Textures and compositions of clinopyroxene in an Fe skarn with implications for ore-fluid evolution and mineral-fluid REE partitioning

Abstract

Clinopyroxene is a common phase in a variety of hydrothermal ore deposits, notably in skarn deposits where it typically occurs as a major constituent. It commonly displays variations in texture and composition due to changing physicochemical conditions and contains relatively high concentrations of rare earth elements (REEs), making it an important petrogenetic indicator. In this study, we integrate textural and geochemical investigations of clinopyroxene from the Baijian Fe skarn deposit (eastern China) to constrain the evolution of ore-forming fluids and provide significant new insights into partitioning of REEs between the clinopyroxene and hydrothermal fluids.In the Baijian Fe skarn, endoskarn clinopyroxene is typically zoned with homogeneous Fe-poor cores and oscillatory zoned Fe-rich rims. The rims are also enriched in Na, Co, Ni, and V and contain primary fluid inclusions with high salinities of 57.7–61.2 wt.% NaCl equiv. The compositional differences between the cores and rims reflect a transition from dilute, Fe-poor fluids to saline, Fe-rich components, likely a result of fluid boiling. In the exoskarn, early formed Fe-poor clinopyroxene is commonly replaced with Fe-rich clinopyroxene through fluid-assisted, coupled dissolution-reprecipitation processes. Overall, the textural features and compositional variations of clinopyroxene reflect a transition from early diffusive metasomatism by low-salinity fluids under low water/rock ratios to later advective metasomatism by high-salinity fluids under high water/rock ratios.When normalized to the modeled composition of the melt-equilibrated fluid, the log light REE and middle REE abundances in the clinopyroxene fit a linear function of the radius parameter (expressed as r0/2 × (ri − r0)2 + 1/3×(ri − r0)3) of the lattice strain model, indicating that crystal lattice strain has exerted a major control on REE clinopyroxene-fluid partitioning. Cores of zoned endoskarn clinopyroxenes show an upward increase in heavy REE (HREE) from Ho to Lu, likely caused by incorporation of those elements both into the eightfold M2 site and the sixfold M1 site of clinopyroxene. The increasing HREE compatibility coincides with elevated Mn contents in the cores of the clinopyroxene grains, implying that Mn has a significant effect on the accommodation of HREE into the sixfold M1 site of clinopyroxene, presumably due to the similar ionic radius between Mn2+ in sixfold coordination (Mn2+ = 0.83 Å) and the HREE (e.g., Lu3+ = 0.861 Å).