NEW HOMOGENIZATION DATA FOR FLUID INCLUSIONS IN FAULT RELATED QUARTZ VEINS IN THE BALD EAGLE FM. IN CENTRAL PA

Fluid inclusion analysis, on samples from the Skytop roadcut, Rt. 322, in central Pennsylvania, was used to evaluate the temperature and history of fluid migration in prominent faults that cut the Bald Eagle Fm there. The current understanding of this locality involves a late Paleozoic mineralization of the fault zone whereby fluid migration occurred through Mississippi Valley Type (MVT) processes. The new data is in conflict with this model. Primary fluid inclusion homogenization temperatures (T_h) from quartz veins in the fault zone were obtained for 108 inclusions. These temperatures ranged from 140°C to > 375°C. T_h for distinct fluid inclusion assemblages (FIA) cluster between 180-220°C. The high temperature inclusions >300°C do not form FIA, but rather are present as solitary, most likely primary, inclusions. The freezing data, from 24 inclusions, indicate involvement of high salinity fluids (melting temperatures in the vicinity of -25) and low salinity fluids (melting temperatures showing as little as 1 degree freezing point depression). Homogenization temperatures in the 150-225 degree range are typical of MVT type processes. However, homogenization temperatures above the 300 and 350 degree marks suggest processes unrelated to MVT processes. Furthermore, these high temperatures are inconsistent with temperatures governed solely by burial and a normal geothermal gradient. The data do not support a single mechanism, but rather process in which both MVT fluids and high temperature fluids migrated through the basin after conventional wisdom suggests it had become quiescent. The relative timing of these events remains unclear. A hot fluid may have been followed by a normal fluid, a normal fluid by a hot fluid, or multiple cycles of hot and normal fluid migration may have occurred. Regardless of the specific nature of the event, the fluid inclusion data have profound implications for the current understanding of the geologic history of central Pennsylvania.