Mineralium Deposita

, Volume 45, Issue 5, pp 419–441 | Cite as

The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes

  • Wolfgang D. Maier
  • Sarah-Jane Barnes
  • Arindam Sarkar
  • Ed Ripley
  • Chusi Li
  • Tim Livesey
Article

Abstract

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 23.23 Mt 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 in Tanzania and Burundi. The igneous bodies are up to about 1 km thick and 4 km long. 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 fine-grained 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) and had pronounced negative Nb and Ta anomalies and heavy O isotopic signatures (δ18O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ18O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ34S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the 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 changed with 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 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 10 s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.

Supplementary material

126_2010_280_MOESM1_ESM.xls (36 kb)
Supplementary Table 1Reproducibility and precision of ICP-MS analyses (XLS 36 kb)
126_2010_280_MOESM2_ESM.xls (54 kb)
Supplementary Table 2Composition of Kabanga olivines (XLS 54 kb)
126_2010_280_MOESM3_ESM.xls (22 kb)
Supplementary Table 3Composition of Kabanga and Luhuma orthopyroxenes (XLS 22 kb)
126_2010_280_MOESM4_ESM.xls (20 kb)
Supplementary Table 4Compositions of Kabanga and Luhuma plagioclase (XLS 19 kb)
126_2010_280_MOESM5_ESM.xls (39 kb)
Supplementary Table 5Composition of Kabanga chromites (XLS 39 kb)
126_2010_280_MOESM6_ESM.xls (128 kb)
Supplementary Table 6Major and trace element concentrations in Kabanga silicate rocks (XLS 156 kb)
126_2010_280_MOESM7_ESM.xls (48 kb)
Supplementary Table 7Composition of Kabanga sedimentary rocks (XLS 48 kb)
126_2010_280_MOESM8_ESM.xls (44 kb)
Supplementary Table 8Modeling of Kabanga magmas (XLS 44 kb)
126_2010_280_MOESM9_ESM.xls (61 kb)
Supplementary Table 9Oxygen and sulfur isotopes of minerals and whole-rocks from ore-bearing intrusions in the Kabanga area (XLS 61 kb)
126_2010_280_MOESM10_ESM.xls (24 kb)
Supplementary Table 10Sulfur and oxygen isotopes of sedimentary country rocks in the Kabanga area (XLS 23 kb)
126_2010_280_MOESM11_ESM.pdf (2.3 mb)
Supplementary Fig. 1Photomicrographs of Kabanga sedimentary rocks. (A) Sulfidic andalusite–muscovite schist, MNB, KN01-05, 538 m. (B) Banded pelite, MNB, KN01-01, 1,645 m. (C) Bleached contact zone in hornfels adjacent to ultramafic rock, Kabanga Main, KN05-01, 138 m. (PDF 2,334 kb)
126_2010_280_MOESM12_ESM.pdf (21 kb)
Supplementary Fig. 2CIPW norms of Kabanga and Luhuma rocks. Silicate rocks from Kapalagulu and Musongati are shown for comparison (Maier et al. 2008a). (PDF 20 kb)
126_2010_280_MOESM13_ESM.pdf (20 kb)
Supplementary Fig. 3(A, B) Compositional variation of Kabanga orthopyroxenes, plotting Al2O3 and Cr2O3 vs Mg#. Dashed line represents field of Uitkomst orthopyroxenes (Maier et al. 2004) and shaded field represents compositional range of Bushveld orthopyroxenes (data from Teigler and Eales 1996, The Lower and Critical Zones of the western limb of the Bushveld Complex, as indicated by the Nooitgedacht boreholes. Geol Surv S Afr Bull 111: 126p). (C, D) Compositional variation of Kabanga chromites plotting Fe# and Ni vs Mg#. Bushveld Lower Zone (LZ) data are from Teigler and Eales (1996). (PDF 20 kb)
126_2010_280_MOESM14_ESM.pdf (16 kb)
Supplementary Fig. 4Variation in trace element ratios and S within the MNB body (as intersected by drill core KN01-08). Yellow-shaded blocks indicate observed ranges within units. See Fig. 7 of paper for legend. (PDF 16 kb)
126_2010_280_MOESM15_ESM.pdf (15 kb)
Supplementary Fig. 5Histogram of whole-rock δ34S in different intrusions at Kabanga and Luhuma, and their sedimentary host rocks. (PDF 15 kb)
126_2010_280_MOESM16_ESM.docx (12 kb)
ESM 1(DOCX 12 kb)
126_2010_280_MOESM17_ESM.docx (13 kb)
ESM 2(DOCX 12 kb)
126_2010_280_MOESM18_ESM.xls (134 kb)
ESM 3(XLS 133 kb)

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Wolfgang D. Maier
    • 1
  • Sarah-Jane Barnes
    • 2
  • Arindam Sarkar
    • 3
  • Ed Ripley
    • 3
  • Chusi Li
    • 3
  • Tim Livesey
    • 4
  1. 1.Department of GeosciencesUniversity of OuluOuluFinland
  2. 2.Sciences de la TerreUniversite du Quebec a ChicoutimiSaguenayCanada
  3. 3.Department of Geological SciencesIndiana UniversityBloomingtonUSA
  4. 4.Barrick Gold CorporationTorontoCanada

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