Spend only mana produced by basic lands to cast Imperiosaur.
5/1/2007 Mana from any source other than a land with the supertype “basic” can’t be spent to cast Imperiosaur.
5/1/2007 Many mana-producing effects (such as Heartbeat of Spring or Utopia Sprawl, for example) add mana to a player’s mana pool whenever a land becomes tapped. This mana is produced by the enchantment, not the land, so it can’t be used to pay for Imperiosaur.
5/1/2007 A small number of effects (Pulse of Llanowar and Hall of Gemstone, for example) state that a land “produces” certain mana instead of what it would normally produce. If the land is basic, then mana produced this way can be spent to cast Imperiosaur.
5/1/2007 If a basic land gains a mana ability (from Riftstone Portal, Joiner Adept, or Rain of Filth, for example), the mana produced this way can be used to cast for Imperiosaur.
5/1/2007 Imperiosaur’s restriction applies to the total cost to cast it. If there are any additional costs (such as from Feroz’s Ban, for example), those costs would have to be paid with mana produced by basic lands as well.