I know I read once in ethnobotany class about the central american indians getting good minerals they wouldn't otherwise get by grinding the corn on stones, but I can't find anything that looks like that article. I found a different article called "ADVENTITIOUS MINERAL ELEMENTS IN HOPI INDIAN DIETS". I tried to get it for you via my my university's online journal system but it's too old. Anyhow the abstract says:
"Both strontium and lead are geologically present in the Hopi area, but only strontium accumulates in culturally important food plants in the agicultural setting. Lead from natural sources is present in traditional ceramics and culinary ash. In native maize corn foods, iron content was elevated several-fold during home processing. The nutritional environment contains unconventional sources of iron, zinc, manganese and other essential elements."
So perhaps some of that iron and whatnot comes from the grinding stone.
Oh I just found another article, "Preparation effects on tortilla mineral content in Guatemala." I can't seem to get this one either but here is the abstract.
"We have previously reported that in Guatemala, the calcium, iron, and zinc contents of tortillas from rural areas are higher that that of tortillas from urban centers. This study examines variation in the calcium, iron, zinc and copper content of tortilla according to the implements used for making tortillas and inquires as to whether preparation effects mediate rural-urban variation in tortilla mineral content. Tortilla samples and information on how the tortillas were prepared were collected from the female heads of a total of 50 households from three rural, two semi-urban and one low income urban community. Samples of lime used for making tortillas were collected from 31 households. To grind masa, a hand mill was found to be used in some rural households whereas a motorized mill predominated in the semi-urban and urban areas. Most women used grinding stones called the "mano y metate" to further refine the texture of the masa. Tortillas prepared with the combined use of the hand mill and "mano y metate" had a significantly (p < 0.05) higher iron content. Use of the "mano y metate" was also associated with a significantly (p < 0.05) higher zinc content. These results suggest that the use of certain grinding implements may mediate rural-urban variation in tortilla iron and zinc content. The cooking surface, pot used for nixtamalization, source of water, and amount of lime used did not significantly account for variation in the content of these minerals."