Glacial dispersal and mode of occurrence of metals in till and esker gravel at Kumpuselka northern Finland

A Cu-Zn-Pb-Au mineralized quartz-carbonate vein within micaceous schist recently exposed in a gravel pit quarried into the Kumpuselka esker (Finland) was the impetus of this study, which examines the glacial dispersal of metals in the nearby Quaternary deposits. The oldest ice flow direction in the area was to the SE-SSE, the younger flow in the ice lobe phase to the ESE and the youngest flow to the ENE. Five till and three esker gravel samples were taken for geochemical and heavy mineral studies. The concentrations of base metals in the conventional fine fraction analysis of the till samples are at background level although the samples had been taken from the very proximal part of the glacial dispersal train. Copper and Pb contents in the fine fraction of esker gravel are at higher levels than those in till fines. Base metal sulphides as chalcopyrite, sphalerite, galena and pyrite were found as some separate grains in the heavy mineral concentrates of till. More often, the Cu, Zn and Pb sulphides occur as tiny inclusions in the coarse pyrite grains. Heavy mineral concentrates of the esker gravel contain abundant pyrite grains, but no visible Cu, Zn or Pb sulphides were recovered. In one heavy mineral concentrate of till (KUM 4), Ni-bearing cobaltite and Co-bearing arsenopyrite along with pyrite, chalcopyrite and sphalerite were found. Sulphide minerals have survived even within the upper C-horizon of the till above the groundwater level and therefore sulphide minerals can be used for prospecting sulphide ores by indicator mineral methods. Some chromite grains were also found in the heavy mineral concentrates of the till samples. Their presence reflects multi-phase, long-distance glacial transport (100 km) of the older till material which have subsequently been inherited by the younger till.