DOI: 10.5593/SGEM2014/B11/S1.007


C. Ciortescu, O.G. Iancu, C. Popa
Wednesday 1 October 2014 by Libadmin2014

References: 14th International Multidisciplinary Scientific GeoConference SGEM 2014, www.sgem.org, SGEM2014 Conference Proceedings, ISBN 978-619-7105-07-0 / ISSN 1314-2704, June 19-25, 2014, Book 1,Vol. 1, 51-62 pp

The present work focuses on the analyses of a selection of heavy mineral
assemblages sampled from the Neagra Șarului River alluvial sediments, in order to determine their provenance and distribution, using their geochemical and physical characteristics. The study area is along a river, located in the north-western part of the Eastern Carpathians, whose bedrocks in its drainage basin are constituted mainly by igneous rocks from Călimani Volcanic Complex in the west, and a small area of low to medium grade metamorphic rocks, part of Crystalline-Mesozoic Zone, in the east. In order to trace the source of the heavy mineral species, the samples were prepared via field separation and subsequent laboratory sieving using 8 different size fractions. An electromagnetic separator (Frantz Isodynamic) was used to separate and classify the heavy minerals species, according to their magnetic susceptibility. Thus prepared, more than 500 grains per sample (from 0.5-1 mm size fraction) were mounted on thin sections and analyzed using electron microprobe with an EDX system. The classification of the minerals and the nature of their inclusions are derived from the major element compositions computed from SEM-EDX analysis. Furthermore, a stereo microscope was used in order to determine complementary properties of the grains, such as: color, degree of roundness and degree of alteration. In order of abundance, the main heavy minerals are magnetite, hematite, pyroxene, pyrite, manganese oxides, garnet, apatite,titanium oxides (ilmenite, titanite and rutile/anatase), chlorite, olivine, epidote, biotite and rhodochrosite. A particularity of the studied area is the presence of an altered magnetite caused first by the hydrothermal alteration and strong weathering of the source rocks and second by the river’s acidity. Manganese oxides are present only in grain fractions greater than 0.25 mm due to higher susceptibility to weathering and dissolution of the manganese aggregates in the river bed. Despite low distribution of the metamorphic units in the river’s studied basin, the garnet’s almandine (Alm 13-88%) and spessartine (Sps 0.5-87%), specific to the medium grade metamorphic rocks, have a relative high frequency. In this study, heavy mineral assemblages generally reflect the composition of primary (augite, almandine) and accessory minerals present in source rocks. The last ones are both primary (apatite) and secondary, which are mainly derived from hydrothermal deposition (e. g. pyrite) and from supergene alterations (e. g. manganese, iron oxides/ hydroxides, and other altered products of magnetite). Therefore, the mineral analyses were not limited only to track the source of each mineral species, but they also revealed the characteristics of their parent rocks.

Keywords:Neagra Șarului, alluvial sediments, heavy mineral, Călimani Volcanic
Complex, distribution, provenance

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