Pine Point; a case history of lead isotope homogeneity in a mississippi valley-type district

Abstract

The lead isotope compositions of 32 galena samples from 20 different deposits in the Pine Point district are essentially identical within error (avg 206 Pb/ 204 Pb = 18.175 + or - 0.008) and indicate a remarkable uniformity of isotope composition over a mineralized area of several hundred square kilometers. The isotope homogeneity could result from an homogeneous source or from thorough mixing during extraction and subsequent precipitation of lead. The homogeneity of the lead isotope data allows an interpretation in terms of a single-stage lead-evolution model and calculation of a model age of mineralization of about 290 Ma (Late Pennsylvanian). This age is substantially younger than the Middle Devonian host rocks (375 Ma), but much earlier than the post-Early Cretaceous ( approximately 100 Ma) tectonic events along the western margin of the Mackenzie basin that have been credited with initiating the regional hydrologic regime that resulted in mineralization. More complex multistage models are possible, but cannot be more precisely defined by present data.The homogeneity and nonradiogenic composition of Pine Point leads is in marked contrast to those reported for Devonian carbonate-hosted Pb-Zn deposits of the Mackenzie platform. This discrepancy may be due in part to the different paleogeographic positions of the two groups of deposits. The more radiogenic, inhomogeneous group is situated on the western edge of the platform adjacent to deep-water shales of the Selwyn basin. Pine Point, in contrast, lies more than 500 km to the east within the carbonate platform.