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Phase I – Hydrothermal Fluids and Basement Alteration in the Richville Shear Zone, Northern New York State:

In the summer of 2008, Selleck and the two student workers spent approximately 2 weeks in the field in northwestern New York State in the vicinity of the Richville Shear Zone near Gouverneur, New York. Field work included: (1) Measurement and documentation the orientation of mineralized veins associated with HTD processes in the basement rocks and overlying Potsdam Sandstone; and (2) Sampling of materials for laboratory analysis. Sampling focused on materials with potential for: evaluating evidence of pulsed fluid flow (e.g. multiply-zoned calcite and dolomite crystals, multiply-floored voids, symmetrically zoned veins); materials for fluid inclusion analyses (e.g. quartz, dolomite, fluorite, calcite) and stable isotopic analyses of carbonates, and; materials for stable isotope geothermometry. Samples of altered basement rock were taken to evaluate Mg loss/gain along fractures and veins.  Additional samples were collected for potential geochronological analyses. 

Thin sections were examined using standard petrographic microscopy and scanning electron microscopy using back-scattered electron detector and energy dispersive X-Ray analysis. Altered and dolomitized basement marbles show evidence of breakdown of Mg-bearing silicate minerals which may form a source for Mg for dolomitization. Basement gneiss in outcrops adjacent to dolomitized basement marbles show significant Mg-depletion along fracture zones.

Fluid inclusion analyses were carried out on calcite and dolomite spar from a number of localities. The materials analyzed represent hydrothermal precipitates from fluids that caused dolomitization of basement marble in the Richville Shear zone system. Homogenization temperatures range from 90-250°C and final water ice melt temperatures range from -25 to -45°C. Many inclusions exhibit anomalous freezing-melting behavior, suggesting the brines are Na-Ca-Cl in general composition. The relatively wide range of homogenization temperatures are consistent with precipitation of calcite and dolomite from seismically pumped fluids that were pumped upward from deeper, hotter portions in the fault system, and contacted cooler and more variable-temperature wall rock.

Samples from the basal Potsdam Formation sandstone from eastern New York State localities were prepared for electron microprobe analyses at University of Massachusetts . 24 samples of altered Potsdam Formation sandstone from the Richville Shear zone were studied using SEM-EDS to search for authigenic monazite and xenotime.  The first analyses of authigenic monazite from the Potsdam Formation was completed in fall, 2008 (Selleck, Williams and Jercinovic, 2008). This proof-of-concept result shows that, for Potsdam Formation arkosic sandstone from eastern New York, a major hydrothermal mineralization event occurred at ca. 440 million years ago, corresponding in time to the main contractional phase of the Taconic Orogeny. 

87 samples were analyzed for stable isotopes of oxygen and carbon at the University of Northern Arizona stable isotope lab. The data, in general, suggest that the fluids involved were isotopically evolved, and that some carbon was provided by breakdown (decarbonation) of organic-sourced hydrocarbon.  The relatively wide range of calculated oxygen isotope values for the hydrothermal fluid, based on homogenization temperature and stable isotope values of carbonate, is also consistent with a seismic pumping model.  In such a system, we would expect variations in the amount of water-rock interaction, which would be manifested in a range of oxygen isotope values for waters, depending on the degree of isotopic exchange with wall rock.

Our results from Phase I are consistent with the following:

1. Down-drawing of seawater-derived fluid during the dilational phase of seismic pumping produced an ‘evolved’ brine in contact with basement rocks.  Rock-water interaction involved isotopic and major element exchange.

2. Expulsion of fluids upward into adjacent basement rocks and overlying Paleozoic sedimentary cover occurred during contractional phases of seismic pumping.  Dolomitization of marble, injection of fluidized sand and precipitation of vein fill of calcite, dolomite and quartz, plus minor minerals, occurred as a result.

3. Fluid inclusion and stable isotopic data suggest a wide range of mineralization temperatures and rock-water interaction, as would be expected in a complex fault zone system.

4. Mineralization involving growth of authigenic monazite in Potsdam Formation sandstone in eastern New York occurred at ca. 448 ma, suggesting that fluid flow was linked to contractional strain during the Taconic Orogeny.

Phase II – Mohawk Valley Faulted Basement and Hydrothermal Veins

During the summer of 2009, Selleck and two student co-workers sampled cores and outcrop in the Mohawk Valley of New York State.  Four cores penetrating the lower Paleozoic sedimentary sequence and underlying basement were sampled for XRF, SEM and fluid inclusion analyses of Paleozoic veins that penetrated basement gneiss.  Outcrops of Utica Shale (Late Ordovician) containing veins of calcite spar were sampled in field exposures in Montgomery County, New York .  The goal of this phase of the study was to assess the alteration of the basement rock beneath the Paleozoic sequence to determine the nature of fluid alteration.  Analytical work on the basement rock included XRF analyses (major and trace elements) of samples taken at intervals below the basal unconformity and SEM study of mineral alteration and porosity and permeability development.  A total of 88 samples from four cores were analyzed, and trends in whole-rock major and trace element variation demonstrate that the basement rocks are depleted in Mg, Ca, and Na, and enriched in Al and K, compared to unaltered precursors.  Trace element patterns likewise show relative depletion in mobile species. Physical alteration of basement rock beneath the unconformity has increased porosity and permeability, suggesting that these rocks might serve as a carbon sequestration target.

Fluid inclusion and stable isotope results from samples of Utica Shale and basement-hosted carbonate veins have been reported in Selleck, Meisner and McDonald, (2009).  These results indicate that the Utica Shale carbonate veins were formed from relatively low TDS fluids that were evolved as compaction waters; carbon isotope patterns suggest derivation of some carbonate from microbial fermentation of hydrocarbons.  Basement-hosted calcite veins were precipitated from very saline (to 30% salinity) Ca-Mg-Na-Cl brines.  These results suggest that the Utica Shale fluids were not an Mg-source for hydrothermal dolomitization of underlying Trenton-Black River carbonates. However, mixing of shale-derived and basement derived fluids would provide the appropriate fluid chemistry for dolomitization.