| Summary: | The St.-Honore alkaline complex is intruded along the Saguenay graben of the lapetan Rift System. Country rocks, in the vicinity, are Pre-Grenvillian syenites belonging to the Central Gneiss Belt of the Grenville Province. The complex has a reniform shape, and consists of a central portion of carbonatitic rocks enclosed in an alkaline syenite. === The carbonatite comprises concentric lenses which evolved from calcitite through dolomitite, to a brecciated core of ferrocarbonatite which hosts REE mineralization, mainly as REE flurocarbonates and monazite. This mineralization forms part of the breccia cement, and is associated with hematite, chlorite, ferroan dolomite, minor thorite, ilmenorutile and pyrite. === Apatite-phlogopite geothermometer yielded magmatic temperatures between 1150 and 800$ sp circ$C for the complex, and for the REE Zone, the temperatures range between 380 and 346$ sp circ$C, and are interpreted to reflect subsolidus conditions. An independent chlorite geothermometer yielded similar temperatures (364 to 321$ sp circ$C) for the REE Zone breccia cement. === A model is proposed in which REE concentration in the magma was initially buffered by the crystallization of pyrochlore and apatite, and was subsequently allowed to build up when these phases stopped crystallizing in the most evolved ferrocarbonatite. Saturation of this magma with water, late in its crystallization history, led to the separation of an acidic aqueous phase into which the REE were partitioned as fluorocomplexes. Fluid overpressure produced hydrobrecciation which led to adiabatic expansion of the fluid and rapid cooling. The precipitation of REE minerals was caused by a combination of the drop in temperature and an increase in pH due to the interaction of the fluid with the dolomite.
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