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

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 incl...

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Bibliographic Details
Published in:Mineralium deposita 2019-02, Vol.54 (2), p.307-328
Main Authors: Alles, Jonas, Ploch, Alexander-Maria, Schirmer, Thomas, Nolte, Nicole, Liessmann, Wilfried, Lehmann, Bernd
Format: Article
Language:English
Subjects:
Analytical methods
Anomalies
Calcite
Carbonates
Cerium
Copper
Diagnostic systems
Earth
Earth and Environmental Science
Earth Sciences
Electron microprobe
Electron probes
Enrichment
Fluorite
Fractionation
Gangue
Geology
Isotopes
Lead
Mesozoic
Microbeams
Mineral inclusions
Mineral Resources
Mineralogy
Mountains
Pattern analysis
Raman spectroscopy
Rare earth elements
Silver
Strontium 87
Strontium isotopes
Veins (geology)
Weight
Yttrium
Zinc
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Summary: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 87 Sr/ 86 Sr i around 0.714 and εNd i of ~ − 9, while carbonate gangue in the Lower Harz systems has 87 Sr/ 86 Sr i around 0.716 and εNd i 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 ( 87 Sr/ 86 Sr i , 0.710–0.718; εNd i , − 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.
ISSN:0026-4598
1432-1866
DOI:10.1007/s00126-018-0847-8