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Giorgio Garuti - One of the best experts on this subject based on the ideXlab platform.
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Zoned Laurite from the Merensky Reef, Bushveld Complex, South Africa: “Hydrothermal” in Origin?
Minerals, 2020Co-Authors: Federica Zaccarini, Giorgio GarutiAbstract:Laurite, ideally (Ru,Os)S2, is a common accessory mineral in podiform and stratiform chromitites and, to a lesser extent, it also occurs in placer deposits and is associated with Ni-Cu magmatic sulfides. In this paper, we report on the occurrence of zoned Laurite found in the Merensky Reef of the Bushveld layered intrusion, South Africa. The zoned Laurite forms relatively large crystals of up to more than 100 µm, and occurs in contact between serpentine and sulfides, such as pyrrhotite, chalcopyrite, and pentlandite, that contain small phases containing Pb and Cl. Some zoned crystals of Laurite show a slight enrichment in Os in the rim, as typical of Laurite that crystallized at magmatic stage, under decreasing temperature and increasing sulfur fugacity, in a thermal range of about 1300–1000 °C. However, most of the Laurite from the Merensky Reef are characterized by an unusual zoning that involves local enrichment of As, Pt, Ir, and Fe. Comparison in terms of Ru-Os-Ir of the Merensky Reef zoned Laurite with those found in the layered chromitites of the Bushveld and podiform chromitites reveals that they are enriched in Ir. The Merensky Reef zoned Laurite also contain high amount of As (up to 9.72 wt%), Pt (up to 9.72 wt%) and Fe (up to 14.19 wt%). On the basis of its textural position, composition, and zoning, we can suggest that the zoned Laurite of the Merensky Reef is “hydrothermal” in origin, having crystallized in the presence of a Cl- and As-rich hydrous solution, at temperatures much lower than those typical of the precipitation of magmatic Laurite. Although, it remains to be seen whether the “hydrothermal” Laurite precipitated directly from the hydrothermal fluid, or it represents the alteration product of a pre-existing Laurite reacting with the hydrothermal solution.
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Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance
Minerals, 2016Co-Authors: Federica Zaccarini, Arifudin Idrus, Giorgio GarutiAbstract:Several chromite deposits located in the in the South and Southeast Arms of Sulawesi, Indonesia, have been investigated by electron microprobe. According to the variation of the Cr# = Cr/(Cr + Fe3+), the chromite composition varies from Cr-rich to Al-rich. Small platinum-group minerals (PGM), 1–10 μm in size, occur in the chromitites. The most abundant PGM is Laurite, which has been found included in fresh chromite or in contact with chlorite along cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with amphibole, chlorite, Co-pentlandite and apatite. Small blebs of irarsite (less than 2 μm across) have been found associated with grains of awaruite and Co-pentlandite in the chlorite gangue of the chromitites. Grains of olivine, occurring in the silicate matrix or included in fresh chromite, have been analyzed. They show a composition typical of mantle-hosted olivine. The bimodal composition and the slight enrichment in TiO2 observed in some chromitites suggest a vertical zonation due to the fractionation of a single batch magma with an initial boninitic composition during its ascent, in a supra-subduction zone. This observation implies the accumulation of Cr-rich chromitites at deep mantle levels and the formation of the Al-rich chromitites close or above the Moho-transition zone. All of the Laurites are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperature of around 1200 °C and a sulfur fugacity below the sulfur saturation. Irarsite possibly represents a low temperature, less than 400 °C, exsolution product.
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Platinum group minerals in chromitite bodies of the Santa Elena Nappe, Costa Rica: mineralogical characterization by electron microprobe and Raman-spectroscopy
Boletín de la Sociedad Geológica Mexicana, 2010Co-Authors: Federica Zaccarini, Oskar Thalhammer, Giorgio Garuti, Joaquín A. Proenza, Ronald J. Bakker, Thomas Aiglsperger, Lolita Campos, John F. LewisAbstract:Forty-seven grains of platinum group minerals (PGM) associated with small chromitite bodies of the Santa Elena ultramafic Nappe (Costa Rica) were mineralogically investigated with electron microscope, electron microprobe and Raman spectroscopy. The mineralogical assemblage includes sulfides of the Laurite-erlichmanite series (RuS 2 -OsS 2 ), irarsite (IrAsS), osmium, Ir-Rh sulfides containing relevant amounts of Ni, Fe and Cu, and a Ru-As-S compound, possibly ruarsite (RuAsS). Most platinum group element (PGE) sulfides and sulfarsenides represent primary magmatic phases entrapped in chromite at high temperatures, whereas native osmium is probably formed by subsolidus exsolution. The lack of primary PGE alloys suggests relatively high S-fugacity in the chromite forming system. This investigation emphasizes the efficiency of Raman spectroscopy in the identification of PGM of extremely small size, and shows how this technique can be used in revealing distinctive compositional differences among PGM of the Laurite-erlichmanite series and irarsite.
