4.2.3: Hydrothermal Minerals

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4.18.jpg — 4.18 Travertine deposits in Hot Springs State Park, Wyoming

If chemical precipitation occurs at elevated temperatures, we call the process hydrothermal. Travertine and tufa, deposited by hot springs, are examples of hydrothermal deposits. Both are composed of calcite that precipitated from hydrothermal solutions, but tufa is more porous than travertine. The photo shows travertine terrace deposits in Wyoming.

Hydrothermal minerals are also created underground. Occasionally, hot circulating groundwater deposits minerals in sufficient quantity to make a valuable ore deposit. Minerals deposited this way include oxides, sulfides, and some others. Hydrothermal ore deposits vary. In some, ore minerals are concentrated in veins or vugs, in others they are disseminated throughout a body of rock. The table below lists some common hydrothermal ore minerals.

Minerals Common in Hydrothermal Ore Deposits
mineral class	mineral or mineral series	chemical formulas
sulfide	pyrite pyrrhotite chalcopyrite galena sphalerite molybdenite	FeS₂ Fe₁_-xS CuFeS₂ Pbs ZnS MoS₂
tungstate	wolframite	(Fe,Mn)WO₄
oxide	magnetite cassiterite pyrolusite	Fe₃O₄ SnO₂ MnO₂
native elements	gold silver	Au Ag

Many spectacular mineral specimens come from hydrothermal deposits. Hydrothermal minerals are often brightly colored because they contain transition metals. Many are metallic and many form highly symmetrical crystals. The photos below show three examples.