As said before chrome is from cylinders or ram usually. Unusual to get tin in hydraulics but a possible source may be from bearings in the pump although another metal ie lead would be expected. Please post if you do find a positive source in the unit in question.
Since both your chromium and tin levels have gone high, I would suspect a bearing problem somewhere in your system, possibly in your pump. Zinc is normally an anti wear additive. could it be possible that someone topped off your tank with new fluid?