Sorry, but there 5 peaks over 6000m in the area. Cerro Acotango 6052m is this 5th and is located on the border (Chile - Bolivia) eastwards of Guallatiri.
I had the pleasure to summit this volcano one year ago (july 25th 2005).
Just southwards and in between Guallatiri and Cerro Acotango, there is also an "almost" 6000er named Cerro Elena Capurata 5990m.
All three are also known as "Quimsachatas" (triplets).
actually if you ll read closely, i said in pn lauca, and that info is correct to my understanding.
pomerape, parinacota, guallatire, and acotango. if there is another in the park, im unaware of it. please let me know which ones you meant.