Composition of garnet from the Yongping Cu skarn deposit, South China: Implication for ore-fluid evolution

Abstract

Garnet typically occurs as a major constituent in skarn deposits. It commonly displays variations in texture and composition due to changing physicochemical conditions. In this study, we integrate textural and geochemical studies of garnet from the Yongping Cu skarn deposit (South China) to constrain the nature and evolution of the ore-forming fluids. In the Yongping deposit, garnet shows red or brown colors in proximal skarns, and shows green color in distal skarns. All types of garnet grains show a core-rim texture. The cores generally have higher proportions of Fe and lower proportions of Al than the rims. In red or grown garnet grains, the Fe-rich cores have relatively high contents of REE, typically with an upward-facing parabola pattern with a peak at Ce or Pr. The rim domains have lower contents of REE, in particular La and Ce, than the cores. The compositional differences between the cores and rims reflect a transition from oxidized, Fe3+- and REE-rich fluids to fluids with lower contents of Fe3+ and REE. In green garnet, the Fe-rich cores have obviously lower contents of REE than the red/brown garnet cores, and their REE patterns resemble those of ore-hosting marbles. It indicates that the distal fluids were relatively poor in REE, and partial REE in garnet may be sourced from the ore-hosting marbles. The rims of green garnet usually have obviously higher contents of REE than the cores, which may imply ingress of new pulses of REE-rich, magmatic-hydrothermal fluids during the garnet growth process. In both proximal and distal skarns, the prograde garnet grains may be partially replaced by Al-rich domains in the retrograde skarn alteration stage. The replaced domains generally have higher contents of F, Ti, HREE and lower contents of LREE than the original domains. It indicates that the metasomatic fluids were relatively reducing, and have higher contents of F, Ti and HREE than the earlier-stage fluids. In general, the composition of garnets can record multiple pluses of fluid activity and the complicated physic-chemical evolution paths of the fluids. The influence of wall-rocks on the compositions of minerals is also indicated. Thus, garnet from skarns can preserve a wealth of information regarding mineralization processes in their crystal growth stratigraphy.