A minion created with Minion of Chaos is able to select Psyker as one of the traits, however Psy Rating (which is needed to pick up any powers, relevant talents and so on which got anything to do with psykers) is a tier 3 talent, which means you're not able to get it.

As far as I know you don't get Psy Rating 1 automatically when you get the Psyker trait. Why is the trait even there, if so? Kinda hard to make a wyrd sorcerer with Minion of Chaos (which it implies you're able to create, according to the talent description).

It's because KHORNE does not like Psykers.

For me explanation for this is that you need Psyker trait for psyniscience skill (it doesn't say so in the description but that may be because it's mainly ctrl+c, ctrl+v from other books ant there you could only select psyniscience skill from psyker-like class).
So your Minion of Chaos can detect immaterium. Or it's a bug not corrected in errata.

ShadowRay said:

This question has been asked several times before on this board, and the consensus seems to be that you're right. (Normal) Minion of Chaos can get the Psyker trait which enables him to use the Psyniscience skill, but to have an actual psyker minion, you have to go with Greater Minion of Chaos Talent.

Which is kinda logical considering how powerful and rare psykers are.

K0balt said:

Undoubtedly, a psyker minion is well worth the cost of a Tier 3 Talent.

At the same time, being able to take a normal minion with Psyniscience is also quite useful if you're not a psyker yourself. All in all, this aspect of minion rules seems alright with me.