are not assessed, further studies on some of these Precambrian ophiolitic chromitites are needed.
The observed range of PGE content in Precambrian chromitites is very large, from 11 ppb (Pampean Ranges, Argentina; Proenza et al., 2008) to 10 ppm (Al'Ays, Saudi Arabia; Prichard et al., 2008; Table 2), still, most orebodies have ΣPGE at ppb levels (Fig. 14b). The observed enrichment in PGE in Al'Ays chromitites (Saudi Arabia; 131-10463 ppb; Prichard et al., 2008) or in Sayan chromitites (Russia; 88-1189 ppb; Kiseleva and Zhmodik, 2017) is anomalous and could be related to postmagmatic processes such as metamorphism. Laurite is the most important PGM in most chromitites (Table 2), followed by Os alloys and erlichmanite, indicating an enrichment in IPGE regarding PPGE, which is typical in mantle-hosted SSZ chromitites (e.g., González-Jiménez et al., 2014b). Even though a relationship between the Cr# of chromite and the PGE content is typically observed (e.g., González-Jiménez et al., 2014b), we do not observe a clear trend in the reviewed Precambrian chromitites (Fig. 14b). To the authors' knowledge, mineral associations nominally indicative of UHP or SuR conditions have not been described in the reviewed Precambrian ophiolitic chromitites listed in Table 2, except for the Zunhua Archean chromitites, where Kusky et al. (2019) reported UHP polymorphs of chromite and rutile (TiO 2 -II). On the other hand, most chromitites formed in a supra-subduction zone setting, but fore-arc or back-arc locations have not been determined, strengthening the need of further studies on these chromitites.
In terms of Cr# (Fig. 14a), the composition of the studied Bou Azzer chromitites is very similar to Neoproterozoic chromitites from Egypt (e. g., Ahmed, 2007;Ali et al., 2020), Sudan (Abdel Rahman et al., 1990) and Saudi Arabia (e.g., Pallister et al., 1988). The corresponding ophiolitic bodies belong to the Arabian-Nubian shield and are related to the Panafrican orogeny as much as the Moroccan Anti-Atlas. However, in north-western African ophiolites, chromitites have only been described in Bou Azzer and not in other ophiolites such as the Khzama sequence in the Sirwa inlier (e.g., Hodel et al., 2020;Chaib et al., 2021) and this may be related to the scarcity of chromitites in Precambrian ophiolites as stated above (e.g., Stowe, 1994 Marques et al., 2003;Arena et al., 2018;Paixão et al., 2008;Paixão, 2009;Hodel et al., 2019;Hackspacher et al., 2000;Tassinari et al., 2001) and Argentina (Pampean Ranges: 647 Ma; Proenza et al., 2008). Even though some authors (Paixão et al., 2008) attempted to correlate the Neoproterozoic Araguaia belt in Brazil (Quatipuru ophiolite) with the NW African Mauritanide-Basseride-Rockelide belts, the well-studied ophiolitic chromitites in South America have compositions richer in Al 2 O 3 than the Bou Azzer chromitites (Table 2). These differences, coupled with a general lack of geological data regarding the geodynamic  N. Pujol-Solà et al. Ore Geology Reviews 134 (2021) 104166 setting of the ophiolites and the Neoproterozoic suture zones, makes it very difficult to relate the South American ophiolites to the Moroccan Bou Azzer ophiolite. Arenas et al. (2020) have recently proposed a geodynamic model for the Neoproterozoic-Devonian margin of Gondwana in which the generation of accreted oceanic lithosphere seems to occur at 100 Ma intervals, as recorded by the age of obducted Neoproterozoic to Devonian ophiolites in Morocco and Iberia. In this model, all these ophiolites were collectively generated in the supra-subduction peri-Gondwanan realm during the opening of fore-arc and back-arc basins. Other authors have interpreted the formation of the Bou Azzer ophiolite in an intra-oceanic arc setting (e.g., Gasquet et al., 2005;El Hadi et al., 2010;Walsh et al., 2012;Hodel et al., 2020). However, there is still debate on whether there is one or multiple magmatic arcs (Admou et al., 2013;Soulaimani et al., 2018;Triantafyllou et al., 2018Triantafyllou et al., , 2020 and whether the ophiolite formed in a fore-arc (e.g., Saquaque et al., 1989;Naidoo et al., 1991;Ahmed et al., 2005;El Hadi et al., 2010;Walsh et al., 2012;Arenas et al., 2020) or a back-arc setting (e.g., Bodinier et al., 1984;Triantafyllou et al., 2018;Hodel et al., 2020).

