A consideration of some hidden hydromorphic effects on the fines of glacial tills in the Kuhmo-Suomussalmi area, eastern Finland

The silt and clay-size (minus 0.063 mm) fractions of 766 samples of glacial tills from the Kuhmo-Suomussalmi area in eastern Finland were analyzed for 8 trace and minor elements (Co, Cr, Cu, Mn, Ni, Pb, Zn, Fe) by AAS after hot 7 mol/I HNO, dissolution treatment. The results were treated statistically. Threshold values were calculated separately for those tills overlying gneissic basement on the one hand and greenstone belt on the other. The results show that the overall element levels for the metals in the area are lower than elsewhere in Finland by a factor which varies between 1 and 46. The reason for the differences may be the absence of preglacial weathering material in the till fines which elsewhere enhance element levels. Marked differences in the element levels over each bedrock subunit show up in the overlying tills. Correlation, regression and factor analyses show that the elements have high mutual correlations suggestive of hydromorphic alteration, even though the macroscopically hydromorphic samples were omitted from the statistical analyses. The effect is weak, however, and does not suppress the primary correlation, which results in the emergence of two factors in factor analysis of the geochemical data of the greenstone belt tills, the Co-Cr-Cu-Mn-Ni-Fe and Pb-Zn factors, the latter corresponding to one of the two known types of mineralization encountered in the underlying bedrock.