Hydrothermal deposits
Hot, salty water is the planet's great metal transporter. It dissolves metals from huge volumes of rock and dumps them wherever it cools, boils, mixes or reacts. Most of the world's copper, zinc, lead and gold was concentrated this way.
- Explain why hot water can carry metals, and name the five triggers that make it drop them
- Say where hydrothermal fluids come from: magmas, metamorphism, basins and seawater
- Explain what alteration halos are, and why they are a bigger target than the ore itself
- Name the main deposit types (porphyry, epithermal, skarn, VMS, SEDEX, orogenic gold, IOCG and MVT) and say where on the plate-tectonic map each one forms
- Describe Indian examples: Rampura Agucha zinc, and the Kolar and Hutti gold fields
Why hot water carries metals
A hydrothermal fluid is hot water, from about 50 °C to over 500 °C, moving through rock underground. Pure water is a poor solvent for metals. Three things change that:
- Heat. Like sugar in tea, most minerals are far more soluble in hot water than in cold. Deep underground, high pressure keeps water liquid (or as a dense fluid) far above 100 °C.
- Salt. Many ore fluids are brines, several times saltier than seawater. Chloride ions latch onto metal ions to form dissolved complexes, small charged clusters that keep copper, zinc, lead, silver and iron in solution.
- Sulphur. Gold is carried mostly by complexes with reduced sulphur (hydrosulphide, HS⁻), not chloride. That is why gold often behaves differently from base metals.
A fluid becomes an ore fluid when it moves through a large volume of rock, picking up metal, and is then focused into a narrow channel: a fault, a fracture network, a permeable bed. The ore forms where something suddenly makes the metal insoluble. These are the traps of the source–transport–trap framework from What makes an ore deposit. There are five main ones.
- Cooling. Solubility falls as temperature drops; copper solubility collapses below about 400 °C. Typical result: chalcopyrite in porphyry copper deposits.
- Boiling. As fluid rises and pressure falls, it boils. Steam carries off H2S and CO2, breaking up the gold–sulphur complexes. Typical result: high-grade gold in shallow epithermal veins.
- Mixing. A hot metal-rich fluid meets cold seawater or groundwater, or a fluid carrying sulphur meets one carrying metal. Typical result: black smokers and seafloor sulphide mounds.
- Pressure drop. A fault slips and opens space; pressure collapses suddenly and minerals crash out. Typical result: quartz–gold veins along shear zones.
- Wall-rock reaction. Acidic fluid reacts with the rock around it. Limestone neutralises acid; iron- or carbon-rich rock changes the chemistry. Typical result: skarns and carbonate-hosted zinc–lead.
A hot, metal-bearing fluid rises up a fracture from 6 km down, starting at 600 °C. As it rises it loses heat to the cooler rock around it. Each metal stays dissolved until the fluid cools below the temperature at which it can no longer hold it, then drops out. Change how fast the fluid flows and see where the metals end up.
Slow flow: the fluid cools quickly near the base. Fast flow: it stays hot almost to the surface.
A deliberately simple model. Each metal's solubility falls about tenfold for every 55–70 °C of cooling (copper first, then zinc–lead, then gold), and the fluid gives up the excess as it cools. In the top kilometre or two, if the fluid is hotter than the boiling point of water at that depth it boils: it is held on the boiling curve, and loss of H2S makes gold fifty times less soluble. The boiling curve is schematic below about 1 km: it rises faster than the real one, so that boiling stays near the surface. Limestone neutralises the acid fluid, cutting copper and zinc solubility sharply. Real fluids also mix, react with the rock and vary in salinity. Bars show how much of each metal drops out at each depth, scaled separately for each metal.
Where the water comes from
Water released by magma
Magmas at subduction zones are wet, typically a few percent water. As they crystallise, they can't hold it, and it escapes as a hot, salty, metal-rich fluid. This drives porphyry, skarn and many epithermal deposits.
Water squeezed from rock
When rocks are buried and heated during mountain building, minerals like micas and chlorite break down and release water and CO2. The fluid rises along major faults. This drives orogenic gold.
Ancient brines in sedimentary basins
Water trapped in sediments, often made very salty by dissolving evaporite beds, is heated as the basin deepens and is driven out by compaction or tectonics. This drives SEDEX and MVT zinc–lead.
Surface water that went deep
Seawater seeping into hot ocean crust drives black smokers. Rainwater (meteoric water) circulating through hot volcanic rock mixes with magmatic fluid in geothermal systems.
