FLUID EVOLUTION AND TRAPPING CONDITIONS IN THE NORTHERN PART OF THE LACKAWANNA SYNCLINORIUM

Earlier work examined fluid inclusion microthermometry of fluid inclusions in quartz veins from Pennsylvanian rocks of the Southern and Middle Anthracite Fields, and supports a deep burial model of coalification in favor of focused orogenic hot fluid flow. High-temperature (250 to 255 °C) trapping of CH₄ ± CO₂ saturated aqueous fluids and CH₄ ± CO₂ fluid inclusions indicate fluid trapping at depths of 11.5 to 13.4 km under a cover of Pennsylvanian to Permian(?) syntectonic load. In the folded rocks to the south of the Southern Anthracite belt near the Blue Mountain Structural Front, CH₄ ± CO₂ fluid inclusions indicate a sediment load that was up to 16.3 km thick.

The Lackawanna Synclinorium, known as the Northern Anthracite Field, is located 50 to 100 km to the north of the Southern Field and is an over 120 km long synclinal trough that has been interpreted as a salt-collapse structure. Preliminary analysis of CH₄ ± CO₂ fluid inclusions in quartz veins from the Pottsville Fm. give minimum homogenization values (T_hH) of -100.8 to -80.0 °C and contain 12 to 15% CO₂ for one vein and 7.0 to 10.0 % CO₂ for another. The CH₄ ± CO₂ fluid inclusions examined so far indicate fluid tapping at conditions less than lithostatic, suggesting hydrofracturing of the fluid system and rapid drop in pore-fluid pressure resulting of degassing of formation fluids. Aqueous fluid inclusions from the Pottsville Fm. have homogenization values (T_hA) of 172.3 to 178.5 °C with ice melting temperatures (T_mIce) of -7.7 to -5.7 (8.8 to 11.3 wt. % NaCl equiv. salinity). These salinities are much higher than those determined from the Pennsylvanian rocks in the more southern Anthracite fields which range from 0 to 3%. Fluid trapping depths are estimated to be 6.2 to 7.8 km. Published vitrinite reflectance values in Pennsylvanian rocks in the northern part of the Lackawanna Synclinorium are <4.0 (<235 °C).

Aqueous fluid inclusions from rare quartz veins the Devonian-Mississippian Spechty Kopf Fm. have T_hA values of 122.1 to 131.6 °C and T_mIce of -9.8 to -5.7 (8.8 to 13.7 wt. % NaCl equiv. salinity). Pressure-corrected trapping temperatures are 150 to 275 °C and indicate trapping depths of 5.2 to 12.8 km. At this point, it is uncertain whether coalification in the northern part of the Lackawanna Synclinorium area is due to burial or focused hot fluid flow.