Abstract

The State Line talc deposit, in Lancaster County, Pennsylvania, near the Maryland border, occurs at the boundary between a serpentinite body and surrounding metamorphosed pelitic rocks. Blackwall-type zones are found in the talc body. The zones are composed of variable amounts of tremolite, chlorite, biotite, and dolomite. However, trace element chemistry indicates that the zones are not a product of simple blackwall metasomatic processes. The ratios of Al 2 O 3 /Cr, Zr/Cr, Ti/Cr, and Al 2 O 3 /Ni in the blackwall-type zones are intermediate between the ratios found in the serpentinite and the pelite, supporting the hypothesis that these zones result from the sedimentary mixing of pelitic and serpentinite-bearing material on the sea floor, with later modifying metasomatism.The distribution of tremolite, a contaminant that affects significantly the market value of a talc deposit, is the result of small-scale metasomatic reactions that modified the original layers of serpentinite-bearing and pelitic detritus; and larger scale metasomatism involving silica, water, and carbon dioxide that fixed tremolite as the saturating calcium phase in the proximity of the pelitic country rock, and dolomite in the proximity of the serpentinite.

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