Volker Möller

 

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Möller, V. and Williams-Jones A.E., 2014.

Hydrothermal controls on the mineralogy of the Basal Zone, Nechalacho Rare Metal Deposit (Canada) (SEG Conference 2014 Keystone)

 

The 2,176.0 ± 2.7 Ma Nechalacho rare metal deposit represents one of the world's largest resources of rare earth elements and niobium, with 269.36 Mt grading 1.4 wt. % total rare earth oxide (TREO) and 0.4 wt. % Nb2O5. The Basal Zone alone comprises a measured resource of 12.56 Mt grading 1.71 wt. % TREO with 0.38 wt. % heavy rare earth oxide (HREO), and an indicated resource of 49.33 Mt grading 1.62 wt. % TREO with 0.35 wt. % HREO, and 0.41 wt. % Nb2O5. Despite the fact that the Basal Zone was formed by magmatic processes, in particular fractional crystallization and accumulation of eudialyte (a complex REE-, Nb- and Ta-rich zirconosilicate) in layers, the mineralogies of the Basal Zone and its foyaite host rocks were intensely modified at the hydrothermal stage. Magnetite formed at the expense of aegirine, biotite and chlorite replaced K-feldspar, illite replaced sodalite, ankerite and fluorite occur in veins and pervasively, and late hematization affected aegirine and magnetite. Pseudomorphic recrystallization of eudialyte produced zircon, fergusonite-(Y), aeschynite (Y), columbite-(Fe) and columbite-(Mn) and the REE-fluorocarbonates, bastnäsite-(Ce), synchysite-(Ce) and parisite-(Ce) together with the gangue minerals, quartz, phlogopitic biotite, chlorite and ankerite. Hydrothermal zircon contains a fine dusting of fergusonite-(Y) micro-inclusions. The alteration is most intense in the central, upper part of the Basal Zone where coarse-textured HFSE minerals in the pseudomorphs, likely reflect greater fluid-rock interaction. The alteration assemblage was produced by a reduced, low pH fluid, which we conclude is represented by methane-bearing fluid inclusions in late magmatic and hydrothermal fluorite. Application of oxygen isotope thermometry to hydrothermal minerals yielded temperatures from 140 to 500 °C, which overlaps with the solidus temperature interpreted for the host rock, and is consistent with a magmatic source for the fluid and its subsequent cooling. Based on quartz-magnetite δ18O thermometry and fluid inclusion microthermometry, the pressure is estimated to have been 2.6 kbar. The δ13C and δ18O values of hydrothermal ankerite are consistent with the exsolution of late magmatic carbon-rich fluids during the cooling of the intrusion. Corresponding values in the T-Zone pegmatite immediately to the north of the Nechalacho deposit fall along the extension of the δ13C and δ18O trend for the latter. Mass transfer calculations indicate that appreciable proportions of magnesium were added to the system from an external source. The magma crystallizing the Nechalacho deposit was highly enriched in fluorine and carbon, which were partitioned into the fluids that exsolved during the crystallization of the Basal Zone. These fluids pooled in the central dome of the host intrusion, partially interacted with the surrounding basement rocks and re-circulated into the Nechalacho deposit. Carbon-rich, late magmatic fluids also escaped into the T-Zone to form a REE-rich pegmatite. Late stage hematization of the Basal Zone is interpreted to be an overprint by surface-derived, oxidized fluids. The hydrothermal modification of the Nechalacho deposit was a key factor in producing an extractable REE-Nb resource.

 

 

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