This chest is by Thunderhead Keep mission. Just look at the map.
Exit out to Ice Floe from Thunderhead Keep. Then follow the map.
The Black 'rectangle' means that the chest can spawn anywhere near there. I just did this run 5 minutes ago to make this map. Also, I believe no one has this chest run anywhere. Hope I get in, thanks.
Since I cannot yet reply to posts, I will say that yes, it is guarded by mursaat and jades(by this time, you should already be infused because it IS at the thunderhead keep mission), but it is very easy to get through them. Unless you run a crappy warrior build, you will make it. Before posting this, I ran there like 4-5 times to check on drops. I had a couple blue items drop, but not Purple or Rare items.
Anywho, this chest is easily reachable if you run a good warrior build. Rangers may be able to do it too, maybe. I will have to try with my ranger.
Also, I have no idea if this has already been posted. I hope not. :/