Application of Stable Isotopic Studies to Problems of Magmatic Sulfide Ore Genesis With Special Reference to the Duluth Complex, Minnesota

Abstract

Analyses of the isotopic composition of S, C, 0, and H in mafic igneous rocks may provide valuable information regarding the extent of contamination by continental crust material. Partial melting and devolatilization of country rocks are important processes in the generation of many Cu -Ni deposits associated with mafic igneous rocks. Sulfur isotopes provide a means of evaluating the possibility of extraneous sulfur addition to a melt. Variability and distribution of o34g values may also provide data relating to the timing and mechanisms of sulfur production and incorporation into the melt. Oxygen, hydrogen, and carbon isotopes also can be sensitive indicators of isotopic contamination, whether by partial melting, devolatilization, or solid state exchange. Zones of contamination within an igneous sequence are likely to be areas where factors that control sulfide solubility (T, fS2, /°2, melt composition) have been perturbed, and are thus key sites for ore generation.Stable isotopic studies of two deposits within the Duluth Complex have highlighted the importance of country rock contamination in the generation of Cu -Ni ores. For the Dunka-Road deposit devolatilization of country rocks has resulted in essentially in situ contamination, and the formation of ore that is variable in both its spatial distribution and o34g values. Partial melting and major element contamination are restricted to areas near the margins of xenoliths. At the Babbitt deposit, contamination via both partial melting and devolatilization has been significant. Sulfur isotopic distribution between igneous and metasedimentary rocks suggests that sulfur must have been derived prior to or during magma ascent. Partial melting of country rocks and oxygen isotopic exchange may have occurred either before or after magma emplacement. Chemical diffusion, as well as fluid dynamic properties of mixing are thought to control the isotopic inhomogeneity that characterizes sulfide ore zones in the Duluth Complex.

Metal Deposits in Relation to Plate Tectonics

“…The evaporites are thought to have played an important role in ore formation in terms of providing a source of sulfur for the mafic magmas (Naldrett 1981 b). The incorporation of sulfur from sedimentary sources has also been demonstrated as a factor in the development of copper-nickel mineralization near the base of the Duluth Complex, Minnesota (Mainwaring and Naldrett 1977;Ripley 1986). The incorporation of sulfur from sedimentary sources has also been demonstrated as a factor in the development of copper-nickel mineralization near the base of the Duluth Complex, Minnesota (Mainwaring and Naldrett 1977;Ripley 1986).…”

Mantle-Derived Magmas and Magmatic Ni-Cu-(PGE) Deposits

“…Early segregation of sulfide liquid can result from assimilation of felsic rock such as granite or detrital sediment, which add Si and alkalis to produce a hybrid magma in which the sulfur solubility is lower (e.g., MacLean, 1969). However, the most efficient and most direct way of inducing sulfide saturation in sulfide-undersaturated magmas is by adding S. By assimilating sulfide or sulfate-bearing sedimentary or volcanic rocks, the S contents of the magma can be driven from levels well below sulfide saturation to the levels at which sulfide segregates (Gorbatchev and Grinenko, 1973;Green and Naldrett, 1981;Lesher et al, 1984;Grinenko 1985;Ripley, 1986;Lightfoot et al, 1994;Naldrett, 1992Naldrett, , 1996Naldrett, , 1999Naldrett et al, 1992Naldrett et al, , 1996Lightfoot and Hawkesworth, 1996).…”

Understanding Mineral Deposits

“…For the Sudbury ores, the narrow range of (j 34S values close to 0%0 suggests an entirely magmatic source ofthe sulfur (Naldrett 1984b). On the other hand, the spread of (j 34S values and their correspondence with the (j 34S values of the associated country rocks have been considered as evidence for a significant contribution of crustal sulfur for gabbroid-class deposits associated with flood basalts -sulfate-bearing evaporites in the case of the Noril'sk-Talnakh deposits (Godlevsky & Grinenko 1963, Grinenko 1985 and sulfidebearing sediments (the Virginia Formation) in the case of Duluth deposits (Mainwaring & Naldrett 1977, Ripley 1986a, Lee & Ripley 1995. The (j 34S values of the komatiite-hosted nickel sulfide ores fall within the magmatic range, but so do the (j 34S values of the associated sediments.…”