Rare-earth-element enrichment in post-Variscan polymetallic vein systems of the Harz Mountains, Germany

Abstract

Carbonate gangue in historically important post-Variscan polymetallic Pb-Zn-Cu-Ag vein systems in the Upper Harz Mountains has elevated rare earth element contents (ΣREE+Y, 1500 ± 340 ppm; n = 16) in bulk samples. This enrichment is due to the occurrence of abundant micrometer-sized synchysite inclusions in calcite, identified via Raman spectroscopy. Calcite bulk analyses have roof-shaped PAAS-normalized REE patterns with a Eu peak but without Ce or Y anomalies and with a steeply dipping HREE pattern. Microbeam analysis (electron microprobe and LA-ICPMS) identifies these patterns as dominated by synchysite-(Ce–Nd) with strong Eu enrichment in the weight percent range and LREE/HREE fractionation (La/Lu ~ 100, PAAS-normalized). The synchysite component in calcite gangue is detected in polymetallic veins all over the Harz Mountains, which seems to be a diagnostic feature of the region and suggests a large-scale Mesozoic fluid system. However, the Upper (western) Harz systems with no fluorite have more elevated REE content, compared with the Lower (eastern) Harz fluorite-rich systems with less REE content. Carbonate gangue in the Upper Harz systems has homogeneous ⁸⁷Sr/⁸⁶Sri around 0.714 and εNdi of ~ − 9, while carbonate gangue in the Lower Harz systems has ⁸⁷Sr/⁸⁶Sri around 0.716 and εNdi of − 2 to − 5. Fluorite in the Lower Harz Mountains has strong positive Y anomalies and variable Eu anomalies at generally low REE abundances and no REE-mineral inclusions. Both Sr and Nd isotope compositions in the fluorite are very variable and indicate an open system (⁸⁷Sr/⁸⁶Sri, 0.710–0.718; εNdi, − 3 to − 17). The synchysite MREE enrichment in calcite gangue in the Upper Harz Mountains compares favorably with carbonatite-related LREE-dominated bastnäsite from China and elsewhere and allows an interesting perspective as a by-product of Pb–Zn mining.