Heavy mineral compositions of lake placer sands in northern Finland

Abstract

The heavy mineral (d > 2.89 g/cm3) composition of 33 samples of black lake shore placer sands from a number of localities in northern Finland were analysed. The heavy fraction of the sample was obtained by treatment with bromoform (d = 2.89) or Clerici's solution of the same density. Identifications using stereo-, polarizing and ore microscopes were made separately on both the loose material and polished impregnated thin sections. A check on the identifications was performed by applying the Debye-Scherrer X-ray powder camera method for both grains picked from the loose material and grains in the thin sections. The grain-size distribution of the loose material was determined by standard sieving techniques and the amount of magnetite by removal with a magnetic separator. The results show fairly uniform grain-size (d50 ~ 0.2 mm) and marked sorting for most of the samples. Among the minerals amphiboles are by far the most significant averaging 63.9 % of the non-magnetic heavy fraction. Magnetite is also abundant, especially in those samples that are poor amphiboles, averaging 6.2 % of the total heavy fraction. Next in order of abundance are ilmenite, hematite, epidote, pyroxenes and garnet. The similarity of the mineral composition of many of the samples implies that the local bedrock is only rarely, and in certain special cases, reflected in their main mineral composition. The exceptions are those from localities where large areas of the bedrock have some significant characteristic such as high epidote in the basement gneiss, and garnet in granulite. From the point of view of the bedrock geology the results indicate that some para-amphibolites intercalated with arenaceous sediments might be the result of a similar kind of sedimentary process but, on the other hand, a process which advanced no further than this would have hardly any significance in terms of the origin of the iron-formations.