Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus

Hulbert, L.; Gruenewaldt, G. von

doi:10.2113/gsecongeo.80.4.872

Cited by 151 publications

(43 citation statements)

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“…Field observations and petrography indicate that the chromitites form immediately below a horizon of major magma replenishment, supporting open-system behaviour in the Dawros magma chamber. Evidence is also preserved in the chromitite crystal size distribution (CSD) data for sub-solidus textural coarsening; an observation consistent with the behaviour of chromitite seams elsewhere (Hulbert & Von Gruenewaldt, 1985;Higgins, 2010;O'Driscoll et al 2010). Cr-spinel compositions in the Dawros Peridotite seams suggest high degrees of melting of an already depleted mantle source, and support the emplacement of the Dawros Peridotite parental magmas within a supra-subduction zone, in line with magma emplacement immediately prior to island arc collision (cf.…”

Section: Introductionsupporting

confidence: 58%

Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

2011

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Chromium-spinels have been widely used as petrogenetic indicators to infer parent melt compositions and the tectonic setting of their formation. This study integrates petrographic, quantitative textural and geochemical analyses of Cr-spinel seams within the Dawros Peridotite, NW Connemara, Ireland to determine the composition of their parental magmas. Calculation of Cr no. (Cr/(Cr + Al)) (0.50-0.77) values and TiO 2 (0.18-0.36 wt %) contents of the Cr-spinel seams, coupled with an estimation of the Al 2 O 3 and TiO 2 contents (∼11.86 wt % and ∼0.39 wt %, respectively) of their parental melts, indicates that they probably formed from boninitic melts sourced from a highly depleted mantle. This implies that the Cr-spinel seams formed in a supra-subduction zone undergoing high degrees of partial melting. The Cr-spinel data support tectonic models for the formation of the Dawros Peridotite (and Connemara Metagabbro-Gneiss Complex) during island arc collision, immediately prior to Grampian orogenesis at ∼470 Ma. The occurrence of the Dawros chromitite seams at the approximate transition between the lower harzburgite sequence and the upper lherzolite sequence bears marked similarities to the positions of such seams in larger anorogenic layered mafic-ultramafic intrusions, and implies that the Dawros Peridotite behaved as an open-system magma chamber.

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Section: Introductionsupporting

confidence: 58%

Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

2011

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“…Some of the detrital grains of magnesiochromite display very high levels of Mg, attaining 16.6 wt % MgO (with 48.6% Cr 2 O 3 , Table 1). This level of magnesium enrichment is unusual and even exceeds the highest contents reported from the Lower Zone in the Bushveld complex, South Africa (up to~14% MgO and 57% Cr 2 O 3 [28] or from the Monchepluton layered complex, Russia (~14% MgO and 56% Cr 2 O 3 ) [29]. On the other hand, these grains are substantially enriched in Ti (up to 1.3% TiO 2 ; Table 1), which could reflect the presence of exsolved, submicrometric lamellae rich in the ulvöspinel component.…”

Section: Potential Provenance Of Pgm In the Sisim Placer Zonementioning

confidence: 60%

PGE–(REE–Ti)-Rich Micrometer-Sized Inclusions, Mineral Associations, Compositional Variations, and a Potential Lode Source of Platinum-Group Minerals in the Sisim Placer Zone, Eastern Sayans, Russia

2018

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Abstract:We report the results of a mineralogical investigation of placer samples from the upper reaches of the Sisim watershed, near Krasnoyarsk, in Eastern Sayans, Russia. The placer grains are predominantly Os-Ir-(Ru) alloys (80%) that host various inclusions (i.e., platinum-group elements (PGE)-rich monosulfide, PGE-rich pentlandite, Ni-Fe-(As)-rich laurite, etc.) and subordinate amounts of Pt-Fe alloys. Analytical data (wavelength-and energy-dispersive X-ray spectroscopy) are presented for all the alloy minerals and the suite of micrometer-sized inclusions that they contain, as well as associated grains of chromian spinel. The assemblage was likely derived from chromitite units of the Lysanskiy mafic-ultramafic complex, noted for its Ti-(V) mineralization. In the Os-Ir-(Ru) alloys, the ratio Ru/Ir is ≤1, Ir largely substitutes for Os, and compositional variations indicate the scheme [Ir + Ru] → 2Os. In contrast, in the laurite-erlichmanite series, Ir and Os are strongly and positively correlated, whereas Ir and Ru are negatively correlated; Ru and Os are inversely correlated. These compositions point to the scheme [Os 2+ + 2Ir 3+ + ] → 4Ru 2+ or alternatively, to Os 2+ + Ir 2+ → 2Ru 2+ . We deduce a potential sequence of crystallization in the parental rock and address the effects of decreasing temperature and increasing fugacity of sulfur and arsenic on the assemblage. Inclusions of Ti-rich minerals in the alloy grains are consistent with the Lysanskiy setting; the complete spectrum of chromite-magnesiochromite compositions indicates that an important part of that complex was eroded. A localized fluid-dominated micro-environment produced the unique association of laurite with monazite-(Ce), again considered a reflection of the special attributes of the Lysanskiy complex.

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“…After Benoit et al (1999), Bosch et al (2004) and Python et al (2007). Naldrett, 2008) and may be sintered (Hulbert & von Gruenewaldt, 1985). The latter process may explain the absence of entrapped basaltic glass in the host chromite.…”

Section: à3mentioning

confidence: 98%

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

Borisova¹,

Ceuleneer²,

Kamenetsky³

et al. 2012

Journal of Petrology

109

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To decipher the petrogenesis of chromitites from the MohoTransition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of $500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1^50 mm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr þAl) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f O2 , of $3 log units below the quartz^fayalite^magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 9508C at subsolidus conditions, were overgrown by more oxidized host chromite (f O2 % QFM) in association with microcrystal inclusions of silicates (plagioclase An 86 , clinopyroxene, and pargasite) that were formed between 950 and 10508C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to $0·18C a À1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional

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Textural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Potgietersrus

Cited by 151 publications

References 0 publications

Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

PGE–(REE–Ti)-Rich Micrometer-Sized Inclusions, Mineral Associations, Compositional Variations, and a Potential Lode Source of Platinum-Group Minerals in the Sisim Placer Zone, Eastern Sayans, Russia

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

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