Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades,with current indicated mineral resources of 23.23Mt at 2.64%Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008). The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt inTanzania and Burundi. The igneous bodies are up to about 1 kmthick and 4 kmlong. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of finegrained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) relatively homogenous distribution of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changedwith time. The heavy δ18O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light δ18O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny and associated synkinematic granite plutonism. This caused solidstate mobilization of ductile sulfides into shear zones, notably along the base of the intrusionswhere sulfide-hornfels breccias and lenses and layers ofmassive sulfidesmay reach a thickness of >10mand can extend for several 10 s to >100maway from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.