MAGNETITE-APATITE MINERALIZATION IN THE NEW JERSEY HIGHLANDS: APATITE AS COUPLED GEOCHRONOMETER AND GEOCHEMICAL INDICATOR

Hundreds of magnetite-apatite ore deposits are hosted throughout the Mesoproterozoic rocks of the New Jersey Highlands. Iron oxide-apatite deposits like these are a global phenomenon, yet the origins of ore mineralization in the context of large, long-lived collisional orogens remains controversial. The exhumed metamorphic core of the Grenville orogen in the New Jersey Highlands provides an opportunity to study these processes.

We have sampled and carried out petrographic characterization of four magnetite-apatite ore deposits. The Dickerson, Canfield Phosphate, and High Ledge mines are located within the magmatic arc rocks of the Losee Suite (ca. 1300-1350 Ma). The Mariot’s ore is hosted in supracrustal rocks that were deposited in a back-arc basin prior to the Ottawan continental collision (ca. 1050 Ma). We present new combined U-Pb TIMS dates, ⁸⁷Sr/⁸⁶Sr TIMS analysis, and ICP-MS trace element analysis (TEA) all from the same chemically separated aliquots of apatite, providing a novel method for utilizing apatite as a coupled geochronometer and sensitive indicator of magmatic and hydrothermal conditions.

Concordant apatite ²⁰⁶Pb/²³⁸U data from Mariot’s and High Ledge reveal a spread in ages, from ~930 to 820 Ma, which nominally post-date the Grenvillian orogeny, consistent with field observations suggesting magnetite-apatite dikes cross-cut regional metamorphic fabrics. Dickerson and Canfield Phosphate U-Pb data are more complex. Samples yield normally and reversely discordant ages, respectively, which exhibit strong linear correlation in concordia space, indicative of post-crystallization metamorphism. ⁸⁷Sr/⁸⁶Sr ratios and geochemistry from apatite are distinct between different ore deposits, with initial ⁸⁷Sr/⁸⁶Sr ranging from .7048-.7050 at High Ledge to .7214-.7261 at Dickerson. These data identify unique magmatic or hydrothermal fluid origins for each deposit, with contributions from both crust and mantle sources. When paired with detailed petrographic analysis of the ore and surrounding host lithologies, in addition to ongoing TIMS and in-situ zircon geochronology, we hope to gain more insight into the duration and mechanisms of magnetite-apatite ore genesis in the context of the Grenville orogen.