Alteration halos: the bigger target
A hot, acidic fluid doesn't just deliver metal. It also reacts with every rock it passes through, swapping elements in and out and turning the original minerals into new ones. This is hydrothermal alteration. Because the fluid passes through a much bigger volume than the part where ore drops out, the altered zone, or alteration halo, can be ten times wider than the ore body. For an explorer, that is good news: it is far easier to hit a halo several kilometres across than a buried ore body a few hundred metres wide.
Around a porphyry (named after a texture of large crystals in a fine-grained matrix that these intrusions often have) the halo forms concentric zones, from hottest and innermost to coolest and outermost:
Potassic
Potassium added: new K-feldspar and biotite, often with magnetite. Much of the copper ore sits at its outer edge.
Phyllic (sericitic)
Acid fluids turn feldspar into fine white mica (sericite) with quartz and lots of pyrite. Magnetite is destroyed, which shows up as a magnetic low.
Argillic / advanced argillic
Acid fluids turn the rock to clays (kaolinite and smectite): argillic alteration. Where the acid is strongest, at the top, it leaches the rock further to alunite and porous "vuggy" silica: advanced argillic alteration. This bleached "lithocap" can sit above a porphyry and host epithermal gold.
Propylitic
Mild, widespread alteration to green chlorite, epidote and calcite. It can extend kilometres from the centre and is often the first clue an explorer sees.
A schematic magmatic–hydrothermal system above a subduction zone. Fluid (blue arrows) escapes from the top of a crystallising granite stock and cools as it rises. Drag the slider to strip away the top few kilometres, as millions of years of erosion would, and see what an explorer would find at the surface.
- Lithocap with high-sulphidation veins. Acid magmatic fluid bleaches the rock to clay and silica; gold–silver veins form where it boils, at about 150–300 °C.
- Low-sulphidation veins. Further out, near-neutral, hot-spring-like water deposits gold–silver veins.
- Porphyry Cu–Mo ore shell. Copper and molybdenum sulphides drop out where the fluid cools through about 400 °C, at the outer edge of the potassic zone.
- Skarn. A second cupola of the pluton releases fluid into a limestone bed, which reacts and is replaced by garnet, pyroxene and copper, zinc, iron or tungsten ore.
Temperatures on the blue arrow are for the fluid rising above the stock, not for the surrounding rock, which is cooler. Because the deposits are stacked by depth, the erosion level decides which one you find at the surface today: erode to level A and you meet gold veins in a clay cap; erode to level B and you meet the porphyry copper that may lie beneath them. Erode too far and the copper has gone.
The main deposit types
Each type below is a different combination of fluid source, pathway and trap. The tectonic setting largely decides which one you get, which is why knowing Plate tectonics helps you predict where to look.
- VMS at mid-ocean ridges and back-arc basins: seawater heated by magma vents as black smokers.
- Porphyry, epithermal and skarn under arc volcanoes: water released by wet magmas above the subducting plate.
- SEDEX in rift basins: hot basinal brines rise up the basin-bounding faults and vent into the muddy floor.
- MVT in limestone platforms beside mountain belts: brines driven out from beneath the mountains flow tens to hundreds of kilometres through the limestone.
- Orogenic gold in collisional mountain belts: metamorphic fluids squeezed from heated rock rise along steep shear zones.
Porphyry copper (Cu–Mo–Au)
Huge, low-grade bodies (often 0.3–1% Cu, up to billions of tonnes) around stocks fed by wet arc magmas. They supply most of the world's copper and much of its molybdenum. The Andes are the classic province: Chuquicamata and Escondida in Chile are among the largest copper mines on Earth.
Epithermal gold–silver
Formed within about 1–1.5 km of the surface, at 150–300 °C, often by boiling. High-sulphidation types form from acidic magmatic fluid in the clay cap right above a porphyry (Yanacocha, Peru). Low-sulphidation types form from near-neutral, hot-spring-like water and can produce spectacular "bonanza" veins (Hishikari, Japan).
Skarn
Where an intrusion touches limestone or marble, hot acidic fluid dissolves the carbonate and replaces it with garnet, pyroxene and ore minerals. Limestone is chemically reactive, so it is an excellent trap. Skarns are mined for copper, zinc, iron, tungsten and gold (Antamina, Peru).
