Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals from the Havana-Matanzas Ophiolite, Cuba

Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 1Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / How to cite this article: Farré-de-Pablo, J., Pujol-Solà, N., TorresHerrera, H., Aiglsperger, T., GonzálezJiménez, J. M., Llanes-Castro, A. I., Garcia-Casco, A., Proenza, J. A., 2020, Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals from the Havana-Matanza Ophiolite, Cuba: Boletín de la Sociedad Geológica Mexicana, 72 (3), A110620. http://dx.doi. org/10.18268/BSGM2020v72n3a110620 Peer Reviewing under the responsibility of Universidad Nacional Autónoma de México. This is an open access article under the CC BY-NC-SA license(https://creativecommons.org/licenses/by-nc-sa/4.0/) RESUMEN La ofiolita de Habana-Matanzas contiene uno de los pocos ejemplos de cromititas ricas en minerales del grupo del platino (MGP) asociadas a ortopiroxenitas de la sección mantélica de complejos ofiolíticos. Las cromititas ocurren como venas encajadas en bandas de ortopiroxenita dentro de la peridotita mantélica. Las peridotitas son mayoritariamente harzburgitas con cromita accesoria rica en Al (#Cr [Cr/(Cr+Al), cociente atómico] = 0.39–0.50), lo cual es típico de espinelas en peridotitas abisales. Por otro lado, la cromita perteneciente a las venas de cromitita y a la ortopiroxenita encajante es rica en Cr (#Cr = 0.72–0.73 y 0.62–0.69, respectivamente) y con bajo #Mg [Mg/(Mg+Fe2+), cociente atómico]. Esto sugiere que tanto las venas de cromitita como la ortopiroxenita se formaron a partir de fundidos de afinidad boninítica. Las abundantes inclusiones de MGP encontradas en las cromititas son principalmente granos de laurita ricos en Os, lo cual también es propio de cromititas formadas a partir de magmas con afinidad boninítica. Por lo tanto, proponemos que las venas de cromitita y las bandas de ortopiroxenitas se formaron contemporáneamente en un contexto de ante-arco en un arco intra-oceánico durante el proceso de subducción. El conjunto cromitita-ortopiroxenita de la ofiolita de HabanaMatanzas se formó por la reacción de fundidos ricos en Si con afinidad boninítica y la harzburgita mantélica. Las bandas de ortopiroxenitas sería la huella química que habrían dejado estos fundidos boniníticos al infiltrarse por el manto. El volumen reducido de las cromititas que se formaron maximizó la eficiencia del proceso de recolección mecánica de los MGP que se formaban en el fundido parental, dando lugar al enriquecimiento de MGP primarios como inclusiones en las cromititas. Palabras clave: cromititas, Cuba, MGP, ofiolita, ortopiroxenita. ABSTRACT The Havana–Matanzas Ophiolite contains one of the few examples of ophiolitic platinum group minerals (PGM)-rich chromitites associated with orthopyroxenites in the mantle section of ophiolitic complexes. The chromitites occur as veins hosted by orthopyroxenite bands within mantle peridotites. The peridotites are mostly harzburgites and their accessory chromite shows high-Al compositions (Cr# [Cr/(Cr+Al), atomic ratio] = 0.39–0.50), which are typical of spinels in abyssal peridotites. Conversely, chromite from the chromitite veins and their host orthopyroxenite are high-Cr (Cr# = 0.72–0.73 and 0.62–0.69, respectively), with lower Mg# [Mg/(Mg+Fe2+), atomic ratio]. This suggests that both the chromitite and the orthopyroxenite formed from melts with boninitic affinity. The abundant PGM inclusions found in the chromitites are mainly Os-rich laurite grains, which is also characteristic of chromitites formed from magmas with boninitic affinity. Therefore, we propose that the chromitite veins and the orthopyroxenite bands probably formed contemporaneously in the fore-arc setting of an intra-oceanic arc during subduction. The chromitite-orthopyroxenite pair of the Havana-Matanzas Ophiolite could form after the reaction of a Si-rich melt with boninitic affinity and mantle harzburgite, with the orthopyroxenite bands preserving fingerprints of the infiltration of boninitic-affinity melts within the mantle. The small volume of forming chromitite could maximize the efficiency for the mechanical collection of the PGM forming in the parental melt of these rocks, resulting in the anomalous enrichment of primary PGM in the chromitites. Keywords: chromitite, Cuba, ophiolite, orthopyroxenite, PGM. 1 Departament de Mineralogia, Petrologia i Geologia Aplicada. Facultat de Ciències de la Terra, Universitat de Barcelona. C/ Martí i Franquès, s/n, 08028, Barcelona, Spain. 