- © The Mineralogical Society Of America
Early in the 20th Century epidote was readily recognized as a common rock-forming mineral of metamorphic and hydrothermal processes (Becke 1903; Grubenmann 1904; Van Hise 1904; Goldschmidt 1911; Eskola 1915). Its distribution is widespread in the Earth’s crust, including metamorphic environments of pumpellyite-prehnite, greenschist, epidote-amphibolite, and blueschist facies (Seki 1972; Liou 1993). In lower-pressure hydrothermal environments epidote is a common mineral in skarns, in propylitic altered volcanic rocks and in late-stage veins related to silicic intrusions (Lindgren 1933; Coats 1940; Nakovnik 1963). Within obducted segments of oceanic crust (ophiolites) and in large igneous provinces epidote is found in veins and replacement bodies (epidosites, cf. Dana 1875) associated with intrusion of dolerite dikes and gabbros (Coleman 1977). It was not until the 1960’s that epidote was first discovered in drill hole samples from active geothermal systems (Naboko and Piip 1961; Sigvaldason 1963; White et al. 1963; Steiner 1966; Keith et al. 1968; Marinelli 1969). Geothermal drill holes provided the first samples of epidote-altered rocks and coexisting hydrothermal fluids at measured temperatures and pressures (White and Sigvaldason 1963; Naboko 1964). Formation of epidote in such low-pressure geologic environments was initially questioned (Rusinov 1966), due in part to geologic observations (Korzhinskiy 1963) and to the sluggish nature of epidote synthesis at low pressures and temperatures (Fyfe et al. 1958; Coombs et al. 1959; Merrin 1960; Fyfe 1960). Epidote is now recognized as a key index mineral related to temperature, permeability, and fluid composition in geothermal systems worldwide (Browne 1978; Giggenbach 1981; Henley and Ellis 1983; Bird et al. 1984; Reyes 1990; Absar 1991; Reed 1994; Muramatsu and Doi 2000).
In general, hydrothermal epidote exhibits a wide range in octahedral …