When a survey team makes a successful find minerals roll, which is based on their skill level, a random mineral is selected with Duranium being twice as likely to be selected as any other mineral. Once the specific mineral is known, the same mineral generation check that was used during system generation is used again. If the result indicates the selected mineral has a higher amount or accessibility than the current deposit, the deposit is updated accordingly, with a limit of 100,000 extra (200,000 for Duranium)
So while a team's chance of carrying out a mineral generation check is based on their skill, the actual chance of finding something during that check is based on the same parameters used during system generation.
Steve