| dc.description.abstract | Accessory rare metal mineralization has been investigated in seven
lithologies in intrusive Centres I and III of the Coldwell alkaline complex. All
units contain minerals that are enriched in a suite of granitophlle elements,
which typically include Nb, REE, Y, Th, U and Zr. Mineral abundances,
composition, and mode of occurrence differ between units.
Centre III is characterized by crystallization of subhedral-to-euhedral
chevkinite, pyrochlore and monazite from late-stage melts or residual pore
fluids in the more-evolved quartz and ferro-edenite syenites. These minerals
are invariably altered to fluorocarbonate or recrystallized by later F‘ and
C032-bearing deuteric fluids. The Centre I, ferroaugite syenite minerals
exhibit similar morphological and replacement textures to those present in
Centre III. In contrast, the Craddock Cove syenite is mildly K and
Fe-metasomatised with incipient replacement of plagioclase and amphibole by
K-feldspar, zircon, fluorocarbonate, Nb-rutile (?), allanite, and rare chevkinite.
Fe-rich fluids under oxidizing conditions are believed to have precipitated
Fe3+ bearing fluorocarbonate in which one third of the (REE)F layers are
replaced by Fe3+ layers.
Most Centre I rare earth minerals are enriched in the HREE relative to those
from centre III, in particular pyrochlore, fluorocarbonate, allanite in the
eastern contact pegmatites and the quartz syenite dykes. Compositional data
for adjacent syntaxial intergrown domains of bastnaesite, synchysite, and
parisite indicate that HREE-enrichment may, in part, be influenced by the Ca
content of the species.
The highest contents of Ce (4193 ppm), Zr (1613 ppm), V (650 ppm), Th (223
ppm) and U (428 ppm) were found in the quartz syenite dykes intruding the
Craddock Cove syenite and Port Munroe megaxenolith. The emplacement of the
quartz syenite dykes and the introduction of the metasomatizing fluids of the
Craddock Cove syenite may be temporally related to the differentiation of
residual fluids in the apical zone of the Centre I magma chamber. Complexing
of F and CO32- with rare metals may have permitted their concentration,
transportation and precipitation in structurally favourable settings. The
megaxenoliths have been susceptible to brittle fracturing and should be
considered primary targets for further exploration. The Craddock Cove syenites,
although intruded by the dykes, may have been hot during dyke emplacement and
therefore not as prone to brittle fracturing. | |
