Zhaoshan Chang, professor of economic geology at the Colorado School of Mines, revealed during his presentation at the Road to proEXPLO 2023 seminar that they are largely composed of tungsten and tin. They also contain minerals such as gold, copper, zinc, lead, molybdenum and iron, as in the case of Antamina.

Metals such as silver, bromine, beryllium, bismuth, cobalt, fluorine, rare earths and uranium are found in smaller quantities.

The mineralization of these skarns is divided into two parts: porphyry deposits and greisen deposits. According to carbonate replacement deposits (CRD), these can be of the Carlin type (disseminated gold deposits hosted in sediments) and/or epithermal.

The specialist stated that the genesis of metamorphic skarn deposits is derived from calcium silicate hornfels and reaction skarns; industrial minerals such as wollastonite. Metamorphic hydrothermal skarns are made of tungsten and magmatic hydrothermal skarns generally have a large magma source and limestone wall linings.

"This is not limited to the contact zone. Depending on its proximity, it surrounds the causative intrusion. Depending on its distance, it is controlled by faults, has a considerable perimeter between different types of rocks, including pre-existing intrusive rocks - carbonate limit," he said.

Skarn-type deposits are developed in five phases:

• Metamorphic phase: calcium-silicate hornfels, reaction skarn and marble.
• Prograde phase: Garnet, pyroxene, wollastonite, plagioclase, scapolite. Rich in iron. Zoned. The intrusion may have sodium.
• Retrograde phase A: Epidote, amphibole, scapolite, vesuvianite, ilvaite, quartz; hematite, magnetite, mushketovite, wolframite and scheelite.
• Retrograde phase B: Chlorite, quartz, carbonate, biotite, Ksp, fluorite, sulfides, sulfosalts, gold and cassiterite.
• Retrograde phase C: Calcite veins (without metals).

Some of the factors that affect the formation of skarns include:

• Redox state gradient between magma and wall rock.
• Causal magma: volatiles, degree of fractionation and redox state.
• Wall rock: Composition, redox state and permeability.
• Depth of formation.
• Distance from magma.

Finally, Chang indicated that no single metal zoning pattern can fit all skarns, considering that many examples of continuous transition from distal to proximal skarn alteration and mineralization prove distal systems are part of a skarn (up to 4.5 km; Chitudian zinc-lead skarn). "Large deposits have all parts uncovered," he concluded.

proEXPLO 2023: Descubriendo la Minería del Futuro