Mineralogy and geochemistry of the fine and the clay fractions of till in northern Finland

Abstract

The mineralogy and geochemistry of the fine and clay fractions of till in different moraine types and in different bedrock areas in northern Finland have been studied. A total of 250 till samples from 140 test pits were studied mineralogically and geochemically. X-ray diffraction, differential thermal and thermogravimetric analyses, transmission electron microscopy, X-ray fluorescence and atomic absorption spectrometry were used as analytical methods.

In the clay fraction of till quartz, plagioclase, microcline and amphibole are the primary minerals occurring. The clay minerals proper include vermiculite, chlorite, illite, swelling-lattice vermiculite and mixed-layer clay minerals. Kaolinite occurs most abundantly in the clay fractions of till in the Kittilä, Jerisjärvi, Kaaresuvanto and Pulju areas. In the clay fraction of Kittilä and Jerisjärvi illite is dioctahedral type, but in their of other study areas both di- and trioctahedral types occur. Kaolinite and dioctahedral illite are evidence of the mixing of the weathered bedrock material into the till matrix. In the fine fraction of till most abundant minerals are primary minerals and clayminerals are in a minor role.

In the clay fraction of till the content of primary minerals are at higher and secondary minerals are at lower level in the Granitic and Archaean gneiss areas than in the Greenstone Belt, Svecokarelian schists and gneiss and Granulite areas. Amphibole, microcline and plagioclase occur in very low amounts or are totally destroyed by chemical weathering in the clay fraction of the till in the Kittilä area. The mineral composition of fine and clay fractions in the tills of northern Inari gives an indication that there occur much more mafic volcanites than is known today. The mineralogical compositions of fine fraction of the tills correlates quite well with the underlying bedrock in all study areas, but clay fraction does not.

Geochemical results are in accordance with the mineralogical composition of both fractions. In the fine fraction of the till Si, Ca and Na contents are higher than in the clay fraction. Clay fraction is enriched in Al, Fe, Mg, K and trace elements as compared to the fine fraction. Present study material points out that the distribution of chemical elements in the clay fraction of the till does not correlate with the composition of the underlying bedrock, but fine fraction does so with a few exceptions. The chemical composition of till in Kaaresuvanto and Inari does not fully correspond to the composition of the underlying bedrock as known today. In northern Inari and Kittilä the results give an indication that there are more mafic volcanites and/or sulphide mineralizations occurring in these areas than is known at the present time.

The most important factors controlling the mineralogical and geochemical composition of the fine and clay fractions 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 type.