Edziza Provincial Park is about 5 km at its closest to the 2008 pit which probably hasn't changed much with the new 2013 pit.

IIRC, the park was expanded in size fairly recently with the Land and Resource Management Planning (LRMP) in 2000.

http://archive.ilmb.gov.bc.ca/slrp/lrmp/smithers/cassiar/plan/files/CIS-LRMP-November-2006.pdf

I started to write a long spiel about the politics of park creation in BC but it gets too complicated. In short, BC went thru a cathartic stage of park establishment that has largely slowed for much of the province as of about 5-10 years ago culminating in Ecosystem Based Management on the north and central coast & Haida Gwaii. BC now enjoys some of the highest representation of parkland in the world. I think only Costa Rica and Alaska are better in terms of % area in protected park. Alaska had the luxury of barren ice fields to boost their percentage a wee bit.

As for expanding Edziza now, I highly doubt there is any interest in it. Edziza doesn't get that many visitors and does a great job of capturing a vast area of spectacular landscape and unique features like the Edziza Volcano Complex. I haven't heard any buzz at all about more preservation on the west side where Schaft Creek is and no reason for it in terms of tourism, biodiversity, fish, watersheds, etc. Mess Lake and much of Mess Creek are already inside the park.

Expanding the park to the west would only gain the park a very thin strip of vegetated land and waters before the big ice fields start in Teckland. Lots of lost economic landbase for little gain.