Complex magmatic and subsolidus compositional trends of columbite-tantalite in the beryl-columbite Sejby granitic pegmatite, Czech Republic: role of crystal-structural constraints and associated minerals

Journal of Geosciences

Milan Novák^1¹  Štěpán Chládek^2² 

[1] Department of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic^1;Department of Mineralogy and Petrology, Comenius University, Ilkovičova 6, 842 15 Bratislava, Slovakia^2

关键词: granitic pegmatite;    columbite-group minerals;    tapiolite;    compositional trends;    Nb-Ta and Fe-Mn fractionation;   

DOI  :  10.3190/jgeosci.269

学科分类：自动化工程

来源: Ceska Geologicka Spolecnost / Czech Geological Society

【 摘 要 】

The simply zoned Šejby pegmatite of beryl-columbite subtype is enclosed in migmatitized gneisses - country rocks of the southern part of the Central Moldanubian Plutonic Complex, Bohemian Massif, Czech Republic. The columbite-group minerals (CGM) occur mostly in blocky K-feldspar unit. Primary magmatic CGM [columbite-(Fe)—tantalite-(Fe)], show an interesting compositional and textural evolution. Subhomogeneous cores of columbite-(Fe)—tantalite-(Fe) (A) are surrounded by heterogeneous intermediate zones (B) and (C) with irregular to oscillatory zoning and mutually comparable compositions. Zone (B) contains grains of tapiolite-(Fe) and inclusions of pyrite decomposed into a mixture of secondary jarosite-group minerals. In the next zone (C), irregular Mn-enriched patches occur and in the outermost thin zone (D) CGM exhibit fine oscillatory zoning typically developed at crystal terminations. The late, subsolidus CGM include patchy-zoned Mn-enriched columbite-(Fe) and late columbite-(Fe) veinlets both formed at the expense of their magmatic precursors in the zone C; the veinlets are also present in the zone D. Similar but fine patches and veinlets are developed around jarosite pseudomorphs after pyrite. The compositional evolution of CGM shows a slight increase in Ta/Nb and Mn/Fe in the zones A and B followed by a reverse trend to elevated Nb and Fe in the zone C and, in particular, in the zone D. Complex subsolidus fluid-melt interactions that generated patches and veinlets caused only minor changes in Ta/Nb and slight Mn-enrichment. Tantalum depletion in veins close to jarosite pseudomorphs after pyrite indicates higher mobility of Ta in acidic low-temperature fluids.

【 授权许可】

CC BY