So, I just upgraded all my copper mines to level 3 built on fast deposits. With production times of around 2 mins, that means they should go down around 90 per hour, meaning each mine should last approximately 9 hours.... However, 3 hours later and the faster ones have exhausted with all the rest down by well over two thirds capacity. Either my maths is way off or there's a bug somewhere!
EDIT:
Just looked more closely at mine behavior and the deposits are depleting by 9 ore per production cycle instead of 3.