no the iMac can only output 60hz, here are the options:
An external 6K display with a resolution of 6016 x 3384 pixels at 60 Hz with support for one billion colours or
Two external 6K displays with a resolution of 6016 x 3384 pixels at 60 Hz with support for one billion colours (requires Radeon Pro 5700 or Radeon Pro 5700 XT)
or
One external 5K display with a resolution of 5120 x 2880 at 60 Hz with support for one billion colours or Two external 4K UHD displays with 3840 x 2160 pixels at 60 Hz with support for one billion colours or Two external 4K displays with 4096 x 2304 pixels at 60 Hz with support for millions of colours