Barentsburg is a Russian mining settlement in the Svalbard archipelago, a short distance away from Longyearbyen. It's a Russian village with a population of 500 and it has a Norwegian post office. Svalbard is Norwegian territory but they allow other nations to have mining settlements there.
There is also a larger 'Barentsburg' cachet on the back of the envelope, similar to the one in blue on the front. The postmark is not very clear though. The image on the postmark is a Svalbard grouse.