PARAGENESIS OF MESOZOIC FAULT ZONE MINERALS AND LEAD ISOTOPE ANALYSIS OF ASSOCIATED GALENA IN CENTRAL AND WESTERN CONNECTICUT

Mesozoic dip- and oblique-slip normal faults bound the Hartford Basin and result in the formation of numerous tilted fault blocks. Similar faults occur throughout the metamorphic rocks of western Connecticut. These faults were fluid conduits, allowing the passage of mineralizing brines and hydrocarbons. For this study, we examined the mineral paragenesis along three faults; the Eastern Border Fault at Middletown, CT, the Western Border Fault near Loudville, MA, and an intrabasinal fault in New Britain, CT. At Middletown, repeated layered sequences of quartz ± pyrite ± sphalerite ± galena indicate fault reactivation and multiple fluid events. At Loudville, mineralization occurred within a breccia with the sequence barite ± quartz ± pyrite ± sphalerite ± galena ± chalcopyrite ± yellow quartz ± iron/manganese oxide ± goethite/ limonite ± cerrusite ± pyromorphite ± malachite ± chalcanthite(?). At New Britain, the sequence is barite ± quartz ± pyrite ± galena ± chalcopyrite ± iron oxide ± dolomite ± calcite, with massive bitumen that pre-date the sulfide minerals. Two-phase fluid inclusions in the quartz and calcite have homogenization temperatures of 70 to 200 °C with salinities of 1.0 to 11.7 wt. % NaCl equiv. These wide variations may be the result of episodic fluid influx from multiple reservoirs and seismic pumping.

To better understand the origin of the mineralizing fluids, we analyzed Pb isotopes from galena samples from the above three faults, as well as six additional sites from central and western Connecticut (Bristol, Meriden, Naugatuck, Long Hill, East Granby, and Thomaston). The data cluster in three groups, the Naugatuck and Long Hill sites have 206Pb/204Pb values of 18.4245 to 18.4502 and 207Pb/204Pb values of 15.6451 to 15.6473. Sites Bristol, Meriden, and East Granby have 206Pb/204Pb values of 18.5380 to 18.5805 and 207Pb/204Pb values of 15.6261 to 15.6603. The remaining sites (Loudville, Middletown, New Britain, and Thomaston) have 206Pb/204Pb values of 18.6004 to 18.6325 and 207Pb/204Pb values of 15.6578 to 15.6882. Pb-Pb dates for each of these three groups are Jurassic, Late Triassic, and Early Cretaceous respectively. This suggests that these faults were active throughout the Mesozoic, and except for New Britain, the galena was undeformed, indicating that the dates were of the last fault reactivation.