I can't tell from the pix if the barrels are brazed or soldered. If brazed, the source of the copper could be the brazed joint and the contamination could be transmitted by a leaky braze joint allowing some fluids (possibly even rusting solution) to come thru the porous joint.
If the barrels are soldered, I'd wire up that barrel and solder the porosity shut. It doesn't take long and it assures no more leakage.
The other thing to consider is that porosity could be telling you something. There could be rust brewing under the ribs. A strip and re-lay of the ribs is the only way to know. Not what you wanted to hear, but always a possibility.
