If you decide on a PI or Pulse machine like the Lorenz Z1 great. That should take care of your mineralization. Personally I like the Makro Deep Hunter or Golden King. Yes one of the complaints on the Internet forums has been said that mineralization affects the Golden King. However there are things one can do to assist the detector in it's processing of detecting deeply buried objects. Much of this "mineralization issue" is due to operator error or inexperience.
Here's somethings to do when the machine begins to be affected by the ground. #1 manually ground balancing #2 turning the sensitivity to 70% or even 50%. #3 You can also try turning off your iron so the machine rejects or ignores iron. #4 Swing the machine steady and even 4-8 cm above the ground #5 using the reset switch as ofter as you need it.
Once you believe you found a target, swing and check the spot several times from different angles. If they all check out, dig. Hope that helps.