Shear zone control on mineral deposits in the State-line serpentinite, Pennsylvania Piedmont

Abstract

The secondary redistribution and, in some cases, the primary formation of mineral deposits in the State-line serpentinite were controlled by shear zone processes. The State-line serpentinite is a layered mafic intrusion with a basal serpentinized peridotite with cumulate chromite layers, rhythmically layered gabbro and quartz diorite in the upper section. There are three generations of shear zones that have affected the serpentinite including early thrust faults, conjugate high-angle reverse faults and late dextral strike-slip faults with complex temporal, spatial and angular relations. The thrust faulting emplaced the State-line serpentinite onto the Peters Creek metasediments and represents the major faulting event. The conjugate reverse faulting may have been a late feature of the thrust faulting or related to the strike-slip faulting. The strike-slip faulting produced extensive mylonitization in the serpentinite and is the latest deformation in the area. Chromite is the most economic mineral resource in the State-line serpentinite. Most of the cumulate chromite deposits are lenticular to lensoidal in shape as the result of physical reorientation in thrust and strike-slip shear zones. Talc and magnesite were also mined from the State-line serpentinite. Talc occurs adjacent to pegmatite dikes but both talc and magnesite formed in mylonite where magnesite also fills the matrix of late-stage cataclasite and extensional veins. Some of the long fiber variety of asbestos formed in cross-fiber veins and slip-fiber slickensides in all brittle faults. The asbestos was mined from a few small prospects. The metamorphic reactions within the active shear zones during talc and magnesite mineralization reflect fluid evolution to lower temperatures and higher concentrations of CO 2. Comparison with experimental results on the metamorphism of serpentinite indicates a decrease in temperature from approximately 500° to < 350°C and an increase in CO 2 content from 0 to 3%. The shear zones acted as fluid conduits during deformation, recorded the evolution of metamorphic conditions in their mineralogies and controlled the distribution of talc and magnesite deposition.