If I copy a spell that has a gold cost with Mastering the Elements, can I use Library of Rebirth's reaction for this part "pay Gold equal to its base Gold Cost:" on MtE?
Library of Rebirth and Mastering the Elements
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
If I copy a spell that has a gold cost with Mastering the Elements, can I use Library of Rebirth's reaction for this part "pay Gold equal to its base Gold Cost:" on MtE?
No, sorry.
"Paying for a Spell" means paying the Gold cost for a Spell to enter play.
When you pay the Gold as instructed by Mastering the Elements' ability, you are paying the cost of an action ON a Spell, not paying for any Spell itself.