Mastery of Elements
The archmage can alter an arcane spell when cast so that it utilizes a different element from the one it normally uses. This ability can only alter a spell with the acid, cold, fire, electricity, or sonic descriptor. The spell’s casting time is unaffected. The caster decides whether to alter the spell’s energy type and chooses the new energy type when he begins casting. This ability costs one 8th-level spell slot.
You add ice or snow to your spell's components to make them more powerful.
Prerequisite: Con 13.
Benefit: If you add a handful of snow or ice as an additional material component to a spell when you cast it, the spell gains the cold descriptor. This does not actually change the nature of the spell you cast; a fireball cast with this feat still deals fire damage, but since it also carries the cold descriptor, it can be augmented by a number of feats listed in this chapter, such as Cold Focus and Frozen Magic.
If you add a handful of snow or ice as an additional material component to a spell when you cast it and that spell already has the cold descriptor, you increase the effective level of the spell being cast by +1.
Adding this additional material component requires you to spend a move action immediately before the spell is cast to gather fresh snow or ice from the surrounding environment. This snow or ice can be magically created by a conjuration spell, but no other ice manifested by a spell will do. You may take no other action between gathering the snow or ice and casting the spell (Frostburn, p. 50)
So since it is changing the spell's energy type (regardless of the original energy type) if the spell has a cold descriptor, then I guess any spell that is using "energy" would apply if is combined with Snowcasting? Here are some examples I can think of that are not using elemental energy originally:
Disrupt Undead (You direct a ray of positive energy... if the ray hits an undead creature, it deals 1d6 points of damage to it.)
Magic Missile (A missile of magical energy darts...)
Waves of Exhaustion (Waves of negative energy...)
So I assume a random archmage necromancer may combine these to make a Waves of Exhaustion [cold] that uses sonic energy to penetrate his opponent's Death Ward, since Death Ward only protects the victim from Negative Energy Effects?
The subject is immune to all death spells, magical death effects, energy drain, and any negative energy effects. (Death Ward)
Also, since the Mastery of Elements didn't actually specialize the available energy types, can positive/negative energy be chosen (e.g. Positive Energy Fireball)?