Mineralogy and geochemistry of the clay fraction of till in northern Finland

Abstract

The mineralogy and geochemistry of the clay fraction of till in different moraine types and in different bedrock areas in northern Finland have been studied. Moraine types include ground moraine, Rogen moraine, Pulju moraine, Sevetti moraine and Kianta moraine. The bedrock is composed of granite, granite gneiss and mafic volcanite. X-ray diffraction, differential thermal and thermogravimetric analyses, transmission electron microscopy, X-ray fluorescence and atomic absorption spectrometry were used as analytical methods. Quartz, feldspars and amphibole are the primary rock-forming minerals occurring in nearly all the samples of each study area. The clay minerals proper include kaolinite, vermiculite, swelling-lattice vermiculite, chlorite, illite and mixed-layer minerals. Kaolinite occurs most abundantly only in till in the Kittiläarea, where a preglacial weathering crust has been the source rock. Poorly crystalline or amorphous Fe oxides also abound in till at Kittilä. Sodium, Ca and Si have been depleted and Al, Mg, Fe, K, Ti, P and 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 granitic areas and the contents of Cu, Co, Ni and Zn are higher in the mafic 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 Kittiläarea. High base metal values in the Inari area suggest the possibility of sulphide mineralizations there and high As values in the Kittiläarea indicate Au potential there. The most important factors controlling the mineralogical and geochemical composition of the clay fraction of the tills in northern Finland are the composition of the bedrock and the possible occurrence of an old weathering crust. The final grain size composition of the tills and consequently the quantitative proportions of the different minerals are often related to the last glacial quarrying and sorting processes; therefore the mineralogical composition of the tills is to a certain extent bound also to the respective moraine types.