MINING HISTORY OF THE SERPENTINITES IN PENNSYLVANIA AND MARYLAND

The varied geologic history of the ultramafic rocks in southeastern Pennsylvania and adjacent parts of Maryland resulted in a wide range of mineral and stone resources. Fractional crystallization of a basaltic magma resulted in chromite ore and ultramafic dunite and peridotite cumulates. Subsequent low-grade metamorphism altered the ultramafic rocks to serpentinite. Emplacement in and reaction with quartz-rich metasediments, intrusion of pegmatites, hydrothermal and supergene alteration produced additional mineral deposits including talc, asbestos, magnetite, corundum, vermiculite and magnesite.

Mining and quarrying span the time period from the pre-Columbian era to the present. Indians quarried and carved soapstone for use as cooking vessels and ornaments. Chromite was discovered in 1810 at Bare Hills, MD near Baltimore. Isaac Tyson recognized the association of chromite with serpentinite and quickly controlled the industry. The region quickly became the largest producer of chromite in the world which lasted until the mid-19^th century. Sporadic, minor production continued up to World War I. The chromite was used by the chemical industry for the production of pigments (chrome yellow). The serpentinite region was also the major source of magnesite for production of magnesia and Epsom salt in the early and mid-19^thcentury.

In addition to mineral production, the serpentinites have been quarried for aggregate, building and decorative stone and for industrial uses. Building stone was widely used in the Philadelphia area in the 19^th century, but use declined because the serpentinite deteriorated rapidly when exposed to pollution. Decorative ‘green marble’ or verde antique from Cardiff, Md. was quarried until the early 1970s. In addition, talc and soapstone were also quarried and used for refractory linings, ceramics, washtubs and as filler for paint. Today serpentinite is quarried for use as crushed stone.