Uranium Transport in F-Cl-Bearing Fluids and Hydrothermal Upgrading of U-Cu Ores in IOCG Deposits

Abstract

Uranium mineralization is commonly accompanied by enrichment of fluorite and other F-bearing minerals, leading to the hypothesis that fluoride may play a key role in the hydrothermal transport of U. In this paper, we review the thermodynamics of U(IV) and U(VI) complexing in chloride- and fluoride-bearing hydrothermal fluids and perform mineral solubility and reactive transport calculations to assess equilibrium controls on the association of F and U. Calculations of uraninite and U 3 O 8 (s) solubility in acidic F-rich (Cl : F = 100 [ppm-based]) hydrothermal fluids at 25–450°C, 600 bar, show that U(IV)-F complexes (reducing conditions) and uranyl-F complexes (oxidizing conditions) predominate at low temperature (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mi>T</mml:mi><mml:mo>&lt;</mml:mo><mml:mtext>~</mml:mtext><mml:mn>200</mml:mn></mml:math>°C), while above ~250°C, chloride complexes predominate in acidic solutions. In the case of uraninite, solubility is predicted to decrease dramatically as U(IV)Cl 2 2+ becomes the predominant U species at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mi>T</mml:mi><mml:mo>&gt;</mml:mo><mml:mn>260</mml:mn></mml:math>°C. In contrast, the solubility of U 3 O 8 (s) increases with increasing temperatures. We evaluated the potential of low-temperature fluids to upgrade U and F concentrations in magnetite-chalcopyrite ores. In our model, an oxidized (hematite-rich) granite is the primary source of F and has elevated U concentration. Hydrothermal fluids (15 wt.% NaCl equiv.) equilibrated with this granite at 200°C react with low-grade magnetite-chalcopyrite ores. The results show that extensive alteration by these oxidized fluids is an effective mechanism for forming ore-grade Cu-U mineralization, which is accompanied by the coenrichment of fluorite. Fluorite concentrations are continuously upgraded at the magnetite-hematite transformation boundary and in the hematite ores with increasing fluid : rock (F/R) ratio. Overall, the model indicates that the coenrichment of F and U in IOCG ores reflects mainly the source of the ore-forming fluids, rather than an active role of F in controlling the metal endowment of these deposits. Our calculations also show that the common geochemical features of hematite-dominated IOCG deposits can be related to a two-phase process, whereby a magnetite-hematite-rich orebody (formed via a number of processes/tectonic settings) is enriched in Cu ± U and F during a second stage (low temperature, oxidized) of hydrothermal circulation.