Experimental Data Bearing on the Origin of Carbonatites, with Particular Reference to the Mountain Pass Rare Earth Deposit

Abstract

Carbonatites associated with alkalic igneous complexes are derivative magmas, not primary melts from the mantle. Experimental phase equilibrium studies have demonstrated that there are differentiation paths from high temperature silicate melts to low temperature carbonatite melts, and that for more alkali-rich melts segregation by liquid immiscibility may occur. Recent experiments in CaO-MgO-CO_2-H_2O show conditions for the coprecipitation of calcite, dolomite and periclase from melts at 650°C (with Fe present, magnetite would substitute for periclase). Addition of P_2O_5 and CaS has demonstrated limited solubility of P and reduced S in carbonatite magmas, and defined the conditions for coprecipitation of apatite and calcite. Addition of La(OH)_3 shows that the light rare earth elements (REE) are highly soluble in carbonatite magmas. A synthetic mixture (E) was selected to approximate the composition and components of the (REE)-rich carbonatite at Mountain Pass, California. The Mountain Pass rock is variable, but dominated by calcite (40-75%), barite (15-50%) and bastnaesite (5-15%). E is estimated to be fairly close to the eutectic in the system CaCO_3-Ca(OH)_2-BaSO_4-CaF_2. The join E-La(OH)_3 was studied at 1 kbar. The liquidus has a minimum at 18% La(OH)_3, 625°C; the solidus is at 543°C ; bastnaesite, (RE)FCO_3, and other fluocarbonates are approximated by their hydroxy equivalents. The solidus is about 35°C below the upper stability limit of bastnaesite. This is a strong indication that bastnaesite could crystallize with calcite and barite, from a similar melt with suitable proportions of CO_2, H_2O, and F, which supports an igneous origin for this mineral in the ore body of the Sulphide Queen Carbonatite at Mountain Pass. The experiments may reveal processes obscured by post-magmatic events in the ore-body.

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