RARE-EARTH ELEMENT AND ARSENIC GEOCHEMISTRY OF THE HYDROTHERMAL CALCITE HOSTED IN THE LACUSTRINE FORMATIONS OF THE NEWARK BASIN: IMPLICATION TO THE FLUID MIGRATION AND QUALITY OF GROUNDWATER

The Rare Earth Element and Yttrium (REY) composition of hydrothermal calcite, hosted in the small fractures in the Triassic lacustrine formations, is low (14.4 to 271.8 ppm) despite the fluid-rock interaction and longer residence time of the fluid as suggested by carbon-oxygen isotope data. The presence of calcite as the only major mineral phase in the veins suggests the predominance of carbonate species/complexing agents (CO₃^2-, HCO₃^-) in the hydrothermal fluid. Despite the presence of these effective complexing agents, the fluid had low REY concentration. The hydrothermal fluid had leached REY poor- lacustrine Triassic rocks, resulting in low REY concentration of calcite. The hydrothermal fluid, which precipitated calcite, was originally REY-depleted meteoric water as deduced from fluid inclusion and C-O stable isotope data.

Chondrite- normalized REY pattern distribution of calcite shows slight LREE enrichment relative to HREE (La/Yb)_N = 1.2 — 7.5). Europium anomalies (Eu/Eu*_N = Eu_N/(Sm²_N x Tb_N)⅓) are overall negative (<1) for the analyzed hydrothermal calcite samples. This negative Eu anomaly reveals that the fluid’s temperature is lower than 250°C, in agreement with fluid inclusion data (Th ~ 153 °C). The Y/Ho ratios (29 to 47) and the plot of data on the Tb/Ca vs Tb/La diagram are indicative of hydrothermal system, and is also consistent with fluid inclusion data. Cerium anomalies (Ce/Ce*_N = Ce_N/(Pr_N (Pr_N/Nd_N)) are overall slightly negative, indicating that the fluid was slightly oxygen-rich.

The very low total REY concentration in calcite and the overall slightly negative Ce anomalies suggest that the original fluid was oxidized meteoric water. The latter descended to deeper parts of the Newark basin where it became hot. Along its migration, these hydrothermal fluids remained poor in REY because they interacted with the REY-poor rocks of the Triassic lacustrine formations

Compared to the lacustrine formations, pyrite, and hematite, the arsenic concentration in calcite veins is extremely low. It follows that the calcite veins are not the source of the groundwater contamination.