| 1. |
- Farré-de-Pablo, Júlia, et al.
(författare)
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Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals from the Havana-Matanzas Ophiolite, Cuba
- 2020
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Ingår i: Boletín de la Sociedad Geológica Mexicana. - : Universidad Nacional Autónoma de México. - 1405-3322 .- 1405-3322. ; 72:3
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Tidskriftsartikel (refereegranskat)abstract
- The Havana–Matanzas Ophiolite contains one of the few examples of ophiolitic platinum group minerals (PGM)-rich chromitites associated with orthopyroxenites in the mantle section of ophiolitic complexes. The chromitites occur as veins hosted by orthopyroxenite bands within mantle peridotites. The peridotites are mostly harzburgites and their accessory chromite shows high-Al compositions (Cr# [Cr/(Cr+Al), atomic ratio] = 0.39–0.50), which are typical of spinels in abyssal peridotites. Conversely, chromite from the chromitite veins and their host orthopyroxenite are high-Cr (Cr# = 0.72–0.73 and 0.62–0.69, respectively), with lower Mg# [Mg/(Mg+Fe2+), atomic ratio]. This suggests that both the chromitite and the orthopyroxenite formed from melts with boninitic affinity. The abundant PGM inclusions found in the chromitites are mainly Os-rich laurite grains, which is also characteristic of chromitites formed from magmas with boninitic affinity. Therefore, we propose that the chromitite veins and the orthopyroxenite bands probably formed contemporaneously in the fore-arc setting of an intra-oceanic arc during subduction. The chromitite-orthopyroxenite pair of the Havana-Matanzas Ophiolite could form after the reaction of a Si-rich melt with boninitic affinity and mantle harzburgite, with the orthopyroxenite bands preserving fingerprints of the infiltration of boninitic-affinity melts within the mantle. The small volume of forming chromitite could maximize the efficiency for the mechanical collection of the PGM forming in the parental melt of these rocks, resulting in the anomalous enrichment of primary PGM in the chromitites.
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| 2. |
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| 3. |
- García-Tudela, Matías, et al.
(författare)
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The chromitites of the Herbeira massif (Cabo Ortegal Complex, Spain) revisited
- 2024
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Ingår i: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 170
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Tidskriftsartikel (refereegranskat)abstract
- The ultramafic rocks of the Herbeira Massif in the Cabo Ortegal Complex (NW Iberia) host chromitite bodies. The textural and compositional study of the host rocks and the chromitites classified them into: (1) Type-I chromitites, forming massive pods of intermediate-Cr chromite (Cr# = 0.60–0.66) within dunites; and (2) Type-II chromitites forming semi-massive horizons of high-Cr chromite (Cr# = 0.75–0.82) interlayered with dunites and pyroxenites. Minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V and Sc) contents in the unaltered chromite cores from both types show patterns very similar to fore-arc chromitites, mimicked by the host dunites and pyroxenites. Calculated parental melt compositions suggest that Type-I chromitites crystallized from a melt akin to fore-arc basalt (FAB), while Type-II chromitites originated from a boninite-like parental melt. Both melts are characteristic of a fore-arc setting affected by extension during rollback subduction and have been related to the development of a Cambrian-Ordovician arc. These chromitites are extremely enriched in platinum-group elements (PGE), with bulk-rock PGE contents between 2,460 and 3,600 ppb. Also, the host dunites and pyroxenites exhibit high PGE contents (167 and 324 ppb, respectively), which are higher than those from the primitive mantle and global ophiolitic mantle peridotites. The PGE enrichment is expressed in positively-sloped chondrite-normalized PGE patterns, characterized by an enrichment in Pd-group PGE (PPGE: Rh, Pt and Pd) over the Ir-group PGE (IPGE: Os, Ir and Ru) and abundant platinum-group minerals (PGM) dominated by Rh-Pt-Pd phases (i.e. Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides). The PGM assemblage is associated with base-metal sulfides (mostly pentlandite and chalcopyrite) and occurs at the edges of chromite or embedded within the interstitial (serpentinized) silicate groundmass. Their origin has been linked to direct crystallization from a S-As-rich melt(s), segregated by immiscibility from evolved volatile-rich small volume melts during subduction. At c. 380 Ma, retrograde amphibolite-facies metamorphism occurred during the exhumation of the HP-HT rocks of the Capelada Unit, which affected chromitites and their host rocks but preserved the primary composition of chromite cores of the chromitites. This event contributed to local remobilization of PGE as suggested by the negative slope between Pt and Pd and high Pt/Pd ratios in the studied chromitites, and host dunites and pyroxenites. In addition, it promoted the alteration of primary PGM assemblage and the formation of secondary PGM. Nanoscale observations made by focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) analysis of a composite grain of Rh-bearing arsenide with PGE-base-metal bearing alloys suggest the mobilization and accumulation of small nanoparticles of PGE and base-metals that precipitated from metamorphic fluids forming PGE-alloys. Finally, we offer a comparison of the Cabo Ortegal chromitites with other ophiolitic chromitites involved in the Variscan orogeny, from the Iberian Peninsula to the Polish Sudetes. The studied Cabo Ortegal chromitites are similar to the Variscan chromitites documented in the Bragança (northern Portugal) and Kraubath (Styria, Austria) ophiolitic massifs.
