THE GENESIS OF THE NEW GALENA MISSISSIPPI VALLEY-TYPE PB-ZN ORE DEPOSIT: CONSTRAINTS FROM SULFUR ISOTOPE STUDIES

The New Galena Pb-Zn mine is hosted in the organic matter-rich Lockatong Late Triassic formation in the Newark basin, located in northern New Jersey. The ore occurs along the NE-SW-trending faults that are roughly parallel to the regional basinal faults.

The δ³⁴S values range from 17.1 to 17.3‰ and from 15.2 to 16.0‰ respectively for sphalerite and galena. Although, the sulfur isotopic signature of sulfides is similar to that of the Triassic-Jurassic seawater (~ 13 - 20‰), the latter is not the source of sulfur for sulfides owing to the fact that the host rocks were lacustrine rather than marine rocks. Sulfur of sulfides did not derive from magma because the sulfur isotopic composition of sulfides does not fall into the range of that of the igneous rocks (-1.0 to +4.0‰). Dissolved sulfate was likely derived from the dissolution of sulfates carried by streams to the lake. Sulfur derived either through Bacterial Sulfate Reduction (BSR) in a closed system at low temperature and/or through Thermochemical Sulfate Reduction (TSR) at higher temperature in the presence of hydrocarbons. Considering the high temperature of the ore-forming fluid (>100 °C) and the presence of organic matter-rich Lockatong formation that acted as a reducing agent, the TSR is the main mechanism that generated large amount of sulfur for the ore.

An ascending deep-seated, hot metal-rich fluid migrated upward along major regional faults and mixed with a cooler, shallow, sulfur-rich fluid during the Late Triassic-Jurassic extensional tectonic activity. This fluid mixing led to the precipitation of sulfides (sphalerite and galena) in the organic matter-rich Lockatong Formation.