PALEO-FLUID EVOLUTION AND MINERAL PARAGENESIS ALONG MESOZOIC FAULTS: HARTFORD BASIN

Mesozoic dip-slip normal faults and oblique-slip faults are common within the Hartford Mesozoic rift basin and break up the rocks within the basin into numerous tilted fault blocks. In addition, the basin is bounded be normal dip-slip faults. These faults were fluid conduits, allowing the passage of brines, hydrocarbons, and mineralizing fluids.

Intrabasinal faults strike 345° - 030°, and dip 67°- 90° with slip lineations raking 12° - 90° from the south. The faults are typically filled with blocky to euhedral quartz, dolomite/siderite calcite, and/or barite. Also present may be blocky to euhedral chalcopyrite, galena, and bitumen found in interstices in healed mineral fractures, joint surfaces and faults breccia cement. For the intrabasinal faults, aqueous inclusions in calcite and quartz occur in two groups based on homogenization temperature: T_h= 100 to 155 °C (105 to 188 °C, pressure corrected), with 3.5 to 14.0 wt. % NaCl equiv. salinity and T_h = 162 to 188 °C (170 to 240 °C, pressure corrected), with 0.0 to 9.0 wt. % NaCl equiv. salinity. Two-phase liquid hydrocarbon inclusions in quartz fluoresce blue to blue-green, indicating 30-35 API gravity oils. Single-phase brown bitumen inclusions are also common.

The western border fault at Loudville, MA has a complex paragenetic sequence of barite ± quartz ± pyrite ± sphalerite ± galena ± chalcopyrite ± yellow quartz. Homogenization temperatures of aqueous inclusions in quartz decrease over time, with the earliest having T_h = 219 to 259 °C (pressure corrected values of 255 to 305 °C) and salinity 3.4 wt. % NaCl equiv. and the latest with T_h = 117 to 137 °C (120 to 140 °C, pressure corrected) and salinity of 17.0 wt. %. The eastern border fault at Middletown, CT has a paragenetic sequence of quartz ± pyrite ± sphalerite ± galena. Homogenization temperatures of aqueous inclusions in quartz have T_h = 165 °C (190 °C pressure corrected).

Pb isotope data from galena samples from intrabasinal faults have ²⁰⁶Pb/²⁰⁴Pb values of 18.5380 to 18.5805 and ²⁰⁷Pb/²⁰⁴Pb values of 15.6261 to 15.6603 and model ages of 85 - 150 mya (Late Jurassic - Early Cretaceous). Galena from the border faults has ²⁰⁶Pb/²⁰⁴Pb values of 18.6004 to 18.6149 and ²⁰⁷Pb/²⁰⁴Pb values of 15.6816 to 15.6882 and model ages of 120 - 140 mya (Early Cretaceous).