Abstract

The Fe-Cu-Co-Zn-Ni mineralization of the Sykesville district occurs in a narrow 16 km-long belt in the Maryland Piedmont of the Appalachian orogen. The mineralization occurs within a banded iron-formation and consists dominantly of magnetite, quartz, chalcopyrite, siegenite, sphalerite, and pyrite with lesser hematite and bornite. The mineralized zones occur near the contact between serpentinized ultramafic rocks and the metasediments of the Morgan Run melange. Intimate mixing of pelitic and ultramafic material indicates that the ultramafic rocks were present on the sea floor during sedimentation.The banded iron oxide-quartz rocks are everywhere in contact with ultramafic rocks; both of these units, including the blackwall and the mineralized rock, contain detrital chromite and exhibit soft-sediment deformation. The detrital chromite contains anomalously high concentrations of ZnO, up to 19 wt percent, in their corroded cores. Textural evidence suggests that the paragenetic sequence is sphalerite-chalcopyrite-pyrite-siegenite. This paragenesis and the chemical zoning of the chromite suggest that the introduction of zinc represents the earliest stage of mineralization.Cobalt and nickel were most likely derived from the ultramafic rocks during the highest temperature stage of a sea-floor hydrothermal event. The high Cu + Zn/Pb ratio in the mineralized rock and the occurrence of ultramafic material suggest that mineralization occurred on the sea floor with little or no involvement of continental crust. Copper and zinc were most likely derived from underlying basalt. The ultramafic rocks and Pb-poor sulfide mineralization occur along a narrow belt tens of miles in length which is interpreted to represent the trace of a subducted spreading center. The presence of pelitic detritus suggests that the spreading center was proximal to continental crust during the time of mineralization.

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