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

摘要

Numerous sediment-hosted base-metal and barite occurrences are found within the post-Caledonian Jameson Land Basin in East Greenland. A study was undertaken to evaluate the sources of lead in several base-metal occurrences by comparing the lead isotope signatures of sedimentary and intrusive rocks from the basin with ore lead signatures established in earlier studies. To differentiate mineral phases and simulate various degrees of natural hydrothermal processes the samples were leached sequentially with HCl, H_2O_2 (only in the case of organic-rich samples), HNO_3 and HF + HNO_3.The lead isotopic compositions of the sedimentary rocks were found to vary according to mineralogical paragenesis, and with origin and geographical position of the adjacent basement. All HF-soluble lead (after leaching with HCl and HNO_3) in coarse-grained arkoses and conglomerates plotted on well-defined mixing lines in both uranogenic and thorogenic diagrams, whereas all other analyses were more radiogenic. Since feldspar is the dominant lead-carrying phase in the HF fraction of the coarse-grained clastic rocks, the mixing lines are referred to as the feldspar-lead trends and are interpreted to reflect the presence of detrital feldspars from two well-defined basement sources. The radiogenic end of the feldspar-lead trend (high ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb ratios) is believed to represent feldspars derived by the breakdown of Caledonian granites on account of isotopic overlap with these. The lower end of the feldspar-lead trend more probably represents pre-Caledonian metasediments.Lead isotopes from the base-metal occurrences in the western part of the Jameson Land Basin plot on the feldspar-lead trends, strongly suggesting that feldspars from the thick sequence of Devonian to Lower Permian continental clastic deposits are the source of ore lead. The position on the feldspar-lead trend of the individual west Jameson Land mineral occurrences varies according to the local geological environment. The Mesters Vig Pb-Zn veins plot nearest the low ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb end of the feldspar-lead trends in accordance with the scarcity of Caledonian granites in the nearby basement, whereas mineralization further to the south has more 'Caledonian 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 Caledonian lead component. The east Jameson Land ore lead defines a trend extending from the upper end of the feldspar-lead trends towards more radiogenic values. This trend away from the feldspar-lead fields probably reflects the fact 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 Jameson Land ore lead is interpreted as being derived from various local source rocks.