English: Magnetite skarn from the Jurassic of Pennsylvania, USA. (~9.5 centimeters across at its widest)

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). The mineralogy of skarns is highly variable, depending on the chemistries of the host rock and the intruding magma.

This particular rock is a magnetite skarn from Pennsylvania's French Creek Mines. The sample is dominated by magnetite (= Fe3O4, iron oxide) and mixed with some greenish-gray serpentine. The magnetite component in the area's skarns are rich enough to have economic significance. The French Creek Mines, active from the early 1700s to the early 1900s, targeted the magnetite for its iron content.

The magnetite has mostly replaced Proterozoic-aged graphitic marble intervals within a succession of high-grade metamorphics, dominated by biotite gneiss. The original sedimentary rocks were altered by regional metamorphism during one or more Appalachian Mountains orogenic events (Paleozoic). During the Early Jurassic, the supercontinent Pangaea broke apart, and in eastern America, this continental rifting resulted in the formation of many diabase intrusions. Contact metamorphism along one such diabase body formed magnetite skarns at French Creek, and many other areas in the Applachians.

Diabase (also known as dolerite, or microgabbro) is a mafic, crystalline-textured, intrusive igneous rock. It has crystals larger than basalt but smaller than gabbro. The principal minerals are plagioclase feldspar and pyroxene. The intrusion in the French Creek area is tholeiitic diabase.

Some consider the diabase intrusions to be Late Triassic in age. Continental rifting and associated igneous activity occurred during both the Late Triassic and Early Jurassic.

Locality: unrecorded/undisclosed site in the French Creek Mining District, southeast of Harmonyville & northeast of Knauertown, northern Chester County, southeastern Pennsylvania, USA

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Some info. from:

Robinson (1988) - Base- and precious-metal mineralization associated with igneous and thermally altered rocks in the Newark, Gettysburg, and Culpeper early Mesozoic basins of New Jersey, Pennsylvania, and Virginia. Developments in Geotectonics 22: 621-648.

Naslund (1998) - Trip B3 - Cornwall-type iron ores of Pennsylvania. Field Trip Guide for the 70th Annual Meeting of the New York State Geological Association: 105-121.

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Locality info. at:

www.mindat.org/loc-4077.html