Metal transport, partition and fixation in drainage waters and sediments in carbonate terrain in Southeastern Ontario

Lake sediment composition as an indicator of mineralization within the catchment area has found widespread application in recent years, particularly in Canada. Results have indicated, however, the existence of varying relationships between lake sediment composition and mineralization resulting from local features of the limnological environment. Accordingly it was considered appropriate to examine the nature of metal transport in the lake and stream environment, the partitioning of metal between the stream waters and stream sediments and between lake waters and lake sediments to obtain some understanding of the factors that affect the lake sediment-mineralization relationship. This investigation was carried out over an area containing Pb-Zn occurrences of supposed “Mississippi-Valley type” in Grenville and Paleozoic bedrock in southeastern Ontario.The headwater drainage systems comprise active streams, swamps, beaver ponds and small lake-bog systems giving way downstream to open lakes. The beaver swamps and seasonal swamps act as drainage sinks for metals, restricting the extent of geochemical dispersion in drainage systems adjacent to mineralization. Selective extraction analysis of bog, stream and lake sediments indicates that metals are preferentially concentrated with amorphous iron oxides, which readily adsorb and complex lead and zinc and are stable in the alkaline environment common in swamps adjacent to carbonate-hosted lead-zinc mineralization. The accumulation of lead and zinc with amorphous iron oxides combined with the adsorbing and chelating action of organic matter on lead and zinc makes organic-rich sediments from these small swampy areas an excellent sample medium for reflecting local mineralization. Down drainage anomalies of these elements can be accentuated by selective analysis for the amorphous iron oxide-held metal, involving selective extraction techniques.In contrast, within larger lake systems, the analysis of water samples indicates that geochemical dispersion in surface waters in the high pH environment (pH = 8.0) associated with the carbonate-hosted lead-zinc deposits is extremely restricted. In this environment, anomalous metal contents in lake water were not evident in lakes adjacent to mineralization, while anomalous lake sediment compositions exist only in lakes immediately adjacent to Pb-Zn mineralization and do not extend down the drainage system. The restricted dispersion necessitates basing geochemical reconnaissance surveys on collection and analysis of samples from the headwater organic-rich swamps at a higher sample density and resulting higher cost than in areas where a lower sample density is acceptable due to a wider dispersion.