Feldspar mineralogy and rare-earth element (re)mobilization in iron-oxide copper gold systems from South Australia: a nanoscale study

Nanoscale characterization (TEM on FIB-SEM-prepared foils) was undertaken on feldspars undergoing
transformation from early post-magmatic (deuteric) to hydrothermal stages in granites hosting the Olympic Dam
Cu-U-Au-Ag deposit, and from the Cu-Au skarn at Hillside within the same iron-oxide copper-gold (IOCG)
province, South Australia. These include complex perthitic textures, anomalously Ba-, Fe-, or REE-rich
compositions, andREE-flourocarbonate + molybdenite assemblages which pseudomorph pre-existing feldspars.
Epitaxial orientations between cryptoperthite (magmatic), patch perthite (dueteric) and replacive albite
(hydrothermal) within vein perthite support interface-mediated reactions between pre-existing alkali-feldspars
and pervading fluid, irrespective of micro-scale crystal morphology. Such observations are consistent with a
coupled dissolution-reprecipitation reaction mechanism, which assists in grain-scale element remobilization via
the generation of transient interconnected microporosity. Micro-scale aggregates of hydrothermal hyalophane
(Ba-rich K-feldspar), crystallizing within previously albitized areas of andesine, reveal a complex assemblage of
calc-silicate, As-bearing fluorapatite and Fe oxides along reaction boundaries in the enclosing albite-sericite
assemblage typical of deuteric alteration. Such inclusions are good REE repositories and their presence supports
REE remobilization at the grain-scale during early hydrothermal alteration. Iron-metasomatism is recognized by
nanoscale maghemite inclusions within ‘red-stained’ orthoclase, as well as by hematite in REE-fluorocarbonates,
which reflect broader-scale zonation patterns typical for IOCG systems. Potassium-feldspar from the contact
between alkali-granite and skarn at Hillside is characterized by 100–1000 ppm REE, attributable to pervasive
nanoscale inclusions of calc-silicates, concentrated along microfractures, or pore-attached. Feldspar replacement
by REE-fluorcarbonates at Olympic Dam and nanoscale calc-silicate inclusions in feldspar at Hillside are both
strong evidence for the role of feldspars in concentrating REE during intense metasomatism. Differences in
mineralogical expression are due to the availability of associated elements. Lattice-scale intergrowths of
assemblages indicative of Fe-metasomatism, REE-enrichment and sulfide deposition at Olympic Dam are
evidence for a spatial and temporal relationship between these processes.