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CHROMIAN SPINEL COMPOSITION AND PLATINUM-GROUP ELEMENT MINERALOGY OF CHROMITITES FROM THE MILIA AREA, PINDOS OPHIOLITE COMPLEX, GREECE
The Canadian Mineralogist, 2009Co-Authors: Argyrios Kapsiotis, Tassos A. Grammatikopoulos, Basilios Tsikouras, Konstantin Hatzipanagiotou, Federica Zaccarini, Giorgio GarutiAbstract:The chromitites of the Milia area in the Pindos ophiolite complex, in Greece, were investigated for their platinum-group mineral (PGM) content. The chromitites are massive, more rarely disseminated in texture and occur as small pods. They are composed of magnesiochromite crystals with Cr# [Cr/(Cr + Al)] between 0.80 and 0.84, and Mg# [Mg/(Mg + Fe 2+ )] between 0.54 and 0.72. The total platinum-group-element (PGE) contents in chromitites are relatively low (≤170 ppb), although they may locally be higher (up to 1059 ppb). The IPGE (Os, Ir and Ru) predominate over the PPGE (Rh, Pt and Pd). The PGM assemblage, consistent with the geochemical data, is dominated by Laurite and Os–Ir–Ru alloys that occur both as single and composite grains, generally less than 15 μm in size. Laurite has a wide range of Os-for-Ru substitution [Ru/(Ru + Os): 0.42–0.99], whereas primary alloys are enriched in Ru (up to 73.80 wt.%). Some Laurite crystals exhibit an anomalous pattern of zoning. Such zoning requires an inversion of the normal T– f (S 2 ) trend in magmatic systems, and is herein considered to be due to postmagmatic processes. Some Ru-rich alloy grains contain relatively high Rh and Pt abundances, similar to those of residual sulfides in mantle peridotites. This feature suggests that these alloys may represent residual PGM phases from earlier episodes of melt extraction from the severely depleted mantle unit of the Pindos ophiolite complex. Combined compositional data indicate that the Milia chromitites formed from a hydrous boninitic melt in a suprasubduction-zone environment.
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The podiform chromitites in the Dagküplü and Kavak mines, Eskisehir ophiolite (NW-Turkey): Genetic implications of mineralogical and geochemical data
Geologica Acta, 2009Co-Authors: Ibrahim Uysal, Oskar Thalhammer, Federica Zaccarini, M. Tarkian, M.b. Sadiklar, Giorgio GarutiAbstract:Mantle tectonites from Eskisehir (NW-Turkey) include high-Cr chromitites with limited variation of Cr#, ranging from 65 to 82. Mg# ratios are between 54 and 72 and chromite grains contain up to 3.71 wt% Fe2O3 and 0.30 wt% TiO2. PGE contents are variable and range from 109 to 533 pbb. Chondrite-normalized PGE patterns are flat from Os to Rh and negatively sloping from Rh to Pd. Total PGE contents and low Pd/Ir ratios (from 0.07 to 0.41) of chromitites are consistent with typical ophiolitic chromitites. Chromite grains contain a great number of solid inclusions. They comprise mainly of highly magnesian (Mg# 95-98) mafic silicates (olivine, amphibole and clinopyroxene) and base-metal sulfide inclusions of millerite (NiS), godlevskite (Ni7S6), bornite (C5FeS4) with minor Ni arsenides of maucherite (Ni11As8) and orcelite (Ni5-xAs2), and unnamed Cu2FeS3 phases. Heazlewoodite, awaruite, pyrite, and rare putoranite (Cu9Fe,Ni9S16) were also detected in the matrix of chromite as secondary minerals. Laurite [(Ru,Os)S2] was the only platinum-group minerals found as primary inclusions in chromite. They occur as euhedral to subhedral crystals trapped within chromite grains and are believed to have formed in the high temperature magmatic stage during chromite crystallization. Laurite has limited compositional variation, range between Ru0.94Os0.03Ir0.02S1.95 and Ru0.64Os0.21Ir0.10S1.85, and contain up to 1.96 at% Rh and 3.67 at% As. Close association of some Laurite grains with amphibole and clinopyroxene indicates crystallization from alkali rich fluid bearing melt in the suprasubduction environment. The lack of any IPGE alloys, as well as the low Os-content of Laurite, assumes that the melt from which chromite and Laurite were crystallized had relatively high fS2 but never reached the fS2 to crystallize the erlichmanite. The presence of millerite, as primary inclusions in chromite, reflects the increasing fS2 during the chromite crystallization.
