OK, sort of a random set of thoughts, here...
Resources becoming available before a use is found for them is silly in that things such as Saltpeter weren't considered valuable until knowledge of what could be done with it was discovered.
Aluminum was worth nothing to a nation until it became a usefull resource. Until then, it was merely a curiosity, nothing more.
How many cities would build themselves near a source of some worthless metal when there was coal and game aplenty in a different location?
Having resources buildable after a peroid of time makes sense for some resources, but you simply can't produce metal or coal out of nothing. Horses, wheat, grapes...these are things that can be reproduced, if a quantity of them and the technological concepts for their reproduction become available. Then there is the consideration of quality to take into account. Sure, Washington can produce wine, but how good is it compared to wine from other places?
The more complicated the calculations added to the system, the larger the game, and the slower the game play. But, then again, games become larger and larger as bigger, better, faster computers become more commonly available.
Coal, Iron, Aluminum...these things get used up. Some are reuseable, while others are completely gone, once they've been used.
Having a limit, based on usage, makes sense, but then more resources would have to be spread around, or the ability, to recycle, rebuild, or simulate resources would have to be added to the game system, as appropriate for any given resource, else all resources would eventually become exhausted, and then the game would run down due the lack of the needed resources to build military units.
Having only some resources become visible upon the discovery of its value, while having the rest remain undiscovered would make the addition of Prospectors or the Prospect order for Workers a sensible one, but having a Prospector able to discover a resource merely by looking might imbalance the game by allowing one civ to successfully discover Iron, Coal, Uranium, and Aluminum all in the same place, while another civ fails time and time again to find any resources at all.