On the Ornate Heorot Idol for the Primalist, this affix does not appear to trigger.
“+x% chance to cast an avalanche boulder at a nearby enemy on SPELL cast”
I had 2 of the idols equipped for a total of 15% chance to drop on avalanche on a spell cast. I was casting ice thorns every 0.9 seconds and casting maelstrom every 1.8 seconds, both being spells, and never saw any avalanche boulders drops during an entire monolith run. Statistically I would expect to see an avalanche about every 6-8 seconds at that rate of casting my other spells. Avalanche is on my bar and I have 20 points in the tree, 4 points in dead of winter, 1 in Frost, 1 in frost touch, 3 in explosive impact, 1 in mountain peak, 1 in precision, 1 in wild path, 4 in naturally occurring, 1 in hailstorm, and 3 in unyielding storm. I also wan not triggering it manually for the purposes of the test. I did not test the other affix that is the same as this one, just for melee attacks (instead of spells) , so I am only speaking of the one that is supposed to trigger of my casting of spells.