Overview

Epithermal mineralization occur in areas where hydrothermal fluids were active over large areas often saturating rock volumes more than 100 cubic kilometers in area. They occur in areas of high heat flow along minor crustal structure, plate margins, grabens, rifts, and along major crustal structures. They form where meteoric water can circulate to great depths within major structures over extended periods of time often exceeding millions of years. Geothermal drilling encounters open fractures at depths greater than 2 kilometers that are held open by fluids under extreme high pressure. Open structures become less abundant deeper in the earth due to lithostatic load and tectonic pressures which tend to squeeze openings closed. Fluid inclusion measurements indicate formation temperatures occur below 350 degrees C and pressures less than 1 kbar (100 MPa) with few exceptions. Geothermal well blowouts mimic natural fault movement releasing pressure on the hydrothermal system. These high pressure fluids often exsolve a gas phase, create breccia bodies from explosive hydrofracking, deposit vein material and flash rapidly to the surface sometimes moving upwards hundreds of meters in minutes. Epithermal veins are often upper level manifestations of deeper higher temperature disseminated deposits, stock work zones, replacement deposits, and other types of gold deposits. Open structures and vein deposits are more common within 2 kilometers of the surface. Gold bearing veins have a wide range of genetic origins, fluid sources, chemistry, source rocks, host rocks, and mineralogy. They often display similar processes of formation and mechanisms that initiate mineral deposition.

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