CONTROLS ON THE MOBILIZATION AND TRANSPORT OF HFSE IN ORE-FORMING MAGMATIC-HYDROTHERMAL SYSTEMS

Abstract:

The genesis of economical rare metals (Zr, Nb, REE) ore deposits in largegranitic complexes (e.g., Strange Lake and Thor Lake Nechalacho deposit, Canada; Galineiro complex, Spain) is related to the intrusion of alkaline halogen-rich magmatic bodies. Although the role of exolved magmatic fluids in the mobilization and transport of HFSE is widely recognized, the physico-chemical conditions and atomic-scale mechanisms that control the formation of the ore deposits remain poorly understood.

We present new experimental constraints on behavior of HFSE during the exsolution of aqueous fluids from peralkaline granitic magmas at crustal conditions. In situ partitioning and speciation studies of Zr in the haplogranite-(F)-H₂O systems using synchrotron X-ray spectroscopies provide evidence for large controls of fluid chemistry and temperature on the mobilization and transport of HFSE in crustal settings. At shallow crustal pressure conditions (> 800 °C and 0.3 GPa), Zr preferentially partitions into the exolved aqueous fluid in the presence of fluorine (D^f/m_Zr = 1.40 ± 0.10) as previously reported for Nb in F- (and Cl-) bearing metaluminous granitic systems at similar conditions. The reverse partitioning of HFSE (Zr and Nb) into the aqueous phase at temperature above 800 °C contrast with the behavior observed at lower temperatures, where the D^f/m_Zr remain lower than 1 at all pressures. The enrichment of the aqueous phase in HFSE (Zr, Nb) in the earlier stages of the magmatic evolution is likely related to the enhanced peralkalinity of low pressure (< 0.4 GPa), F-bearing aqueous fluid coexisting with granitic melts as temperature increases. This particular fluid chemistry provides the favorable conditions for the mobilization of HFSE via the formation of HFSE-O-Si/Na clusters in the fluid as shown by the in situ Zr speciation data. Our results show that the exsolution of highly alkaline early magmatic fluid at pressures below 0.4 GPa has the potential to extract HFSE from F-rich granitic melts and thus play a chief role in the formation of rare metal deposits. Moreover, the potential mobilization and redistribution of HFSE by high T-low P (T > 700 – 800 °C; 0.05 < P < 0.3 GPa) fluids in volcanic arcs may have critical implications for the interpretation of geochemical signature of arc magmas as will be discussed.