Hydrothermal alteration of magmatic zircon related to NaCl-rich brines: Diffusion-reaction and dissolution-reprecipitation processes

Abstract

Magmatic zircon from altered gabbros adjacent to the ∼1.07 Ga Lala Fe-Cu deposit, SW China was modified by NaCl-rich brines related to the Fe-Cu mineralization. The modified zircon grains are composed mostly of both inclusion-free and porous domains, and some grains also have overgrowth/rims. The inclusion-free domains, commonly overgrown by the porous domains, are roughly homogeneous under BSE imaging but display oscillatory or sector zoning under CL imaging. In contrast, the porous domains are distinctly mosaic-like under CL imaging, and contain abundant pores and mineral inclusions such as thorite, xenotime, REE-rich phases, actinolite, albite, biotite and calcite.

The inclusion-free domains have concentrations of “non-formula” elements (for example Al, Ca, and Fe) much higher than the magmatic zircon, and are thus interpreted to be products of interaction between magmatic zircon (possibly metamictized) and the fluids via a diffusion-reaction process. However, these resultant domains have retained the Th and U contents, REE patterns, U-Pb ages and Hf isotopes of the precursor, magmatic zircon. On the other hand, the porous domains have stoichiometric end-member compositions (that is lowered Th, U, REE, Y, and P), and given that they are porous and inclusion-rich, we proposed that they have formed from the precursor, element-rich inclusion-free domains or magmatic zircon via a fluid-induced dissolution-reprecipitation process. It is notable that the porous domains retained the ¹⁷⁷Hf/¹⁷⁶Hf ratios of the precursors, but possess modified and meaningless U-Pb ages. Instead, a meaningful U-Pb age (1006 ± 62 Ma), similar to the timing of the Lala deposit, is well recorded by the overgrowth/rims with distinctly high LREEs (La = 50–700 ppm) and elevated Hf isotopic ratios. Our new results reveal that in the presence of the NaCl-rich brines, the nature of the precursor, magmatic zircon (that is high element budgets and/or metamictization) plays a key role on hydrothermal alteration of zircon.