Ni-Co sulphide segregation in the Mamb pyroxenite intrusion, Cameroon

Abstract

The amphibole metapyroxenite intrusion from Mamb (Cameroon) consists of enstatite, diopside-augite, edenite, and traces of plagioclase, biotite and rutile. It contains notable amounts of sulphides (pyrrhotite, pentlandite, chalcopyrite, pyrite and Co-rich violarite). The sulphide phase segregated during magma crystallization as immiscible droplets ( 1 mm) included in growing ferromagnesian minerals or aggregated in the silicate crystal mush to form an interstitial phase. However, observed sulphide assemblages are likely to result from low-T ( 300 8C) re-equilibration of high-T monosulphide solid solutions, more particularly during the Pan-African metamorphic event. The Mamb amphibole pyroxenite intrusion is tentatively considered as a potential source for Ni and Co mineralization.

Petrography and geochemistry of weathered pyroxenite boulders from Mamb, Ngong and Nkolmbong (Pan-African Yaoundé group, Cameroon): tracking the Ni, Co, Cr and Cu mineralization processes during the alteration of ultramafic rocks

“…The weathered metric to decametric pyroxenite boulders studied here, outcrop in the High Pressure-Medium Temperature garnet micaschists associated with metagabbro at Mamb and Ngong (Fig. 2a), (Nkoumbou et al 2009;Toteu et al 2006) and the migmatitic gneisses at Nkolmbong (Fig. 2b) metamorphosed at HP-HT in the granulite facies (Barbey et al 1990).…”

“…3d). The accessory minerals are biotite, plagioclase and opaque minerals (sulphides, according to Nkoumbou et al 2009); the latter can be found as inclusions in pyroxene or as interstitial crystals between pyroxene and amphibole (Fig. 3d).…”

“…However, the weathering processes at the scale of the mafic and ultramafic boulders are still poorly understood. Pyroxenite rocks are the suitable materials for such a study and are found in Cameroon in the Paleoproterozoic greenstone belt of the Nyong unit (Ebah Abeng et al 2012) and in the Neoproterozoic formations of the Yaoundé group (Nkoumbou et al 2009(Nkoumbou et al , 2014.…”

Trace-Element Geochemistry of Sulfides in Upper Mantle Lherzolite Xenoliths from East Antarctica

“…Such veinlets contain minerals such as apatite, calcite, Fe-rich dolomite, ferroan magnesite, Ba-Ti-rich and Ba-Ti-poor mica, and humite-group minerals (see Kogarko et al [9]). We found that such veinlets also contain a sulfide phase that is significantly different from the BMS in the lherzolite xenolith matrix, i.e., skeletal crystals of Co-millerite + Co-violarite (see Hughest et al [40], and Nkoumbou et al [46]). The formation temperature for such assemblages was just above 350 °C [52], which was close to the blocking temperatures of the subsolidus reactions in the BMS from the lherzolite marix.…”

“…Compositions of the studied BMS from the lherzolite matrix lie within both the mss and the mss + Liq fields of the Fe-(Ni + Co)-S system (Figure 6). After mss (Fe-rich and Ni-poor) begins to crystallize at 1190 • C from Ni-Cu-Fe sulfide liquid, residual sulfide liquid gradually enriches in Ni relative to Fe [43][44][45][46]. Therefore, in the S-rich of the studied lherzolites system at the suggested ambient temperatures suggested for most varieties of those lherzolites (1030-1180 • C), mss of an overall pyrrhotite composition is the only possible solid phase coexisitng with the Ni-Cu-Fe sulfide liquid [21].…”