Supergene neoformation of Pt-Ir-Fe-Ni alloys: multistage grains explains nugget formation in Ni-laterites

Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as 'floating chromitites' within limonite (above the Mg discontinuity). Both chromitite types have similar iridium-group PGE (IPGE)-enriched chondrite normalized patterns; however, chromitites included in limonite show a pronounced positive Pt anomaly. Investigation of heavy mineral concentrates, obtained via hydroseparation techniques, led to the discovery of multistage PGE grains: (i) Os-Ru-Fe-(Ir) grains of porous appearance are overgrown by (ii) Ni-Fe-Ir and Ir-Fe-Ni-(Pt) phases which are overgrown by (iii) Pt-Ir-Fe-Ni mineral phases. Whereas Ir-dominated overgrowths prevail in chromitites from the saprolite, Pt-dominated overgrowths are observed within floating chromitites. The following formation model for multistage PGE grains is discussed: (i) hypogene platinum-group minerals (PGM) (e.g. laurite) are transformed to secondary PGM by desulphurization during serpentinization; (ii) at the stages of serpentinization and/or at the early stages of lateritization, Ir is mobilized and recrystallizes on porous surfaces of secondary PGM (serving as a natural catalyst) and (iii) at the late stages of lateritization, biogenic mediated neoformation (and accumulation) of Pt-Ir-Fe-Ni nanoparticles occurs. The evidence presented in this work demonstrates that in situ growth of Pt-Ir-Fe-Ni alloy nuggets of isometric symmetry is possible within Ni-laterites from the Dominican Republic

Nanoscale Structure of Zoned Laurites from the Ojén Ultramafic Massif, Southern Spain

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 (<50 um across) within larger crystals of unaltered chromite form the Ojén 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 (10–20 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’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 ºC), 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.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 was provided by the Ramón y Cajal Fellowship RYC-2015-17596 granted by the Spanish MINECO to JMGJ. A. Jiménez-Franco is supported with a postdoctoral grant (CVU 350809) from the National Council on Science and Technology (CONACYT) of Mexico

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15 presentations in scientific congresses. These activities enhance teamwork skills anddemonstrate that students can develop high quality research during the whole of theirformation.Peer ReviewedPostprint (published version

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15presentations in scientific congresses. These activities enhance tea

Fibrous Platinum-Group Minerals in “Floating Chromitites” from the Loma Larga Ni-Laterite Deposit, Dominican Republic

This contribution reports on the observation of enigmatic fibrous platinum-group minerals (PGM) found within a chromitite body included in limonite (“floating chromitite”) from Ni-laterites in the Dominican Republic. Fibrous PGM have a Ru-Os-Ir-Fe dominated composition and are characterized by fibrous textures explained by grain-forming fibers which are significantly longer (1–5 µm) than they are wide (~100 nm). Back-scattered electron (BSE) images suggest that these nanofibers are platinum-group elements (PGE)-bearing and form &lt;5 µm thick layers of bundles which are oriented orthogonal to grains’ surfaces. Trace amounts of Si are most likely associated with PGE-bearing nanofibers. One characteristic fibrous PGM was studied in detail: XRD analyses point to ruthenian hexaferrum. However, the unpolished fibrous PGM shows numerous complex textures on its surface which are suggestive for neoformation processes: (i) features suggesting growth of PGE-bearing nanofibers; (ii) occurrence of PGM nanoparticles within film material (biofilm?) associated with PGE-bearing nanofibers; (iii) a Si-rich and crater-like texture hosting PGM nanoparticles and an Ir-rich accumulation of irregular shape; (iv) complex PGM nanoparticles with ragged morphologies, resembling sponge spicules and (v) oval forms (&lt;1 µm in diameter) with included PGM nanoparticles, similar to those observed in experiments with PGE-reducing bacteria. Fibrous PGM found in the limonite may have formed due to supergene (bio-)weathering of fibrous Mg-silicates which were incorporated into desulphurized laurite during stages of serpentinization

Fibrous platinum-group minerals in 'floating chromitites' from the Loma Larga Ni-laterite deposit, Dominican Republic

This contribution reports on the observation of enigmatic fibrous platinum-group minerals (PGM) found within a chromitite body included in limonite ("floating chromitite") from Ni-laterites in the Dominican Republic. Fibrous PGM have a Ru-Os-Ir-Fe dominated composition and are characterized by fibrous textures explained by grain-forming fibers which are significantly longer (1-5 µm) than they are wide (~100 nm). Back-scattered electron (BSE) images suggest that these nanofibers are platinum-group elements (PGE)-bearing and form <5 µm thick layers of bundles which are oriented orthogonal to grains' surfaces. Trace amounts of Si are most likely associated with PGE-bearing nanofibers. One characteristic fibrous PGM was studied in detail: XRD analyses point to ruthenian hexaferrum. However, the unpolished fibrous PGM shows numerous complex textures on its surface which are suggestive for neoformation processes: (i) features suggesting growth of PGE-bearing nanofibers; (ii) occurrence of PGM nanoparticles within film material (biofilm?) associated with PGE-bearing nanofibers; (iii) a Si-rich and crater-like texture hosting PGM nanoparticles and an Ir-rich accumulation of irregular shape; (iv) complex PGM nanoparticles with ragged morphologies, resembling sponge spicules and (v) oval forms (<1 µm in diameter) with included PGM nanoparticles, similar to those observed in experiments with PGE-reducing bacteria. Fibrous PGM found in the limonite may have formed due to supergene (bio-)weathering of fibrous Mg-silicates which were incorporated into desulphurized laurite during stages of serpentinization

Transmission electron microscopy applied to nanomineralogical studies, an innovative approach

#LatinXChem Twitter Conference 2020, México. 7th of September, 202

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15 presentations in scientific congresses. These activities enhance teamwork skills anddemonstrate that students can develop high quality research during the whole of theirformation.Peer Reviewe

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15 presentations in scientific congresses. These activities enhance teamwork skills anddemonstrate that students can develop high quality research during the whole of theirformation.Peer Reviewe

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15presentations in scientific congresses. These activities enhance tea