Here's why:

* Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions.

* Calcareous tufa forms from the precipitation of calcium carbonate (CaCO3) from mineral-rich water, often in areas with springs or waterfalls. This process is similar to how stalactites and stalagmites form in caves.

Therefore, calcareous tufa forms directly from the precipitation of minerals, making it a sedimentary rock.