Applications of Stable and Radiogenic Isotopes to Magmatic Cu-Ni-PGE Deposits: Examples and Cautions

Measurements of S, O, and radiogenic isotope ratios are all potentially powerful tracers of magma interaction with country rocks and the importance of assimilation processes in the genesis of magmatic Ni-Cu-PGE deposits. Sulfur isotope measurements of deposits such as those in the 1.1 Ga Duluth Complex, the Permo-Triassic intrusions of the Noril'sk area, and the 1.4 Ga Kabanga intrusions provide evidence for the derivation of S from both sulfide-and sulfate-bearing country rocks. The 1.3 Ga Voisey's Bay deposit provides an example where δ34S values of the ores commonly fall within the accepted mantle range of 0 ± 2%, but detailed studies of Proterozoic metasedimentary country rocks show that their weighted average δ34S value is also within this range. A thorough knowledge of the isotopic compositions of potential contaminants is essential for a proper evaluation of the role of country rock derived S in ore formation. When O and radiogenic isotopic measurements are employed as tracers to evaluate magma-country rock interaction, it is essential to provide evidence that open system processes have not perturbed the isotopic systematics. Low-temperature hydrothermal processes can mask evidence of high-temperature processes in the oxygen isotope system and in radiogenic systems such as Re/Os and Pb where involved elements may be mobile under hydrothermal conditions, or host phases may close to exchange and uptake at different temperatures. Careful petrographic observation and analyses of individual minerals may be required before the models involving the contamination of magmas by country rocks can be meaningfully applied.