Original paper

The BeTa-rich granite of Seiffen (eastern Erzgebirge, Germany): accessory-mineral chemistry, composition, and age of a late-Variscan LiF granite of A-type affinity

Förster, H.J.; Rhede, D.

Abstract

The mildly peraluminous granite of Seiffen, in the eastern Erzgebirge of Germany, is exposed by drillcores and associated with an abandoned Sn mine. The granite is of Stefanian age, with overlapping Th-U-total Pb monazite (302 ± 4 Ma) and K-Ar siderophyllite ages (301 ± 5 Ma). It is among the youngest granites in the Erzgebirge, emplaced in an extensional setting. The medium-grained, equigranular granite classifies as high-F, low-P Li-mica granite of A-type affinity. It is spatially associated with a high-Si rhyolitic microgranite, documenting the shallow intrusion level of this igneous association. Zircon, monazite-(Ce), and xenotime-(Y) constitute important radioactive accessory minerals in the granite, hosting the major proportions (>80-90%) of the bulk-rock budgets of the REE, Y, and Th. A significant percentage of U (40-50%) may reside within unidentified phases or precipitated along grain boundaries. The most uncommon accessory phase is late-magmatic ytterbian xenotime-(Y) containing up to 11.2 wt% Yb2O3, in addition to 7.3 wt% Er2O3 and 7.9 wt% Dy2O3.The Seiffen granite (εNd(300) = −4.6) is geochemically evolved and rich in Sn (23-63 ppm) and W (11-14 ppm). It contains elevated to high concentrations of incompatible lithophile elements such as F, Li, Ga, Rb, Y, Nb, Cs, REE, Th, and U, thus having much in common chemically with subvolcanic ongonites. The most prominent compositional feature is the strong enrichment (in ppm) in Be (51-55) and Ta (23-28). The granite exhibits flat chondrite-normalized REE patterns (LaN/LuN = 1.35-1.48) and a moderate negative Eu anomaly (Eu/Eu* = 0.12-0.13). Indications for alteration-induced, postmagmatic disturbances of initial elemental abundances are weak and mainly relate to the ore-forming elements Sn and U.

Keywords

li-f graniteth-u-pb monazite ageberylliumtantalumytterbian xenotime-(y)ongonitepost-collisional setting