TACONIC FAULTS AND MINERALIZATION: OUTCROP ANALOG FOR ORDOVICIAN HYDROCARBON PLAYS IN THE APPALACHIAN BASIN

The Cambro-Ordovician section in the Mohawk Valley (east-central New York State) provides an analog for hydrocarbon plays associated with normal faults. The Mohawk Valley section is cut by normal faults with up to 450 m of stratigraphic offset. The faults were associated with fluid migration that resulted in mineralization that occurred in two phases. Continued analysis of our data, discussed below, has allowed us to develop a tectono-stratigraphic model that relates each phase of mineralization to Ordovician-age faulting.

Mineralization phase 1 was contemporaneous with passage of the North American plate over the Taconic peripheral bulge; phase 1 mineralization is characterized by dolomitization of the Tribes Hill Formation near certain faults. Mineralization phase 2, coincident with the main phase of the Taconic Orogeny, formed vugs in the Cambrian to Middle Ordovician carbonate units, and veins throughout the entire Precambrian to lowermost Upper Ordovician section. Phase 2 vugs and veins occur near most faults and cross-cut dolomitization textures in the Tribes Hill Formation.

Multiple veining episodes during phase 2 mineralization are evidenced by cross-cutting relationships, differing fill minerals and different vein orientations. A scanline across the only exposed fault in the field area shows an increase in the frequency of veins of the dominant set toward the fault. Lithology exerted a control on both phases of mineralization. Phase 1 dolomitization is most pervasive in the thick-bedded, non-shaly portions of the Tribes Hill Formation. Phase 2 vugs are most abundant in primary dolostones. Veins and joints show distinct variations in orientation and frequency in contrasting lithologies.

Stable isotope analyses, mineral textures and previously published conodont alteration indices indicate a hydrothermal origin for phase 2 mineralization. Carbon and oxygen isotope ratios for phase 2 mineralization are similar to Mississippi Valley-type hydrothermal carbonates. Trace and rare-earth elements in phase 2 secondary carbonates indicate a basement source for the species in the mineralizing fluids, indicating a crustal scale fluid-flow system. This scale is consistent with the mapped Taconic faults that affect the Precambrian basement as well as the Cambro-Ordovician cover sequence.