mineral systems
The archetype: a shallow granitoid porphyry, an exsolving magmatic fluid, kilometres of altered rock, and copper spread thinly through an enormous volume. It supplies most of the world’s copper and the model that describes it says many of its controls are still poorly understood.
About: Porphyry copper system
Porphyry copper deposits consist of disseminated copper minerals and copper minerals in veins and breccias, relatively evenly distributed through large volumes of rock. The host rocks are altered granitoid porphyry intrusions and the wall rocks adjacent to them, related to one another geographically, temporally and genetically.[1,2]
The scale of the class is the reason it matters: it accounts for more than 60 percent of annual world copper production and about 65 percent of known copper resources, at grades the model characterises as low to moderate, in the range 0.3 to 2.0 percent copper.[1,2]
The generalized model involves an interplay between magmatic metal sources and hydrothermal fluid systems: the metals and the fluid both come from the magma. Fluid-phase separation fractionates water, metals, sulfur and other volatiles out of the crystallising melt.[1,2]
Depth decides what leaves. Above about 1.4 kilobars — deeper than roughly five kilometres — the exsolved fluid is a single supercritical phase of moderate to low salinity, about 3 to 5 weight percent. As pressure falls it separates into a high-salinity brine and a lower-salinity vapour, and the metals partition between them.[1,2,3]
The alteration types recognised in these systems are potassic, sericitic, advanced argillic, intermediate argillic, propylitic, sodic-calcic and sodic, greisen, and skarn. They occupy far more rock than the mineralisation does.[1]
Potassic alteration is the one directly associated with the ore-bearing stockwork. It adds potassium feldspar and biotite — feldspar dominant in silicic wall rocks, biotite in more mafic ones — with bornite, chalcopyrite and pyrite, along with magnetite, molybdenite, anhydrite and calcite, and abundant quartz-rich stockwork veins.[1]
Roles here are contextual, and Lithoen stores them that way. Chalcopyrite is the ore mineral of this system; pyrite is gangue in it and the ore mineral in others; quartz forms the veins that carry the metal and is never itself the metal.[1]
Alteration minerals have distinctive absorption features in the visible near-infrared, short-wave infrared and thermal infrared, so sericitic, argillic, advanced argillic and propylitic assemblages, iron oxides and hydroxides, and silica can all be mapped remotely.[1]
The model is candid about the limits of what that buys. The regional, deposit-scale and local-scale environments of these deposits are diverse, and despite much scientific study many features are poorly understood in regard to their relevance as controls of ore formation. This is the best-studied mineral system on Earth.[1]
Often said
“Porphyry deposits are rich; that is why they dominate copper supply.”
What the evidence supports
The model characterises them as low- to moderate-grade, 0.3 to 2.0 percent copper. They dominate supply because they are very large and relatively evenly mineralised, not because they are concentrated.
Supporting records: pc-grade-range · pc-supply-share
Often said
“The alteration halo marks the deposit.”
What the evidence supports
The alteration halo is far larger than the mineralisation, is produced by hydrothermal systems that never mineralised, and is detected by remote sensing as mineralogy without regard to its cause.
Supporting records: pc-remote-expression · pc-alteration-types
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-07.