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**Unveiling the World of Hydrothermal Ore Deposits: From Porphyry Copper to Deep-Sea VMS**
Hydrothermal ore deposits form when hot, corrosive, mineral-rich fluids escape from cooling magma and interact with surrounding rock, leaving behind valuable minerals. The specific type of deposit created depends on the path the fluid takes and the rock it encounters. When the fluid is trapped within the magma chamber, it forms pegmatite deposits, known for their large crystals. If it escapes into fractures in the rock, it creates concentrated veins characteristic of orogenic deposits. However, when the fluid diffuses through the rock itself, it forms more diffuse deposits. If it permeates non-reactive igneous rock, it creates a **porphyry deposit**, which is a primary source of copper today. These deposits are low-grade, meaning the metal is spread thinly, making them economically viable only with modern, large-scale mining techniques like massive open-pit operations. Interestingly, the stone „porphyry” was prized in antiquity for its decorative purple hue, giving us the phrase „born to the purple,” but its metallic content was too diffuse to be mined until recently.
When the same acidic hydrothermal fluid encounters reactive carbonate rock, a vigorous chemical reaction occurs, forming a **skarn deposit**. This process often concentrates a wider variety of metals, including tin, tungsten, copper, and gold, into more economically viable grades than porphyry deposits. The unique chemistry of skarn formation also makes it a prime source for gemstones like garnet, topaz, and even rubies and sapphires (corundum). If the hydrothermal fluid escapes the rock entirely and vents into the ocean, it creates famous „black smokers” on the seafloor. The minerals that precipitate from these plumes settle over geologic time to form **Volcanogenic Massive Sulfide (VMS) deposits**. These are rich sources of sulfide ores for copper, zinc, lead, and silver, and have been mined since ancient times, but many remain on the deep seabed, representing a potential future frontier for mining.
Two other related, but less common, underwater deposits are SedEX and Mississippi Valley Type (MVT) formations. **SedEX (Sedimentary Exhalation) deposits** are fine-grained lead-zinc deposits formed when hydrothermal fluids pass through sedimentary rock before meeting seawater. **Mississippi Valley Type (MVT) deposits** are also lead-zinc focused but form in carbonate rocks in shallow inland seas, often with the help of organic matter to supply sulfur. While VMS deposits are globally significant, SedEX and MVT are more niche. The formation of these deposits is a geological lottery, dependent on specific combinations of rock, fluid, and environment, with MVT deposits likely being unique to Earth due to their specific formation requirements.
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Forrás: https://hackaday.com/2025/10/21/ore-formation-return-of-the-revenge-of-the-fluids/.