Distribution and origin of columbite and (Y,REE,U,Th)-(Nb,Ta,Ti) oxide minerals in alluvial soils and river sediments in the area of the Grantcharitsa tungsten deposit, Western Rhodopes, Bulgaria

Abstract

Columbite-(Fe) and (Y,REE,U,Th)-(Nb,Ta,Ti) oxide minerals – ishikawaite (samarskite group), euxenite-(Y) and uranopolycrase (euxenite group) and minerals of the pyrochlore group (betafite subgroup) were found in the heavy fractions of alluvial soils and river sediments in the area of the Grantcharitsa tungsten deposit. Columbite-(Fe) is the most common mineral among them and is characterized by weak compositional variations (Ta/(Ta+Nb) = 0.04–0.21, Mn/(Mn+Fe) = 0.33–0.5) and lack of fractionation trends. All (Y,REE,U,Th)-(Nb,Ta,Ti) oxide minerals are metamict due to high uranium content (UO2 17–32 wt%). Preserved relationships between minerals in some grains, some features of their chemical composition (chemical similarity: Nb >> Ta, high content of Y, Ti, U) suggest their common origin from one rock source – NYF granitic pegmatites and/or genetically associated granitoids of mantle-dominated origin. It is shown that columbite-(Fe), ishikawaite, euxenite-(Y) and uranopolycrase form zoned crystals and are of magmatic origin, while the betafite subgroup minerals are secondary phases formed under hydrothermal conditions.