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Friday, October 16, 2009

Gold Mining in Pennsylvania

In Pennsylvania, gold has been produced in significant quantities only at the Cornwall iron mine,
about 5 miles south of Lebanon in the southern part of Lebanon County. Mining of the Cornwall deposit began in 1742 and was still active in 1959. Iron has been the chief commodity, but small
amounts of copper, gold, and silver have also been recovered. The first gold production recorded was in 1908, when about 35 ounces was refined from copper concentrates. The total gold output of the Cornwall mine through 1959 was 37,459 ounces, most of which was produced after 1937.
The oldest rocks of the area, according to Geyer and others (1958), are of Cambrian age and consist of five limestone and dolomite members of the Conococheague Formation. These are overlain by four formations of the Beekmantown Group of Ordovician age, and by the Annville, Myerstown, and Hershey Limestones and Martinsburg Formation, also of Ordovician age. In the immediate vicinity of the Cornwall deposit are two rock units of doubtful age: one of these is known as the Mill Hill slate and may be an outlier of Martinsburg Shale; the other is conglomerate or breccia called "blue conglomerate" by the miners. The entire Paleozoic section was folded into a recumbent synclinorium that trends east-northeast. Considerable thrust faulting accompanied the folding. The Paleozoic rocks are unconformably overlain by shales, sandstones, and conglomerates of the Gettysburg Formation of Triassic age. These rocks are warped into a homocline that dips north. The contact between the two series of rocks trends east and in many places is a fault. Large sills, dikes, and plugs of diabase of Triassic age intrude all the sedimentary rocks, and in part of the area a sill separates the Triassic and Paleozoic rocks. The Cornwall deposit is a contact metasomatic deposit at the contact of a diabase dike and Cambrian limestone. The dike has an elliptical outcrop pattern but at depth becomes sheet like, which creates a trough where the ore bodies of replaced limestone are situated. The ore consists chiefly of magnetite
and actinolite and lesser amounts of chalcopyrite, pyrite, diopside, phlogopite, chlorite, and serpentine. Small amounts of gold are recovered from copper concentrates, which suggests that it occurs with chalcopyrite.
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