The speaker is alone in winter, and the poem is built around that loneliness. The opening image is bleak: winter is "a mine of nothing" and "a great iron book," as if the season were a heavy, closed thing that holds no riches and can't be read. The cold feels hard and metallic, something to be dug through rather than enjoyed.
The middle lines turn strange. The speaker dines "with the lord of paint" and sends "to Africa for gold ore." Judging by the imagery, these look like attempts to fill the emptiness with color, art, and wealth, a hunt for something precious. But the last lines reveal what is really being sought: "I do not find my fair / late wife." The gold and the painting can't replace her.
The mood is quiet, stark, and sorrowful. The speaker never says "I grieve." The short, plain closing sentence, and the way it follows all that searching, lets the loss land with force.