In the last edition of our ongoing series on how planets get ore– those wonderful rocks rich in industrial minerals worth mining– we started talking about hydrothermal fluid deposits. Hydrothermal fluid is the very hot, very salty, very corrosive water that sweats out of magma as it cools underground and under pressure.

We learned that if the fluid stays in the magma chamber and encourages the growth of large crystals there, we call that a pegmatite deposit. If it escapes following cracks in the surface rock, it creates the characteristic veins of an orogenic deposit. What if the fluid gets out of the magma chamber, but doesn’t find any cracks?

Perhaps the surrounding rock is slightly permeable to water, and the hydrothermal fluid can force its way through, eating away at the base rock and remineralizing it with new metals as it goes. That can happen! We call it a porphyry deposit, particularly in igneous rock. It’s not exactly surprising that a hydrothermal fluid would find igneous rock: the fluid is volcanic in origin, after all, just like igneous rock. (That’s the definition of igneous: a rock of volcanic origin.) Igneous rocks, like granite, tend not to be terribly reactive so the fluid can diffuse through relatively unchanged.

Igneous rocks aren’t the only option, though. If the hydrothermal fluid hits carbonates, well, I did mention it’s acidic, right? Acid and carbonates are not friends, so all sorts of chemistry happens, such that geologists give the resulting metamorphic formation a special name: skarn. Though similar in origin, skarns are often considered a different type of deposit, so we’ll talk about the simpler case, diffusion through non-reactive rocks, before getting back to the rocks that sound like an 80s fantasy villain. (Beware Lord Skarn!)

Continue reading “Ore Formation: Return Of The Revenge Of The Fluids” →