Well I have seen similar things in Colorado when I lived there. They were used to bake bricks. However that looks too small to bake many bricks, so I am going out on a limb here and say it's a kiln for smelting precious metals. Maybe there was an assay office there? Can't tell if there was a building there or not.
If not that then my second guess is that was part of the mine's kitchen and it was an oven perhaps used for baking bread for the miners.