Metal enrichment in refertilized subcontinental lithospheric mantle: insight from the ultramafic xenoliths from the volcanic rocks of the Oku Volcanic Group (Cameroon)

Sulfides hosted by peridotites from Befang (Oku Volcanic Group, Cameroon) xenolith suite can play an important role in tracking migration of strategic metals such as Au, Ag, or Cu through the subcontinental lithospheric mantle (SCLM) beneath the Central African Orogenic Belt. Most peridotites are lherzolites, which are subdivided into two main groups differing by crystallographic preferred orientation (CPO) and rare-earth element (REE) composition of clinopyroxene. Group I is characterized by light REE (LREE)-depleted clinopyroxene (re-)crystallized during percolation of metasomatic melt. Group II contains LREE-enriched clinopyroxene with the CPO representing deformation before percolation of the melt (Tedonkenfack et al., 2021). Lherzolites of group I  are interpreted to be metasomatized by MORB-like melts coming from  Depleted MORB Mantle (DMM). Peridotites of  group II are interpreted to be a protolith for the group I ones.

The sulfides form oval to slightly elongated grains enclosed usually in orthopyroxene, or rarely in clinopyroxene and olivine. They are composed of pyrrhotite (Po), pentlandite (Pn), and chalcopyrite (Ccp). Pyrrhotite is mostly predominant, whereas Pn forms exsolution lamellae in Po or massive crystals separating Po from Ccp. Chalcopyrite is present on the rims of grain or penetrates through the entire grain, occasionally containing cubanite exsolutions. The Group I lherzolites contain more sulfides (up to 0.031 vol.‰), with larger grains (range: 14−250 µm, 57 µm on average) compared to the Group II sulfides (up to 0.002 vol.‰, range: 12−45 µm, 27 µm on average respectively). Sulfides from Group I are richer in Po, and especially Ccp (Po₇₇Pn₁₂Ccp₁₁ on average) compared to Group II (Po₇₂Pn₂₃Ccp₄ on average). Ni/(Ni+Fe) in pyrrhotite from Group I (0.14–0.43) is more heterogeneous compared to group II (0.20–0.37).

Enrichment in Po and Ccp in the Befang Group I xenoliths suggests a significant role of melts in transporting sulfur and metals. Observed refertilization by DMM-derived melts may affect the chalcophile and highly siderophile metal budget of the SCLM. The degree of refertilizaton seems to depend on temperature and therefore is moderate in Befang (up to 0.031 vol.‰) with moderate temperatures of orthopyroxene-clinopyroxene equilibration (938–997°C; Tedonkenfack et al, 2021). In lower temperatures of Opx-Cpx equilibration (810–970°C), we observe higher sulfide abundances (up to 0.062 vol.‰), whereas in higher temperatures (1010–1120°C) lower sulfide abundances (up to 0.00048 vol.‰; Mazurek et al., 2021).

 

This study was supported by the Diamond Grant project 093/DIA/2020/49.

 

References

Mazurek, H., Ciazela, J., Matusiak-Małek, M., Pieterek, B., Puziewicz, J., Lazarov, M., Horn, I., Ntaflos, T.: Metal enrichment as a result of SCLM metasomatism? Insight from ultramafic xenoliths from SW Poland., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15992, https://doi.org/10.5194/egusphere-egu21-15992, 2021

Tedonkenfack SST., Puziewicz J., Aulbach S., Ntaflos T., Kaczmarek M-A., Matusiak-Małek M., Kukuła A., Ziobro M.: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line – a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Contributions to Mineralogy and Petrology, 176: 37.