Hmmm, I have said this before, but a slightly simpler compromise system.
Yes, resources come in 'quantities', but not in 'barrels', 'tonnes' or whatever-just a simple 'size' characteristic-from 1 to 10, for instance.
This size factor will help determine the likelihood of finding the resource, in a inversely proportional fashion. So, for example, a size 8 resource is much rarer than a size 4 resource, and so the former will appear on a map much less often than the latter. 'Size' also effects the 'disappearance' ratio. Therefore a large resource will have less chance of vanishing than a small one. Lastly, it effects how much of a resource you can trade to other civs. So, for instance, if you had a size 8 resource, you could keep 4 for yourself, and trade 2 each to 2 other civs!
The other side of resource disappearance, of cource, is demand. Therefore, the more cities you have, the more you are trading to other civs, and the more units and improvements you have which require that resource, then the more likely the resource is to disappear!
Yours,
Aussie_Lurker.