Tertiary mineralization and magmatism, East Greenland: lead isotope evidence for remobilization of continental crust

Well-defined lead isotope signatures of sulphides from mineral occurrences associated with Tertiary igneous rocks in East Greenland fall in two distinct groups: (1) sulphide samples from the region south of Scoresby Sund (Kangerlussuaq igneous complex) have lead characterized by retarded uranogenic evolution and slightly elevated thorogenic isotopic ratios; (2) sulphide samples from the region north of Scoresby Sund (Werner Bjerge, Kap Simpson and Kap Broer Ruys igneous complexes) have lead compositions typical of modern average terrestrial lead. These distinct signatures in both conventional and 208Pb/204Pb–207Pb/204Pb diagrams lie within linear trends defined by previously published lead isotope data for unmineralized Tertiary basalts and intrusive rocks. The linear trends are mixing lines between the lead in the Tertiary mantle-derived magmas and lead remobilized from continental crust. Tertiary magmas in the region south of Scoresby Sund remobilized and incorporated lead from low-μ, high-208Pb/204Pb crustal sources similar in isotope composition to Archaean high-grade metamorphic gneisses exposed in the area. Non- to sub-parallel, distinct lead mixing lines in the 208Pb/204Pb–206Pb/204Pb diagram for the Tertiary igneous complexes in this region suggest strong heterogeneity of the remobilized crust. More than one-half of the lead from sulphide mineral occurrences within the Kangerlussuaq alkaline intrusive complex may be derived from local continental crust. In the region north of Scoresby Sund the Tertiary magmas remobilized moderately radiogenic lead from local basement and sedimentary basins: Palaeoproterozoic–Mesoproterozoic gneisses, Neoproterozoic–Ordovician sediments, Caledonian granites and Palaeozoic–Mesozoic sediments. These crustal rocks lack the very low 206Pb/204Pb and 207Pb/204Pb ratios typical of the gneisses from the Kangerlussuaq region, and mixing trends between mantle-derived and crustal lead are therefore much less extensive. However, because the mantle-derived magmas had very low total lead contents, even small additions of crustal lead are traceable. Sulphide-mineralized alkaline intrusive complexes are shown to be highly contaminated with crustal lead, and it is further shown that the pre-Tertiary rocks of North-East Greenland are likely local crustal sources of lead for contaminated Tertiary plateau basalts. Many stratabound, sediment-hosted hydrothermal base metal occurrences in the Palaeozoic–Mesozoic Jameson Land Basin are thought to be Tertiary in age, but do not appear to contain Tertiary, igneous-derived lead. On the contrary, the hydrothermal alteration zones associated with several of the mineralized Tertiary igneous complexes attest to extensive fluid/rock mass transfer; lead and other base metals were leached from the country rocks, homogenized, and incorporated into sulphide minerals.