Comparing orthomagmatic and hydrothermal mineralization models for komatiite-hosted nickel deposits in Zimbabwe using multiple-sulfur, iron, and nickel isotope data
|Author(s)||Hofmann Axel1, Bekker Andrey2, 3, Dirks Paul4, Gueguen Bleuenn5, 6, Rumble Doug3, Rouxel Olivier6|
|Affiliation(s)||1 : Univ Johannesburg, Dept Geol, ZA-2006 Auckland Pk, South Africa.
2 : Univ Manitoba, Dept Geol Sci, Winnipeg, MB R3T 2N2, Canada.
3 : Carnegie Inst Sci, Geophys Lab, Washington, DC 20015 USA.
4 : James Cook Univ, Sch Earth & Environm Sci, Townsville, Qld 4812, Australia.
5 : Univ Brest, UEB, European Inst Marine Studies, F-29280 Plouzane, France.
6 : IFREMER, Ctr Brest, F-29280 Plouzane, France.
|Source||Mineralium Deposita (0026-4598) (Springer), 2014-01 , Vol. 49 , N. 1 , P. 75-100|
|WOS© Times Cited||21|
|Keyword(s)||Komatiite-hosted nickel deposit, Zimbabwe craton, Archean, Sulfur isotopes, Iron isotopes, Nickel isotopes|
|Abstract||Trojan and Shangani mines are low-grade (<0.8 % Ni), komatiite-hosted nickel sulfide deposits associated with ca. 2.7 Ga volcano-sedimentary sequences of the Zimbabwe craton. At both mines, nickel sulfide mineralization is present in strongly deformed serpentinite bodies that are enveloped by a complex network of highly sheared, silicified, and sulfide-bearing metasedimentary rocks. Strong, polyphase structural–metamorphic–metasomatic overprints in both the Trojan and Shangani deposits make it difficult to ascertain if sulfide mineralization was derived from orthomagmatic or hydrothermal processes, or by a combination of both. Multiple S, Fe, and Ni isotope analyses were applied to test these competing models. Massive ores at Shangani Mine show mass-dependent fractionation of sulfur isotopes consistent with a mantle sulfur source, whereas S-isotope systematics of net-textured ore and disseminated ore in talcose serpentinite indicates mixing of magmatic and sedimentary sulfur sources, potentially via post-magmatic hydrothermal processes. A restricted range of strongly mass-independent Δ33S values in ore samples from Trojan Mine likely reflects high-temperature assimilation of sulfur from supracrustal rocks and later superimposed low-temperature hydrothermal remobilization. Iron isotope values for most Ni-bearing sulfides show a narrow range suggesting that, in contrast to sulfur, nearly all of iron was derived from an igneous source. Negative Ni isotope values also agree with derivation of Ni from ultramafic melt and a significant high-temperature fractionation of Ni isotopes. Fe isotope values of some samples from Shangani Mine are more fractionated than expected to occur in high-temperature magmatic systems, further suggesting that hydrothermal processes were involved in either low-grade ore formation (liberation of Ni from olivine by sulfur-bearing hydrothermal fluids) or remobilization of existing sulfides potentially inducing secondary Ni-sulfide mineralization.|