As I have advanced in the game I found it difficult to keep up with bronze sword production to supply the adventures I want/need to do. The bottleneck has been copper production. With only 6 copper mines available there is only so much copper you can create in a given time. Fortunately rebuilding a copper mine is very quick, only 5 minutes for a geo find the resource, 3 minutes to build a mine and 2.5 minutes to upgrade to level 2.
Now, I could just upgrade my copper mines higher and extract the copper faster and rebuild mines more frequently. However, the tool cost of upgrading mines makes for diminishing returns with this strategy. I have upgraded a geologist to get 15% more copper in my deposits for 827 copper ore per mine. I have also upgraded him so that 40% of my mines can be mined faster, 1.5 minutes compared to 3 minutes. (This helps a lot!). I have run some numbers on this to quantify the dimensioning returns of upgrading copper mines. This chart shows the theoretical production values of running all 6 deposits at each level.
Level 1 mines yields 1800 copper in 12 hours
Level 2 mines nets 1582 copper over level 1 in 12 hours
Level 3 mines net s 494 copper over level 2 in 12 hours
Level 4 mines nets -2553 copper over level 3 mines in 12 hours
Level 5 mines net -9083 copper over level 4 mines in 12 hours
*assumes no travel time for production and no time between mine collapse and rebuild.
Upgrading mines to level 4 or 5 actually costs you copper in the long run. The benefit of upgrading to level 3 is negligible when you consider that you will need the same amount of coal as copper to create the bronze needed for tool production. You would need 5.5 coking plants to supply the need coal for level 3 mines vs level 2 mines.
There are 3 things I conclude from this. You should only upgrade copper mines to level 2 and purchase any additional copper you need on the TO. There are other resources that have less impact on production that have similar or higher value in the TO. Second, I think the game developers should add a few more copper deposits on the map. Considering that endless copper mines help solve this bottleneck problem, I don't think this will happen. Finally, I need to reexamine my overall weapon production strategy.