Garnet-pyroxene-molybdenite-scheelite skarn (Late Cretaceous, 92 Ma; Reilly Mine, Osgood Mountains, Nevada, USA) 1
Skarn from the Cretaceous of Nevada, USA. (field of view ~2.0 centimeters across)
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.
The rock seen here is an andradite-diopside skarn from Nevada having 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 (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 latitude, 117° 15’ 03” West longitude)
Garnet-pyroxene-molybdenite-scheelite skarn (Late Cretaceous, 92 Ma; Reilly Mine, Osgood Mountains, Nevada, USA) 1
Skarn from the Cretaceous of Nevada, USA. (field of view ~2.0 centimeters across)
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.
The rock seen here is an andradite-diopside skarn from Nevada having 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 (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 latitude, 117° 15’ 03” West longitude)