Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia

Leblanc, Marc

doi:10.2113/gsecongeo.90.7.2028

Cited by 55 publications

(30 citation statements)

References 29 publications

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“…In contrast, the majority of the supra-Moho chromitites are Al-rich and characterized by a variable content of TiO 2 (0.15 wt % to 0.8 wt %). Confirming the previous work by [8,[14][15][16][17]21], the composition of the Alapevsky chromite indicates the coexistence of the Al-rich and Cr-rich chromitites, a common feature of many ophiolitic belts worldwide [22,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50].…”

Section: Chromite Composition and Silicate Inclusion: Their Applicatisupporting

confidence: 85%

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

et al. 2016

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An electron microprobe study has been carried out on platinum-group minerals, accessory phases, and chromite in several chromite deposits of the Alapaevsk ophiolite (Central Urals, Russia) namely the Bakanov Kluch, Kurmanovskoe, Lesnoe, 3-d Podyony Rudnik, Bol'shaya Kruglyshka, and Krest deposits. These deposits occur in partially to totally serpentinized peridotites. The microprobe data shows that the chromite composition varies from Cr-rich to Al-rich. Tiny platinum-group minerals (PGM), 1-10 µm in size, have been found in the chromitites. The most abundant PGM is laurite, accompanied by minor cuproiridsite and alloys in the system Os-Ir-Ru. A small grain (about 20 µm) was found in the interstitial serpentine of the Bakanov Kluch chromitite, and its calculated stoichiometry corresponds to (Ni,Fe) 5 P. Olivine, occurring in the silicate matrix or included in fresh chromite, has a mantle-compatible composition in terms of major and minor elements. Several inclusions of amphibole, Na-rich phlogopite, and clinopyroxene have been identified. The bimodal Cr-Al composition of chromite probably corresponds to a vertical distribution in the ophiolite sequence, implying formation of Cr-rich chromitites in the deep mantle, and Al-rich chromitites close to the Moho-transition zone, in a supra-subduction setting. The presence of abundant hydrous silicate inclusions, such as amphibole and phlogopite, suggests that the Alapaevsk chromitites crystallized as a result of the interaction between a melt enriched in fluids and peridotites. Laurite and cuproiridsite are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at high temperatures. The sulfur fugacity was relatively high to allow the precipitation of Ir-bearing sulfides, but below the Os-OsS 2 buffer. The alloys in the system Os-Ir-Ru are classified as secondary PGM, i.e., formed at low temperature during the serpentinization process. The (Ni,Fe) 5 P phase is the first occurrence of a Ni-phosphide in terrestrial samples. Its composition indicates that it may be a new mineral. However, the small size has, so far, prevented a crystallographic study to support this conclusion.

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Section: Chromite Composition and Silicate Inclusion: Their Applicatisupporting

confidence: 85%

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

et al. 2016

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“…Frequently highCr and high-Al chromitites are found to coexist within a single ultramafic massif containing variably depleted peridotites (e.g., Leblanc 1995;Economou-Eliopoulos 1996;Melcher et al 1997;Proenza et al 1999;Gervilla et al 2005).…”

Section: Introductionmentioning

confidence: 99%

Genesis of chromitites from Korydallos, Pindos Ophiolite Complex, Greece, based on spinel chemistry and PGE-mineralogy

Kapsiotis¹

2013

Jour. Geosci.

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The Pindos Ophiolite Complex, located in northwestern Greece, hosts various chromite deposits of both metallurgical (high-Cr) and refractory (high-Al) type. In Korydallos are encountered both types of chromitites. These are podiform chromitites that have small dimensions and occur sub-concordantly to the hosting peridotites. The Cr-rich chromitites contain magnesiochromite with high Cr# [Cr/(Cr + Al): 0.65-0.68] and low platinum-group element (PGE) contents (294.1 ppb), whereas the Al-rich ones host spinel with low Cr# (0.44-0.48) and elevated total PGE grades (28830 ppb). The former display a nearly flat C1-normalized PGE-pattern, whereas the latter show a positively sloped normalized PGE-pattern. The in situ mineralogical investigation of the Cr-rich chromitites revealed a platinum-group mineral (PGM) assemblage dominated by small (≤ 3 μm) sperrylite, laurite and erlichmanite grains (determined by recalculated qualitative analytical data). Textural relations suggest crystallization under conditions of high fS 2 and fAs and/or low T. The in concentrates mineralogical study of the Al-rich chromitites showed that the PGM assemblage that they host is dominated by Pd-Cu and Pd-Au-Cu alloys. The vast majority of these alloys is associated with abundant secondary BMS (base-metal sulfides) and BMA (base-metal alloys), thus confirming that a sulfide melt scavenged the PGE + Au of the silicate magma from which chromian spinel had already started to crystallize. Both assemblages were affected by an invasion of an oxidizing aqueous fluid in the investigated chromitites. Combined data indicate that the chromitites from the Korydallos area crystallized from a progressively differentiating MORB/IAT melt, produced in a small backarc basin in a supra-subduction zone setting.

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“…However, the presence of both Al-rich and Cr-rich chromitites within one single ophiolite complex has been documented worldwide [26,27,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. This coexistence can be explained with the following models as summarized in [52]: (1) intrusions of different melts derived from differently depleted mantle sources (i.e.,: MORB versus boninite), during the geodynamic evolution of the oceanic lithosphere from the MOR towards the SSZ; (2) mixing of several magmas produced by multi-stage melting of a partially re-fertilized and fluid-metasomatized residual source, in the SSZ; (3) the reaction of a single intrusion of a boninitic parental melt with country-rock peridotites characterized by variable residual composition; (4) fractional precipitation during differentiation of a single batch of magma with initial high-Cr boninitic composition, in the SSZ; and (5) bimodal distribution and vertical zoning with the Cr-rich chromitite located in the deep mantle section and the Al-rich chromitite occurring higher in the succession, close or above the Moho transition.…”

Section: Parental Melt and Chromite Composition: Tectonic Implicationsmentioning

confidence: 99%

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

2016

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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 + Fe 3+ ), 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 TiO 2 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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Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia

Cited by 55 publications

References 29 publications

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

Genesis of chromitites from Korydallos, Pindos Ophiolite Complex, Greece, based on spinel chemistry and PGE-mineralogy

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

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