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Hydrothermal Alteration of Proterozoic ‘Grenville’ Marble and Cambrian Potsdam Sandstone by Seismically-Pumped Fluids, Adirondack Lowlands of Northwestern New York State

Bruce W. Selleck, Department of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346, [email protected]

Well-exposed NE-trending strike-slip shear zones in the Adirondack Lowlands of northeastern New York State were active during burial diagenesis of the Cambrian Potsdam Sandstone. Hydrothermal fluids interacted with basement marble to produce widespread dolomitization, hydrothermal leaching and alteration of silicate minerals. Cambrian sandstone overlying the basement was injected as fluid slurry into corrosion pipes and tunnels and cemented by quartz, illite, chlorite, apatite and minor authigenic phases. Fluid inclusion data suggest the mineralizing system was generally 120-180ºC with fluid salinities 12-18 wt. % NaCl equivalent. Stable isotope data on hydrothermal carbonate minerals are consistent with evolved basinal brines (δ 18OSMOW in the range +8 to +11 ‰) as the mineralizing fluid. Patterns of dissolution and void development in marble and paragenesis of minerals within the marble and sandstone suggest fluctuating fluid composition in terms of carbonate saturation and oxidation state. Multiple zonation of void fill and alternating ferric-ferrous mineral precipitates argue for rhythmic fluid pumping within the shear zone/hydrothermal system framework.

The tectonic, structural and stratigraphic setting of this mineralizing system forms an excellent model for the basement-cover root zone of HTD reservoir systems. Seismic pumping of locally-derived connate waters best explains the scale and distribution of hydrothermal dolomite. Hydraulic connection between sheared, altered basement rock and overlying carbonates undergoing hydrothermal dolomitization is critical to allow sufficient flux of Mg to promote dolomitization. Active seismic pumping of fluids during wrench faulting was likely a key element in the development of HTD reservoirs.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York