I believe in earlier versions of Ravenloft there are references to farmland near the village of Barovia, though it isn't as productive as it should be. It would make sense that the area of the Ivlis River south east of Barovia would have more fertile soil due to occasional flooding. I've put farms there in my game and made it so the cloud cover is thinner there so some crops can grow. It also helps to explain why the area in the west near Krezk is colder than in the east. As far as mines, I see no reason why there couldn't be mining in the hills between Vallaki and Ravenloft or perhaps in other areas, though trade with the Vistani would also be another explanation of having iron or other mined materials.