Geochemical indicators of buried sulphide mineralisation under sedimentary cover near Talbot Lake, Manitoba.

摘要

The Paleoproterozoic Talbot volcanogenic massive sulphide (VMS) Cu-Zn occurrence is located near Talbot Lake, Manitoba, about 80 km south of Snow Lake. The mineralised system is unconformably overlain by more than 100 m of Paleozoic dolomite sequences and Quaternary glacial till. The clay fraction of the till contains pronounced anomalies of Zn, Cu, Ag, Pb, Au, Mn, Hg, Cd, Co, Bi and Se, which is ascribed to the formation of clay-humic complexes that have carbon-isotopic compositions of delta13C -20‰. A VMS origin of the near-surface anomaly is confirmed by distinctly low 206Pb/204Pb ratios, compared to a more radiogenic till signature. The overall low 206Pb/ 204Pb ratios in the organic surface media black spruce bark, moss and humus also indicate input from a magmatic sulphide source, but likely reflect windblown material from the Flin Flon smelter stack at 160 km to the northwest. Black spruce tree rings that pre-date industrial sources of atmospheric metal input are used to measure historical element distributions because they record the local geochemistry at the time of heartwood growth and their root systems tap into large volumes of soil. At the Talbot prospect, the presence of secondary Zn from the till in black spruce tree cores is reflected in the Zn/Ca ratio, which eliminates the effect of dolomite-derived Zn. The process of element migration from buried mineralisation to the surface is complex and includes oxidation of sulphides, mobilisation and complexation of metal ions, physical upward transport, and deposition of indicator elements near the surface in accessible sampling media. To account for a significant influx of indicator elements into the relatively recently deposited till blanket, post-glacial groundwater upwelling is proposed to act as a major mechanism driving upward element migration.