Genesis of an exotic platinum-group-mineral-rich and Mg-poor chromitite in the Kevitsa Ni-Cu-platinum-group-elements deposit

• Published in: Mineralogy and petrology Vol. 115; no. 5; pp. 535 - 555
• Main Authors: González-Pérez, Igor, Gervilla, Fernando, González-Jiménez, José M., Kojonen, Kari
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.10.2021; Springer Nature B.V
• Subjects: Amphiboles; Apatite; Augite; Barite; Biotite; Calcium magnesium silicates; Chromite; Copper; Crystallization; Crystallography; Diopside; Earth and Environmental Science; Earth Sciences; Galena; Geochemistry; Ilmenite; Inorganic Chemistry; Iridium; Lamellae; Lava; Magma; Mineral inclusions; Mineralogy; Minerals; Nickel ores; Original Paper; Osmium; Oxides; Pentlandite; Petrogenesis; Platinum; Pyroxenes; Pyrrhotite; Ruthenium; Saturation; Silicates; Solid solutions; Stringers; Mafic-ultramafic layered intrusion; Chromitite; Platinum-group minerals; Kevitsa intrusion; Xenolith
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-021-00751-1

This paper offers the first-ever study of a local chromitite stringer (~ 2 cm wide) enriched in platinum-group elements (PGE) from the Kevitsa Ni-Cu-PGE deposit. Chromite forming this chromitite is rich in Cr 2 O 3 (29.06 ̶ 37.17 wt%) and FeO (FeO total  = 45.88 ̶ 56.06 wt%) and low in MgO (0.04 ̶ 0.91 wt%). The chromitite host an unusual set of mineral inclusions including: (1) platinum-group minerals of the laurite-erlichmanite (RuS 2 -OsS 2 ) solid solution series, irarsite (IrAsS), sperrylite (PtAs 2 ), hollingworthite (RhAsS), anduoite [(Ru, Os)As 2 ], kotulskite (PdTe) and As-, Te-, Os-, and Ru- bearing unidentified phases; (2) exsolution-lamellae of ilmenite and ulvöspinel along with chromite (111) and (100) planes, respectively; (3) silicates including crystallographically-oriented biotite, euhedral pyroxene and irregular-shaped amphiboles which tend to follow chromite growth planes, and (4) Fe-oxides, pyrrhotite, pentlandite, galena, barite, apatite, and native gold. Intercumulus minerals are microcrystalline Mg-rich biotite and minor amphibole. Pyroxenes (diopside) located immediately above the chromitite stringer exhibit higher CaO (23.02 ̶ 25.42 wt%) and Na 2 O (0.16 ̶ 0.55 wt%) than pyroxenes (augite) located below. We suggest that chromite saturation in the mafic melt was caused by local assimilation of a small pelitic xenolith. The xenolith also provided K and H 2 O explaining the occurrence of K-and volatile-rich intercumulus phases. Crystallization of chromite in this environment promoted preferential sequestering of Ir and Ru, and to a lesser extent Os, from the parental magma giving rise to an anomalous iridium-group-PGE (IPGE: Ir, Ru, and Os) enrichment.