Bulletin of the Geological Society of Finland (Jun 1992)
Relationship between mineralogy and the physico-chemical properties of till in central Finland
Abstract
The chemical and physical characteristics of the fine fraction of till were examined in the area of the Raahe-Ladoga metallogenic belt in central Finland. Till, weathered bedrock and rock samples were collected from the area, where element concentrations of till abruptly ranged from high in an anomalous zone (NE) to low in a non-anomalous zone (SW). The grain size distribution, specific surface area and unit weight, and the leachability of elements measured with several chemical methods were determined from the fine fraction (<0.06 mm) of till. The mineralogical composition of the fine fraction of till and weathered bedrock was established with the X-ray diffraction method (XRD), and the chemical composition of rock samples with the X-ray fluorescence method (XRF). The main factor affecting the increased element concentrations in the anomalous zone is the variation in mica and clay mineral types. The abundance of trioctahedral micas (biotite) and vermiculitic clays carrying most of the trace metals dissolved in aqua regia, has affected to the geochemical pattern of till in the study area. The concentrations of trace metals in the iron precipitates extracted with acid oxalate were too low to be used as evidence of the hydromorphic origin of the anomalous zone. The abundance of main silicates (quartz, feldspars) does not vary much in the fine fraction of till throughout the study area. The absence of the swelling smectite component from the surface layers of till indicates that the source was not the underlying weathered bedrock. The till with a vermiculitic mixed-layer mineral in the anomalous zone contains more material from old sediments weathered during interstadial or interglacial time than does the till with a low clay content in the non-anomalous zone. The abrupt change in the mineralogical and physical properties of till in the study area is suggested to be attributed to the difference in the transport and accumulation dynamics of overburden during deglaciation.
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