Lead isotope signatures of sedimentary rocks as a tool for tracing ore lead sources: a study of base-metal and barite occurrences in Jameson Land Basin, central East Greenland

Numerous sediment-hosted base-metal and barite occurrences are found within the post-Caledonian Jameson Land Basin in East Greenland. A study was unde rtaken to evaluate the sources of lead in several base-metal occurrences by comparing the lead isotope signatures of sedimentary and intrusive rocks f rom the basin with ore lead signatures established in earlier studies. To d ifferentiate mineral phases and simulate various degrees of natural hydroth ermal processes the samples were leached sequentially with HCl, H2O2 (only in the case of organic-rich samples), HNO3 and HF + HNO3. The lead isotopic compositions of the sedimentary rocks were found to vary according to mineralogical paragenesis, and with origin and geographical po sition of the adjacent basement. All HF-soluble lead (after leaching with H Cl and HNO3) in coarse-grained arkoses and conglomerates plotted on well-de fined mixing lines in both uranogenic and thorogenic diagrams, whereas all other analyses were more radiogenic. Since feldspar is the dominant lead-ca rrying phase in the HF fraction of the coarse-grained elastic rocks, the mi xing lines are referred to as the feldspar-lead trends and are interpreted to reflect the presence of detrital feldspars from two well-defined basemen t sources. The radiogenic end of the feldspar-lead trend (high Pb-207/Pb-20 4 and Pb-208/Pb-204 ratios) is believed to represent feldspars derived by t he breakdown of Caledonian granites on account of isotopic overlap with the se. The lower end of the feldspar-lead trend more probably represents pre-C aledonian metasediments. Lead isotopes from the base-metal occurrences in the western part of the Ja meson Land Basin plot on the feldspar-lead trends, strongly suggesting that feldspars from the thick sequence of Devonian to Lower Permian continental elastic deposits are the source of ore lead. The position on the feldspar- lead trend of the individual west Jameson Land mineral occurrences varies a ccording to the local geological environment. The Mesters Vig Pb-Zn veins p lot nearest the low Pb-207/Pb-204 and Pb-208/Pb-204 end of the feldspar-lea d trends in accordance with the scarcity of Caledonian granites in the near by basement, whereas mineralization further to the south has more 'Caledoni an granite-lead' incorporated. Mineral occurrences in east Jameson Land are slightly more radiogenic than those of west Jameson Land, showing a strong affinity towards the Caledonia n lead component. The east Jameson Land ore lead defines a trend extending from the upper end of the feldspar-lead trends towards more radiogenic valu es. This trend away from the feldspar-lead fields probably reflects the fac t that the Carboniferous-Permian sequence in the east is much thinner than in the west owing to the overall architecture of the basin. The east Jameso n Land ore lead is interpreted as being derived from various local source r ocks.