SKARN
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¡ 11Õ 21Ó North latitude, 117¡ 15Õ 03Ó West
longitude).
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.
ILVAITE-HEDENBERGITE SKARN
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).
Info. from:
Tony Peterson (pers. comm.)
Gauthier & Albandakis (1991) - Minerals of the
Seriphos Skarn, Greece. Mineralogical Record 22: 303-308.