2 Fundación Universitaria del Area Andina – Sede Valledupar. Transv 22 Bis 4-105, Valledupar, Colombia. 3 Department of Civil Engineering and Natural Resources, Luleå University of Technology. SE 97187, Luleå, Sweden. 4 Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada. Avda. Fuentenueva, s/n, 18002, Granada, Spain. 5 Departamento de Petrología y Mineralogía, Instituto de Geología y Paleontología. Vía Blanca 1002, San Miguel del Padrón, Cuba. 6 Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR). Avda. de las Palmeras 4, E-18100 Armilla, Granada, Spain. * Corresponding author: (J. Farré de Pablo) jfarredepab[email protected] ABSTRACT Júlia Farré-de-Pablo1,*, Núria Pujol-Solà1, Harlison Torres-Herrera2, Thomas Aiglsperger3, José María González-Jiménez4, Angélica Isabel Llanes-Castro5, Antonio Garcia-Casco4,6, Joaquín A. Proenza1 Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals from the Havana-Matanzas Ophiolite, Cuba Vetas de cromitita en ortopiroxenita anómalamente enriquecidas en minerales del grupo del platino de la ofiolita Habana-Matanzas, Cuba Manuscript received: May 11, 2020 Corrected manuscript received: June 5,2020 Manuscript accepted: June 10,2020 Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 2 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 2 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 INTRODUCTION 1. Introduction Chromitite bodies from the mantle section of ophiolites are frequently enveloped by dunites hosted in mantle harzburgites (e.g., Leblanc and Nicolas, 1992; González-Jiménez et al., 2014, and references therein). The formation of these mantle chromitites is commonly associated with basaltic melts reaching the mantle–crust transition zone of the oceanic lithosphere through percolation processes and their consequent melt-rock reaction in the upper mantle (e.g., Arai and Miura, 2016, and references therein). Typically, high-Al chromitites (Cr# [Cr/(Cr+Al), atomic ratio] < 0.60) are located near the Moho Transition Zone, while high-Cr (Cr# > 0.60) occur deeper in the upper mantle levels from the ophiolitic sequence (e.g., Proenza et al., 1999). High-Al mantle chromitites are usually enriched in TiO2 (~0.5 wt.%) and depleted in platinum group elements (PGE: Os, Ru, Ir, Pt, Pd, Rh), whereas high-Cr chromitites are depleted in TiO2 (<0.25 wt.%) and enriched in PGE (e.g., Gervilla et al., 2005; González-Jiménez, 2011a). The mineralogical expression of PGE enrichment is the presence of abundant micro inclusions of platinum group minerals (PGM) in chromite, especially members of the solid solution series laurite (RuS2) – erlichmanite (OsS2), Os-Ir alloys and Ir-sulfarsenides (e.g., Zhou et al., 1996; Ahmed and Arai, 2002). Even if chromitites are globally associated with dunite bodies hosted in harzburgite, chromitites are locally associated with orthopyroxenites, such as in the Bay of Islands ophiolites in Newfoundland (Canada) (Bédard and Hebert, 1998), in the northern part of the Oman ophiolite (Ahmed and Arai, 2002), and the Ouen Island in New Caledonia (González-Jiménez et al., 2011b). In the Bay of Islands ophiolites, high-Cr chromitite replaces orthopyroxenite, interlayered with peridotite (mostly dunite), along its rim. According to Bédard and Hebert (1998), the composition of chromite from their studied chromitites depends more on the nature of the minerals being assimilated than on the composition of the magma. In Oman, high-Cr chromitite pods with unusually high IPGE (Os, Ru, Ir) contents are described in the deep mantle section related to orthopyroxenite bands within harzburgite (Ahmed and Arai, 2002). Their origin is linked to trapped refractory mafic melts derived from high-degree partial melting of peridotite in a suprasubduction zone (SSZ) environment (Ahmed and Arai, 2002). In New Caledonia, the chromitites from Ouen Island formed from small-volume hydrous silicate melts and are high-Al and very PGE-rich. They are associated to orthopyroxenite pockets within wehrlitic units (González-Jiménez et al., 2011b). Another good example of the chromitite–orthopyroxenite association is from the subcontinental mostly lherzolitic massif of Ronda (Spain), where high-Cr and high-Al chromitites are spatially associated with clinopyroxenites and orthopyroxenites (Gervilla et al., 2019). Here, chromitite occurs as entwined and contorted veins replacing pyroxenites with a clear correlation between the composition of chromite grains and the type of pyroxene relicts found in the them, orthopyroxene in high-Cr versus clinopyroxene in