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!)
Porphyry: Born to the Purple
In terms of ore deposits, humans have only started to exploit porphyry deposits relatively recently. Quite a few metals can be laid down, but a mine digging into a porphyry deposit is almost certainly chasing copper, to feed the industrial machine’s voracious appetite for the red metal. There’s generally going to be gold mixed in, and make no mistake; it’s not going to get left in the ground, but these are first and foremost copper mines.
Indeed, the gold, and lead, zinc, silver, and molybdenum that can also be present, are too diffusely mixed in with the copper to be left alone even if you wanted to. The copper, too, is very diffuse; these ores are low grade, with concentrations better measured in ppm than percent. That’s a consequence of the hydrothermal fluid spreading out through the rock, rather than concentrating its metals inside small veins.
They don’t make ’em this big for fun. The Caterpillar 797 can haul 362 tonnes of low-grade ore at once, and Emperor Constantine had nothing like it. Lechhabmed, CC-BY-4.0Skarn: Ugly Name, Pretty Rocks
Skarns can look as pretty as they don’t sound. This sample is composed of blue calcite, green augerite, and orange garnets, and is more likely to end up in a museum than a mill.. Sim Sepps, CC-BY-3.0.The extra chemistry going on to create skarn deposits make them a different story; there you can find decent concentrations of things like tin, tungsten, manganese, copper, gold, zinc, lead, nickel, molybdenum and iron. Apparently the name comes from what they called waste rock in an old Swedish iron mine. The interesting chemistry — remember: acid fluid meeting basic, carbonate rock — also makes skarn deposits a good place to look for certain gemstones, like garnet, tourmaline, topaz, beryl, and even corundum– the mineralogist’s name for emeralds and sapphires. Just as quartz comes in many colours depending on what trace elements are contaminating the basic crystal of SiO2, corundum, or Al2O3 , can take different colours as well. Rubies are red due to chromium contamination, for example.
So that’s what happens when the hydrothermal fluid gets loose and oozes through the base rock. What if it gets loose from the base rock entirely? Well, on land that’s a geyser and I’m not aware of any ore deposits directly formed by geysers. (Associated with, yes, but formed by? No.) Underwater it’s a different story: a plume of hot water coming into the ocean from beneath is famously known as a “black smoker” and that black smoke is mineralogically interesting.
VMS : When Alvin Goes Prospecting
This is making an ore deposit. Who knew Alvin was a prospector? NOAA, via Wikimedia.The hot water hitting the cold sea water causes all sorts of things that were happily dissolved in the fluid to stop being happy, and stop being dissolved. In the short term, this leads to the delightfully creepy lightless ecosystems feasting on the chemical potential of the sulfides in the water around the Black Smoker. In the long term– the very, very long term, the geologic time long term, that is–the mostly-sulfide particulates in the “smoke” settle down into the local sediment to create “Volcanogenic Massive Sulfide” deposits, more commonly known by the acronym VMS. (I always misremember the V as “vented” , which is actually handy as it keeps me from getting confused from flood-basalt-generated sulfide melt deposits like Norilsk.)
Strictly speaking, the pretty picture of the black smoker spewing sulfide-particle smoke is not necessary: the cold seawater intermixing with hydrothermal fluids can happen entirely underground and the same reactions will occur. Either way, you can guess this sort of deposit is going to be restricted to watery worlds like Earth, Europa and other icy moons, or just possibly Mars.
The big problem with VMS deposits is that, as we do not typically want to do major mining operations on the sea floor, yet, they require the ocean to go away. This can happen through changes in sea level, or uplift of the rocks to some point above sea level. That’s not always going to happen, so there’s an idea out there that most VMS deposits will be underwater, and that this may represent a new frontier in mining. That idea deserves its own article someday, but for now, what would these hypothetical underwater miners be after?
You can find MVTs all over the globe, but probably only on this globe, at least in our solar system.And geology is a little bit like a gatcha system, when you think about it. You might know roughly what kind of rock types a given kind of ore deposit is found in, but until you make the draw – or drill core, as the case may be – you never know what you’re going to get. While this is the last article that’s going to cover hydrothermal ore deposits, there remains one last family of ore formation processes– quaternary processes, those that occur on surface and are ongoing in the present day. Maybe you’ll enjoy it, maybe not; that too, is a bit like gatcha. Regardless, that’s what we’re going to cover in the last work in the series. Stay tuned.