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| 4. |
- González-Jiménez, José M., et al.
(författare)
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Precious metals in magmatic Fe-Ni-Cu sulfides from the Potosí chromitite deposit, eastern Cuba
- 2020
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Ingår i: Ore Geology Reviews. - : Elsevier. - 0169-1368 .- 1872-7360. ; 118
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Tidskriftsartikel (refereegranskat)abstract
- The Moa-Baracoa ophiolite in eastern Cuba is one of the few known ophiolites that display sulfide mineralization attributable to a magmatic origin in association with podiform-chromite ores hosted in the mantle-crust transition. These sulfide ores chiefly consist of Fe-Ni-Cu sulfides, namely pyrrhotite, pentlandite, chalcopyrite and cubanite partly altered to valleriite. The sulfide mineralization is located along the contact between the podiform-like chromite ores and intruding pegmatitic gabroic dykes. The detailed mineralogical study of the sulfide mineralization coupled with the first ever laser ablation ICP-MS analysis reveals that this sulfide mineralization show contents of the precious metals (Os, Ir, Ru, Pt, Re, Au, Ag) and other (semi)-metals (Co, Ni, Cu, Se, Te, Bi, Pb, As Sb) comparable to those sulfides from the magmatic sulfide deposits associated with mafic complexes hosted in the continental crust. The results obtained from this study confirm that Fe-Ni-Cu sulfides at Potosí are magmatic in origin, and very likely derived from the solidification of droplets of sulfide melt segregated by immiscibility from the intruding mafic melts once they interacted with the pre-existing chromitite at the mantle-crust transition zone of the ophiolite. The immiscibility of sulfide melt was achieved as a result of a progressive increase of fS2, very likely triggered by a set of circumstances, including the progressive fractionation of the intruding mafic melt leading to increase of aSiO2 and accumulation of volatiles as well as the crystallization of oxides. Two main generations of pentlandite were observed. One generation is primary in origin and it was locally exsolved along with pyrrhotite from monosulfide solid solution (MSS) during low-temperature cooling. The second type of pentlandite resulted from the reaction of MSS with coexisting droplets of Cu-and Ni-rich sulfide melt. LA-ICP-MS analysis reveals that most precious metals (Ru, Os, Ir, Re, Au, Ag) were concentrated along with the base-metal sulfides (BMS), although their distribution among the different BMS (pyrrhotite, pentlandite, chalcopyrite and cubanite) does not strictly follow the expected distribution according to the known melt-solid and solid-solid partition coefficients. Unlike the other analyzed PGEs, Pt was not preferentially concentrated in BMS but as discrete micrometer-sized sperrylite grains. The crystallization of sperrylite took place before and contemporaneous to sulfide segregation, and Pt-As nanoparticles probably played an important role in the Pt uptake as nucleation seeds for the formation of micron-sized sperrylite grains. These observations highlight the open-system nature of the ore forming system as well as the important role of arsenic in concentrating PGE in high-temperature silicate and sulfide melts.
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| 5. |
- Hernández-González, Juan S., et al.
(författare)
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Petrology and geochemistry of high-Al chromitites from the Medellín Metaharzburgitic Unit (MMU), Colombia
- 2020
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Ingår i: Boletín de la Sociedad Geológica Mexicana. - : Universidad Nacional Autónoma de México. - 1405-3322 .- 1405-3322. ; 72:3
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Tidskriftsartikel (refereegranskat)abstract
- The Medellin Metaharzburgitic Unit (MMU), emplaced onto the western continental margin of Pangea during Triassic time, is located in the Central Cordillera of Colombia and consists of metaharzburgites, minor metadunites and chromitite bodies (Patio Bonito and San Pedro ore deposits). The ultramafic rocks contain relicts of mantle-derived olivine, chromian spinel and minor orthopyroxene, and a later metamorphic mineral assemblage composed by tremolite, chlorite, talc, fine-grained recrystallized olivine, serpentine-group minerals, magnetite, and secondary chromian spinel, formed during the thermal evolution of the unit. The Cr# [Cr/(Cr+Al) atomic ratio] of the accessory primary chromian spinel in the metaperidotites ranges from 0.58 to 0.62 and overlaps those of supra-subduction peridotites from ophiolites. According to textural and compositional variations, the accessory chromian spinel in the metaperidotites can be classified into three groups: i) partially altered chromian spinel with an Al-rich core, ii) porous, Cr-Fe2+-enriched and Al-Mg-depleted chromian spinel, and iii) homogeneous Fe3+-rich chromian spinel. These variations can be related to superimposed medium-T metamorphism that reached amphibolite facies (ca. 600 ºC). Chromitite bodies associated with the metaperidotites have massive and semi-massive textures, and mainly consist of chromian spinel crystals, which show large unaltered cores surrounded by thin alteration rims of ferrian chromian spinel and chlorite. Chromitites are Al-rich (#Cr <0.6) and strongly depleted in platinum group elements (ΣPGE <41 ppb). The primary petrological and geochemical characteristics preserved in the metaperidotites and chromitites indicate that the MMU formed at shallow levels of a suboceanic lithospheric mantle related to a supra-subduction zone (back-arc basin/incipient arc scenario), and that the chromitites crystallized from a tholeiitic magma (back-arc basin basalt type).
