Abstract
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.
Highlights
The platinum-group elements (PGE) are a group of precious metals (Os, Ir, Ru, Rh, Pt, and Pd) which are referred to strategic metals because of their extreme scarcity within the Earth’s crust combined with the increasing need for PGE in advanced technologies and jewellery [1]
This supply situation has motivated the search for alternative sources of PGE, and large oxidized ore deposits associated with near surface modification of ultramafic rocks, such as ophiolite-related
Samples were collected in the ophiolite-related Loma Larga Ni-laterite ore deposit which forms part of the Falcondo mining area located in the central part of the Dominican Republic (Figure 1)
Summary
The platinum-group elements (PGE) are a group of precious metals (Os, Ir, Ru, Rh, Pt, and Pd) which are referred to strategic metals because of their extreme scarcity within the Earth’s crust combined with the increasing need for PGE in advanced technologies (e.g., autocatalyst industries; electronics) and jewellery [1]. The vast majority (~80%) of the worldwide PGE demand is supplied by magmatic ore deposits located in only two countries, i.e., South Africa and Russia [2]. This supply situation has motivated the search for alternative sources of PGE, and large oxidized ore deposits associated with near surface modification of ultramafic rocks, such as ophiolite-related. Ni-laterites from the northern Caribbean are systematically enriched in the limonite and reach values of up to 0.6 ppm total PGE [6] These increasing PGE contents toward the surface are mainly explained by residual accumulations of PGE-bearing minerals. The presented contribution reports on the observation of enigmatic fibrous PGM found close to the surface in Ni-laterites from the Dominican Republic and aims (i) to give a general overview of the prevailing features of fibrous PGM; (ii) to provide detailed textural as well as mineralogical data of one characteristic fibrous PGM and (iii) to discuss a possible origin of fibrous PGM
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