Volcanogenic massive sulphide (VMS)
Mounds of nearly solid sulphide built by black smokers on the seafloor (see the diagram below), later buried and preserved in ancient volcanic belts. The word "copper" comes from the Latin for "metal of Cyprus", whose ancient mines worked VMS deposits. Kidd Creek (Canada) and the Iberian Pyrite Belt (Spain, Portugal) are major examples.
SEDEX / sediment-hosted zinc–lead
Hot basinal brines vent into the bottom of a basin or soak into its muddy sediments, where sulphur from bacteria traps zinc and lead as fine-grained layered sulphides. Rampura Agucha in Rajasthan, usually classed in this group, is one of the world's largest zinc mines. Elsewhere in Rajasthan, Zawar has been worked for zinc for many centuries.
Orogenic gold
During mountain building, metamorphic fluids rich in CO2 and sulphur rise along crustal-scale shear zones and drop gold in quartz veins when faults slip and pressure drops. The Kolar gold fields in Karnataka were worked for over a century to depths of more than 3 km before closing in 2001; the Hutti mine is India's main working gold mine. Both sit in ancient greenstone belts of the Dharwar craton.
Iron-oxide copper–gold (IOCG)
Large, hematite- or magnetite-rich breccias with copper, gold and often uranium and rare earths. Their origin is debated, probably involving both magmatic and basinal fluids. Olympic Dam in South Australia is one of the world's largest deposits of copper and uranium. Rajasthan's Khetri copper belt has been compared to IOCG systems.
Mississippi Valley-type (MVT)
Warm (about 75–200 °C) basinal brines migrate long distances through limestone and dolomite and drop zinc and lead where they meet sulphur, filling cavities and dissolution breccias. No magma is involved. Named after deposits along the Mississippi River in the USA.
Black smokers: ore forming today
Most hydrothermal deposits formed long ago, out of sight. The exception is on the seafloor. At mid-ocean ridges and above some subduction zones, seawater seeps into the cracked volcanic crust, is heated by magma below to around 350 °C, and leaches metals and sulphur from the hot basalt. It rises and gushes out of chimneys. The moment it meets near-freezing seawater, metal sulphides precipitate as a black cloud of tiny particles. These black smokers were first seen in 1979 and are the modern equivalent of the VMS deposits mined on land.
- Recharge. Cold seawater enters cracks in the seafloor and seeps down through the crust.
- Reaction zone. Near the magma chamber it is heated to about 400 °C and leaches Cu, Zn, Fe and S from the basalt.
- Upflow. Hot, buoyant fluid rises fast through a network of veins.
- Discharge. Fluid at about 350 °C meets 2 °C seawater and sulphides precipitate as black "smoke" and build chimneys on a sulphide mound.
- Later. Buried under lava and sediment, the mound is sealed from oxygen and preserved as a VMS ore deposit.
Plumbing: why structure matters
Fluids can only form an ore deposit if they are focused. Rock is mostly impermeable, so fluids flow along the paths of least resistance: faults, shear zones, fold hinges, fractured contacts and porous beds. Many of the world's gold camps line up along a single major fault for tens of kilometres. The structures themselves are covered in Folds, faults & deformation. In practice, structural geology is one of the most valuable skills an exploration geologist has.
Faults do more than provide a pipe. Deep in a mountain belt, a fault can act as a valve. Between earthquakes it is sealed by its own minerals, and fluid pressure builds beneath it. Eventually the pressure helps the fault fail. It slips, a gap opens along a bend, fluid rushes in, and the sudden pressure drop makes quartz and gold crash out. The gap then cements shut and the cycle begins again. This is fault-valve behaviour, proposed by the structural geologist Richard Sibson. Each cycle leaves one thin band of quartz, so an orogenic gold vein built over thousands of earthquakes has a layered, crack-seal texture.
Step through the earthquake cycle. Each full cycle adds one band to the vein and one tooth to the pressure record.
Hydrothermal systems make much of the world's copper, gold, zinc and lead, so reading alteration is one of the main tools in mineral exploration. The halo of altered rock around a deposit is far bigger than the ore itself, and mapping how the alteration minerals change across it points towards the centre.
Hydrothermal alteration is fluid–rock reaction, the same family of chemistry as weathering. Basalt that has been altered by hot fluids, for example to chlorite, epidote, clays or calcite, has already lost some of its fast-reacting minerals and will weather differently from fresh basalt. Recognising alteration is part of characterising a feedstock.
The reaction can also be used on purpose. In Iceland, the CarbFix project dissolves CO2 in water and injects it into basalt, where most of it turned into carbonate minerals within about two years. That is a deliberately engineered hydrothermal system.