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Material World
Deposit type

Oxidised zone

The weathered cap above a sulfide ore body, where air and water have rebuilt the ore into carbonates and oxides — and where the brightly coloured minerals come from.

Ore

Sulfide minerals are stable underground and not at the surface. Where an ore body is exposed to air and groundwater, the sulfides oxidise, sulfuric acid is generated in place, and the metals are dissolved and reprecipitated as an entirely different set of minerals — carbonates, oxides, silicates and sulfates.

That zone is where nearly every brightly coloured mineral in a museum case comes from. Malachite's green, azurite's blue, vanadinite's orange and wulfenite's yellow are all products of a sulfide deposit rotting, and none of them forms any other way.

How it forms

Pyrite does the work. It is present in almost every sulfide deposit and oxidises readily, producing sulfuric acid and iron sulfate, and that acid then attacks everything else. Copper, zinc and lead go into solution; iron stays behind as the oxides and hydroxides that stain the outcrop rust-red.

What happens next depends on the metal and the rock. Copper carried downward reprecipitates below the water table as a supergene enrichment blanket, several times richer than the primary ore — which is why so many great copper mines began with a few decades of unusually easy production. Lead barely moves at all, and reprecipitates almost where it started as carbonates and sulfates.

Where the wall rock is limestone the acid is neutralised quickly and the metals drop out as carbonates: malachite and azurite for copper, cerussite for lead. That is why the showiest specimens come from carbonate-hosted deposits in dry climates, where the zone develops deeply and is not washed away.

Economic significance

The oxidised zone was historically the whole mine. It is softer, richer, and — crucially — smeltable without the roasting a sulfide needs, so pre-industrial copper came almost entirely from carbonate ore. A great many ancient workings stop exactly where the sulfides begin, because that is where the metallurgy of the time ran out.

The same boundary decides a modern mine's flowsheet, in reverse. Oxidised ore dissolves in acid and goes to leaching and electrowinning; the sulfide beneath it goes to flotation and a smelter. One deposit, two plants, and the depth of weathering decides how much of each gets built.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is composed of

  • Malachite — mineral · the green copper carbonate, and the mineral pre-industrial copper was almost entirely smelted from — no roasting needed
  • Azurite — mineral · the blue one, less stable than malachite and slowly altering to it, which is why green outcrops outnumber blue
  • Vanadinite — mineral · orange hexagonal crystals in the lead-bearing oxidised zones of dry climates, where the zone develops deeply and is not washed away
  • Wulfenite — mineral · the yellow lead molybdate, from the same setting and for the same reason
  • Pyrolusite — mineral · manganese oxide precipitated where dissolved manganese met air, and the commonest ore of the metal
  • Hematite — mineral · as the iron staining that makes a gossan visible from a distance — iron is what stays behind when everything else has gone into solution

is sourced from

  • Porphyry copper deposit — deposit type · the weathered cap of a sulfide body rather than a deposit in its own right: the same metal, rebuilt in place by air and water
  • Pyrite — mineral · pyrite is what drives it — oxidising to sulfuric acid in place, which then attacks everything else in the deposit

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Where it comes from, and what it becomes

Follow Oxidised zone back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Oxidised zone → is sourced from (the weathered cap of a sulfide body rather than a deposit in its own right: the same metal, rebuilt in place by air and water) → Porphyry copper deposit → is sourced from (an intrusion of granitic composition, fractured and altered by fluid separating out of it as it crystallised) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
  • Oxidised zone → is composed of (as the iron staining that makes a gossan visible from a distance — iron is what stays behind when everything else has gone into solution) → Hematite
  • Oxidised zone → is sourced from (pyrite is what drives it — oxidising to sulfuric acid in place, which then attacks everything else in the deposit) → Pyrite
  • Oxidised zone → is composed of (the green copper carbonate, and the mineral pre-industrial copper was almost entirely smelted from — no roasting needed) → Malachite
  • Oxidised zone → is composed of (the blue one, less stable than malachite and slowly altering to it, which is why green outcrops outnumber blue) → Azurite
  • Oxidised zone → is composed of (orange hexagonal crystals in the lead-bearing oxidised zones of dry climates, where the zone develops deeply and is not washed away) → Vanadinite

These are the most distinct paths back. Oxidised zone can be traced through others besides.