Texture and composition evolution of sphalerite in metamorphosed deposits: An example from the Keketale Pb-Zn(-Ag) deposit, NW China

Sphalerite hosts high amounts of Fe, Mn and Cd, and critical metals such as Ga, Ge and In within undeformed hydrothermal deposits, but the impact of deformation and metamorphism on trace element concentrations remains poorly understood. In this study, we combined with the microscopic observation, electron microprobe analysis and laser ablation -inductively coupled plasma -mass spectrometer to constrain the textural and compositional evolution of sphalerite from the metamorphosed Keketale Pb-Zn(-Ag) deposit, NW China. Five types sphalerite were identified, namely, the syn-VMS type (Sp1) and deformation -metamorphism related types including deformation (Sp2), peak- (Sp3), retrograde- (Sp4), and late -metamorphism (Sp5) types, with Sp2 and Sp3 formed under solid -phase and Sp4 and Sp5 in fluid access environment. We found three different patterns for the trace element behaviors in sphalerite during deformation -metamorphism. The first pattern is represented by a group of elements (Cu, Pb, Ag, In, Cd and Sb), the contents of which decrease from Sp1 to partially recrystallized Sp2 and Sp3. It suggests the elemental remobilization under deformation and metamorphism with possible correlation with metamorphic grade. These elements may have been released through intragrain diffusion and fluid -mediated liberation. The In contents are relatively high in Sp4 and Sp5. This is owing to that the liberated In would concentrate in the fluids and is hard to access to other sulfides other than sphalerite. The second pattern is represented by Mn, the content of which increases from Sp1 to Sp2 and Sp3. The Mn, possibly sourced from co -existing pyrrhotite, could reincorporate into sphalerite during metamorphism. The great decrease of Mn from Sp4 to Sp5 suggests Mn would enter into the early formed sphalerite and remain low contents in the late -metamorphic hydrothermal fluids. The third pattern is represented by Hg, the content of which is relatively constant in original and metamorphosed sphalerite. This may indicate that Hg could be steady in the lattice during the deformation -metamorphism process. Such a process can also influence the elemental substitution mechanisms in sphalerite. The dominant substitution for Ag in Sp1 is 2Zn2+<-> Ag++Sb3+, but in Sp2 and Sp3, some Ag are involved in the substitution of 2Zn2+<-> (Cu, Ag)++(Sb, In)3+. The unequal release of substitution element couples is likely to account for the substitution mechanism variations. Temperature conditions of the evolution processes can be constrained by the sphalerite geothermometors with the VMS origination at - 350-330 degrees C (Sp1) and fluid access during the retro-metamorphism process at - 360-310 degrees C (Sp4 and Sp5). The prograde and peak metamorphic temperatures recorded by the Sp2 and Sp3 were not robust due to extensive release of thermometer related trace elements (i.e., Ga, Ge and In). The trace elements in the Keketale sphalerite combined with other metamorphosed deposits confirm that the elemental abundances are mainly controlled by the combination of genetic type and metamorphic grade, with possible other controlling factors to be explored.