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Platinum-group elements (PGEs) occur in ophiolitic chromitite in the Dominican Republic as platinum-group minerals (PGMs) in spatial association with hydrothermal uvarovite and chromian clinochlore. Bulk-rock total PGE content in a single analyzed chromitite sample is of 6.54 g/t. Three main PGM types are distinguished: euhedral magmatic laurite completely encased in chromite, subhedral to euhedral Ru-Os-Fe-(Ir) compounds partially encased in chromite, and anhedral Pt-Fe–Ni-rich grains exclusively embedded in uvarovite or chromian clinochlore. The Ru-Os-Fe-(Ir) compounds are interpreted as magmatic Ru-Os sulfides that experienced desulfurization during hydrothermal alteration of the chromitites, whereas the Pt-Fe–Ni-rich grains are hydrothermal in origin. We propose a model in which the Pt-Fe–Ni-rich PGMs formed via the accumulation of nanoparticles directly precipitated from the hydrothermal fluids. An estimation of the temperature of crystallization of uvarovite and chromian clinochlore suggests hydrothermal alteration of the chromitite within the thermal range of 150–350 °C. Thermodynamic modeling shows that, within this range of temperature, Pt could be mobilized as aqueous bisulfide complexes (HS−) by S-poor, highly reducing hydrothermal fluids originated during serpentinization of the host chromitite rock. The crystallization of Ni sulfides in the chromitite would drop the S concentration of the fluid, causing the precipitation of Pt as native element. Ultimately, this process contributes to constrain the conditions for the genesis of hydrothermal PGE mineralizations in ophiolitic chromitites.

This research was supported by Spanish projects: RTI2018-099157-A-I00, CGL2015-65824-P and CGL2016-81085-R granted by the "Ministerio de Ciencia, Innovacion y Universidades" and "Ministerio de Economia y Competitividad" (MINECO), respectively. Additional funding for chemical analysis was provided by the Ramon y Cajal Fellowship RYC-2015-17596 to JMGJ. Maria del Mar Abad, Isabel Sancez Almazo and Rocio Marquez Crespo (CIC, University of Granada) are acknowledged for her assistance with HRTEM, and HR-SEM and FESEM analysis respectively. We are also indebted to Miguel Angel Hidalgo Laguna from CIC of University of Granada and Xavier Llovet from the Centres Cientifics i Tecnologics of the Universitat of Barcelona (CCiTUB) for their careful help with EMPA. This paper was written during the lockdown provoked by the pandemic COVID-19 and the authors would like to dedicate this manuscript to the memory of those who lost their lives., Porphyry Cu can contain significant concentrations of platinum-group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd). In this study, we provide a comprehensive in situ analysis of noble metals (PGE, Au, Ag) for (Cusingle bondFe)-rich sulfides from the Elatsite, one of the world's PGE-richest porphyry Cu deposits. These data, acquired using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), indicate that Pd was concentrated in all the (Cusingle bondFe)-rich sulfides at ppm-levels, with higher values in pyrite (~6 ppm) formed at the latest epithermal stage (i.e., quartz–galena–sphalerite assemblage) than in bornite and chalcopyrite (, Spanish projects - "Ministerio de Ciencia, Innovacion y Universidades" RTI2018-099157-A-I00 CGL2015-65824-P CGL2016-81085-R, Spanish projects - "Ministerio de Economia y Competitividad" (MINECO) RTI2018-099157-A-I00 CGL2015-65824-P CGL2016-81085-R, Spanish Government RYC-2015-17596

Podiform chromitites occur in mantle peridotites of the Late Triassic Puerto Nuevo Ophiolite, Baja California Sur State, Mexico. These are high-Cr chromitites [Cr# (Cr/Cr + Al atomic ratio = 0.61-0.69)] that contain a range of minor- and trace-elements and show whole-rock enrichment in IPGE (Os, Ir, Ru). That are similar to those of high-Cr ophiolitic chromitites crystallised from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction-zone mantle wedges. Crystallisation of these chromitites from S-undersaturated melts is consistent with the presence of abundant inclusions of platinum-group minerals (PGM) such as laurite (RuS2)-erlichmanite (OsS2), osmium and irarsite (IrAsS) in chromite, that yield TMA ≈ TRD model ages peaking at ~ 325 Ma. Thirty-three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages (2263 ± 44 Ma to 278 ± 4 Ma) and Hf-O compositions [ɛHf(t) = − 18.7 to + 9.1 and 18O values < 12.4 ] that broadly match those of zircons reported in nearby exposed crustal blocks of southwestern North America. We interpret these chromitite zircons as remnants of partly digested continental crust or continent-derived sediments on oceanic crust delivered into the mantle via subduction. They were captured by the parental melts of the chromitites when the latter formed in a supra-subduction zone mantle wedge polluted with crustal material. In addition, the Puerto Nuevo chromites have clinopyroxene lamellae with preferred crystallographic orientation, which we interpret as evidence that chromitites have experienced high-temperature and ultra high-pressure conditions (< 12 GPa and ~ 1600 °C). We propose a tectonic scenario that involves the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago, deep-mantle recycling, and subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an extensional stage of the Vizcaino intra-oceanic arc ca. 221 Ma ago. The TRD ages at ~ 325 Ma record a partial melting event in the mantle prior to the construction of the Vizcaino intra-oceanic arc, which is probably related to the Permian continental subduction, dated at ~ 311 Ma.

