TEXTURAL AND GEOCHEMICAL EVIDENCE FOR SECONDARY TH AND REE MOBILIZATION VIA CARBONATE-SULFATE FLUIDS IN THE BUENA VISTA MAGNETITE-APATITE SYSTEM (NEVADA)

Kiruna-type magnetite apatite (MtAp) systems are currently an important source of iron and a potential future resource for Rare Earth Elements (REE) and other rare metals. A number of MtAp systems in Europe and Asia contain high concentrations of REE-bearing mineral phases which appear to form as a result of the secondary recrystallization of apatite and/or remobilization of REEs during late-stage hydrothermal overprinting. While this interpretation is well supported by textural evidence, questions remain as to the character of the fluids involved in this process. Recent studies which focus on the nature and evolution of ore forming fluids in MtAp systems in the USA and Chile indicate that carbonate-sulfate melts and aqueous fluids are present throughout the paragenesis of the MtAp systems and are likely linked to the formation of ore-stage mineralization. Here we present textural and geochemical data of ore-stage apatite from the Buena Vista system. Buena Vista apatite shows evidence for the simultaneous recrystallization of apatite and leeching/mobilization of REEs and Th via a carbonate-sulfate fluid. Experimental evidence indicates that carbonate and sulfate melts strongly mobilize REEs and Th. Similarly, recent studies have shown that SO₄^2- and CO₃^2- act as strong complexing agents for REEs and Th in aqueous solutions over a range of conditions relative to chloride solutions. Together, this data indicates that the carbonate-sulfate fluids associated with ore-stage iron mineralization in MtAp deposits also are responsible for the secondary enrichment of REEs and critical metals in these systems. In addition, similar hydrothermal upgrading of REE and rare metal concentrations via aqueous-carbonic fluids and sulfate-bearing aqueous solutions is observed in carbonatite and evolved granitic systems. This suggests a common genetic process involved in the formation of REE mineralization over a range of geologic settings.