Damn... "thanks a bunch" = a sarcastic/ironic version of saying thank you. 
Oh boy, and I often used it to thanks people here on twitter for their comments. 
Sorry!! english is not my first language
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It's actually perfectly fine lol, it all depends on context
yeah, finally I think so too. I just googled it up and the first results gave me the sarcastic and ironic meaning behind it. thanks for clearing it up! 