Kreshimir Nenadovitch Malitch - One of the best experts on this subject based on the ideXlab platform.
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Sulfur isotope composition of secondary Ru-Os-Ir sulfides and sulfoarsenides from the Verkh-Neivinsk dunite-harzburgite massif (Middle Urals)
Vestnik of Geosciences, 2020Co-Authors: V. V. Murzin, Kreshimir Nenadovitch Malitch, Geochemistry Ub Ras, I. Yu. Badanina, A. V. Ignatiev, T. A. VelivetskayaAbstract:Limited data on the isotopic composition of sulfur of platinum-group minerals (PGM) are reported in the literature. The LA-ICP-MS method was used to study the S-isotopic composition of Ru-Os-Ir sulfides and sulfoarsenides forming secondary corrosion and superimposed PGM assemblages from placer deposit of the East Shishim river occurring within the Verkh-Neivinsky dunite-harzburgite massif. Sulfides from corrosion PGM assemblage (Laurite, As-containing Laurite) replace the grains of primary native Os and Ru minerals, forming rims on them. PGM from superimposed assemblage (Laurite, irarsite, tolovkite, etc.) originated on the surface of Laurite rims that form part of corrosion PGM assemblage. Variations in 34S values of secondary PGM (from –4.6 to 7.6 ‰) are more pronounced than those for Laurite and erlichmanite from primary PGM assemblage (i. e., 0.2—2.3 ‰). The results are consistent with a model of the ultramafic massif ascending to the surface and subsequent water metamorphism during the tectonic flow and crust-mantle interaction. At the initial stage of metamorphism, mantle sulfur is present, isotopically lightened under oxidation conditions and temperature reduction. The final stage is marked by sulfur of the metamorphic fluid formed by the involvement of isotopically-heavy sulfur derived from the host sedimentary rocks.
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Laurite and zircon from the Finero chromitites (Italy): New insights into evolution of the subcontinental mantle
Ore Geology Reviews, 2017Co-Authors: Kreshimir Nenadovitch Malitch, Vladimir V Knauf, I. Yu. Badanina, William L. Griffin, Suzanne Y. O'reilly, Elena Belousova, Norman J. PearsonAbstract:Abstract Chromitites enclosed within metasomatised Finero phlogopite peridotite (FPP) contain accessory platinum-group minerals, base metal (BM) sulfides, baddeleyite, zircon, zirconolite, uraninite and thorianite. To provide new insights into mantle-crustal interaction in the Finero lithosphere this study evaluates (1) the mineral chemistry and Os-isotope composition of Laurite, (2) the crystal morphology, internal structure, in-situ U-Pb, trace-element and Hf-isotope data of zircon from two chromitite localities at Alpe Polunia and Rio Creves. The osmium isotope results reveal a resticted range of ‘unradiogenic’ 187 Os/ 188 Os values for Laurite at Alpe Polunia (0.1247–0.1251, mean 0.1249 ± 0.0001). Re-Os model ages (T RD ) of Laurite reflect an Early Paleozoic partial melting event ( ca 450 Ma or older), presumably before the Variscan orogeny. The Os isotopic composition of Laurite/chromitite probably preserves their mantle signature and was not affected by later metasomatic processes. U-Pb and Hf-isotope data show that the Finero chromitites have distinct zircon populations with peculiar morphology, internal cathodoluminescence textures, trace-element composition and an overall U-Pb age span from ∼310 Ma to 190 Ma. Three age peaks at Rio Creves (220 ± 4 Ma, 234.2 ± 4.5 Ma and 277.5 ± 3.2 Ma) are consistent with a prolonged formation and multistage zircon growth, in contrast to the common assumption of a single metasomatic event during chromitite formation. The trace-element signatures of zircons are comparable with those of mantle-derived zircons from alkaline ultramafic rocks, supporting the carbonatitic nature of the metasomatism. Hf-isotope compositions of the Finero zircons, with eHf values ranging mainly from −3 to +1, are consistent with crustal input during metasomatism and could indicate that the parental melts/fluids were derived from a relatively old source; the minimum estimates for Hf model ages are 0.8–1.0 Ga. Our findings imply that mantle rocks and metasomatic events at Finero have a far more complex geological history than is commonly assumed.
