Investigating the Influence of Crustal Contamination on the Stillwater Complex, Montana Using Sr, Nd, and Pb Isotopes.

This article focuses on the origin and isotopic characteristics of pegmatoid bodies within the Archean Stillwater Complex, a layered mafic-ultramafic intrusion in Montana, USA. Using whole-rock Rb-Sr, Sm-Nd, and Pb-Pb isotopic analyses of samples from the Stillwater Complex, its host rocks, and the underlying metamorphic hornfels country rock, the study finds that isotopic ratios in the intrusion overlap with those of the country rock, indicating modest isotopic heterogeneity and a shared source region. The data suggest that crustal contamination, including assimilation of country-rock-derived fluids released during contact metamorphism, contributed to isotopic variability, particularly in fluid-mobile elements like Sr and Pb, while Nd isotopes reflect magmatic source heterogeneity or crustal assimilation. Unlike the Bushveld Complex in South Africa, where country rocks are isotopically distinct from the intrusion, the isotopic similarity at Stillwater complicates distinguishing contamination effects, but supports a model involving limited fluid contamination (<1 wt %) and a gradual decrease in crustal influence through the stratigraphy.