Skarns are calc-silicate rocks formed during contact metamorphism and metasomatism between an intrusive body and carbonate-rich sedimentary rocks. The hot fluids expelled from the intrusive contain iron, silica, magnesium, aluminium and a variety of incompatible elements. These fluids dissolve carbonates and precipitate calc-silicates in their place. The most common minerals that comprise skarns are epidote, garnet, idocrase, pyroxenes, wollastonite, tremolite, and magnetite or hematite. Calcite and quartz are generally also present. Mineralized skarns may be enriched in one or more of the following; Au, Ag, Cu, Zn, Pb, Fe, Mo, Sn, W. Au and Cu skarns are some of the most common type, others contain polymetallic sulfides (+/-Ag) such as the Nuestra Senora Ag-Zn-Cu-Pb skarn in Mexico, or Sn and W as the Ok Tedi and Pine Creek mines in California.
Exoskarns represent the alteration zone outside the intrusive and endoskarns are formed within the intrusive body. Fluids for exoskarns are derived from the intrusive. Fluid inclusions in ore or gangue minerals often contain solid salt crystals, indicating that the mineralizing fluids were highly saline.
In Petrographic Reports on skarns it is important to distinguish between endo and exoskarns, to characterize mineral zonation and to estimate the temperature gradient from the alteration assemblages. A series of detailed studies on skarns by GeoConsult includes the Nuestra Senora Zn-Ag mine in Mexico and the Rosemont Cu-Mo-Ag deposit in Arizona, US.
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Nuestra Senora Ag-Cu-Zn Skarn, Mexico. SCORPIO MINING INC.
(x-axis of photos: 1.6mm)
Skarns Petrographic Services