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closed system behaviour of the re os isotope system recorded in primary and secondary platinum group mineral assemblages evidence from a mantle chromitite at harold s grave shetland ophiolite complex scotland
Ore Geology Reviews, 2016Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard Lord, E A Belousova, Thomas MeiselAbstract:This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of Laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by Laurite, Osrich Laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting Laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained Laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.
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Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland)
Mineralogy and Petrology, 2013Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard A. Lord, Thomas C. MeiselAbstract:In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by Laurite and osmian iridium, with subordinate Laurite + osmian iridium + iridian osmium and rare Laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated Laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002 ) providing unequivocal information on conditions of their genesis, with the upper thermal stability of Laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f (S_2) of 10^−0.39–10^−0.07.
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Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold's Grave (Shetland Ophiolite Complex, Scotland)
Mineralogy and Petrology, 2013Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard Lord, Thomas MeiselAbstract:In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by Laurite and osmian iridium, with subordinate Laurite + osmian iridium + iridian osmium and rare Laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated Laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of Laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S2) of 10−0.39–10−0.07.
Thomas Meisel - One of the best experts on this subject based on the ideXlab platform.
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closed system behaviour of the re os isotope system recorded in primary and secondary platinum group mineral assemblages evidence from a mantle chromitite at harold s grave shetland ophiolite complex scotland
Ore Geology Reviews, 2016Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard Lord, E A Belousova, Thomas MeiselAbstract:This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of Laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by Laurite, Osrich Laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting Laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained Laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.
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Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold's Grave (Shetland Ophiolite Complex, Scotland)
Mineralogy and Petrology, 2013Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard Lord, Thomas MeiselAbstract:In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by Laurite and osmian iridium, with subordinate Laurite + osmian iridium + iridian osmium and rare Laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated Laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of Laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S2) of 10−0.39–10−0.07.
Inna Yu. Badanina - One of the best experts on this subject based on the ideXlab platform.
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closed system behaviour of the re os isotope system recorded in primary and secondary platinum group mineral assemblages evidence from a mantle chromitite at harold s grave shetland ophiolite complex scotland
Ore Geology Reviews, 2016Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard Lord, E A Belousova, Thomas MeiselAbstract:This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of Laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by Laurite, Osrich Laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting Laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained Laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.
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Closed-system behaviour of the Re-Os isotope system recorded in primary and secondary platinum-group mineral assemblages : evidence from a mantle chromitite at Harold's Grave (Shetland Ophiolite Complex, Scotland)
'Elsevier BV', 2016Co-Authors: Inna Yu. Badanina, Malitch, Kreshimir N, Lord, Richard A, Belousova, Elena A, Meisel, Thomas CAbstract:This study evaluates in detail the mineral chemistry, whole-rock and mineral separate Os-isotope compositions of distinct platinum-group mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of Laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by Laurite, Os-rich Laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' ¹⁸⁷Os/¹⁸⁸Os values for coexisting Laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' ¹⁸⁷Os/¹⁸⁸Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk ¹⁸⁷Os/¹⁸⁸Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained Laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re-Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a supra-subduction zone setting shortly before obduction.12 page(s
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Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland)
Mineralogy and Petrology, 2013Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard A. Lord, Thomas C. MeiselAbstract:In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by Laurite and osmian iridium, with subordinate Laurite + osmian iridium + iridian osmium and rare Laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated Laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002 ) providing unequivocal information on conditions of their genesis, with the upper thermal stability of Laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f (S_2) of 10^−0.39–10^−0.07.
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Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold's Grave (Shetland Ophiolite Complex, Scotland)
Mineralogy and Petrology, 2013Co-Authors: Inna Yu. Badanina, Kreshimir Nenadovitch Malitch, Richard Lord, Thomas MeiselAbstract:In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by Laurite and osmian iridium, with subordinate Laurite + osmian iridium + iridian osmium and rare Laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated Laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of Laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S2) of 10−0.39–10−0.07.
Peter Charles Fisher - One of the best experts on this subject based on the ideXlab platform.
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Inclusions in an isoferroplatinum nugget from the Freetown Layered Complex, Sierra Leone
Mineralogical Magazine, 2018Co-Authors: John F. W. Bowles, Hazel Margaret Prichard, Saioa Suárez, Peter Charles FisherAbstract:ABSTRACTInclusions of platinum-group minerals (PGM) within alluvial isoferroplatinum nuggets from the Freetown Peninsula, Sierra Leone, are aligned with their shape determined by the structure of their host. The edges of the majority of the inclusions lie at 0°, 45° or 90° to external crystal edges of the nugget which shows that the inclusions are not randomly oriented earlier minerals incorporated within their host. The inclusions are later infills, probably formed at the surface of the nugget during growth and subsequently enclosed by the growing nugget. PGM on the present surface of the nugget represent the last stage of this partnership. A single nugget containing abundant inclusions is described here but similar features are observed in other nuggets from the same area. The inclusions contain Laurite (RuS2), irarsite–hollingworthite (IrAsS–RhAsS), Pd–Te–Bi–Sb phases, Ir-alloy, Os-alloy, Pd-bearing Au, an Rh–Te phase, Pd–Au alloy and Pd–Pt–Cu alloy. PGM found on the nugget surface include Laurite, irarsite and cuprorhodsite (CuRh2S4). The Pd–Te–Bi–Sb phases may include Sb-rich keithconnite (Pd20S7) and compositions close to the kotulskite–sobolevskite solid-solution series (PdTe–BiTe). Textural evidence suggests that formation of the nuggets began with the isoferroplatinum host and the voids were filled starting intergrowths of Laurite and irarsite–hollingworthite with both Laurite and irarsite–hollingworthite often showing compositional zonation and each of them replacing the other. Filling of the voids probably continued with Pd-Cu-bearing gold, Sb-rich keithconnite (Pd,Pt)20.06(Te,Sb,Bi)6.94, keithconnite, telluropalladinite Pd9(Te,Bi)4, RhTe and finally Ir-alloy and then Os-alloy. The nuggets are thought to be neoform growths in the organic- and bacterial-rich soils of the tropical rain forest cover of the Freetown intrusion. The mineralogical assemblage in the layered gabbros of the intrusion has been previously shown to differ from the alluvial assemblage in the rivers and these inclusions, not seen in Pt3Fe in the unaltered rocks, add a further item to the catalogue of differences.
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Laurite and Associated PGM In the Stillwater Chromitites: Implications For Processes of Formation, and Comparisons With Laurite In the Bushveld and Ophiolitic Chromitites
The Canadian Mineralogist, 2017Co-Authors: Hazel Margaret Prichard, Sarah-jane Barnes, Peter Charles Fisher, Philippe Pagé, Michael L. ZientekAbstract:Abstract Chromitites found in layered intrusions and ophiolite complexes are generally enriched in platinum-group elements (PGE), especially IPGE ( i.e ., Ir, Os, Ru)-bearing platinum-group minerals (PGM), and the chromitites are usually poor in base metal sulfide (BMS) minerals. The most common PGM observed is Laurite [Ru(Os,Ir)S 2 ], but how the Laurite formed is not clearly understood. To address this problem we compare the differences in the composition and shape of PGM in the nine chromite layers (A to K) in the Stillwater Complex, Montana and then extend the study to examine Laurites from the Bushveld Complex and ophiolites. The most common PGM in the Stillwater chromitites is Laurite, predominantly enclosed in chromite grains. In a few cases the Laurite is accompanied by rarer and smaller PGM, including malanite [CuPtRh(±Ir)S] and Pt-Pd-sulfides. Interstitial to the chromite grains the PGM assemblage is quite different, dominantly PPGE ( i.e ., Pt, Pd, and Rh)-bearing, including Pd-Pb, Pt-Pd tellurides, sperrylite, platarsite, minor Laurite, and one grain of Pd-Ge. The PGM grains enclosed in chromite formed by a different mechanism to the PGM grains outside chromite. During the crystallization of the chromite the magma was sulfide undersaturated and Ru, Os, Ir, and Rh partitioned into chromite thereby enriching the chromitite layers in IPGE. As the cumulate pile cooled, the fractionated silicate liquid became saturated in a BMS liquid and this migrated among the chromite grains. With further cooling the chromite grains sintered to form larger grains and in some case incorporated small grains of the BMS, which was converted to Laurite by the exchange of IPGE and Fe plus Ni between the chromite and the BMS. In contrast, the BMS that was not included in chromite exsolved to form pentlandite, pyrrhotite, and PGM. The shape and composition of the PGM within the chromite grains in the Stillwater chromitite layers is not uniform. Upper and lower layers