At a fundamental level, a problem with impurities in water that touches heat exchagers is that it is possible for a layer of the impurities to form on the heat exchanger. The higher temperature can make molecules in water precipitate out and attach to the heat exchanger. What this does is basically form an insulative layer around the heat exchanger which is the last thing you want because you want the heat to move as freely as possible. Some might scoff at this, but when I was briefly exposed to boilers in a different career, I found that a layer of deposits only milimeters thick could make a boiler tube burst. While goethermal is not running steam we don't need to worry about bursting, but I would guess you would lose a lot of efficiency.

That is all theoretical - there may be something that open loop geo systems do to ensure that these deposits do not form. I only state this because in the original post there is a question as to what impurities can do to the geo system.

One of the things we do that is not true of steam boilers is put our flow solenoids downstream of the heat pump as positive pressure helps mitigate build-up. But you are correct in that extra maintenance is required with open loop systems because of build up. Called "flushing" different chemicals are used depending on what is in the water. The good geo contractor will test or have H/O test water first to try to predict how much of a problem this will pose.
MI has bacterial iron here and there, not much is nastier to deal with.
J