Mana provided by default: no such thing.
There's no such thing as lands producing mana by default. They only produce mana according to the abilities given to them. It's entirely possible to have lands which generate no mana at all: consider Dark Depths, for example.
You're probably now thinking "how do Islands work then? They have no abilities at all." However, basic lands — Islands, Mountains, Plains, Swamps and Forests — all implicitly, because of having that basic land type, have an ability of the form:
{T}: Add {color} to your mana pool.
Where {color} is blue, red, white, black or green depending on the type of land. So if a land is a Forest, being a forest implicitly means it has the ability:
{T}: Add {G} to your mana pool.
Lands of more than one basic land type, such as Steam Vents which is an Island Mountain, implicitly have this ability for each of their types, but you can only activate one. Hence the reminder text at the top of the card's description:
({T}: Add {U} or {R} to your mana pool.)
It really has these two abilities implicitly:
{T}: Add {U} to your mana pool.
{T}: Add {R} to your mana pool.
But tapping a card doesn't mean activating all tap abilities on it. You pick an ability (just one), and tap it to pay the cost of that ability. When a card like this has two tap abilities, that limits you to only being able to activate one at a time. (Unless you can untap it somehow, such as with Kiora's Follower.)
Radiant fountain has no basic land types (it's not an Island, Forest, Plains, Swamp or Mountain). It has exactly one mana ability, which adds 1 colorless mana to your mana pool. That is the only mana it produces.