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We report new results of a combined focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) investigation of two grains of platinum-group minerals (PGM) corresponding to the laurite (RuS)-erlichmanite (OsS) solid solution series. The grains are included within unaltered chromite and the serpentine-dominant matrix of the mantle-hosted chromitite body of Monte Bueno in eastern Cuba. At the micron-scale, the grain hosted in unaltered chromite preserves magmatic features including a complex oscillatory zoning defined by a core of erlichmanite surrounded by alternating bands of Os-rich/Os-poor laurite, revealed by conventional field emission scanning electron microscopy (FE-SEM) and X-ray mapping by electron microprobe analyzer (EPMA). Combining high-magnification (HMEM), high angle-annular dark field (HAADF) images and the corresponding Energy Dispersive Spectra (EDS), the elemental mappings acquired using HRTEM showed that the oscillatory zoning occurs at both the micron and nano scales. A specific feature highlighted by HRTEM observations is the rather complex structure of the erlichmanite core, consisting of an aggregation of much smaller cores (, 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 for chemical analysis was provided by the Ramón y Cajal Fellowship RYC–2015–17596 to JMGJ. 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 . This paper was written during the lockdown caused by the global COVID–19 pandemic and the authors would like to dedicate this manuscript to the memory of those who lost their lives to the coronavirus.

The tin-rich polymetallic epithermal deposit of Poop&oacute, of plausible Late Miocene age, is part of the Bolivian Tin Belt. As an epithermal low sulfidation mineralisation, it represents a typological end-member within the &ldquo, family&rdquo, of Bolivian tin deposits. The emplacement of the mineralisation was controlled by the regional fault zone that constitutes the geological border between the Bolivian Altiplano and the Eastern Andes Cordillera. In addition to Sn and Ag, its economic interest resides in its potential in critical elements as In, Ga and Ge. This paper provides the first systematic characterisation of the complex mineralogy and mineral chemistry of the Poop&oacute, deposit with the twofold aim of identifying the mineral carriers of critical elements and endeavouring to ascertain plausible metallogenic processes for the formation of this deposit, by means of a multi-methodological approach. The poor development of hydrothermal alteration assemblage, the abundance of sulphosalts and the replacement of l&ouml, llingite and pyrrhotite by arsenopyrite and pyrite, respectively, indicate that this deposit is ascribed to the low-sulphidation subtype of epithermal deposits, with excursions into higher states of sulphidation. Additionally, the occurrence of pyrophyllite and topaz has been interpreted as the result of discrete pulses of high-sulphidation magmatic fluids. The &delta, 34SVCDT range in sulphides (&minus, 5.9 to &minus, 2.8&permil, ) is compatible either with: i. hybrid sulphur sources (i.e., magmatic and sedimentary or metasedimentary), or ii. a sole magmatic source involving magmas that derived from partial melting of sedimentary rocks or underwent crustal assimilation. In their overall contents in critical elements (In, Ga and Ge), the key minerals in the Poop&oacute, deposit, based on their abundance in the deposit and compositions, are rhodostannite, franckeite, cassiterite, stannite and, less importantly, teallite, sphalerite and jamesonite.

Platinum-rich nanonuggets (s.l., nanoparticles) are commonly produced in experiments attempting to quantify the solubility or partitioning of noble metals in silicate and sulfide melts. However, it has been thought that these represent artifacts produced during quenching of the experimental runs. Here, we document nanoparticles (~ 20–80 nm) of Pt-rich alloys and arsenides dispersed in high-temperature metasomatic silicate glasses and in base-metal sulfides (BMS) entrained in them, found interstitially between minerals of mantle peridotite xenoliths from southern Patagonia. Pt-rich nanoparticles found in the interstitial silicate glasses are frequently attached to, or in the proximities of, oxides (ilmenite or Cr-spinel) suggesting a close link between the formation of the oxides and the Pt-rich nanoparticles. The interstitial glasses in the studied xenoliths correspond to quenched alkaline basaltic melts that infiltrated the subcontinental lithospheric mantle (SCLM) at > 1000 °C at an oxygen fugacity (fO2) near the fayalite–magnetite–quartz (FMQ) buffer. Experimental works indicate that at these conditions the crystallization of oxides such as ilmenite or Cr-spinel may lower fO2 to promote the precipitation of Pt-rich nanoparticles. The investigation of four Pt-rich nanoparticles hosted in two different pentlandite grains using a combination of focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) show that these nanoparticles consists of polycrystalline aggregates, The authors are grateful to D. Kamenetsky, Dave Holwell and an anonymous reviewer for their criticism that greatly improved our manuscript. We also acknowledge Associate Editor Chris Ballhaus for careful editorial handling. This research was supported by spanish projects: RTI2018-099157-A-I00 and CGL2015-65824-P granted by the “Ministerio de Ciencia, Innovación y Universidades” and Ministerio de Economía y Competitividad” (MINECO), respectively. Additional funding for LA–ICP–MS analysis was provided by the Ramón y Cajal Fellowship RYC-2015-17596 to JMGJ. This study also contributes to the DID-UACh project #S-2015-52 to M. Schilling and the PhD project (S. Tassara) at University of Chile supported by the CONICYT Award scholarship #21170857. A. Jiménez is supported with a postdoctoral grant from the National Council on Science and Technology (CONACYT) of Mexico. Jesús Montes is acknowledged for preparation of thin sections. Trifon Trifonov (Polytechnic University of Catalonia), Rocío Márquez and María del Mar Abad (CIC of the University of Granada) and Macolm P. Roberts (CMCA of the University of Western Australia) and Rosanna Murphy (GAU of Macquarie University) are acknowledge for their assistance with FIB, FESEM, HRTEM and EMPA, respectively.

We report the first results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of zoned laurite (RuS2)-erlichmanite (OS2) in mantle-hosted chromitites. These platinum-group minerals form isolated inclusions (&lt, 50 &micro, m across) within larger crystals of unaltered chromite form the Oj&eacute, n ultramafic massif (southern Spain). High-magnification electron microscopy (HMEM), high angle-annular dark field (HAADF) and precession electron diffraction (PED) data revealed that microscale normal zoning in laurite consisting of Os-poor core and Os-rich rims observed by conventional micro-analytical techniques like field emission scanning electron microscope and electron microprobe analysis (FE-SEM and EPMA) exist at the nanoscale approach in single laurite crystals. At the nanoscale, Os poor cores consist of relatively homogenous pure laurite (RuS2) lacking defects in the crystal lattice, whereas the Os-richer rim consists of homogenous laurite matrix hosting fringes (10&ndash, 20 nm thickness) of almost pure erlichmanite (OsS2). Core-to-rim microscale zoning in laurite reflects a nonequilibrium during laurite crystal growth, which hampered the intra-crystalline diffusion of Os. The origin of zoning in laurite is related to the formation of the chromitites in the Earth&rsquo, s upper mantle but fast cooling of the chromite-laurite magmatic system associated to fast exhumation of the rocks would prevent the effective dissolution of Os in the laurite even at high temperatures (~1200 &deg, C), allowing the formation/preservation of nanoscale domains of erlichmanite in laurite. Our observation highlights for the first time the importance of nanoscale studies for a better understanding of the genesis of platinum-group minerals in magmatic ore-forming systems.

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.

[eng] The Santa Fe mining district is located in the Central Andean tin belt of Bolivia and contains several Sn-Zn-Pb-Ag deposits. From the economic point of view, the most important deposits of the district are Japo, Santa Fe and Morococala. Beyond the traditional metal commodities, the Central Andean Tin Belt could become an exploration target for indium, owing to the potential of the ore-bearing paragenesis with high concentrations of this technology-critical element. In the Santa Fe mining district, the ore occurs as two main types: (a) Sn-rich cassiterite-quartz veins, and (b) Zn-Pb-Ag veins with sphalerite, galena and stannite mineral phases. The In content in igneous rocks is between 1.5 and 2.5 ppm, whereas in the ore concentrate it attains up to 200 ppm. The 1,000×In/Zn ratio in concentrate ranges from 25 up to 4,000. Exceptionally high In values were found in sakuraiite from Morococala deposit (2.03 wt%). Sakuraiite in this deposit shows evidences for a link between stannite and kësterite trend of solid solutions. There is a noteworthy exploration potential for strategic metals in this district and even in similar deposits elsewhere in the Central Andean tin belt., [spa] El distrito minero Santa Fe se encuentra ubicado en la Faja Estannífera de la cordillera Central Andina de Bolivia, que aloja diversos depósitos de Sn-Zn-Pb-Ag. Desde el punto de vista económico, los depósitos más importantes en este distrito son los de Japo, Santa Fe y Morococala. Más allá de los productos metálicos tradicionales, la Faja Estannífera podría convertirse en un blanco de exploración para elementos como el indio, por el potencial de su paragénesis mineral con altas concentraciones de este elemento, crítico para el desarrollo de tecnología. Los yacimientos minerales del distrito de Santa Fe se presentan en dos tipos principales: (a) depósitos ricos en Sn, constituidos por vetas de cuarzo con casiterita, y (b) depósitos diseminados de Zn-Pb-Ag, con esfalerita, galena y estannita. El contenido de In en las rocas ígneas del distrito, varía entre 1,5 y 2,5 ppm, mientras que en el concentrado de los minerales de mena alcanza hasta 200 ppm de In. Asimismo, la razón 1.000×In/Zn en concentrados de mineral oscila entre 25 y 4.000. Fases de sakuraiíta del yacimiento de Morococala contienen valores excepcionales de In (hasta 2.03% en peso). La sakuraiíta en el distrito minero de Santa Fe muestra evidencia de ser el vínculo mineralógico entre la tendencia general de las soluciones sólidas de estannita y kësterita. Existe un potencial de exploración obvio para metales estratégicos en este distrito e incluso en depósitos similares en otras zonas de la Faja Estannífera en la cordillera Central Andina.

The present study determined the mineralogy and thermal properties of kaolin from Acoculco (Puebla), at the eastern Trans-Mexican Volcanic Belt and compared it with the nearby deposits of Agua Blanca (Hidalgo) and Huayacocotla (Veracruz). The mineralogy of the kaolins was determined by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. Thermal behaviour was studied by differential thermal analysis, dilatometry and hot-stage microscopy. The Acoculco deposit is composed mainly of kaolinite and SiO2minerals. In the case of Agua Blanca and Huayacocotla, alunite is abundant in places and minor anatase is also present locally. The Acoculco kaolins are Fe-poor and relatively rich in some potentially toxic elements (Zr, Sb, Pb). They undergo a relatively small amount of shrinkage (∼3–4 vol.%), during firing at 20–1300°C and cooling down to 20°C, except when >10 wt.% alunite is present. These kaolins are a suitable raw material for the ceramics industry. Other applications (pharmaceuticals, cosmetics) would require an enrichment process to eliminate impurities such as Fe oxides.