Geodynamic setting and tectonic implications
The composition of unaltered chromite cores from the Bou Azzer chromitites reveals the presence of two compositional groups of chromitites: intermediate-Cr (Cr# = 0.60-0.74) and high-Cr (Cr# = 0.79-0.84) chromitites (Fig. 7). This distinction was not reported in previous studies of the Bou Azzer chromitites (e.g., Ikenne et al., 2005;El Ghorfi et al., 2007;Ahmed et al., 2009) but it is highly relevant because it translates into two types of parental melts for the studied chromitites. Intermediate-Cr chromitites yield parental melts similar to MORB but with lower TiO 2 contents (the low Ti content in the chromitites is also observed in the trace elements in Fig. 9a) corresponding to FAB melts, whereas high-Cr chromitites yield boninitic parental melts. This association of melts only occurs in fore-arc regions during subduction-initiation (Reagan et al., 2010;Whattam and Stern, 2011;Torró et al., 2017;Shervais et al., 2019;Liu et al., 2019;Pandey et al., 2019;Whattam et al., 2020). In this setting, extension of the upper plate due to rollback of the sinking plate contributes to a stronger interaction of the mantle flowing to the nascent mantle wedge with fluids from the subducting plate, triggering a higher degree of melting, which is translated into a depletion of Ti, and possibly into an increase in the PGE content in the released melts when compared to typical MORB (Reagan et al., 2010;Shervais et al., 2019). In the Izu-Bonin Mariana (IBM) forearc, FAB occur stratigraphically below and <2 Ma older than the associated boninite sequence (Reagan et al., 2010(Reagan et al., , 2013. In Bou Azzer, intense deformation, alteration and metamorphism obscure the stratigraphy of the volcanic sequence. Trace elements contents in the studied unaltered chromite cores show patterns very similar to fore-arc chromitites (Fig. 12). Additionally, the magmatic rocks that form the Bou Azzer ophiolitic crustal sequence (gabbroic rocks and undifferentiated volcanic rocks and dikes) show characteristics of a subduction-initiation ophiolite formed in a fore-arc environment (Arenas et al., 2020). Therefore, the origin of the Bou Azzer chromitites is conceptualized within a subduction-initiation ophiolite model (Whattam and Stern, 2011). In this context, chromitites formed in the fore-arc during the early stages of a Neoproterozoic intra-oceanic arc developed ahead the West African Craton margin. Similar studies have also interpreted the formation of chromitite bodies with different compositions in a subduction-initiation geodynamic setting (e.g., Moghadam et al., 2015;Zhang et al., 2016Zhang et al., , 2020Uysal et al., 2018). According to the subduction-initiation model, from early to more mature stages of subduction, melts evolve from FAB to boninites and island arc tholeiites to calc-alkaline arc magmas (Reagan et al., 2010;Whattam and Stern, 2011;Ishizuka et al., 2014;Shervais et al., 2019;Whattam et al., 2020). In this setting, during early stages of intraoceanic subduction (Fig. 15a) intermediate-Cr chromitites formed from FAB melts that were generated by initial spreading of oceanic crust and melting of the nascent mantle wedge. At this stage, there is little or no mass transfer from the subducting slab ( Fig. 15a; Liu et al., 2019). Later, as subduction proceeded, high-Cr chromitites precipitated from hydrated melts with boninitic affinity that migrate through the mantle wedge in the fore-arc ( Fig. 15b; e.g., Shervais, 2001;Dilek and Furnes, 2014;Moghadam et al., 2015).
Our interpretation of the geodynamic setting of formation of the Bou Azzer ophiolite in the fore-arc clearly differs from the recent proposal of a back-arc SSZ setting by Hodel et al. (2020). In the studied North Ait Ahmane ophiolitic sequence (Fig. 1), high-Cr chromitites (Filon 60 deposit) are located in between chromitites with intermediate-Cr composition (Inguijem and Ait Ahmane deposits). This interesting feature can be related to imbrication of the ophiolitic sequence during the obduction onto the WAC passive margin (El Hadi et al., 2010), hence repeating the serpentinized harzburgite sequence as suggested by Arenas et al. (2020) based on its unusual thickness.

Concluding remarks
We interpret that the Bou Azzer chromitites formed in a subductioninitiation geodynamic setting with two-stages of evolution, including formation of intermediate-Cr chromitites from fore-arc basalts (FAB) in the early stage and formation of high-Cr chromitites from boninitic melts in the ensuing stage. The studied chromitites are enriched in IPGE with respect to PPGE as observed in the magmatic PGM: Os-poor laurite and Os-Ir alloys. Unusual mineral phases in the studied chromite grains include clinopyroxene lamellae, moissanite, native Cu, diaspore, and zircon, and these were formed during shallow magmatic crystallization or via different stages of post-magmatic alteration related to serpentinization, rather than having UHP origin. Precambrian ophiolitic chromitites are quite rare compared to Phanerozoic ophiolitic chromitites and tend to be richer in Cr, while the PGE content is highly variable and clear trends related to the ophiolites age cannot be defined. The studied chromitites are very similar to Neoproterozoic ophiolitic chromitites of the Arabian-Nubian shield, also related to the Panafrican orogeny.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper