Ilmenite generations in kimberlite from Banankoro, Guinea Conakry

Xu, Jinyao; Melgarejo, Joan Carles; Castillo-Oliver, Montgarri; Arqués, Laia; Santamaría, Joan

doi:10.1127/njma/2018/0096

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“…In any case, all the rutile types tend to be replaced by geikielite (ilmenite-group mineral 4), which also replaces the above categories of ilmenite. The composition of the replacing ilmenite in terms of minor elements inherits the composition of the replaced minerals, as noted in other kimberlites [37,39]. The site 73 kimberlite emplacement age has been estimated to be 155 Ma using zircon fissiontrack dating and 190 Ma using K-Ar of phlogopite [8].…”

Section: Paragenetic Sequencementioning

confidence: 99%

Magma Mingling in Kimberlites: Evidence from the Groundmass Cocrystallization of Two Spinel-Group Minerals

Melgarejo

et al. 2020

Minerals

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We present the results of a detailed petrographic study of fresh coherent samples of the Menominee kimberlite sampled at site 73, located in Menominee County, MI, USA. Our objective is to account for its unusual and complex paragenetic sequence. Several generations of olivine, ilmenite, and spinel-group minerals are described. Early olivine and ilmenite are xenocrystic and were replaced or overgrown by primary minerals. Zoned microcrysts of olivine have a xenocrystic core mantled by a first rim in which rutile, geikielite, and spinel s.s. (spinel sensu stricto) cocrystallized. The in situ U–Pb dating of a microcryst of primary rutile yielded 168.9 ± 4.4 Ma, which was interpreted as the age of emplacement. The groundmass consists of olivine, spinel s.s., a magnesian ulvöspinel–ulvöspinel–magnetite (MUM) spinel, calcite, and dolomite. An extremely low activity of Si is suggested by the crystallization of spinel s.s. instead of phlogopite in the groundmass. The presence of djerfisherite microcrysts indicates high activities of Cl and S during the late stages of melt crystallization. The occurrence of two distinct spinel-group minerals (spinel s.s. and qandilite-rich MUM) in the groundmass is interpreted as clear evidence of the mingling of a magnesiocarbonatitic melt with a dominant kimberlitic melt.

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Section: Paragenetic Sequencementioning

confidence: 99%