The FAQ for A Fistful of Cards has entries about the Abandoned Mine card that are very confusing. I finally realized the problem: there is a mistranslation in the card:
Abandoned Mine wrote:
Nella propria fase 1, se possibile, il giocatore pesca dalla pila degli scarti; in tal caso, per quel turno scarta sempre coperto sul mazzo.
In phase 1 of your turn, you must draw from the discard pile if possible. If you can, then for the whole turn you
play discard cards face down on the draw deck.
During Abandoned Mine, cards that you play and use normally go onto the discard pile as normal. Cards that you discard go onto the draw pile face down; this would include excess cards discarded at the end of your turn, cards discarded because of Duel, Indians, etc.
I don't speak Italian (though I do speak Spanish), but I compared the English and Italian versions of the rules for the base game. I noticed on p. 3 that the three phases Draw, Play and Discard in English are Pescare, Giocare and Scartare in Italian. In the quote above, "scarta" should have been translated "discard", not "play".
The discussion of the card in the FAQ makes sense now!