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Advanced radio mobile systems at millimeter waves using satellites in highly inclined orbits: assessment of the propagation impairments based on 20 to 50 GHz data measured in Italy
No full textComposition and mineralogy of PGE-rich chromitites in the Nurali Lherzolite-Gabbro complex, southern urals, Russia
No full textWe have investigated two subeconomic bodies of chromitite in the Nurali Iherzolite-gabbro complex, in the southern Urals, Russia, with regard to the composition of the chromian spinel and the distribution and mineralogy of the platinum-group elements (PGE). The bodies of chromitite, referred to as CHR-I and CHR-II, occur as small concordant lenses located at two stratigraphic levels within layered wehrlite and clinopyroxenite, overlying the Iherzolitie mantle tectonite. The chromian spinel is Al-rich, showing an increase of Cr/(Cr + Al), Fe2+/(Fe2+ + Mg) and TiO2 from CHR-I to CHR-II. The total PGE contents vary from 1.26 to 11.61 ppm, and show increase in (Pt + Pd)/(Os + Ir + Ru) from 0.1 to 52.2 as a result of the appearance of magmatic sulfides in the upper chromitite. The PGM assemblage shows a drastic change from laurite-erlichmanite-dominated to enriched in Pt Pd sulfides and alloys. Laurite is the first PGM to crystallize, and its composition typically reflects the Ru/Os ratio of the primitive mantle, indicating that the parent melt of the chromitite did not undergo fractionation during ascent. The Nurali chromitites are rather unusual as they have characteristics in common with chromitites associated with ophiolitic cumulates, layered intrusions, Alaskan-type complexes, and the subcontinental orogenic mantle
Rare and new mineralogical phases in the Ni-Cu-Sb-As system from the Gomati ophiolite, Northern Greece
No full textThe Gomati ultramafic body (Chalkidiki peninsula, Northern Greece) is located in the Serbo-Macedonian
Massif, one of the geotectonic terranes composing the Hellenides orogenic belt. Ophiolite occurrences in
this domain have an unclear origin, and consist of altered peridotites hosting scattered chromitite bodies with
massive, schlieren and disseminated textures. These ultramafic bodies are enclosed in the Vertiskos unit,
an alternation of Silurian gneisses and schists, and are sometimes in contact with late Cenozoic granites.
The present work focuses on several accessory minerals in the Ni-Cu-Sb-As system, found in a chloritized
clinopyroxenite in contact with chromitite. The composition of these accessory minerals was determined
through electron microprobe analyses. Well known mineralogical phases are represented by orcelite (Ni5As2)
and breithauptite (NiSb), while other detected phases have been either not well described or never reported.
The chemistry of the Gomati minerals clusters around the following ideal stoichiometries: (Ni,Cu)7(Sb,As)3,
(Ni,Cu)2(Sb,As), (Ni,Cu)11(Sb,As)8, Ni3As, Ni5(As,Sb)2 and Ni7(As,Sb)3. As orcelite (Ni5As2) is a nonstoichiometric
mineral, (Ni,Cu)7(Sb,As)3, Ni5(As,Sb)2 and Ni7(As,Sb)3 may correspond to Cu and/or Sb-rich
terms of this phase. A mineral phase corresponding to the (Ni,Cu)2(Sb,As) stoichiometry was first described in
the Alaskan-type Tulameen complex of Canada as unknown phase by Nixon and Cabri (1990). A phase with
stoichiometry Ni3As is reported by Tredoux et al. (2016), from Bon Accord oxide body (South Africa), and
corresponds to the mineral dienerite, known only from one loose crystal found in Austria in 1921 and recently
discredited my IMA. (Ni,Cu)11(Sb,As)8 probably represents a Cu-rich Sb analogue of the mineral maucherite
(Ni11As8).