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| 6. |
- López-Males, Gladys G., et al.
(författare)
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New mineralogical data on platinum-group minerals from the Río Santiago alluvial placer, Esmeraldas province, Ecuador
- 2020
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Ingår i: Boletín de la Sociedad Geológica Mexicana. - : Universidad Nacional Autónoma de México. - 1405-3322. ; 72:3
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Tidskriftsartikel (refereegranskat)abstract
- Mineralogical studies on platinum-group minerals found in placer deposits from the Río Santiago (Ecuador) are scarce. In this investigation, one sample collected from the Río Santiago alluvial placer was studied via a multi-disciplinary approach, including optical microscopy, scanning electron microscopy, electron microprobe, and Raman spectroscopy. Whole-rock geochemistry data of the sample confirms elevated Au and platinum-group elements contents and the chondrite-normalized pattern reveals pronounced positive Ir and Pt anomalies. Free grains of platinum-group minerals were separated via hydroseparation techniques and identified as: i) Pt-Fe alloy (Pt3Fe), ii) tulameenite (Pt2FeCu) and iii) hongshiite (PtCu). The most abundant platinum-group mineral is Pt-Fe alloy (85%) that occasionally hosts cuprorhodsite (CuRh2S4) inclusions. Although the primary source remains unknown, the geochemical and mineralogical data suggests that the source of platinum-group minerals in the Río Santiago alluvial placer is a mafic-ultramafic Ural-Alaska type complex. Possible primary sources are the mafic and ultramafic rocks found in the mafic basement of the coastal region and the Western Cordillera (Piñón, San Juan and Pallatanga units), which derive from the Late Cretaceous Caribbean Colombia Oceanic Plateau (CCOP).
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| 7. |
- Pujol-Sola, Nuria, et al.
(författare)
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Olivine orthopyroxenite-hosted chromitite veins in the ophiolitic mantle, Havana-Matanzas, Cuba
- 2019
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Ingår i: Proceedings of the 15th SGA Biennial Meeting, 27-30 August. - : University of Glasgow Publicity Services. ; , s. 569-572
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Konferensbidrag (refereegranskat)abstract
- The Havana-Matanzas ophiolite contains an example of chromitite veins hosted in olivine orthopyroxenites within mantle peridotites. Accessory Cr-spinei in the harzburgite and in the Ol-orthopyroxenite, and chromite in the chromitite vein show a wide range of composition (Cr# 0.39-0.75). However, the chromite in the vein and the Cr-spinel in the Ol-orthopyroxenite show similar composition with Cr#>0.7 (Cr-rich), probably related to crystallization from boninitic-like mantle melts. The study of platinum-group minerals hosted in the chromite shows that Os-rich laurite is the main mineral phase, as it is typically observed in chromitites that formed from magmas of boninitic affinity. The Ol-orthopyroxenite bands are believed to represent melt channels that formed after replacement of peridotites by a Si-rich melt with boninitic affinity. This is supported by the Cr-spinel composition of the pyroxenites (Cr#>0.7) and the low Al, Ca and Cr content of orthopyroxene. The interaction between a Si-rich melt with harzburgite/dunite formed Ol-orthopyroxenite and a Cr-saturated melt, which also circulated through the pyroxenite channels and formed the chromitite veins.
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| 8. |
- Tobón, Mónica, et al.
(författare)
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Geochemistry of Platinum-Group Elements (PGE) in the Cerro Matoso and Planeta Rica Ni-Laterite deposits, Northern Colombia
- 2020
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Ingår i: Boletín de la Sociedad Geológica Mexicana. - : Universidad Nacional Autónoma de México. - 1405-3322 .- 1405-3322. ; 72:3
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Tidskriftsartikel (refereegranskat)abstract
- 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
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