MINERALOGY AND GEOCHEMISTRY OF THE CLAY FRACTION OF TILL IN NORTHERN FINLAND

The mineralogy and geochemistry of the clay fraction of till in differ ent moraine types and in different bedrock areas in northern Finland h ave been studied. Moraine types include ground moraine, Rogen moraine, Pulju moraine, Sevetti moraine and Kianta moraine. The bedrock is com posed of granite, granite gneiss and mafic volcanite. X-ray diffractio n, differential thermal and thermogravimetric analyses, transmission e lectron microscopy, X-ray fluorescence and atomic absorption spectrome try were used as analytical methods. Quartz, feldspars and amphibole a re the primary rock-forming minerals occurring in nearly all the sampl es of each study area. The clay minerals proper include kaolinite, ver miculite, swelling-lattice vermiculite, chlorite, illite and mixed-lay er minerals. Kaolinite occurs most abundantly only in till in the Kitt ila area, where a preglacial weathering crust has been the source rock . Poorly crystalline or amorphous Fe oxides also abound in till at Kit tila. Sodium, Ca and Si have been depleted and Al, Mg, Fe, K, Ti, P an d trace elements have been enriched in the clay fraction compared with the fine fraction. The contents of Si, K and Na are higher in the gra nitic areas and the contents of Cu, Co, Ni and Zn are higher in the ma fic volcanite areas. Preglacial weathering has led to the depletion of Si, Mg, Ca, K and Na and to the enrichment of Al, Fe, Ti, P and trace elements in the clay fraction of till in the Kittila area. High base metal values in the Inari area suggest the possibility of sulphide min eralizations there and high As values in the Kittila area indicate Au potential there. The most important factors controlling the mineralogi cal and geochemical composition of the clay fraction of the tills in n orthern Finland are the composition of the bedrock and the possible oc currence of an old weathering crust. The final grain size composition of the tills and consequently the quantitative proportions of the diff erent minerals are often related to the last glacial quarrying and sor ting processes; therefore the mineralogical composition of the tills i s to a certain extent bound also to the respective moraine types.