ENIG frequency response

I vaguely recall that the ENIG (Electrolytes Nickel/Immersion Gold) finish on a PCB starts to have a lot of attenuation at high frequencies (from the skin loss in nickel or the thin gold layer?). Can anyone point me to documentation about that effect, or tell me what frequency range it affects? I'm supporting a 10 Gbps PCB design and unfortunately our experienced RF engineers are long gone.

Thanks,