As noted, malt brings in significant magnesium and phosphate content into the wort. In addition, it brings potassium into the wort. Unfortunately, most of that content is bound in molecules and is not ionic. Therefore, its not appropriate to assume a flavor contribution from the malt with regard to ion flavor.
PS: Yeast require calcium and magnesium in the wort to provide good yeast health and enhance flocculation performance. Both of those ions are provided in malt worts. In the case of calcium, additional calcium provided in the water is helpful. It doesn't appear that additional magnesium is needed in a malt wort since yeast only need about 5 ppm for their health. Magnesium should only be added to brewing liquor when desired for its taste effects...a sour, bittering. That is welcome in a hoppy ale, but may not be so welcome in a malty lager.