Rare earth elements as indicators of groundwater environment changes in a fractured rock system: evidence from fracture-filling calcite

Rare earth element (REE) abundances in core samples from Precambrian crystalline rocks at the Samkwang Mine site provide evidence of the solution chemistry involved in precipitation of calcite on fractures. The rock types collected in core samples are mainly banded-gneisses, with mineral assemblages dominated by biotite, K-feldspar, quartz and plagioclase. Calcite, chlorite, muscovite and sericite occur as secondary minerals, with calcite being the main filling material in fractures. In general, the core samples from 4 boreholes are enriched in light REE (LREE) and depleted in heavy REE (HREE), with negative Eu anomalies. However, positive Eu anomalies also occur at specific depths within 3 boreholes. Variation of chondrite-normalized REE patterns results from the fracture-filling calcite in core samples. Calcite fracture fillings provide a record of paleo-hydrology, where Eu has been reduced and selectively concentrated in the solutions from which calcite has precipitated.