A preliminary petrogenetic grid for REE fluorocarbonates and associated minerals

Abstract

The bulk of the world's economic LREE reserves occur as fluorocarbonate minerals, notably bastnaesite. However, despite the importance of these minerals, very little is known about the physicochemical conditions controlling their formation. In this paper we attempt to partly redress this deficiency by qualitatively determining P- T and compositional relationships for part of the system Ln(CO 3)F-CaCO 3-F 2(CO 3) −1-H 2O, including the minerals fluorite, calcite, bastnaesite, parisite, synchysite and fluocerite. This degenerate ( n + 3)-phase multisystem has 23 possible base P- T topologies, plus their mirror images and trivial conjugates, from which we have been able to select a single probable stable topology using a combination of published experimental phase equilibrium data, molar volume and entropy estimates and natural assemblage data. Compositional relationships in the system have been established by constructing log ( a ca 2+ · a F −2 ) vs. log ( a F −2 a CO 3 2− ) diagrams for each of the stable divariant regions shown on the P- T net. Important conclusions of the study with respect to P- T relationships are 1. (1) that all of the above REE-fluorocarbonate minerals can form at comparatively low pressure and temperature; 2. (2) that bastnaesite + fluorite is a low-temperature assemblage and, in the presence of synchysite or calcite, is also restricted to low or high pressure, respectively; 3. (3) that parisite + fluorite is stable to higher temperatures; 4. (4) that bastnaesite + synchysite + calcite is restricted to high P- T conditions; 5. (5) that parisite reacts to form bastnaesite and calcite at high temperatures (<620°C at 1 kb); and 6. (6) that bastnaesite-(La) decomposes by a decarbonation reaction at temperatures <750°C at 1 kb and at lower temperatures with decreasing ionic radius of the lanthanide. The principal conclusions with respect to compositional relationships are 1. (1) that transformations among the REE fluorocarbonates cannot occur through changes in F − activity alone, but can result from variations in either Ca 2+ or CO 3 2− activities, and 2. (2) that fluocerite can coexist with any of the fluorocarbonate minerals at high F − or low CO 3 2− activity depending on the activity of Ca 2+, pressure and temperature. Fuller understanding of the genesis of REE fluorocarbonate deposits will require extensive fluid inclusion and related studies and systematic determinations of phase relations through well-constrained, reversed experiments.