high-Al chromitite, which would reflect the Cr2O3/Al2O3 ratio of the reacting mafic rock (González-Jiménez et al., 2013). In this work, we study an unprecedented case of chromitite veins hosted in orthopyroxenite bands in the mantle tectonites from the Havana–Matanzas Ophiolite, in Cuba. These chromitites are characterized by being extremely rich in platinum-group mineral (PGM) inclusions. We provide detailed petrographic and geochemical description of the host peridotites, orthopyroxenites, and chromitite veins and their PGM inclusions in order to shed some light on the genesis of the chromitites and the possible genetic relations with their orthopyroxenite host. 2. Geological setting The northern margin of the island of Cuba contains the largest and more abundant ophiolite bodies of the Caribbean region, extending ca. 1000 km from SE to NW (Figure 1A; Iturralde-Vinent, 1996, 1998; Garcia-Casco et al., 2006; Lewis et al., 2006; Iturralde-Vinent et al., 2016). These ophiolites are Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 3Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / 3 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / GEOLOGICAL SETTING part of the Cuban orogenic belt, and represent slices of oceanic lithosphere (~136 to 70 Ma; RojasAgramonte et al., 2010) obducted onto the North American continental paleo-margin in Late Cretaceous to mid Eocene times during the collision of the Caribbean volcanic arc with the Jurassic-Cretaceous passive margins of the continental Maya block and the Bahamas platform (Iturralde-Vinent, 1996, 1998; Garcia-Casco et al., 2008; Iturralde-Vinent et al., 2008, 2016). The ophiolite assemblages are highly tectonically dismembered, but sections of large volumes of mantle tectonites and a Moho Transition Zone (dunite, wehrlite, plagioclaseand clinopyroxene-bearing peridotites, sills and dikes of gabbro) associated with a crustal sequence (layered gabbros, mafic volcanic rocks and sediments) can be recognized. Field relations, petrography, mineral chemistry, and whole rock compositions indicate a predominance of oceanic lithosphere formed above subducting plates (i.e., SSZ-type ophiolites) (e.g., Proenza et al., 1999; Iturralde-Vinent et al., 2016). These SSZ ophiolites from the basement of the Pacific (Farallon)-derived Caribbean plate are intimately related to the origin and evolution of the Caribbean volcanic arc (~135–70 Ma). The Havana–Matanzas Ophiolite is located at the west-central part of the island (Figure 1A). It consists of a complete but tectonically dismembered ophiolitic sequence with a predominance of upper mantle rocks over crustal rocks (Llanes-Castro et al., 2015, and references therein). As part of a tectonic mélange, the Havana-Matanzas Ophiolite are tectonically imbricated with Mesozoic sedimentary rocks of the southern paleo-margin of North America, Cretaceous intra-oceanic volcanic arc units, and Late Cretaceous to Eocene synorogenic sedimentary sequences (Figure 1B; Llanes-Castro et al., 2015). The tectonic blocks/ slices of ophiolitic rocks crop out as bodies that extend up to 15–20 km long and 2–3 km wide, although the sequence may reach 4 km in thickness (Iturralde-Vinent, 1996). The reconstructed Havana-Matanzas Ophiolite sequence consists of a lower unit of upper mantle peridotites, the Moho Transition Zone, and a crustal section composed of massive (isotropic) gabbros, dolerites dykes, pillow lavas with island-arc tholeiite (IAT) and boninitc affinities, and volcano-sedimentary rocks on top (Fonseca et al., 1985; Kerr et al., 1999; Llanes-Castro et al., 2015). The mantle peridotites are plagioclase lherzolites and highly serpentinized harzburgites with porphyroclastic fabric (Fig. 1C), which contain ubiquitous orthopyroxenite veins and minor sub-concordant dunite layers (Lewis et al., 2006; Llanes-Castro et al., 2015). The Havana-Matanzas Ophiolite contains a total of 10 chromitite occurrences (Llanes-Castro et al., 2015, and references therein). These chromitites occur as small bodies (around 0.5 m wide and 2 m long) with tabular to lenticular shapes. They are spatially related to mantle tectonites from the Moho Transition Zone and are enclosed in deformed serpentinized dunites within highly serpentinized harzburgite (Llanes-Castro et al., 2015). The majority of these chromitites are concordant with the foliation of their host peridotites. They display predominantly massive texture, but also nodular to disseminated textures at the margin of some chromitite bodies (Llanes-Castro et al., 