Investigate the processes that concentrate metals and minerals into economic ore deposits, including but not limited to magmatic, hydrothermal, sedimentary and weathering processes that operate in the Australian geological setting

What this dot point is asking

NESA wants you to explain how ordinary crustal rock, in which valuable metals are spread thinly, becomes an economic ore deposit where those metals are concentrated enough to mine at a profit. You need to name and describe the main concentration processes (magmatic, hydrothermal, sedimentary and weathering) and link each to a real Australian example.

The answer

An element such as copper or gold makes up only a tiny fraction of average crustal rock. A deposit becomes an ore only when a geological process concentrates that element well above its background crustal abundance. The factor by which it must be concentrated is the enrichment factor, and it varies from a few times (iron) to thousands of times (gold).

Magmatic processes

As a body of magma cools, minerals crystallise at different temperatures. Dense, early-formed crystals such as chromite or magnetite sink and accumulate at the base of the magma chamber by gravity settling, a process called fractional crystallisation. Immiscible sulfide liquids can also separate from silicate magma and gather nickel, copper and platinum. The nickel and copper sulfides of the Kambalda region in Western Australia formed this way, settling out of komatiite lavas.

Hydrothermal processes

Hot, mineral-rich water moving through the crust dissolves metals from surrounding rock and re-deposits them when temperature, pressure or chemistry changes. The water may come from cooling magma, from heated groundwater or from seawater driven down at mid-ocean ridges. As the fluid cools or reacts with wall rock, metals precipitate in veins, fractures and porous beds. Many of Australia's gold deposits, including those of the Victorian goldfields around Bendigo and Ballarat, are hydrothermal quartz-vein systems. Olympic Dam in South Australia, one of the largest known deposits of copper, uranium and gold, formed from hydrothermal fluids within a giant iron-oxide breccia.

Sedimentary processes

Moving water and wind sort grains by size and density. Dense, durable, weathering-resistant minerals such as gold, cassiterite (tin) and zircon are left behind and concentrated as placer deposits in river channels and beaches. The heavy-mineral sand deposits along the New South Wales and Queensland coasts, mined for rutile, ilmenite and zircon, are placers. Chemical sedimentary processes also matter: the banded iron formations of the Pilbara in Western Australia precipitated from iron-rich Precambrian seawater and now host the world's largest iron-ore mines at Mount Whaleback and the Hamersley Range.

Weathering processes

Prolonged chemical weathering in warm, wet climates strips soluble elements out of rock and leaves insoluble metals behind in the soil profile, a process called residual concentration. Australia's deep, ancient weathering profiles have produced extensive bauxite (the ore of aluminium) at Weipa in Queensland and at Gove in the Northern Territory, where leaching removed silica and concentrated aluminium oxides. Weathering can also enrich existing deposits: in supergene enrichment, descending groundwater leaches copper from near the surface and re-deposits it at depth, raising the grade.

Why this matters for Australia

Australia's resource wealth reflects an old, stable continent that has experienced many cycles of magmatism, hydrothermal activity, sedimentation and deep weathering. Broken Hill in New South Wales, the source of the company that became BHP, is a metamorphosed lead-zinc-silver deposit whose original metals were concentrated on an ancient sea floor and later remobilised. Recognising which process formed a deposit guides exploration geologists toward similar settings elsewhere.

Try this

Q1. Explain why the same body of rock might be classified as ore in 2025 but as waste in 2005. [3 marks]

• Cue. Link the cut-off grade to rising metal prices and improved processing technology that lower the economic threshold.

Q2. Compare the processes that concentrated the Pilbara iron ores with those that concentrated the Weipa bauxite. [4 marks]

• Cue. Pilbara: chemical precipitation from Precambrian seawater (banded iron formation). Weipa: residual concentration by deep chemical weathering that leached silica and left aluminium oxides.