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.

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.

Peg It as a Pegmatite

This can create what’s called a pegmatite deposit. Strictly speaking, “pegmatite” refers to rock with a specific texture; when we’re talking about ore, we’re almost always referring to granitic pegmatites: that is, granite rocks with this texture. That texture is big crystals: centimeter size or bigger. Crystals grow large in a pegmatite deposits in part because of the slow cooling, but in part because of the action of the hydrothermal fluid that is squeezed out of the slowly-cooling rock.

Again, we’re talking about a fluid that’s hundreds of degrees Celsius: seriously supercritical stuff. It can carry a lot of ions. Circulating through the magma chamber, this ion-rich fluid brings each crystal all the metal ions it needs to grow to its full potential. Maybe that’s a garnet the size of your fist, or feldspar crystals like pink playing cards. The ions in the fluid can be leftovers from the earlier melt, but may also include material scoured from surrounding rocks.

Aside from the spectacular granite counter tops and semiprecious gems that sometimes come out of these deposits, granitic pegmatites come in two types: lithium-rich and rare-earth element rich. The lithium rich pegmatites are often called LCT deposits, the letters standing for Lithium, Cesium and Tantalum, the metals of interest. Those–especially the first and last–are not exactly metals of low consequence in this electronic era. That goes double for the rare-earth elements. Especially in North America, there’s a great deal of active prospecting searching for these increasingly valuable deposits.

Mines have been sunk to extract boron, fluorine, tin, and uranium from pegmatite deposits as well. Of particular note to Hackaday readers would be the mineral Muscovite, a course-grained mica often found in pegmatites, among other locales. Muscovite mica has excellent dielectric properties and fractures easily into thin sheets, making it very useful in capacitors and high voltage applications. The high thermal stability and voltage tolerance of mica capacitors makes them invaluable even today in niche applications, even though ceramics have taken over most of their original uses.

One thing to note about these deposits is that they are not necessarily going to be restricted to Earth. Don’t let the “hydro” in “hydrothermal” fool you– this process is occurring deep underground, in a magma chamber with no access to any surface water. The H2O involved is coming up from the mantle, and the mantle of every rocky body does contain trace water. That even holds true for the Earth’s moon; while older sources will declare that no hydrothermal processes are possible there, newer work has led to a reevaluation of how “wet” lunar rock really is, and re-opened the possibility of lunar pegmatites. Given that, there’s no reason not to expect the process to be at work on every rocky body in the solar system. Look for granitic rock, and you might find an interesting pegmatite.

Orogenic Ores

If the hydrothermal fluid stays put in a magma chamber, it can create pegmatite deposits, but if it breaks free, you’ll find something completely different. Running through faults, fissures, and cracks in the surrounding rock, the somewhat-lower-temperature fluid will have a different mineral content depending both on the melt and the host rock. These hydrothermal vein deposits are sometimes called orogenic ore deposits, because they are often associated with mountain building, which geologists call orogeny.

That doesn’t mean you need to look near mountains: the gold fields of Kirkland Lake, mentioned last time, are actually an orogenic deposit, and Kirkland Lake sits near the middle of the Canadian Shield, as far from any (modern) mountains as you are likely to find. There may have been mountains there, once, but they were eroded away by the time the Dinosaurs walked the Earth. What you will find there are shocking white veins of quartz shooting through the granite of the Canadian Shield– evidence of the hydrothermal fluid’s ability to carry dissolved silica through fissures of the rock– interspersed with flecks and pebbles of gold. Most gold started in hydrothermal deposits like this one, but in an ironic twist, most of the gold humans have mined is actually from a different type of deposit we’ll get to later. For now we’ll say there are secondary processes at work on this planet and leave it at that.

Gold isn’t the only thing to be found in these hydrothermal veins: native silver and copper mines have also been found chasing quartz veins. Cobalt, Molybdenum, even Tin and Tungsten may be found, though not necessarily in native form. To a geologist, note that the word “native” has nothing to do with tribal affiliation, and everything to do with elemental composition. “Native” metals are just that: metals. Native copper is a lump of Cu, not chemically bound into any mineral.

As you might imagine, native metals are among the most desirable of ores, as they often require very little by way of refining. For that reason, until perhaps Greenland or Antarctica’s melting glaciers expose new lands to prospecting, you’re not likely to ever see a new mine producing native copper.

The redox conditions of the fluid are hugely important here: as you might imagine, native metals aren’t going to precipitate from an oxidizing fluid. Redox reactions are hard enough in chemistry class, though; bring them into the world of geochemistry and it gets hugely complicated. Nature is a messy system with too many variables to easily predict.

That’s something many a prospector has found out to his chagrin, for not every vein of quartz will bear metals. On the other hand, enough quartz veins do that “look for veins of quartz” was common advice for prospectors once upon a time. Not all metal-bearing veins may not be entirely quartz, either; many contain quite a lot of carbonate minerals like calcite. The hydrothermal fluid may start out with different amounts of metals dissolved within it, depending on the source magma; it may also scour more or different minerals from the host rocks it flows through. Veins may go on for miles of nothing but quartz before something in composition of the rock, or its temperature, or the pH causes the fluid to start depositing valuable minerals. Geology can be a crapshoot like that.

Of Course It’s More Complex

The above description is somewhat misleading as it makes it sound like vein deposits can only be produced from hydrothermal fluid coming from magma, but that is untrue. It is also possible that surface water (called “meteoric” water by geologists who want to confuse you into thinking about space rocks) can trickle down through fractured rocks until it

reaches a hot-zone and picks up elements by dissolving minerals. A mix of meteoric and “crustal” water (that is, water from magma) may be present in a balance that changes over time. It should also be noted that this water can form a convective circuit, down to the hot zone (or melt) to pick up new minerals, then circulate upwards to deposit them in colder rock. Because this circulating fluid is cooler than in the case of fluids coming directly from a melt (“only” three or four hundred degrees Celsius) , they are sometimes called “epithermal” fluids, and the resulting veiny deposits can be called “epithermal” deposits. Those temperatures are not too far off from what you might find in geyser country. While I’m not suggesting anyone go digging under Old Faithful right now, it might be an interesting locality in a few million years or so.

Epithermal/orogenic/quartz vein deposits don’t need meteoric water– crustal water can be enough–but I have seen no references suggesting they might be found on the Moon. Mars, on the other hand, seems to have every condition required, so there may well be gold in them thar’ Arean hills. Meteorites believed to have come from Vesta show evidence of quartz veinlets as well, so don’t count out larger planetoids when talking about hydrothermal processes either.

There are other high-temperature hydrothermal deposits other than granitic pegmatites we haven’t yet gotten into; there are also several lower-temperature types that are likely to be exclusive to Earth. This entry in our series is getting long enough, however, so we will return to the theme of hydrothermal ore deposits another day.