FLUID INCLUSION MICROTHERMOMETRY OF FAULT ZONE MINERALS FROM THE HARTFORD BASIN: EVIDENCE FOR FAULT REACTIVATION AND FLUID MIGRATION

Fluid inclusion microthermometry of fault zone minerals is used to infer the syn-deformational fluid history of the Hartford Basin. The western border fault zone consists of a large displacement (100s m) normal dip-slip fault plus numerous strike- and oblique-slip faults that cut the metamorphic rocks of the footwall. The wide variety of fault orientations indicates a long and complex fault history. Quartz mineralization on the faults reveals three primary NaCl-rich fluid types that are correlative with fault orientation: NNE-SSW sinistral faults have inclusions with T_h values of 172° - 311°C and with 0.0 to 12.8 wt. % NaCl equiv. salinity. These faults also contain CO₂ inclusions with T_h values of -5° to 26°C, and H₂O-CO₂ inclusions with T_h values of 229° - 305°C, and <3.0% salinity. Dip-slip faults contain inclusions with T_h values of 163° - 267°C and 0.0 to 5.3 wt. % NaCl equiv. salinity. NE-SW sinistral faults have NaCl-rich inclusions with T_h values between 224° - 293°C and 17.0 - 17.8 wt. % NaCl equiv. salinity. All three fault sets have pseudosecondary inclusions of NaCl-CaCl₂ rich fluids with Th values of 121° - 147°C and 5.5 -12.8 wt. % NaCl equiv. salinity.

NE-SW striking normal dip-slip faults that cut the central part of the Hartford Basin are commonly mineralized with dolomite, calcite, quartz, barite, sulfides and bitumen. Inclusions in early calcite contains NaCl-rich fluids with T_h values of 140° - 158°C and 3.2 - 10.5 wt. % NaCl equiv. salinity. Inclusions in later quartz contains NaCl-CaCl₂ fluids that fall into two groups: 1) T_h values of 63° - 149°C and 0.4 - 12.8 wt. % NaCl equiv. salinity, and 2) T_h values of 71° - 158°C and 16.0 - 23.7 wt. % NaCl equiv. salinity. Liquid hydrocarbon inclusions in quartz that have T_h values of 30° - 192°C.

Late quartz within the eastern border fault zone contains NaCl-CaCl₂ inclusions with T_h values of 134° - 160°C and up to 14.0 wt. % NaCl equiv. salinity.

The distinct difference in the types of fluids in the western border faults vs. fluids within the basin faults, indicate little fluid interconnectivity early during the deformation history of the Hartford Basin. However, later inclusions in quartz along the western border faults and primary inclusions from the eastern border fault indicate reactivation of all of the fault sets, and the mobility and influx of basinal fluids into the border faults.