A.B. Bobrov. THE FIRST FINDING OF SILICIDES AND SILICON IN PALEO-PLACERS OF THE LEONE-LIBERIAN SHIELD (SIERRA LEONE, WEST AFRICA)

https://doi.org/10.15407/mineraljournal.41.03.024

UDC 549.2 + 622.037 (664)

THE FIRST FINDING OF SILICIDES AND SILICON IN PALEO-PLACERS OF THE LEONE-LIBERIAN SHIELD (SIERRA LEONE, WEST AFRICA)

A.B. Bobrov 1,

L.M. Stepanyuk 2, https://orcid.org/0000-0001-5591-5169

S.І. Kurylo 2, https://orcid.org/0000-0003-4466-6851

S.M. Bondarenko 2, https://orcid.org/0000-0001-7948-3583

S.D. Kakaranza 1,

T.B. Iaskevich 2

1 Expert Advice of the Union of Geologists of Ukraine

26/2, Kyrylivska Str., Kyiv, Ukraine, 04070

E-mail: albobrov@ukr.net

2 M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine

34, Acad. Palladin Ave., Kyiv, Ukraine, 03142

E-mail: stepaniuk@nas.gov.ua, iaskevych@ukr,net

Language: Ukrainian

Mineralogical journal 2019, 41 (3): 24-35

Abstract:

In the middle course of the Sewa River, from the conglomerates and gravelites of the second type, represented by loose non-lithiated varieties, metal phases of multicolored silicides (magnesium, iron silicides, iron disilicide, iron, calcium, and native silicon) and diamond satellites were found. These conglomerates contain a pebble component (quartz, tonalite granitoids, and crystalline schists of basic composition, migmatites, aplites, and epidosites) and slightly clayey sandy filler. Silicides are present in the form of large (up to 2 cm) fragments of crystalline aggregates. They are unique mineral formations that have never been described in this region before. We have identified: 1) iron disilicide (FeSi2), which determines the main elements of the microstructure of these mineral composites in polished sections; 2) whitish-blue magnesium silicide (Mg2Si) which cements the FeSi2 phase. Magnesium silicide is significantly minor to it and amounts to about 10—20 % of rock volume; 3) disilicide of magnesium and calcium (MgCaSi2) of pink-cream color. Its content is estimated at 2—6 %. The phase is uniformly dispersed in the main matrix and has the appearance typical of decomposition products of solid solutions; 4) iron silicide (FeSi) that forms separate grains of gray color; 5) native silicon (Si) that forms fine grains in the interstitial spaces between other silicides. The authors are inclined to believe that the probable source of silicides is a gabbro-diorite (layered to pyroxenites / hornblendites) massif found nearby. It shows clear signs of both linear and globular stratification. For these questions to be unambiguously answered , the geological and geophysical mapping of the probable areas of location within the massif of single geological bodies of basic and ultrabasic composition is in progress.

Keywords: silicides, disilicides, native silicon, stratified gabbro-dioritic massif.

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