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.
[Oreg.] [Ariz.] [N.M.] [S.D.] [Tenn.] [S.C..] [N.C..] [Penn.]
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