Such an anomalous mineral assemblage in the Gomati ophiolite is puzzling. While ultramafic rocks contain
Ni and As of magmatic origin, the presence of Sb and Cu could be indicative of a metasomatic enrichment,
probably linked to the presence of fluids emanating from the granite body in contact with the Gomati ophiolite
Chromite composition and platinum-group mineral assemblage in the Uktus Uralian-Alaskan-type complex (Central Urals, Russia)
No full textChromitite segregations in dunites of the Uktus Uralian-Alaskan-type complex (Central Urals, Russia) display large variation of the chromite composition: Cr/(Cr + Al) - 0.46-0.77, Fe2+/(Fe2+ + Mg) = 0.28-0.66, and Fe3+ /(Fe3+ + Fe2+) =0.23-0.59. Three types of PGM assemblages have been recognized, varying in accordance with chromite composition: type I, dominated by Ru-Os-Ir (sulfides), is associated with magnesiochromite having Fe3+ /(Fe3+ + Fe2+) < 0.30, in the southern dunite body. Type 11, containing abundant Pt-Ir (alloys, minor sulfides), is found in magnesiochromite with Fe3+ /(Fe3+ + Fe2+) = 0.40-0.44; type III, consisting of Ir-Rh-Pt-Pd (alloys, sulfarsenides, antimonides) in Fe-rich chromite having Fe2+ /(Fe2+ + Mg) = 0.66 and Fe3+ /(Fe3+ + Fe2+) = 0.59. Positive anomalies of Ir and Pt, and a negative peak of Ru characterize the PGE patterns of chromitites with type II and III PGM assemblages, whereas a positive Pt anomaly is observed in their dunite host. Intensive fractionation of Pt-Fe alloys in the Uktus chromitites reflects the anomalous behavior of Pt which is decoupled from Rh and Pd. Among other factors, the high iron activity and oxygen fugacity in the parent melt appear to exert a major control on precipitation of Pt-Fe alloys, below sulfur saturation. The strong Pt anomaly in chromitites from Uktus may indicate that Uralian-Alaskan-type magmas were derived from a Pt-rich mantle source
Age, geochemistry and petrogenesis of the ultramafic pipes in the Ivrea Zone, NW Italy
Full text linkPipe-like ultramafic bodies, hosting Ni-Cu-PGE sulphide deposits, intrude the Main Gabble and the roof metasediments of the Ivrea Zone, NW Italy. These bodies were emplaced at 287 +/- 3 Ma and represent the last mantle-derived melts associated with an underplating event that largely drove the crustal evolution of this area during the late Carboniferous (similar to 300-290 Ma). nle Pipes are composed of volatile-rich ultramafic locks and gabbros with an alkaline signature simultaneously enriched in both incompatible and the most compatible elements but depleted in elements of intermediate compatibility. The isotope composition of these pipe rocks is E-290Ma(Nd) similar to3.7 to -1.9 and E-290Ma(Sr) similar to0.8-26. In a E-290Ma((Nd)) vs E290(Ma)(Sr) diagram they define a linear array between the unmetasomatized and metasomatized peridotites of Finero, but distinctlj oblique with respect to the trend defined by Balmuccia Peidotites. irhe delta S-34 ranges from 0.0 to + 0.9%(0) and is indicative of a mantle source. We suggest that the pipes represent infiltration of melts derived from a depleted mantle protolith flushed with alkaline metasomatic fluids, probably of juvenile mantle origin, which underwent partial melting as a consequence of the depression of the solidus owing to the increased activity of water and other volatiles. The similarity in age, trace-element, and isotopic signatures indicates that the Pipes were probably produced in the course of the same metasomatic event that affected the Finero ultramafic body. The overall geochemical characteristics of the pipes ale more consistent with magmatism related to a mantle plume than with a subduction setting
Platinum group minerals in ophiolitic chromitites from Tehuitzingo (Acatlan complex, southern Mexico): implications for post-magmatic modification
No full textPodiform chromitite bodies occur in serpentinites at Tehuitzingo (Acatlan complex, southern Mexico). Serpentinite and chromitite are believed to represent a fragment of Paleozoic ophiolitic mantle formed in a supra-subduction zone setting. The ophiolitic mantle sequence is associated with eclogitic rocks, enclosed in a metasedimentary sequence. This association suggests that serpentinites, chromitites and eclogitic rocks underwent a common metamorphic evolution, starting from high pressure (eclogite facies) followed by retrogression (epidote-amphibolite and greenschist facies). The chromitites are strongly altered so that chromite grains are transformed to ferrian chromite; no primary silicates (i.e. of magmatic origin) have been preserved. The chromitites are Al-rich, and contain up to 303 ppb platinum group elements (PGE), with a marked predominance of Os+Ir+Ru over Rh+Pd+Pt, resulting in a characteristic negative-slope of the chondrite-normalized PGE pattern. Consistent with the geochemical data the platinum group minerals (PGM) assemblage is dominated by Ru-Os-Ir minerals, occurring both as single-phase or as composite grains generally less