2015). Chemically, they are classified as both high-Al (Cr# = 0.44–0.46) and high-Cr (Cr# = 0.65–0.74) chromitites (Llanes-Castro et al., 2015). According to these authors, the composition of the parental magmas of the high-Al chromitites had a back-arc basin basalt (BABB) affinity, and the composition of the high-Cr parental magmas had IAT affinity. This is also in accordance with the occurrence of platinum group mineral (PGM) inclusions, mainly laurite, in high-Cr chromitites, which indicates crystallization at high temperatures (~1300ºC) typical of boninitic magmas. Chromitites in Havana-Matanzas Ophiolite also occur as small veins within orthopyroxenite bands (Figure 1D to 1E). The orthopyroxenite bands are up to 1.5 m in thickness and are systematically emplaced in highly serpentinized harzburgites. The chromitite veins hosted by the pyroxenites are centimetric in thickness with a lateral extension of several decimeters. They display massive fabric (> 80 vol.% chromite) which tends to be disseminated (20–80 vol.% chromite) at the contact with the pyroxenite (Figure 1E). Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 4 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 4 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 GEOLOGICAL SETTING Figure 1 A: Geological sketch map showing location of the Cuban ophiolitic belt. The red rectangle marks the position of the Havana– Matanzas Ophiolitic Massif (modified after Iturralde-Vinent et al., 2016). B: Geological map of the Havana–Matanzas Ophiolitic Massif with the location of the studied samples (modified after Llanes-Castro et al., 2019). C: field photograph of harzburgite with porphyroclastic texture. D: field photograph of a chromitite vein and the host orthopyroxenite. E: thin section of a chromitite vein and the host orthopyroxenite. Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 5Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / 5 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / METHODS / PETROGRAPHY 3. Samples and analytical methods Samples of the chromitite veins and their host orthopyroxenite and peridotites were collected from the area indicated in Figure 1B, in the Havana-Matanzas Ophiolitic Massif. We prepared thin and thick polished sections and studied them under optical microscope and scanning electron microscope (SEM), using a Quanta 200 FEI XTE 325/D8395 equipped with an INCA Energy 250 EDS at the Centres Científics i Tecnològics de la Universitat de Barcelona (CCiT-UB). Representative mineral composition of olivine, orthopyroxene, chromite, and platinum group minerals (PGM) were obtained using a JEOL JXA-8230 electron microprobe equipped with five WDS spectrometers and one EDS spectrometer at the CCiT-UB. The analytical conditions were 20 kV accelerating voltage, 15 nA beam current with spot diameter 1-5 µm, and 20 s count time. The calibration standards used for silicates and oxides were: hematite (Fe-LIF-Kα), rutile (Ti-PET-Kα), periclase (Mg-TAP-Kα), rhodonite (Mn-LIF-Kα), Al2O3 (Al-TAP-Kα), Cr2O3 (Cr-PET-Kα), diopside (Si-TAP-Kα), wollastonite (Ca-PET-Kα), albite (Na-TAP-Kα), and orthoclase (K-PET-Lα). The analytical conditions for the PGM were 15 kV accelerating voltage, 10-20 nA beam current, 2 µm spot diameter and 20 s count time. The calibration standards used were: native metals (Os-LIF-Lα, Ir-LIFL-Lα, Ru-PETJ-Lα, Rh-PETJ-Lα, Pt-LIFL-Lα, Pd-PETJ-Lβ, Co-LIFH-Kα, Cr-LIFH-Kα, Ni-LIFH-Kα), FeS2 (Fe-LIFH-Kα, S-PETJ-Kα), chalcopyrite (Cu-LIFL-Kα), and GaAs (As-TAPHLβ). A representative chromitite vein sample (~300 g) was processed in order to obtain heavy mineral concentrates. The sample was crushed, grinded and sieved into different fractions (106–75 µm, 75–53 µm, 53–30 µm, and <30 µm) that were processed at the Hydroseparation Laboratory of the University of Barcelona (www.hslab-barcelona. com). The resulting concentrates were mounted into thick sections which were polished and studied by means of field emission scanning electron microscopy (FE-SEM) using a JEOL JSM-7100 at the CCiT-UB. 4. Petrography 4.1. HARZBURGITE Harzburgites hosting pyroxenite bands with chromitite veins consist of partially serpentinized olivine (65–75 vol.%), orthopyroxene (25–35 vol.%), clinopyroxene (<2 vol.%) and accessory chromian spinel (<1 vol.