Monazite Alteration in H2O ± HCl ± NaCl ± CaCl2 Fluids at 150 ºC and psat: Implications for Uranium Deposits

Antonin Richard,Jean-Marc Montel,Romain Leborgne,Michel Cuney,Chantal Peiffert,Michel Cathelineau

doi:10.3390/min5040518

Abstract

Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some Proterozoic sedimentary basins in close relationship with high-grade uranium (U) deposits. Hence, monazite has been proposed as a viable source for some U deposits. However, monazite alteration remains enigmatic with regard to its high stability in relatively low temperature hydrothermal conditions. Here, the results of batch experiments in which 10 mg of natural monazite grains were reacted with 15 mL of Na-Ca-Cl (6 molal Cl) solutions as well as in pure water at 150 oC and saturated vapor pressure (psat) for one and six months are reported. The influence of pH (pH = 1, 3, 7) and relative molar proportions of Na and Ca (Na/(Na + Ca) = 0, 0.5, 1), were tested. Discrete alteration features (etch pits and roughened surfaces) appear in a minority of the one month experiments and are more developed in the six months experiments, especially at pH = 1 and 3. Although spectacular alteration of monazite, as seen around U deposits, could not be reproduced here, this study shows that monazite is unstable in the presence of fluids analogous to acidic deep basinal brines.

Highlights

Monazite is a Light Rare Earth Element (LREE) orthophosphate, with the formula (LREE)PO4, and a common accessory mineral in magmatic and metamorphic rocks
Because monazite instability may result in the release of U, Th, Pb and REE, it has been considered as a potential source mineral for some U deposits, U oxides being in turn frequently enriched in REE [17,33,34,35,36,37,38]
Experimental conditions, experiment numbers and observations by SEM of monazite grains after one month and six months experiments are reported in Tables 2 and 3 respectively

Summary

Introduction

Monazite is a Light Rare Earth Element (LREE) orthophosphate, with the formula (LREE)PO4 , and a common accessory mineral in magmatic and metamorphic rocks. Basin (Saskatchewan, Canada) [41,42] These U deposits originate from protracted basin-scale circulation of U-bearing fluids, at ca. 1.5–1.2 Ga, through sedimentary and basement rocks concurrent with sediment diagenesis at temperatures of 150–200 ̋ C [43,44,45,46,47]. Monazite Alteration and U Sources in Proterozoic Basins and Basements

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Conclusion