Metal parts that move against other metal parts will leave marks. That's just the way it is - friction - even a small amount - will cause wear.
Move your slide back, and you will find that the lockup area of the barrel is not the only place where the slide contacts the barrel. From what you describe, there does not seem to be anything wrong. Then again, we cannot tell for sure since you have elected to not tell us how far these "marks" go down the length of the barrel.