It will be great to divide mine by type and progression. Actually, all bronze is open to anyone found one and all of them have same minerals. Will be better to divide mine by type and give bronze mine only available by the same mechanic you make for iron (imagine a mine blocked by rocks where you can use "Secure Path" from technology to be able to access bronze mine the same like "Mine" to acces Iron mine.
This way people can be able to make tin item, copper item, prepare bronze age by unlocking bronze item and will be obliged to travel more from different mine and not only one when "Mine is used" 🙂