Ore Formation: Return Of The Revenge Of The Fluids

October 21, 2025 by Tyler August 2 Comments

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!)

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Ore Formation Processes, Part Two: Hydrothermal Boogaloo

September 8, 2025 by Tyler August 2 Comments

There’s a saying in mine country, the kind that sometimes shows up on bumper stickers: “If it can’t be grown, it has to be mined.” Before mining can ever start, though, there has to be ore in the ground. In the last edition of this series, we learned what counts as ore (anything that can be economically mined) and talked about the ways magma can form ore bodies. The so-called magmatic processes are responsible for only a minority of the mines working today. Much more important, from an economic point of view, are the so-called “hydrothermal” processes.

Come back in a few million years, and Yellowstone will be a great mining province.
Image: “Gyser Yellowstone” by amanderson2, CC BY 2.0

When you hear the word “hydrothermal” you probably think of hot water; in the context of geology, that might conjure images of Yellowstone and regions like it : Old Faithful geysers and steaming hot springs. Those hot springs might have a role to play in certain processes, but most of the time when a geologist talks about a “hydrothermal fluid” it’s a lot hotter than that.

Is there a point on the phase diagram that we stop calling it water? We’re edging into supercritical fluid territory, here. The fluids in question can be hundreds of degrees centigrade, and can carry things like silica (SiO2) and a metal more famous for not dissolving: gold. Perhaps that’s why we prefer to talk about a “fluid” instead of “water”. It certainly would not behave like water on surface; on the surface it would be superheated steam. Pressure is a wonderful thing.

Let’s return to where we left off last time, into a magma chamber deep underground. Magma isn’t just molten rock– it also contains small amounts of dissolved gasses, like CO2 and H2O. If magma cools quickly, the water gets trapped inside the matrix of the new rock, or even inside the crystal structure of certain minerals. If it cools slowly, however? You can get a hydrothermal fluid within the magma chamber.

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Ore Formation: Introduction And Magmatic Processes

August 13, 2025 by Tyler August 10 Comments

Hackaday has a long-running series on Mining and Refining, that tracks elements of interest on the human-made road from rocks to riches. What author Dan Maloney doesn’t address in that series is the natural history that comes before the mine. You can’t just plunk down a copper mine or start squeezing oil from any old stone, after all: first, you need ore. Ore has to come from somewhere. In this series, we’re going to get down and dirty into the geology of ore-forming processes to find out from wither come the rocks that hold our elements of interest.

What’s In an Ore?

Though we’re going to be talking about Planetary Science in this series, we should recognize the irony that “ore” is a word without any real scientific meaning. What distinguishes ore from other rock is its utility to human industry: it has elements or compounds, like gems, that we want, and that we think we can get out economically. That changes over time, and one generation’s “rock” can be another generation’s “ore deposits”. For example, these days prospectors are chasing copper in porphyry deposits at concentrations as low as 1000 ppm (0.1%) that simply were not economic in previous decades. The difference? Improvements in mining and refining, as well as a rise in the price of copper. Continue reading “Ore Formation: Introduction And Magmatic Processes” →