The enchantment is producing the mana, but you can't Stifle it. I am curious what the link is you think exists there.
Stifle can't target mana abilities, because in the first place, Utopia Sprawl's ability is never on the Stack. It resolves instantly with the mana ability that triggered it.
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Sprawl has what is called a 'triggered mana ability'. Those simply cannot be targetted.
Rule 605.4a: A triggered mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after the mana ability that triggered it, without waiting for priority.
Stifle can't target mana abilities, because in the first place, Utopia Sprawl's ability is never on the Stack. It resolves instantly with the mana ability that triggered it.
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RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules