Magmatic and hydrothermal ore formation: WACE Year 12 Earth and Environmental Science
“Explain how magmatic differentiation and hydrothermal fluids form economic mineral deposits”
Magmatic ore deposits form when a cooling magma concentrates metals, either as early dense crystals that settle to the bottom of a magma chamber or as an immiscible sulfide melt that scavenges metals such as nickel and copper. Hydrothermal deposits form when hot, mineral-rich fluids move through fractures and react with rock, precipitating metals such as gold in veins as conditions change.
What this dot point is asking
SCSA wants you to explain how ordinary crustal rocks, which contain metals at very low average concentrations, become ore: rock rich enough to mine at a profit. The key idea is concentration. A metal like gold sits at a few parts per billion in average crust, so a process must enrich it thousands of times before it becomes an ore. Magmatic and hydrothermal processes are two of the main concentrating mechanisms, and Western Australia hosts world-class examples of both.
From magma to magmatic ore
Magma is a complex melt that crystallises minerals one after another as it cools. This sequential crystallisation, called fractional crystallisation, can separate metals from the remaining melt.
- Crystal settling. Dense early-formed crystals such as chromite or magnetite sink through the magma and accumulate in layers at the base of the chamber, forming concentrated layers far richer than the bulk magma.
- Sulfide immiscibility. If a mafic magma becomes saturated in sulfur, a separate dense sulfide liquid forms. This liquid is chemically attractive to metals like nickel, copper and platinum, which partition strongly into it. The sulfide droplets sink and pool, producing a magmatic sulfide deposit.
Nickel, copper and the platinum group metals are chalcophile, meaning they prefer to bond with sulfur rather than oxygen. When an immiscible sulfide melt forms in a mafic magma, these metals concentrate into the small sulfide fraction, so a tiny volume of melt can carry most of the deposit's metal.
The nickel sulfide deposits at Kambalda, near Kalgoorlie, formed from ancient komatiite lavas, very hot magnesium-rich magmas that pooled sulfide liquid at the base of lava flows. These supplied much of Australia's early nickel.
From hot fluids to hydrothermal ore
Hydrothermal deposits do not crystallise straight from magma. Instead, hot water-rich fluids, heated by a magma body or by deep crustal circulation, dissolve metals and carry them through fractures.
- The fluid transports metals as dissolved complexes while it is hot, pressurised and chemically suitable.
- When conditions change, for example the fluid cools, depressurises, boils, or reacts with the wall rock, the metals become insoluble and precipitate.
- Repeated precipitation along a fracture builds a mineral vein; broad zones of fluid alteration around an intrusion can build large low-grade deposits.
Ore: rock rich enough in a mineral to mine profitably. Fractional crystallisation: sequential crystallisation of minerals from cooling magma. Immiscible: unable to mix, like oil and water. Hydrothermal fluid: hot, chemically active water-rich solution. Vein: a mineral-filled fracture.
The Golden Mile at Kalgoorlie is one of the world's great hydrothermal gold systems, where gold was deposited from fluids moving through fractured rock during ancient mountain building. Porphyry copper deposits, common around the Pacific Ring of Fire, form where fluids escaping a cooling intrusion deposit copper and molybdenum across a large rock volume, giving large tonnage at low grade.
Comparing the two pathways
Magmatic deposits are tied directly to the cooling of a specific magma and its metal budget. Hydrothermal deposits depend on a fluid pathway, a source of metals and a trigger for precipitation, so they can occur away from the original heat source. Both rely on the same underlying principle the syllabus wants you to articulate: a natural process must concentrate a metal far above its crustal average before extraction makes economic sense.
A question may give crustal abundance and ore grade for a metal and ask why the rock is mineable. Calculate the enrichment factor by dividing ore grade by crustal abundance. A large factor (often thousands) shows a concentrating process, magmatic or hydrothermal, has operated. Then name the likely mechanism: crystal settling or sulfide immiscibility for magmatic, fluid transport and precipitation for hydrothermal.
- Confusing magmatic and hydrothermal
- Magmatic deposits crystallise from melt; hydrothermal deposits precipitate from hot fluids in fractures.
- Forgetting the concentration idea
- The whole point is enrichment above crustal average, so always link the process to concentration.
- Saying gold comes straight from magma
- Most WA gold is hydrothermal, deposited from fluids, not crystallised directly from magma.
- Ignoring the trigger
- Hydrothermal precipitation needs a change such as cooling, boiling or wall-rock reaction.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksGold has an average crustal abundance of about 0.004 grams per tonne, while a mineable hydrothermal deposit grades 6 grams per tonne. Calculate the enrichment factor and explain the process that produced this concentration.Show worked answer →
A 6 mark answer rewards the calculation plus a hydrothermal explanation.
Calculation. Enrichment factor . The gold is concentrated about 1500 times above average crustal abundance.
Process. Such enrichment is achieved by hydrothermal processes. Hot, water-rich fluids (heated by a magma body or deep circulation) dissolve gold as chemical complexes and carry it through fractures. When conditions change, the fluid cools, boils, depressurises or reacts with wall rock, the gold becomes insoluble and precipitates, building up in veins. Repeated precipitation along the same fractures concentrates gold to mineable grade, as at the Kalgoorlie Golden Mile.
Markers reward the correct enrichment factor with working and a valid hydrothermal mechanism including a precipitation trigger.
WACE 2023-style7 marksCompare the formation of a magmatic nickel sulfide deposit with a hydrothermal gold deposit, and explain why each tends to occur in a different geological setting.Show worked answer →
A 7 mark answer needs a structured comparison with settings explained.
- Magmatic nickel sulfide
- Forms within a cooling mafic or ultramafic magma. When the magma saturates in sulfur, an immiscible sulfide liquid forms; chalcophile metals (nickel, copper, platinum) partition into it, and the dense sulfide pools at the base of a chamber or lava flow (for example komatiite-hosted Kambalda nickel). It is tied directly to a specific metal-bearing magma.
- Hydrothermal gold
- Forms when hot fluids transport gold through fractures and precipitate it where conditions change. It depends on a fluid pathway and a precipitation trigger, not on a single magma body, so it can occur in fractured rock away from the heat source (for example Kalgoorlie).
- Different settings
- Magmatic deposits need a suitable mafic or ultramafic magma, so they sit in or near such intrusions and lava flows. Hydrothermal deposits need fractured, permeable rock and a fluid source, so they cluster along fault and shear zones, often in deformed terrains during mountain building.
Markers reward the melt-versus-fluid contrast, the chalcophile partitioning point, and setting linked to the formation requirements.