%). Their texture is characterized by plastic deformation of orthopyroxene porphyroclasts, typical of mantle tectonites deformed at high temperature. Olivine grains (<2 mm in size) occur as partially serpentinized neoblasts that constitute the matrix of the rock (Figure 2A and 2B). Orthopyroxene porphyroclasts are up to 5 mm in size with rounded shapes (Figure 2A and 2B). They are locally replaced by olivine in embayments along their boundaries (Figure 2B). Clinopyroxene grains are scarce and generally altered to amphibole. The accessory chromian spinel grains (referred hereafter as chromite, for convenience) are subhedral to anhedral with size ranging between 10 µm and 1 mm. Locally, they are partially or completely altered to ferrian chromite and magnetite plus chlorite. Mesh and bastite textures were produced during pseudomorphization of previous minerals by serpentine group minerals, mainly lizardite (Figure 2A). 4.2. ORTHOPYROXENITE BANDS Orthopyroxenite bands within harzburgite mainly consist of orthopyroxene (80–90 vol.%) and olivine (up to 18 vol.%), with minor clinopyroxene (<3 vol.%) and accessory chromite (<2 vol.%). Generally, they display equigranular texture with orthopyroxene grains up to 5 mm in size and olivine and clinopyroxene grains around 0.5 mm in size (Figure 2C to 2D). Accessory chromite grains are euhedral and less than 0.2 mm in size. They are hosted both in orthopyroxene and olivine grains (Figure 2D). Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 6 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 6 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 PETROGRAPHY 4.3. CHROMITITE VEINS The chromitite veins in the orthopyroxenite bands show massive textures (> 80 vol.% chromite) with euhedral to subhedral chromite grains which are locally intensely fractured (Figure 3A and 3B). Serpentine fills the fractures and the interstitial spaces between chromite grains. The chromite grains are homogeneous, with no alteration rims, and their sizes vary from larger grains (<0.5 mm) in the middle of the veins, to smaller grains (down to 50 µm) towards the contact with the orthopyroxenite. Chromite grains locally show embayment at the contact with the host orthopyroxenite (Figure 3C), and may host silicate inclusions and PGM. The silicate inclusions are around 10 µm in size with euhedral to round shapes, and they are often aligned along trails crossing the chromite grains (Figures 3D to 3F). These inclusions consist of phlogopite, olivine and orthopyroxene that, in most cases, have been altered to serpentine and bastite, respectively. PGM inclusions are very abundant in the chromitite veins, which is one of their most remarkable characteristics. 4.4. PGM IN THE CHROMITITE VEINS Platinum group minerals (PGM) were identified both in situ in unaltered chromite grains from the chromitite veins (3 PGM grains; Figure 4A and 4B) and in the heavy mineral concentrates from the processed chromitite vein samples (17 PGM Figure 2 Optical microscope images of harzburgite (A, B) and orthopyroxenite (C, D). A: orthopyroxene porphyroclasts in a serpentinized matrix of olivine from a harzburgite sample. B: orthopyroxene porphyroclast with embayments filled by olivine surrounded by a serpentinized matrix of olivine neoblasts in harzburgite. C: orthopyroxene, olivine and clinopyroxene grains showing granoblastic texture in orthopyroxenite. D: accessory chromite associated with both orthopyroxene and olivine in orthopyroxenite. Opx = orthopyroxene; Ol = olivine; Srp = serpentine; Cpx = clinopyroxene; Chr = chromite. Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 7Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / 7 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / PETROGRAPHY Figure 3 Optical microscope (A, B) and BSE (C-F) images of the chromitite veins. A and B: contact of the chromitite vein with the host orthopyroxenite. C: chromite grains at the contact with the host rock. D: trail of subhedral inclusions (olivine, clinopyroxene and serpentine) across chromite grains. E: single elongated olivine inclusion within chromite. F: trail of rounded inclusions (olivine and serpentine). Chr = chromite; Srp = serpentine. Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 8 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 8 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 PETROGRAPHY Figure 4 BSE images of PGM found in the chromitite veins from Havana-Matanzas Ophiolite. A: in situ grain of laurite within a chromite grain. B: in situ laurite grain together with clinopyroxene (Cpx). C: homogeneous euhedral Os-rich laurite. D: homogeneous subhedral Os-rich laurite with crystal growth bands. E: euhedral laurite grain with acicular oriented exsolutions of Ir along crystallographic planes. F: laurite crystal with oscillatory zoning. G: euhedral laurite with nanometric inclusions and Ir on the surface. H: detail of the nanometric inclusions of the laurite crystal shown in G. Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 9Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / 9 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / PETROGRAPHY 5 µm 1 µm 5 µm 5 µm1 µm 5 µm I J K L M N secondary Ir laurite secondary Ir secondary Ir Laurite with corroded surface Cu(Fe)S Ir Ir Laurite with corroded surface Figure 4 (Continuation) BSE images of PGM found in the chromitite veins from Havana-Matanzas Ophiolite. I: subhedral Os-rich laurite with overgrowth of Ir on the surface. J: detail of I showing fibrous texture of Ir. K: detail of I showing aggregate almost dendritic texture of Ir. L: euhedral laurite with corroded surface. M: subhedral laurite with corroded surface and nanoinclusions. N: detail of the nanometric inclusions of laurite shown in M. Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 16 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 16 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 / Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 DISCUSSION relatively high SiO2 contents and are normally saturated in orthopyroxene (Falloon and Danyushevsky, 2000, and references therein). Therefore, we consider that the chromitite veins from the Havana-Matanzas Ophiolite formed from low-Ti high-Mg andesitic melts with boninitic affinity that originated in the fore-arc of an intra-oceanic island arc. This interpretation is in accordance with models of formation of high-Cr ophiolitic chromitites by melt–rock reaction processes (e.g., Zhou and Robinson, 1997; Melcher et al., 1997; Proenza et al. 1999; González-Jiménez et al., 2014, and references therein). On the other hand, it has been proposed that two olivine-saturated liquids, one residual to dunite and the other extracted from harzburgite at higher pressure, could mingle to produce chromite-saturated hybrid melts from which chromitite can form (Ballhaus, 1998). This has been demonstrated by thermodynamic modeling (Abuamarah et al., 2020). In the case of the Havana-Matanzas Ophiolite chromitite veins, we interpret the systematic occurrence of the chromitite within orthopyroxenite and the lack of chromitite compositional variations towards their contact as evidence for the contemporaneous formation of the two types of rock. We infer that a Mg-rich andesitic melt with boninitic affinity interacted with harzburgite upon infiltration, triggering the peritectic reaction Ol+Liquid = Opx+Chr that leaded to consumption of olivine and precipitation of orthopyroxene and chromite (Kelemen et al., 1992, 1995; Wang et al., 2016). Further infiltration of new batches of liquid would mix with the residual liquids to yield secondary chromite-saturated melt which would precipitate chromite to form the chromitite veins (Irvine, 1977). The small volume of the chromitite veins indicates that the volume of parental magma required for their formation is lower than for the rest of chromitite bodies from the Havana-Matanzas Ophiolite. 6.2.2. PGE MINERALIZATIONS AND THEIR GENETIC IMPLICATIONS As mentioned above, high-Cr chromitites are interpreted to crystallize from magmas with boninitic affinity. These magmas are S-undersaturated and normally contain higher PGE contents than tholeiitic magmas, hence explaining the PGE-rich composition of high-Cr chromitites relative to high-Al chromitites (e.g., Peck et al., 1992; Zhou et al., 1998; Saha et al., 2015, and references therein). In the Havana-Matanzas Ophiolite chromitite veins, the abundant PGM inclusions reflect the PGE-rich composition expected for high-Cr chromitites. The main PGM phase found in the studied chromitites is laurite, which is also the main PGM observed in other ophiolitic chromitites worldwide (e.g., Luguet et al., 2007; González-Jiménez et al., 2014, and references therein). This PGE mineralogy also reflects the IPGE (Os, Ir, Ru) enrichment relative to PPGE (Rh, Pt and Pd) typical for mantle-hosted ophiolitic chromitites (e.g., Zhou et al., 1996; Ahmed and Arai, 2002). On the basis of their textural features and composition, the PGM identified in the chromitite veins can be divided into primary and secondary phases. Laurite crystals with homogeneous compositions or displaying magmatic textures (crystal growth bands and oscillatory zoning; Figures 4C to 4F) are considered primary. In contrast, those Figure 6 Composition (at.