Porphyry copper deposit

A porphyry deposit is the opposite of a vein in every way that matters. Instead of a sharply bounded fracture at high grade, it is a body of intrusive rock a kilometre or more across, veined through with a stockwork of hairline fractures, carrying copper at somewhere between a quarter and one per cent.

That is a grade nobody could mine by hand and nobody needed to until the twentieth century. What made it ore was scale: open-pit mining, flotation, and machinery large enough that moving a hundred tonnes of rock to recover half a tonne of copper is economic.

The deposit forms where a water-rich magma crystallising at a few kilometres' depth reaches the point at which it can hold no more dissolved water. The fluid separates violently, fractures the rock around and above it, and deposits metal sulfides through the resulting network as it cools.

Alteration zones ring the centre in a pattern regular enough to navigate by — potassic in the core, phyllic outside it, propylitic beyond — and mapping which zone you are standing in is how the centre is found. That regularity is what made porphyry deposits the first ore bodies to be genuinely modelled rather than merely followed.

Weathering then rearranges the top. Sulfides near the surface oxidise, copper is dissolved and carried down, and it reprecipitates below the water table as a supergene blanket several times richer than the primary rock — which is the part most porphyry mines started on.

Something like three quarters of world copper and most of its molybdenum comes from this one deposit type, concentrated along the subduction margins of the Americas and the western Pacific. Rhenium arrives with the molybdenum, which is why an element with almost no ore body of its own has a supply at all.

The economics are entirely about scale and grade decline. Average copper grades have fallen for a century while production has risen, which works only because equipment has grown and flotation has improved — and it makes energy the dominant input cost of the industry rather than the metal being scarce.