Using U-PB Geochronology to Constrain the Formation of a New Jersey Nelsonite

Abstract

The gneisses in the New Jersey Highlands host a collection of small ore bodies called nelsonites that are primarily comprised of apatite and magnetite. Other minerals present include significant amounts of pyroxenes, amphiboles, and quartz that signify hydrothermal activity. Samples of this unique ore were collected and dated using U-Pb geochronology on zircon grains. Ages that have ~20 Ma spread within each sample indicate a complex formation history. In conjunction with mineral species and grain textures seen in thin section, the geochronologic evidence indicates that these small bodies the highlands were formed by late-stage orogenic magmatic processes that incrementally caused the main ore composition (magnetite and apatite) to separate from the main chamber as fluid separates that then migrated along strike