Geochemistry of amphiboles in
niobium-yttrium-fluorine enriched pegmatites
ABSTRACT
Pegmatites are extremely coarse-grained igneous rocks that form from fluid-rich
magmatic bodies. These fluid-rich magmas allow for the migration of various
rare earth elements into the pegmatite. As a result, pegmatites are host to
a rich diversity of complex and exotic minerals. Due to their high rare element
content, pegmatites are of high economic importance for their use in a diverse
array of industrial and technological applications. Pegmatites occur most
commonly as laminar dikes, sills or veins stemming from a pluton, most likely
granitic in nature. Pegmatites often reflect the geochemical signature of
their host granite and serve as an extension of the fractionation trend seen
in the granite. Genetic models linking pegmatites to their mineral constituents
are critical in the exploration and mining of them. The objective of this
study was to focus on the possible use of the amphibole mineral group as a
genetic indicator reflecting the geochemistry of granitic pegmatites. Amphiboles
are uncommon in granitic pegmatites but do occur in niobium-yttrium-fluorine
(NYF) type pegmatites. The most common types of amphiboles known to occur
are hornblende, arfvedsonite, and riebeckite. Samples of pegmatitic amphiboles
were taken from the Mineral Collection in the Department of Mineral Sciences
and collected in the field from the Adirondacks, NY and from the New Jersey
Highlands, NJ. Twenty amphibole samples in all were extracted and prepared
for chemical analysis. The trace element content was measured using an X-ray
fluorescence spectrometer. Electron microprobe analysis was employed to reveal
the major element content. K/Rb, Al/Ga and Ta/Nb + Ta ratios were used as
fractionation indicators to measure any evolutionary trends within the amphiboles.
A positive correlation was noticed in graphical plots of Al/Ga vs. Zr and
Ta/Nb+Ta vs. Cu. These relationships indicated that there is a geochemical
evolution in the amphibole, much like there is in the pegmatites. However,
more samples from amphibole-bearing pegmatites need to be collected and analyzed
to verify these trends. Additionally, further studies on the rare element
content of the host granites should be pursued to verify the possible correlation
between them and the pegmatites.
This research was supported by a geology grant from the
National Science Foundation's Research Experiences for Undergraduates Program,
award # EAR-9732416.
Letter of Gratitude
