ABSTRACT A set of criteria has been established that we use to assess whether carbonatites are generated as primary mantle melts, or whether they are the products of magma differentiation (crystal fractionation, liquid immiscibility) of a parental, carbonated silicate melt. On the basis of these criteria, in conjunction with the vast amount of published field information and geochemical data from the Kola Alkaline Province (KAP), no clear-cut pattern emerges that favours any of these processes over the others. Any evidence for liquid immiscibility seems to be restricted to some members of the dyke swarms (Kandaguba, Turiy), while robust evidence for the generation of primary carbonatitic magmas seems to be lacking. Potential candidates for primary melts include the carbonatites from Turiy Mys and the older dyke swarms associated with the Kandalaksha Deep Fracture Zone. The spatial and temporal association of most carbonatites with silicate rocks at Kola, along with the presence of olivinites, clinopyroxenites and other cumulate rocks in some complexes favour crystal fractionation as an important process in generating some of the KAP rocks. We are left with the impression that all three processes may be responsible for carbonatite generation, even within the same complex. The isotopic evidence suggests the involvement of at least three distinct mantle sources, one of which is common to all of the complexes and perhaps indicative of a deep-seated, primitive mantle at least 3 Ga old. Overall, we propose an integrated plume-related model for the Devonian alkaline and carbonatitic magmatism that characterizes much of the KAP. Low-degree partial melting within the volatile-rich, and cooler parts of a plume head accompanied by the mixing of small-volume magma batches, their subsequent differentiation, and interaction with entrained materials and continental lithosphere may help explain some of the problems associated with unravelling the genesis of carbonatite magmas.

Abstract Lead isotope ratios for till in the Halfmile Lake area, Bathurst, New Brunswick, have successfully indicated a glacial dispersal train that extends 600 m down-ice from a known sulfide deposit. The contrasting isotopic signatures of Pb in the volcanic massive sulfide (VMS) deposit and in the surrounding country rocks provide a means of evaluating the contributions of Pb from both of these sources to the glacial till. Glacial sediments at Halfmile Lake that lie close to the VMS deposit have low 206 Pb/ 204 pb isotope ratios similar to those that characterize the massive sulfides. The Pb isotope ratios systematically increase toward the east and indicate a dispersal train broadly consistent with the ice-flow direction. Lead isotope ratios, when plotted against Pb abundance data, define curves that are consistent with the mixing of two end members, one represented by the sulfide deposit and the other by the country rocks. Results also show that there are significant variations in the Pb isotope compositions and abundances within vertical sections of the till. Lead isotope compositions reflecting input from the ore are found at or near the bottom of glacial sediments in three of the four pits analyzed. One of the advantages of using Pb isotope ratios, rather than Pb abundance data, is that the Pb isotope signatures provide a means of distinguishing Pb from the VMS deposit and Pb from sources unrelated to mineralization. This cannot be done using Pb abundances alone. Lead isotope ratios thus provide a powerful tool in mineral exploration by fingerprinting anomalous Pb from mineralized sources.