Iron oxide-apatite mineralization in the New Jersey Highlands: Apatite as coupled geochronometer and petrogenetic indicator

Abstract

Orogenic systems drive the concentration of critical ore minerals in the Earth's crust, yet debate continues over the timing and underlying mechanisms of metallogenesis. Here, we address the process of iron-oxide apatite (IOA) mineralization within the New Jersey Highlands, an Appalachian inlier which exposes the exhumed metamorphic core of the ca. 1.4-1.0 Ga Grenville orogeny. We develop and utilize a novel application of U-Pb geochronology by isotope dilution thermal ionization mass spectrometry (U-Pb ID-TIMS) coupled with trace element analysis (TEA) by solution-based inductively coupled plasma mass spectrometry (ICP-MS) to apatite, which enables us to extract high-precision age and geochemical information from the same fraction of a single grain. We apply this method to apatites from four separate IOA deposits at the Canfield Phosphate, Dickerson, Mariot's, and High Ledge mines. Each location yields distinct information about the timing and nature of IOA ore formation. Taken together, apatite U-Pb dates imply a spread of at least 300 Myr in ore-forming processes, likely the result of multiple stages of apatite crystallization and ore mineralization. We suggest that the 200 Myr following the collisional Ottawan phase of the Grenville orogeny, rather than being a period of slow cooling as previously interpreted, experienced multiple metamorphic events resulting in crystallization or reheating of apatite and continued ore mineralization. Meanwhile, a range of initial \(^{87}\)Sr/\(^{86}\)Sr from 0.704850±0.000013 at the High Ledge mine to 0.726125±0.000022 at the Dickerson mine imply both primitive mantle and more evolved sources for ore formation. Apatite TEA tracks variability between ore deposits on a close spatial scale and within the same host lithology, highlighting the complexity of the tectonic processes underlying ore formation. Our data challenge the lower bound of the timescale for post-Ottawan metamorphism in the New Jersey Highlands and highlight the need for continued application of high-spatial resolution data sets and careful interpretation to understand the processes driving ore mineralization during and after orogenesis.