Explain how magmatic differentiation and hydrothermal fluids form economic mineral deposits

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.

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.

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.