contain Laurites with rounded shapes and an Os content of 7–8%. In the sulfide inclusion-poor middle G layer, the Laurites have 5% Os and a predominantly euhedral shape. It is likely that both rounded and euhedral Laurites formed by subsolidus ejection of PGE from the chromite as it cooled and recrystallized. The rounded Laurite formed in a more BMS and S-rich environment, whereas the euhedral Laurite formed in an S-poor environment. Traces of Rh in Laurite, PPGM, and BMS inclusions associated with Laurite are most abundant in the uppermost layer K, suggesting that the upper-layer chromitites contained more PPGE in solid solution on crystallization. The average size of the Laurite grains increases upwards from an average area of 6 μm 2 in layer A to 21 μm 2 in layer K. The larger size of the Laurites from higher layers in the intrusion may be the result of them having had a longer period to cool, further from the basal contact. Rutile inclusions are most abundant in chromitite layer B and could be the result of a greater degree of contamination of the magma in the lower layers. Comparison of the shape of Stillwater Laurites with those in the Bushveld Complex chromitites reveals similarities with the Bushveld chromitites, as they also contain both euhedral and rounded Laurites that are commonly associated with smaller PPGM. Ophiolitic Laurites entirely enclosed in chromite are predominantly euhedral and sometimes zoned. Chromitites from ophiolites are generally PPGE-poor and although ophiolitic Laurites also form composite PGM with other smaller PGM, these are usually Os- and Ir-rich rather than PPGE-rich. These Laurites have a more variable and greater range of Os concentrations than those from the Stillwater and Bushveld Complexes. Most ophiolitic Laurites probably formed by crystallizing directly from magma, but it is possible that some formed by diffusing from the chromite in a low f S 2 environment.
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PGM in the stillwater chromitites and implications for the magmatic processes that formed the ultramafic part of the stillwater complex
2014Co-Authors: Hazel Margaret Prichard, Sarah-jane Barnes, Peter Charles Fisher, Philippe Pagé, Michael L. ZientekAbstract:The distribution of PGM provides insights into the processes forming PGE deposits. PGM have been located in 8 chromitite layers from basal layer A to top layer Kin the ultramafic units below the economic PGE-rich JM reef in the Stillwater complex, USA. Within chromite grains most PGM are Laurite, with rarer, smaller PPGM including malanite [CuPtRh(+/-lr)SJ and Pt-Pd-sulfides. Interstitial to chromite grains PPGM dominate including Pd-Pb, PtPdtellurides, sperrylite, platarsite, minor Laurite and one grain of Pd-Ge. The Laurite shape reflects formation and varies from mainly round in lower and upper layers, to euhedral in the thicker middle G layer. The Os content of Laurite is lowest and silicate inclusions with Laurites are most abundant in layer G. Rutile inclusions are most abundant in layer B. Trace Rh in Laurite and PPGM with Laurite are most abundant in the uppermost layer Kand Laurite size increases upwards. Each layer has a distinct PGM assemblage. Within chromite grains IPGE- and PPGE-PGM are greatest in the upper layers suggesting increasing PGE content in the magma upwards whereas interstitial PGM associated with BMS indicate sulphur saturation occurred between chromite grains in the lower layers.
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Compositional variation of Laurite at Union Section in the western Bushveld Complex
South African Journal of Geology, 1999Co-Authors: Wolfgang Maier, Hazel Margaret Prichard, Peter Charles Fisher, Sarah-jane BarnesAbstract:One hundred and forty five grains of Laurite in polished sections of samples from one borehole through the major chromitite layers and some chromite-bearing silicate rocks of the Lower and Critical Zones of the western Bushveld Complex at Union Section have been located and analysed by scanning electron microscope. Ninety per cent by number of Laurite grains are included within chromite, with the remainder being located on chromite-silicate grain boundaries, and in interstitial silicates and sulphides. The composition of Laurite shows considerable variation within individual samples. Furthermore, there is no apparent correlation between whole-rock Ru and Cr contents in our samples, arguing against a model whereby Laurite exsolved from the chromite lattice. Based on a well-defined correlation between whole-rock S, PPGE (Rh+Pt+Pd), and IPGE (Os+Ir+Ru) contents, we favour a mechanism whereby Laurite crystallized from segregating sulphide melt and was subsequently entrapped by growing chromite grains.