The Huari Huari deposit, Potos&iacute, Department in SW Bolivia, hosts polymetallic stratiform and vein mineralization of Miocene age with significant concentrations of the critical metal indium (In). Vein mineralization records document early crystallization of quartz and cassiterite followed by prominent associations of sulfides and sulfosalts. The earliest sulfide was arsenopyrite, followed by pyrrhotite, and progressively giving way to pyrite as the main iron sulfide, whereas Cu&ndash, Ag&ndash, Pb sulfosalts constitute late hypogene associations. Sphalerite is the chief ore mineral, and its crystallization is extended during most of the mineralization lifespan as evidenced by its initial cocrystallization with pyrrhotine, then with pyrite, and finally with Ag&ndash, Pb sulfosalts. The composition of sphalerite varies from early to late generations with a continuous decrease in FeS that attests to a decrease in temperature, which is constrained to vary from ~450 to &lt, 200 &deg, C, and/or an increase in f(S2), both congruent with the described paragenetic sequence. Indium concentrated mostly in the structure of Fe-rich sphalerite (up to 3.49 wt. %) and stannite (up to 2.64 wt. %) as limited solid solutions with roquesite in the (Zn,Fe)S&ndash, Cu2FeSnS4&ndash, CuInS2 pseudoternary system. In sphalerite, In shows a strong positive correlation with Cu at Cu/In = 1, suggesting its incorporation via a (Cu+ + In3+) &harr, 2Zn2+ coupled substitution, and it does not correlate with Fe. In stannite, In shows a moderate, negative correlation with Cu and Sn, and an In3+ &harr, (Cu+ + &frac12, Sn4+) coupled substitution is suggested. Coexisting sphalerite and stannite yielded the highest In concentrations and crystallized at temperatures between 350 and 250 &deg, C. Copper activity probably played a major role in the accumulation of In in the structure of sphalerite since In-bearing sphalerite coexisted with the deposition of stannite, shows high concentrations of Cu (up to 0.13 atoms per formula unit (a.p.f.u.)) in its structure, and hosts exsolutions of stannite and chalcopyrite. Distribution on the district scale of In suggests an input of hydrothermal fluids richer in Cu in the central position of the mineralizing system, represented by the Ant&oacute, n Bravo vein.

The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) exist in solid solution in accessory base-metal sulfides (BMS) as well as nano-to-micron scale minerals in rocks of the subcontinental lithospheric mantle (SCLM). The latter include platinum-group minerals (PGM) and gold minerals, which may vary widely in morphology, composition and distribution. The PGM form isolated grains often associated with larger BMS hosted in residual olivine, located at interstices in between peridotite-forming minerals or more commonly in association with metasomatic minerals (pyroxenes, carbonates, phosphates) and silicate glasses in some peridotite xenoliths. The PGM found inside residual olivine are mainly Os-, Ir- and Ru-rich sulfides and alloys. In contrast, those associated with metasomatic minerals or silicate glasses of peridotite xenoliths consist of Pt, Pd, and Rh bonded with semimetals like As, Te, Bi, and Sn. Nanoscale observations on natural samples along with the results of recent experiments indicate that nucleation of PGM is mainly related with the uptake of HSE by nanoparticles, nanominerals or nanomelts at high temperature (> 900 °C) in both silicate and/or sulfide melts, regardless of the residual or metasomatic origin of their host minerals. A similar interpretation can be assumed for gold minerals. Our observations highlight that nanoscale processes play an important role on the ore-forming potential of primitive mantle-derived magmas parental to magmatic-hydrothermal deposits enriched in noble metals. The metal inventory in these magmas could be related with the physical incorporation of HSE-bearing nanoparticles or nanomelts during processes of partial melting of mantle peridotite and melt migration from the mantle to overlying continental crust., We thank Laurie Reisberg, Hannah Hughes and an anonymous referee for their criticism, which greatly improved the quality of our manuscript. We also are indebted to Prof. Sisir K. Mondal for Editorial handling of this work and their constructive edits. This research was supported by Spanish projects: RTI2018-099157-A-I00 , CGL2015-65824-P and CGL2016-81085-R granted by the “ Ministerio de Ciencia, Innovación y Universidades ” and Ministerio de Economía y Competitividad ” (MINECO), respectively. Additional funding was provided by the Ramón y Cajal Fellowship RYC-2015-17596 and Junta de Andalucía project B-RNM-189-UGR18 to JMGJ, and the BES-2017-079949 fellowship to ES. This work was also supported by the Mexican research programs CONACYT-Ciencia Básica ( A1-S-14574 ) and UNAM-PAPIIT grant IA-101419 awarded to VC. A. Jiménez-Franco also acknowledge a postdoctoral grant (CVU 350809 ) from the National Council on Science and Technology (CONACYT) of Mexico. Research grants, infrastructures and human resources leading to this research have benefited from funding by the European Social Fund and the European Regional Development Fund. We are grateful to Prof. José Jorge Aranda Gómez who provided the xenolith samples of La Breña (Durango Volcanic Field, Central Mexico). María del Mar Abad, Isabel Sánchez Almazo and Rocío Márquez Crespo (CIC, University of Granada) are acknowledged for her assistance with HRTEM, and HR-SEM and FE-SEM analysis, respectively. We are also indebted to Miguel Ángel Hidalgo Laguna from CIC of University of Granada and Xavier Llovet from the Centres Científics i Tecnològics of the Universitat of Barcelona (CCiTUB) for their careful help with EMPA.