Numerical Modeling of REE Fractionation Patterns in Fluorapatite from the Olympic Dam Deposit (South Australia)

Abstract

Trace element signatures in apatite are used to study hydrothermal processes due to the ability of this mineral to chemically record and preserve the impact of individual hydrothermal events. Interpretation of rare earth element (REE)-signatures in hydrothermal apatite can be complex due to not only evolving f O 2 , f S 2 and fluid composition, but also to variety of different REE-complexes (Cl-, F-, P-, SO 4, CO 3 , oxide, OH − etc.) in hydrothermal fluid, and the significant differences in solubility and stability that these complexes exhibit. This contribution applies numerical modeling to evolving REE-signatures in apatite within the Olympic Dam iron-oxide-copper-gold deposit, South Australia with the aim of constraining fluid evolution. The REE-signatures of three unique types of apatite from hydrothermal assemblages that crystallized under partially constrained conditions have been numerically modeled, and the partitioning coefficients between apatite and fluid calculated in each case. Results of these calculations replicate the measured data well and show a transition from early light rare earth element (LREE)-to later middle rare earth element (MREE)-enriched apatite, which can be achieved by an evolution in the proportions of different REE-complexes. Modeling also efficiently explains the switch from REE-signatures with negative to positive Eu-anomalies. REE transport in hydrothermal fluids at Olympic Dam is attributed to REE-chloride complexes, thus explaining both the LREE-enriched character of the deposit and the relatively LREE-depleted nature of later generations of apatite. REE deposition may, however, have been induced by a weakening of REE-Cl activity and subsequent REE complexation with fluoride species. The conspicuous positive Eu-anomalies displayed by later apatite with are attributed to crystallization from high pH fluids characterized by the presence of Eu 3+ species.

Zircon at the Nanoscale Records Metasomatic Processes Leading to Large Magmatic–Hydrothermal Ore Systems

“…With ever increasing instrumental resolution and analytical precision, nanoscale studies can provide new insights into processes carrying implications for geochronology, ore deposit genesis and mineral exploration. 4. In summary, this study has shown that early metasomatic alteration can be recorded in magmatic zircon and that these patterns can be preserved over geological time, even when superimposed by the effects of metamictization.…”

“…Debate on their genesis includes the relative timing and evolution of fluids during transition from the magmatic to hydrothermal stage. Detailed investigation of magmatic accessory minerals at appropriate scales of observation has enabled fluid-mineral interaction to be fingerprinted (e.g., using apatite [3,4]).…”

Editorial for the Special Issue: Experimental and Thermodynamic Modeling of Ore-Forming Processes in Magmatic and Hydrothermal Systems

“…The reconstruction of the sources of ore components and fluids, physicochemical parameters, and the mechanisms of accumulation and separation of elements during the formation of ore deposits is an important task in addressing the fundamental problems of ore-forming processes. Some of the papers in this issue deal with experimental and thermodynamic modeling [19][20][21][22][23][24][25], while the others are devoted to the analytical geochemistry, geochronology, and genesis of some ore occurrences [26][27][28].…”

“…Krneta and coauthors [23] investigated the REE fractionation in apatites from the Olympic Dam iron oxide-copper-gold deposit (South Australia) with the aim of constraining fluid evolution. The concentrations of trace elements and their variation within hydrothermal minerals can provide valuable information on the fluid parameters and conditions of ore deposition for assemblages.…”

“…The REE signatures of three unique types of apatite from hydrothermal assemblages crystallized under partially constrained conditions were numerically modeled, and the partitioning coefficients between the apatite and fluid were calculated. Article by Krneta et al [23] adds important information to the current knowledge on REE fractionation in apatites.…”