Mathematical analysis of rare earth element patterns of fluorites from theEhrenfriedersdorf tin deposit, Germany: evidence for a hydrothermal mixingprocess of lanthanides from two different sources
T. Monecke et al., Mathematical analysis of rare earth element patterns of fluorites from theEhrenfriedersdorf tin deposit, Germany: evidence for a hydrothermal mixingprocess of lanthanides from two different sources, MINER PETR, 70(3-4), 2000, pp. 235-256
Systematic changes in REE patterns of fluorites with various colourations f
rom the Ehrenfriedersdorf tin deposit, Germany, can be related to the evolu
tion of the mineral forming hydrothermal fluid within the endo- and exocont
act of a related Li-F granite. One possible geochemical model that may expl
ain the fluorite REE patterns assumes that lanthanides derived from two dis
tinct element sources. It is suggested that a mantle- or lower crust-derive
d fluid (LREEsn < HREEsn) participated as one source of elements in the for
mation of the Li-F granitic magmas and in ore deposition. The metamorphic w
all rocks (LREEsn > HREEsn) were involved as a second source of elements du
e to assimilation and fluid-rock interaction (including admiring of crustal
fluids). Minerals that crystallised in the relatively cool and/or oxidisin
g environment of the exocontact incorporated the REEs from the ore-forming
fluid and therefore exhibit REE patterns that result from simple mixing of
the lanthanide distributions of the two element sources. Mineral precipitat
ion in the exocontact vein system led to a successive decrease of the total
lanthanide concentration of the migrating fluid. The fluorite REE patterns
further suggest that the mineral-forming fluid in the exocontact showed a
relative enrichment of Eu that can be attributed to fluid evolution within
the endocontact. The large ionic radius of EU2+ hampered the incorporation
of Eu into minerals crystallising from the fluid-magma system in this high
temperature and/or low oxygen fugacity environment. This process lead to an
enrichment of Eu in the fluid that subsequently entered the exocontact and
resulted in a corresponding Eu depletion of the altered granite.
The lanthanide distributions of three differently coloured fluorite samples
from a single zoned fluorite aggregate out of a cassiterite-bearing quartz
vein of the exocontact are suitable for mathematical modelling because of
their sample location in a sequence of homogeneous wall rocks and their wel
l-constrained relative age relationships. Mathematical modelling has been c
arried out to test whether the geochemical mixing model can indeed explain
the entire REE patterns of the three samples. In accordance with the propos
ed geochemical model, it is assumed that two endmembers with distinct REE p
atterns were mixed and that a process such as mineral precipitation reduced
the total REE content of the mixture. The incorporation of REEs from the f
luid into fluorite is described by a partition coefficient. Model values th
at are in closest possible agreement with the measured fluorite lanthanide
concentrations were derived from the mathematical model by minimising the m
ean squared deviation between the two data sets. It is shown that the globa
l minimum was found by application of the Gauss-Newton method and a special
ly designed evolutionary strategy. Ce and Eu were not considered in a first
analysis because positive anomalies in the fluorite REE patterns indicate
anomalous geochemical behaviour of these elements during the ore forming pr
ocesses. The differences between the model values and the measured lanthani
de concentrations were found to be smaller than the analytical errors of th
e REE measurements. In a second analysis the Ce and Eu concentrations of th
e three fluorites were taken into account and deviations between the model
and measured values for these two elements were observed that could not be
explained by analytical errors only. These findings suggest that the geoche
mical mixing model is a likely explanation for the measured fluorite REE pa
tterns, while the observed Ce and Eu contents do not simply result from mix
ing processes.