Step 1: Composition of dolomite.
Dolomite is a double carbonate mineral with the chemical formula $\mathrm{CaCO_3 \cdot MgCO_3}$. It is an important source of magnesium.
Step 2: Analysis of other options.
Siderite is an ore of iron, calamine is an ore of zinc, and limonite is a hydrated iron oxide. None of these contain magnesium as a major component.
Step 3: Conclusion.
Therefore, the correct ore of magnesium is Dolomite.