During the corse of casting spell's in a battle i see to the left the nbr of cards left in the deck. Question the cards that i discard are they returned into the deck after the battle or are they lost. If so how do i replenish the deck or do i have to buy a new deck?
The regular cards are returned to the deck, there's no need to buy a new deck or restock your cards.
The Treasure Cards (gold cards) are one use, and once you've cast them, they're gone, and you will need to replenish your stock of those cards to your deck.