In March 2025, the USGS published an updated global database cataloguing every known porphyry copper deposit on Earth, a remarkable cartographic exercise for a deposit type that already supplies the planet with about 60% of its copper. What the database does not show is that the same magmatic engine produces a second, very different ore type immediately above: epithermal gold-silver veins, which the corresponding USGS model credits with about 8% of global gold supply. The accompanying infographic shows both halves in cross-section. The deeper story is that this single architecture, replicated along convergent plate margins for hundreds of millions of years, has built the world's largest copper mines and its richest precious-metal veins, often stacked vertically in the same district.

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One Magma, Two Deposit Types

The architecture is unusually elegant. Per the USGS porphyry model, water-rich magma rises from convergent plate margins and stalls in the upper crust. As it crystallizes, it releases hot, metal-bearing fluids that fracture the surrounding rock and precipitate copper, molybdenum, and gold in a dense network of veinlets called a stockwork. This is the porphyry, sitting 1 to 6 km below the paleo-surface. As the same fluids rise into the top 1.5 km, mix with circulating groundwater, and boil, they partition gold and silver into high-grade veins, the epithermal half described by USGS as the source of roughly 8% of global gold and a significant share of global silver.

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Reading Each Half

The two halves write different signatures in the rock. The porphyry's classic alteration sequence runs from a high-temperature potassic core out through phyllic (quartz, sericite, pyrite), argillic (clay-dominated), and propylitic (chlorite, epidote) at the periphery. The epithermal half itself splits into two end-members. High-sulfidation (HS) deposits form from acidic, oxidized magmatic fluids close to the intrusion, hosting gold-copper mineralization in zones of advanced argillic alteration (alunite, kaolinite). Low-sulfidation (LS) deposits form from cooler, near-neutral fluids farther out, hosting high-grade gold and silver in quartz-adularia veins. A lithocap at the surface often marks the bridge between the two halves.

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💡 Did You Know?

Most epithermal gold-silver deposits on Earth are geologically young. Per the USGS, most known deposits are Cenozoic (under 66 million years old), not because they only formed recently, but because the shallow half of the system erodes away quickly. Almost every ancient epithermal vein the planet ever built is gone. The porphyry below, sitting 1 to 6 km deep, often survives. The flashy upper half almost never does.

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Flagship Districts

These deposits do not appear at random. They line up along convergent plate boundaries. A previous MiningVisuals piece traced 34% of world copper to the Andean margin alone, with Chile's Chuquicamata and El Teniente headlining the porphyry half. On the same belt, Peru's Yanacocha district has produced over 37 Moz of gold from HS epithermal deposits since 1993, and Bolivia's Cerro Rico de Potosí has been mined for silver since the sixteenth century. In Utah, the Bingham Canyon Mine is the largest man-made excavation on Earth, with 19 million tons of copper produced. Indonesia's Grasberg is one of the world's most significant copper-gold porphyries.

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Why Both Halves Matter Now

Both halves now sit at the center of strategic mineral conversations. The USGS global porphyry database notes that porphyries also host rhenium, tellurium, and selenium as critical by-products. The USGS epithermal model, by contrast, confirms that epithermal districts are major contributors to global gold and silver supply, the same metals now under structural demand from electronics, solar, and the wider energy transition. The same magmatic engine that supplied colonial empires is being re-examined for what else it can deliver.

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