I just wanted to know is it a good idea to use heavy armor on a mage? I was thinking of making a tanking mage. Just a llot of endurance mixed with willpower and intelligence.

That sounds like a battle mage.Of course it will work.

Right. That's seems like a Battlemage.

If you get skilled at Heavy Armor the Spell Effectiveness Penalty won't matter much, except for those spells that are level limited (eg Frenzy, Command, Turn Undead) which will be ineffective on enemies above level 23 or 24 - but it sounds like that's not a big deal for your character.

Sure it will work. Not the best choice for an illusionist as Savlian pointed out, but for other mages, not a big deal. Due to a battlemage's heavy armor, more enchantment slots can be devoted to magicka and magical protection. Battlemages are built to go toe-to-toe, so I like them to embrace that. The key for a battlemage to me is to embrace the synergy possible by combining magic and melee. This can be as simple as starting a fight with a touch spell like this: Weakness to magic 100 x 10 seconds + soul trap x 10 seconds; then rail away with almost any enchanted weapon.

A couple great choices for battlemages:

Breton with mage birthsign. Focus on improving their puny starting melee attributes and finding items to boost a Breton's magic resist from 50% up toward 100%.

Any other race with atronach birthsign. Focus on finding items to boost the spell absorption from 50% up toward 100%. Atronach can be challenging for newer players, but can be quite powerful for those willing to get hit by enchanted weapons and spells - like battlemages are designed to do.

Not just Battle Mage, but also Spellsword and Sorcerer.