The Kabanga Ni Sulfide Deposits, Tanzania: A Review of Ore-Forming Processes
W. D. Maier, S.-J. Barnes, E. M. Ripley, 2011. "The Kabanga Ni Sulfide Deposits, Tanzania: A Review of Ore-Forming Processes", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current measured and indicated mineral resources of 37.2 million metric tons (Mt) at 2.63 percent Ni and inferred mineral resources of 21 Mt at 2.6 percent Ni (Dec. 2010, Xstrata.com). The sulfides occur in sill-like and chonolithic ultramafic-mafic intrusions that form part of the approximately 500-km long, 1.4-Ga Kabanga-Musongati-Kapalagulu mafic-ultramafic igneous belt, within the Karagwe-Ankole belt in northwestern Tanzania and adjacent Burundi. The intrusions are up to ∼1 km thick and 4 km long and crystallized from several pulses of compositionally distinct magma emplaced into sulfide-bearing pelitic schists. The first magma pulse consisted of siliceous high magnesium basalt with approximately 13 percent MgO. It formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg no. opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotope signatures (δ18O 6–8), consistent with ∼20 percent contamination of primitive picrite with the sulfidic schists. Subsequent magma pulses were more magnesian, containing approximately 14 to 15 percent MgO, and less contaminated (e.g., δ18O 5.1–6.6). They injected into the earlier sills, forming medium-grained harzburgites and orthopyroxenites (Fo83–89, Mg no. Opx86–89), and magmatic breccias consisting of gabbronorite-orthopyroxenite fragments within an olivine-rich matrix.
The Kabanga intrusions contain abundant disseminated sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of the Kabanga North and Kabanga Main intrusions, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. Postemplacement tilting of the intrusions caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.
Compared to other sulfide ores that segregated from magnesian basalts (e.g., Jinchuan, Pechenga, Raglan), most Kabanga sulfides have low Ni (<1–3%), Cu (∼0.1–0.4%), and PGE contents (<<1 ppm), and high Ni/Cu (5–15) ratios. Higher metal contents (∼5% Ni, 0.8% Cu, 10 ppm PGE) are found in only one unit from Kabanga North. The observed metal contents are consistent with segregation of magmatic sulfides from fertile to strongly metal-depleted magmas, at intermediate to very low mass ratios of silicate to sulfide liquid (R factors) of approximately 10 to 400. The sulfides have heavy S isotope signatures (δ34Swr = 10–24) that broadly overlap with those of the country-rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe-Ankolean sedimentary sequence. Based on the relatively homogeneous distribution of disseminated sulfides in many of the intrusive rocks we propose that the 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 changed with time. The early magmas assimilated smaller quantities of country rocks but, in addition, sulfur was selectively assimilated, either by means of a volatile phase or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The unusually large degree of crustal contamination and the low R factors render Kabanga an end member in the spectrum of magmatic Ni sulfide ores.
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Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the