Common sources
This may be a little dated - I have it in my files from a few years ago.
Iron - Cylinders, rotating shafts, valve train and any steel part sharing the oil.
Copper - Brass or bronze parts, bushings, bearings, oil coolers, sacrificial coatings.
Nickel - Valve guides, trace element in steel, some cylinder types.
Chromium - Rings, cylinders, a trace element in steel.
Silver - Sacrificial coatings, a trace element in some types of bearings, bearing cage plating
Magnesium - Engine casings, additives
Aluminum - Pistons, piston pin plugs, bearing overlay, casings.
Lead - Primarily leaded gas blow-by, traces from bearings
Silicon - Abrasive dirt from intake air, silicone sealers and gaskets, sample contamination.
Tin - Bearings, bronze parts (with copper), anti-wear coatings.
Molybdenum - Traces of anti-wear coatings, some cylinder types, and bearings.
Also, I've heard, but not personally seen, that higher than normal copper can be due to using Aeroshell Plus causing some copper plating to come off some internal parts (ie., camshaft) - the copper plating was to protect those areas from nitriding and if this happens it's not really a problem although it might seem like one.