Transport of rare earth elements by hydrocarbon-bearing brines: Implications for ore deposition and the use of REEs as fluid source tracers

Abstract

Low-temperature (100–160 °C) hydrocarbon-bearing brines mobilize REE at concentrations up to several thousand ppm total rare earth elements (ΣREE) at El Hammam, Morocco. The major element chemistry (Na, Ca, Mg) and temperature of the fluids in this system are consistent with those of oil field brines. In contrast, REE concentrations in the fluid suggest a significant element input from a magmatic source. Spatially related lamprophyres predating the formation of El Hammam share a similar trace element signature. Therefore, hydrocarbon-bearing basinal brines seem to have leached some of these elements through fluid–rock interaction from the lamprophyres, supporting the ability of oil-field brines to mobilize not only base metals (as in Mississippi Valley Type [MVT] deposits), but also rare earth elements.The stability constants of REE aqueous complex at temperatures of 100–160 °C, the chemistry of oil-field brines worldwide, and the partitioning behavior of REE between fluorite and water at El Hammam indicate that acetate (± carbonate and fluoride) are likely to be major contributors to REE mobilization in these settings. The direct implications of the low temperature transport of REE by oil-bearing brines is that, in systems where these kind of fluids occur, the REE signature observed may not reflect the source of the fluid as much as a combination of fluid source and fluid-rock interaction. In addition, oil-field brines can efficiently scavenge, transport, and deposit REE leading to anomalously high accumulations such as the El Hammam fluorite deposit and may also contribute to REE mobilization and concentration in other systems.