%) of laurite grains from the chromitite veins. Ru Os Erlichmanite Laurite Ir Laurite from chromitite bodies from HavanaMatanzas Ophiolite (from Llanes-Castro et al., 2015) Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals, Cuba 17Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / 17 http://dx.doi.org/10.18268/BSGM2020v72n3a110620 Boletín de la Sociedad Geológica Mexicana / 72 (3) / A110620 / 2020 / DISCUSSION / CONCLUSIONS PGM grains displaying rugged surfaces or that overgrow the surface of primary laurite (Figures 4L to 4N) are interpreted as secondary in origin, very likely originated by alteration of pre-existing PGM initially included in chromite and by local remobilization and redeposition of PGE during serpentinization (e.g., Garuti and Zaccarini, 1997; Proenza et al., 2008). Regarding primary laurite grains, most authors agree that they form at high-temperature stages from S-undersaturated mafic melts before or coeval with crystallization of chromite. In the studied samples, their predominantly euhedral morphology and their location in unaltered chromite suggests that they crystallized from a magma and were entrapped by growing grains of chromite (e.g., Melcher et al., 1997; González-Jiménez et al., 2009). As mentioned above, the composition of the laurite inclusions from the Havana-Matanzas Ophiolite chromitite veins is generally Os-rich (Table 4). The Ru–Os contents of laurite are strongly influenced by sulfur fugacity (f S2) and temperature (e.g., Brenan and Andrews, 2001; Andrews and Brenan, 2002): the solubility of Os in laurite increases with decreasing temperature and/or increasing f S2 (e.g., González-Jiménez et al., 2009). Nearly stoichiometric laurite (RuS2) has a maximum temperature of crystallization around 1200ºC, at very low sulfur fugacity (log f S2 < -2; Brenan and Andrews, 2001; Andrews and Brenan, 2002). Therefore, the Os-rich composition of laurite from the Havana-Matanzas Ophiolite chromitite veins indicates that they probably crystallized at relatively lower temperatures and/or higher f S2 than stoichiometric laurite. Lower temperatures for the Os-rich laurite formation would be in accordance with the cooling temperatures calculated for the orthopyroxenite (700–1100ºC), which formed contemporaneously with the chromitite veins containing the PGM. Moreover, the oscillatory zoning displayed by some laurite crystals (Figure 4F), with Ru–Os variations, lead to suggest that the PGM crystallized within a system dominated by short-term variations of f S2 and temperature (González-Jiménez et al., 2009). That could be the case of the model proposed for the chromitite formation, in which variations in the physicochemical properties of the parental melt would be a natural consequence of the melt–rock reactions. The chromite-bearing melts from the proposed model could efficiently concentrate high amounts of PGE along with already nucleated chromite. In this context, PGM tend to wet and/ or nucleate along the edges of growing chromite (e.g., Finnigan et al., 2008, and references therein). Therefore, chromite would act as a PGM collector, favoring the PGM concentration process. The efficiency of the mechanical collection is maximized when the magma crystallizing the PGM is in contact with small volumes of chromite (Augé et al., 2005). For this reason, the small and thin chromitite veins from the Havana-Matanzas Ophiolite show extreme enrichment in PGM inclusions. 7. Conclusions Orthopyroxenite bands within the Havana-Matanzas Ophiolite harzburgites formed from magmas with boninitic affinity, which are typical of fore-arc settings in intra-oceanic arcs. Small and thin PGM-rich chromitite veins are hosted within the orthopyroxenite. The chromitites are in equilibrium with the host orthopyroxenite and they also show boninitic affinity. We propose that the orthopyroxenite and the chromitite are formed contemporaneously after the reaction between pre-existing harzburgite and a Si-rich melt, probably of boninitic affinity. This reaction generated orthopyroxenite and a secondary melt that would mix with newly infiltrated liquid to form a Cr-saturated liquid from which chromite would precipitate to form chromitite veins within the orthopyroxenite. The growing chromite would act as a PGM collector due to the PGE affinity for chromite. The small volume of chromitite formed would maximize the efficiency of the mechanical collection, resulting in the high abundance of primary PGM inclusions observed in the chromitite veins from the Havana-Matanzas Ophiolite. 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