Beryllium minerals as monitors of geochemical evolution from magmatic to hydrothermal stage; examples from NYF pegmatites of the Třebíč Pluton, Czech Republic

[摘要] Mineral assemblages of primary and secondary Be-minerals were examined in intraplutonic euxenite-type NYF pegmatites of the Třebíč Pluton, Moldanubian Zone occurring between Třebíč and Vladislav south of the Třebíč fault.Primary magmatic Be-minerals crystallized mainly in massive pegmatite (paragenetic type I) including common beryl I,helvite-danalite I, and a rare phenakite I. Rare primary hydrothermal beryl II and phenakite II occur in miarolitic pockets(paragenetic type II). Secondary hydrothermal Be-minerals replaced primary precursors or filled fractures and secondarycavities, or they are associated with ,,adularia” and quartz (paragenetic type III). They include minerals of bohseite-bavenite series, less abundant beryl III, bazzite III, helvite-danalite III, milarite-agakhanovite-(Y) III, phenakite III, anddatolite-hingganite-(Y) III. Chemical composition of the individual minerals is characterized by elevated contents ofNa, Cs, Mg, Fe, Sc in beryl I and II; Na, Ca, Mg, Fe, Al in bazzite III; REE in milarite-agakhanovite-(Y) III; variationsin Fe/Mn in helvite-danalite and high variation of Al in bohseite-bavenite series. Replacement reactions of primary Be--minerals are commonly complex and the sequence of crystallization of secondary Be-minerals is not defined; mineralsof bohseite-bavenite series are mostly the latest. Beryl usually occurs in pegmatites with rare tourmaline, whereas helvite--danalite bearing pegmatites are tourmaline-rich. Abundant tourmaline in pegmatites with helvite-danalite and its scarcityin beryl-bearing pegmatites indicate that early tourmaline crystallization affected activity of Al in the parental mediumand thus may have controlled formation of primary Be-minerals (beryl – higher Al, helvite-danalite – lower Al) whichcrystallized later. Secondary Be-minerals with dominant minerals of bohseite-bavenite series and milarite suggest highactivity of Ca in fluids. Variations in chemical composition (Al contents) of bohseite-bavenite series were controlled bythe chemical composition of the precursor. High variability of primary magmatic Be-minerals within a single pegmatitedistrict is exceptional and it is constrained by variable activities of Si and mainly Al, divalent cations – Ca, Mn, Fe, Znand Mg, trivalent cations – REEs, Sc, and B, S, and fO2 in the individual pegmatites.

[学科分类] 自动化工程

[关键词] beryl;phenakite;helvite-danalite;milarite- and gadolinite-group minerals;bohseite-bavenite;NYF pegmatites;Třebíč Pluton