As far as I know minerals are only generated either when a body is surveyed, or if an NPR is generated on the body. For an NPR the "homeworld mineral generation" calculation is used which gives a guaranteed High availability of all TNE's but at fairly low reserve. Basically they get the same level of resources as you always start with on earth, there's slight variability of course. You can check yourself using space master mode, go to the system information screen with SM mode on and click any body, there's an option somewhere to regenerate mineral deposits, and another to regenerate homeworld level deposits, or alternatively you can manually define any resources.