No abstract

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 (Cu-Fe)-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 (Cu-Fe)-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 (<5 ppm) from the hypogene quartz–magnetite–bornite–chalcopyrite ores. Likewise, Au is significantly more concentrated in pyrite (~ 5 ppm) than in the (Cu-Fe)-rich sulfides (≤0.08 ppm). In contrast, Ag reaches hundreds of ppm in pyrite and bornite (~ 240 ppm) but is in much lesser amounts in chalcopyrite (< 25 ppm). The inspection of the time-resolved spectra collected during LA-IPC-MS analyses indicate that noble metals are present in the sulfides in two forms: (1) structurally bound (i.e., solid solution) in the lattice of sulfides and, (2) as nano- to micron-sized inclusions (Pd-Te and Au). These observations are further confirmed by careful investigations of the PGE-rich (Cu-Fe)-rich sulfides by combining high-spatial resolution of field emission scanning electron microscope (FESEM) and focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM). A typical Pd-bearing mineral includes the composition PdTe 2 close to the ideal merenskyite but with a distinct crystallographic structure, whereas Au is mainly found as native element. Our detailed mineralogical study coupled with previous knowledge on noble-metal inclusions in the studied ores reveals that noble metal enrichment in the Elatsite porphyry ores was mainly precipitated from droplets of Au-Pd-Ag telluride melt (s) entrained in the high-temperature hydrothermal fluid. These telluride melts could separate at the time of fluid unmixing from the silicate magma or already be present in the latter either derived from deep-seated crustal or mantle sources. Significant enrichment in Pd and Au (the latter correlated with As) in low-temperature pyrite is interpreted as remobilization of these noble metals from pre-existing hypogene ores during the epithermal overprinting.

Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH 4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH 4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350°C and 100 MPa) is consistent with extreme reduction of the fluid to logfO 2 (MPa) = −45.3 (ΔlogfO 2 [Iron-Magnetite] = −6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.

ResumenEn este trabajo se presenta una síntesis de las técnicas analíticas más utilizadas en la caracterización mineral, y su aplicación a la exploración y explotación minera. Las técnicas han sido clasificadas en 2 grupos. El primer grupo incluye a las técnicas de mayor uso ("técnicas convencionales"): (i) difracción de polvo de rayos X y difracción cuantitativa de rayos X, (ii) Microscopio electrónico de barrido con analizador de energías (SEM-EDS), (iii) catodoluminiscencia, y iv) microsonda electrónica (EMPA). El segundo grupo abarca un grupo de técnicas menos accesible, y mucho más caras, ("técnicas no convencionales"): (i) Particle Induced X-Ray Emission (Micro-PIXE), (ii) Secondary Ion Mass Spectrometry (SIMS, (iii) Laser-Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). La mayor parte de la compilación esta dedicada a las técnicas convencionales (DRX, SEM-EDS y EMP), las cuales pueden ser de mayor impacto en el campo de la pequeña minería.Palabras Claves: Caracterización mineral, técnicas analíticas, Difracción de rayos X, microscopio electrónico, microsonda electrónica. Abstract This paper is an overview of the main analytical techniques used in mineral characterization, as well as their application to mining exploration and exploitation. Two groups of techniques have been established. The first group includes more frequently used techniques ("conventional techniques"): (i) X-ray powder diffraction and quantitative XRD, (ii) scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), (iii) cathodoluminiscence, and iv) electron probe microanalysis (EPMA). In contrast, the second group comprises less commonly used and more expensive techniques ("unconventional techniques"): (i) Particle Induced X-Ray Emission (Micro-PIXE), (ii) Secondary Ion Mass Spectrometry (SIMS), (iii) Laser-Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS).The paper covers mainly the fundamentals of the most commonly used analytical techniques in mineral characterization (DRX, SEM-EDS, EMPA), and information on applications of these tools in mining exploration, which can be of major interest in small mining operations.

Platinum-group elements (PGE) are included among the so-called critical metals, and are essential metals for the technological industry. However, there are very few deposits in the world from which these metals can be extracted. The present work investigates three Ni-laterite profiles (hydrous Mg silicate type) formed over the ultramafic rocks of Cerro Matoso and Planeta Rica in Colombia. The main goal is to determine their PGE concentration and distribution, as well as to identify the carrier phases of these noble metals. The highest PGE contents in Cerro Matoso and Planeta Rica are concentrated in the limonite horizon (141–272 ppb), showing a strong decrease towards the saprolite and the underlying serpentinized peridotite (parent rock; < 50 ppb). The highest concentrations correspond to Pt>Ru>Pd and the lowest to Rh<Os<Ir. Such distribution indicates that PGE are mobilized in different proportions by the laterization processes. The high affinity between PGE and Fe favors the formation of PGE-Fe mineral alloys such as the Pt-Ir-Fe-Ni minerals hosted by Fe-oxyhydroxide found in the limonite–saprolite transition zone in Planeta Rica. In addition, in the same zone, nanoparticles of Pt (< 1 µm) were found within framboidal pyrite. Both types of platinum group minerals (PGM) are secondary in origin. In the case of Pt-Ir-Fe-Ni alloys, this interpretation is supported by their morphology and chemical composition, which is comparable with PGE-Fe-Ni alloys found in laterites of Dominican Republic. In the case of Pt nanoparticle, textural relations suggest the neoformation of PGM adhered to the porous edges of altered pyrite. Cerro Matoso and Planeta Rica should be considered as unconventional PGE deposits, if adequate recovery processes can be applied for their recovery as by-products during Ni (+Co) production.