What is Nundorite? A petrological and geochemical study of an unusual rock of the Mount Arrowsmith Volcanics
Date
2022
Authors
Johns-Mead, L. Y.
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Thesis
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Abstract
Nundorite is an enigmatic rock, known only from its type locality within the Mount Arrowsmith Volcanics (MAV), Western NSW. Alkaline igneous systems such as the MAV are an important source of critical mineral resources. Nundorite is compositionally unique, being peralkaline and enriched in rare metals (Zr, Nb and REEs), but with low silica and high alumina. Despite these features, there has been limited research on nundorite.
This thesis represents the most extensive study of nundorite to date. A combination of field observations, petrography, bulk-rock and mineral geochemistry are used to describe samples and develop an understanding of nundorite’s genesis and evolution.
Although nundorite only occurs as a single low outcrop, there is a progression of alteration and deformation over a span of ~250 m across the outcrop. Bulk geochemical composition does not change across this traverse, so this variation is attributed to localised shearing and associated metamorphism.
Nundorite’s mineral assemblage is established as aegirine porphyroblasts in a predominantly sodic zeolite-orthoclase matrix with minor albite and nepheline. The primary zeolite changes from natrolite to analcime with proximity to a shear zone. Nundorite hosts a suite of unusual accessory minerals, including zircon, monazite, schizolite, cerite group minerals, fergusonite, fersmite, galgenbergite, epidote-allanite, as well as unidentified Zr and REE minerals. Similar complex arrays of accessory minerals have been described from other peralkaline volcanic systems that host critical minerals.
Modelling nundorite to felsic alkaline igneous rocks shows that its unique geochemical composition could be achieved through metasomatism of a highly fractionated member of the MAV. Fractionation caused initial enrichment in alkalis and REEs, this was then concentrated by the removal of silica by an alkaline fluid.
There are several avenues for further research on nundorite, including comprehensive mineral identification, further constraining alteration conditions, and evaluating the economic potential of the rock.
School/Discipline
School of Physical Sciences
Dissertation Note
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, YEAR
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