Accessory Minerals in Felsic Igneous Rocks - Part 2

Rare Earth Deposits & Mining

Hans-Jürgen Förster (2018) investigated the composition of monazite-(Ce), xenotime-(Y) and zircon from the multi-stage, strongly peraluminous, P−F-rich Li-mica granite massif of Eibenstock (Erzgebirge−Vogtland metallogenic province, Germany).This data set is the second part of a series reporting chemical data for accessory minerals from felsic igneous rocks. Most data refer to plutonic rocks from the Saxothuringian Zone of the Variscan Orogen (Erzgebirge−Vogtland metallogenic province) in Germany performed between about 1995 and 2005 on surface rocks and borehole samples.

This data set assembles the results of electron-microprobe spot analyses of monazite-(Ce), xenotime-(Y) and zircon from the Li-mica granite massif of Eibenstock. This massif is composed of several, compositionally and texturally distinct sub-intrusions. Least evolved members of the fractionation series are exposed as variably sized enclaves. The pluton is cross-cut by fine-grained aplitic dikes. These late-Variscan (∼ 318−320 Ma) granites are highly evolved, rich in Si (72.4−75.8 wt% SiO₂), F, P, Li, Rb, Cs, and Sn, mildly to strongly peraluminous (A/CNK = 1.14−1.35), of transitional S−I-type affinity, and spatially and genetically associated with coeval significant Sn−W−(Mo) mineralization.

Most notably, a comparatively large population of grains of all three species is distinguished by abnormal composition, reflecting the chemically evolved nature of their hosts. Probe data indicate that the composition of monazite-(Ce) and zircon changes with fractionation-driven evolution of magma chemistry. Monazite-(Ce) composition extends over an abnormally large range. In the course of magma differentiation, mineral chemistry evolves towards enrichment Th and U and development of flattened and kinked chondrite-normalized LREE patterns, with negative anomalies at La or Nd, or both (also known as lanthanide "tetrad" effect). Many grains are so rich in Th that they classify as cheralite-(Ce). The concentrations (in oxide wt%) of the radionuclides Th and U maximizes to 51.7 and 5.3, respectively. The maximum concentration of Y amounts to 4.7 wt% Y₂O₃.

Composition of zircon displays a large variability. A greater number of grains or domains are distinguished by abnormal enrichment in (in oxide wt%) P (up to 9.6), Th (up to 12.2), U (up to 8.7), Hf (up to 5.6) Al (up to 2.2), Sc (up to 2.0), Y (up to 7.0), HREE and Y. Enrichment in these elements is usually associated with low analytical totals, reflecting precipitation from volatile-rich magmas and/or their interaction with, and alteration by, late-magmatic fluids. Xenotime-(Y) chemistry is comparatively little sensitive to changes of Eibenstock-magma composition relative to what has been observed for monazite-(Ce) and zircon. The U concentrations in xenotime-(Y) are generally high and maximize to 6.7 wt% UO₂. Chondrite-normalized MREE and HREE patterns preferentially in xenotime-(Y) from more evolved magma batches mimic that of their host granites in that they are (a) inclined from Tb-Dy towards Lu, (b) partially evolved the lanthanide "tetrad" effect, and (c) display above-CHARAC Y/Ho ratios up to 41.

Other Parts of the work done by Hans-Jürgen Förster: http://bit.ly/2CmiUL8