than 10 mu m in size. The PGM mineralogy includes laurite, osmium, irarsite and Ru-Fe oxide or hydroxide. Based on textural relations, paragenesis and composition, it was possible to establish that Os-rich laurite and irarsite were early liquidus phases, which now occur as inclusions in unaltered chromite. However, most of the PGM are found in the alteration assemblages of the chromitites in close association with ferrian chromite, chlorite, and heazlewoodite. Laurite from the secondary assemblage is Os-poor and commonly shows overgrowths of Os - Ir alloys. Internal zoning of some laurite grains indicates that Os-poor laurite formed from a Os-rich laurite by release of Os and some Ir, that are readily incorporated in the Os - Ir alloys. Such process requires a decrease of sulfur fugacity with decreasing temperature; this is not consistent with the fS(2)-T trend in magmatic systems. It is proposed, therefore, that the magmatic PGM assemblage underwent mineralogical reworking starting from relatively high temperature during metamorphism. Temperatures, estimated from chlorite geothermometry ( 399 - 210 degrees C), possibly reflect effects of low-grade metamorphism. After that the PGM and the associated sulfides started to be oxidized. Although it is difficult to determine the extent of PGE mobilization on the basis of mineralogical observations, our data suggest that the metamorphism affecting the Tehuitzingo chromitites caused only re-distribution of PGE on a small scale. Thus, we conclude that metamorphism modified the primary PGM assemblage without having changed the whole-rock PGE concentration
Platinum-Group Element Geochemistry in Chromitite and Related Rocks of the Bracco Gabbro Complex (Ligurian Ophiolites, Italy)
Full text linkPalladium and gold minerals from the Baronskoe-Kluevsky ore deposit (Volkovsky complex, Central Urals, Russia)
No full textDrill cores from the newly discovered Baronskoe-Kluevsky Pd-Au deposit (Volkovsky massif, Central Urals) have been investigated by reflected-light and electron microscopy, and the ore minerals were analyzed by electron microprobe. The most abundant Platinum-group mineral (PGM) is vysotskite, ideally PdS, characterized by an unusual Pt,Ni-poor composition. Palladium also occurs in kotulskite (PdTe), stillwaterite (Pd8As3 and unknown Pd-As-Te compounds with vincentite-type Pd-3(As,Te), stillwaterite-type Pd-8(As,Te)(3), and Pd-7(As,Te)(2) stoichiometries. The main carrier of Au is Pd-rich electrum, approaching the composition Au75Ag15Pd10, with minor Fe, Cu, Ni and Pt. The precious minerals are closely associated with minute blebs of chalcopyrite + magnetite disseminated throughout serpentinized olivine-apatite host rock. Paragenetic relationships among the ore minerals define a succession of crystallization events in the order: 1) Cu-Pd sulfides + electrum, 2) replacement by Pd-Te-As and late Pd-As PGM, 3) final replacement by magnetite. The paragenesis is tentatively related with cooling of a fluid phase in the late-to post-magmatic stage
The Pb-rich sulfide veins in the Boccassuolo ophiolite: Implications for the geochemical evolution of hydrothermal activity across the ocean-continent transition in the Ligurian Tethys (Northern-Apennine, Italy)
No full textGalena bearing sulfide veins have been discovered coexisting with Fe–Cu–Zn dominated veins in the hydrothermal stockwork of the Boccassuolo ophiolite (External Ligurides, Northern Apennine, Italy). The galena-rich veins cut across a volcanic pile composed of pillow lava flows, pillow breccia, and ophiolitic sandstone. Bulk-ore analyses indicate significant enrichment in Pb giving raise to mantle normalized Pb–Ag–Au–Zn–Cu patterns with unusual negative slope, in contrast with the average flat pattern of most sulfide deposits in the Internal Liguride ophiolites which reflect the Fe–Cu–Zn assemblage of ophiolite-hosted Volcanic-associated Massive Sulfide (VMS) deposits all over the world.
A wide literature shows that, in contrast with the Internal Ligurides, plutonic and volcanic rocks of the External Ligurides display less depleted and even enriched geochemical characters, not consistent with common oceanic crust at mid oceanic ridges (MOR), but probably originated in the ocean–continent transition of the Adria continental margin. In this geodynamic context, pillow basalts become locally enriched in Pb with high Pb/Cu ratios, and other crustal-compatible elements such as Mo and U. The Pb enrichment observed in the veins Boccassuolo is interpreted to be a result of leaching of such anomalous volcanics forming the ophiolitic substrate. The case of Boccassuolo supports the conclusion that the geochemical character of hydrothermal activity evolved from Cu–Zn rich in MOR-type assemblages of the Internal Ligurides, towards composition enriched in Pb in the External Liguride domain, representing the transition from the Ligurian ocean to the Adria continental margin
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