Hydrothermal mineral deposits

Hydrothermal Activity The circulation of water through hot volcanic rocks and magma Forms fumaroles – volcanic vents emitting gases, some charged with dissolved minerals Forms geysers – heated hydrothermal waters under pressure, that intermittently erupts at the surface Provides the source for Geothermal Energy.

Hydrothermal Mineral Deposits

Most of the metallic minerals mined in the world, such as copper, gold, silver, lead, and zinc, are associated with volcanism Magmas may get the metallic components from: Partial melting of the mantle at mid-ocean ridges and subduction zones From the subducted slab Melting of the crust as magma rises through it

Image from the article: The role of Magmas in the formation of Hydrothermal Ore Deposits, by Hendenquist and Lowenstern, Nature, Vol. 370, 1994

Rising magma does not always reach the surface to erupt; instead it may slowly solidify beneath the volcano As the magma body cools, the metallic elements become concentrated in the residual fluids called HYDROTHERMAL SOLUTIONS Groundwater or seawater can also come in contact with the magma body and form hydrothermal solutions

Water enters the ground by infiltration Hydrothermal Systems

Water enters the ground by infiltration Heated water begins to rise Hydrothermal Systems Heated water begins to rise

Water enters the ground by infiltration Hydrothermal Systems Hot Springs: form when heated groundwater reaches the surface Heated water begins to rise

Water enters the ground by infiltration Hydrothermal Systems Hot Springs: form when heated groundwater reaches the surface Geysers: form where a complicated plumbing system allows steam pressure to be built up, causing intermittent eruptions Heated water begins to rise

The magma body heats and circulates the hydrothermal solutions carrying the metallic compounds The metals become concentrated by circulating hot fluids When the fluids become cooler (near surface or away from the magma body), the metallic minerals are deposited to form rich mineral veins

Gold bearing hydrothermal vein deposit Oatman, AZ Gold bearing hydrothermal vein deposit Cutting across the middle of this image is a thicker band of light quartz crystals (a few millimeters high) surrounded by other bands of light quartz, gold, and darker sulfide minerals. This is an epithermal ore deposit from the gold mining district of Oatman in western Arizona. Epithermal deposits form from hydrothermal solutions ranging in temperature from 50° to 200°C and typically occur as veins, where the minerals grow in open fractures (note the well formed quartz crystals) rather than by replacing existing minerals within the rock. Each band of mineralizaton records specific chemical and physical conditions associated with the opening and precipitation of minerals within the vein. Fractures open repeatedly during the pressurization of the hydrothermal system. Mineral deposition is induced by release of pressure as fractures open to allow fluids to escape outward.

Large quantities of sulfide deposits form at mid-ocean ridges and other undersea volcanoes in this manner * Sulfides are the most common of the metallic ores

Some Copper Ores

Kennecott Open Pit Copper Mine, Utah