It's not surprising anymore if Mercatox scam their users because there's many accusations has been posted in this forum:

1. MERCATOX POTENTIAL SCAM !!!!!!
2. mercatox.com - Beware!
3. Mercatox.com is a big SCAM !
4. Blocking access to verified Mercatox account
5. My btc not confirmed for 24HRS/72 H MERCATOX MULTIPLE CASUALTIES SCAM MERCATOX?
6. MERCATOX BTC WITHDRAWAL

I read your problem hasn't been solved for one year and they're scam you for high amount, IMO you can report to police or legal law to process this matter.