Using Rare Earth Elements (REE) to Decipher the Origin of Ore Fluids Associated with Granite Intrusions

Abstract

A practical method is presented to estimate rare earth element (REE) concentrations in a magmatic fluid phase in equilibrium with water-saturated granitic melts based on empirical fluid–melt partition coefficients of REE (kPREE). The values of kPREE can be calculated from a set of new polynomial equations linking to the Cl molality (mClv) of the magmatic fluid phase associated with granitic melts, which are established via a statistical analysis of the existing experimental dataset. These equations may be applied to the entire pressure range (0.1 to 10.0 kb) within the continental crust. Also, the results indicate that light REEs (LREE) behave differently in magmatic fluids, i.e., either being fluid compatible with higher mClv or fluid incompatible with lower mClv values. In contrast, heavy REEs (HREE) are exclusively fluid incompatible, and partition favorably into granitic melts. Consequently, magmatic fluids tend to be rich in LREE relative to HREE, leading to REE fractionation during the evolution of magmatic hydrothermal systems. The maximum kPREE value for each element is predicted and presented in a REE distribution diagram constrained by the threshold value of mClv. The REE contents of the granitic melt are approximated by whole-rock analysis, so that REE concentrations in the associated magmatic fluid phase would be estimated from the value of kPREE given chemical equilibrium. Two examples are provided, which show the use of this method as a REE tracer to fingerprint the source of ore fluids responsible for the Lake George intrusion-related Au–Sb deposit in New Brunswick (Canada), and the Bakircay Cu–Au (–Mo) porphyry systems in northern Turkey.

HFSE-REE Transfer Mechanisms During Metasomatism of a Late Miocene Peraluminous Granite Intruding a Carbonate Host (Campiglia Marittima, Tuscany)

“…The controlling factors for REE mobilization are the formation of aqueous chloride complexes (REECl 2+ , REECl 2+ ), whereas the mobility of Zr mostly depended on the formation of hydroxy-fluoride complexes such as ZrF(OH) 3 and ZrF 2 (OH) 2 [42][43][44]. This process is enhanced by low pH values, which depends on the initial concentrations of Cl and F, temperature (decreasing T correspond to increasing supply of H + ), and the rate of fluid-rock exchange reactions.…”