Skarn is a contact metamorphic rock with a crystalline texture. It forms by heating and addition of elements (metasomatism) to country rock in the immediate vicinity of an igneous intrusion (batholith, stock, sill, dike, laccolith). Carbonate rock skarn is frequently called tactite.
First, a couple skarn rocks from Nevada. These are from an andradite-diopside skarn having light reddish-brown andradite garnets (ideally Ca3Fe2Si3O12), dark greenish diopside (a Ca-Mg pyroxene), silver-colored molybdenite (molybdenum sulfide - MoS2), a little quartz (silicon dioxide - SiO2), and a little scheelite (calcium tungstate - CaWO4). The molybdenite content is high enough for this rock to qualify as a molybdenum ore.
Geology - contact metamorphosed limestones of the Comus Formation (Upper Cambrian-Lower Ordovician) against the Osgood Mountain Stock, an early Late Cretaceous (92 Ma) granodiorite intrusion. The garnet-rich skarn zone around the Osgood Mountain Stock is moderately rich in tungsten (W) and occasional molybdenum (Mo).
Locality - Reilly Mine, located in section 9, T38N, R42E, Potosi District, eastern side of the Osgood Mountains (Osgood Range), eastern Humboldt County, northern Nevada, USA (= approximately 41° 11’ 21” North, 117° 15’ 03” West). See map.
Skarn (field of view ~2.0 cm across) with andradite garnet (reddish brown), diopside pyroxene (green), and molybdenite (silvery gray).
Skarn (above & below; ~4.6 cm across) with andradite garnet (reddish brown), diopside pyroxene (dark green), molybdenite (silver), and scheelite (white). How do I know the white stuff is scheelite? Well, scheelite fluoresces bright blue under ultraviolet (UV) light - see the photo below. The garnet, pyroxene, and molybdenite don't fluoresce under UV light.
Some info. provided by Keith Wood.
Seriphos Island in the Aegean Sea has long been famous for its unusual minerals. Seriphos has well developed skarns (contact metamorphic rocks) surrounding an early Late Miocene (8-10 million years) granodiorite intrusion. The mineral collecting has been so spectacular and so intense that extremely restrictive laws are now in place. The rock shown below is a nice bimineralic skarn having the rare mineral ilvaite (black - CaFe+3(Fe+2)2O(Si2O7)(OH) - calcium iron hydroxysilicate) and hedenbergite pyroxene (greenish-gray - CaFeSi2O6 - calcium iron silicate).
Ilvaite-hedenbergite skarn (cut surface; 11.4 cm across) having ilvaite (black) and hedenbergite (greenish-gray).
Tony Peterson (pers. comm.)
Gauthier & Albandakis (1991) - Minerals of the Seriphos Skarn, Greece. Mineralogical Record 22: 303-308.