The "miners" who literally dug the metal from the earth were almost always slaves. This summary of Roman mining practices describes working rock with iron tools, not steel, by hammering piton-like pegs called gads were hammered into the rock face with iron sledgehammers. The broken rock and ore was collected into baskets--possibly by hand in some cases--was carried away through single-file shafts and cramped galleries. This was not the labor of a free person in Roman society, who relied heavily on slaves for unskilled labor. While some of the aqueducts and waterworks used to service these mines would have surely been done by skilled artisans, the "miners" themselves would have profited very little from their labor.

Of course, the owners of the mines based their financial decisions on the profitability of the ore and the ease of extracting it. This Wikipedia article's references, if you have JSTOR or access to a library, has a lot of information about specific region's mines and the political and economic decisionmaking behind their operation--specifically, the lead and silver mines in Hispania and the diverse mining interests in Britain.