Monday, 23 February, 2026/Edition 99

This week’s Core Elements presents Part Three of a trilogy on mineralization processes (See editions 97 and 98 for our previous coverage).  

Before we dig in, check out the U.S. Geological Survey’s geologic description of 13 minerals used in the Winter Olympics—from fluorspar, mica, and chromium used in figure skating, to gold, silver, copper, and zinc used for Olympic medals. The 2026 Winter Olympics ended yesterday, but the minerals will be used in the Olympic Games for years to come.    

Rasoul Sorkhabi

Editor, Core Elements

Depth and Timing of Ore Crystallization in the Southwestern United States

Santa Rita/Chino open pit mine in New Mexico/ WikimediaCommons 

Porphyry copper-gold-molybdenum deposits form in the upper crust in regions of large magmatic-hydrothermal systems. However, the depth levels and timing of porphyry deposit-forming fluids are subjects of much debate.

Two papers in a recent issue of Economic Geology provide interesting data on these issues.

Study #1: Depth of Magma Crystallization and Fluid Exsolution

Andreas Audétat and colleagues have conducted a study of Santa Rita and Hanover-Fierro plutons in New Mexico’s Central Mining District.

What they did:

• The researchers collected samples from little-altered, pre-mineralization, syn-mineralization, and post-mineralization dikes, sills, and stocks.

• They conducted mineral assemblage thermobarometric analyses of samples from various stages of mineralization magmas:

  • Pre-mineralization magmas (61–60 Ma)

  • Syn-mineralization magmas (60–58 Ma)

  • Post-mineralization magmas (58–57 Ma)

What they found:

• Syn-mineralization magmas give an average pressure of 3.10 kilobars, corresponding to crustal depths of about 12 kilometers. This is interpreted to be the depth of magma fractionation.

• Ore-forming fluids then exsolved and migrated seven kilometers upward to create ore deposits at paleo-depths of five kilometers.

Study #2: Timing of Magma to Hydrothermal Transition

Lawrence Carter and colleagues studied the onset of porphyry copper mineralization across the Yerington batholith in Nevada.

What they did:

• The researchers collected samples from mineralized miarolitic cavities, quartz unidirectional solidification textures, and quartz veins from Ann Mason, Yerington, Bear, and MacArthur porphyry deposits.

• They analyzed the samples using:

  • QEMSCAN petrography

  • Scanning electron microscopy (SEM)

  • Electron probe microanalysis (EPM)

  • Titanium-in-quartz geothermometer

  • U-Pb chemical-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS)

  • Molybdenite Re-OS negative TIMS geochronology

What they found:

• U-Pb dating shows that plutonic stocks and dikes were emplaced within only 100,000 years.

• The onset of copper mineralization was late in the transition from magmatic to hydrothermal conditions and occurred only when the mineralizing fluids cooled below 600 degrees Celsius.

Go deeper: Read the full papers in Economic Geology here and here.

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Subduction Slab Melting and Porphyry Ore Deposits

World's subduction zones and plate subduction tectonics/ Wikimedia Commons

Partial melting of a subducting oceanic plate is considered a main source and formation process for copper-silver-gold-molybdenum deposits in magmatic arcs worldwide. However, exact processes and genetic relationships constrained by empirical data remain poorly understood.

Two studies published in the February issue of Geology offer interesting insights into these problems.

Study #1: Magmatic Anhydrites in the Laramide Magmatic Arc in the Southwestern United States

Magmatic anhydrite background:

• Anhydrite is a rare mineral phase in magmatic rocks because it does not survive and decomposes in the near-surface groundwater zone.

• In 1984, Luhr et al. first discovered magmatic anhydrite in volcanic ejecta from the El Chichón Volcano in Mexico.

• Since then, 33 other cases have been documented in intrusive and extrusive rocks from the Philippines, Indonesia, and the United States.

The new study:

• Andreas Audétat and colleagues report on the presence of magmatic anhydrite inclusions in rock samples collected from igneous stocks and dikes associated with porphyry copper-gold-molybdenum deposits in Arizona and New Mexico.

• The samples come from magma systems of the Laramide orogenic event, dating from 40 to 80 Ma.

Why it matters: Anhydrite saturation in subduction melts indicates oxidizing conditions and high magmatic sulfur contents, which are necessary for the genesis of porphyry chalcophile deposits.

Study #2: Slab Melting and Sulfide Solutions Beneath the Western Aleutian Arc

The Aleutian island arc is a unique tectonic setting because it represents the active subduction of two oceanic plates: the Pacific (and preceding Kula) plate to the south and the oceanic segment of the North American plate. The region thus offers nearly unmodified primitive melt samples.

The new study: Maxim Portnyagin and colleagues report on a study of chalcophile and siderophile elements from rock samples collected from the Western Aleutian Islands.

What they did: The researchers collected subduction-related magnesium-rich rhyodacites, andesite, as well as eclogitized mid-ocean ridge basalts, and analyzed them for trace elements using LA-ICP mass spectrometry and optical petrography.

What they found:

• The study found the presence of anhydrite phenocrysts and hornblende composition, which indicate highly oxidizing conditions of crystallization.

• The rocks exhibit strongly fractionated chalcophile element ratios, such as high Ag/Cu and Bi/Cu ratios.

• No sulfides were present in the mineral assemblage, although sulfide oxidation is critical for porphyry copper deposits.

What it means:

• The researchers suggest that monosulfide solid solution was widely present during slab melting.

• The sulfide-bearing cumulates were subsequently delaminated at deeper lithospheric levels.

Why it matters: When sulfide minerals oxidize, they trigger a series of chemical reactions that influence the speciation and mobility of metals, including copper.

Go deeper: See this paper in Communications Earth & Environment on sulfide oxidation control on and porphyry copper deposits to learn even more.

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