IRON OXIDE-APATITE GENESIS IN THE NEW JERSEY HIGHLANDS: A GEOCHRONOLOGICAL AND PETROLOGICAL APPROACH

The Mesoproterozoic rocks of the New Jersey Highlands host hundreds of iron oxide-apatite (IOA) deposits, which were historically mined for iron ore. The emplacement and metamorphism of the host gneisses occurred during the Grenville orogeny, yet the tectonic processes leading to IOA genesis in the context of the ~400Myr, multi-phase orogenic cycle are unresolved. We employ both high-precision and high-spatial resolution geochronology, combined with detailed petrologic characterization in order to constrain the timing, duration, and mechanisms of IOA formation in the core of a collisional orogen.

Our field observations constrain ore emplacement to post-date the Ottawan, continental-collision phase of the Grenville. Small ore veins crosscut the Ottawan fabric of the host gneisses and are themselves foliated parallel to vein orientation. In addition, U-Pb ID-TIMS dates from ore apatites exhibit a spread over ~200Myr from ca. 1030 Ma (Ottawan) to ca. 820 Ma. These dates overlap with, and trail younger than, both high-precision U-Pb ID-TIMS dates of individual zircon grains and U-Pb LA-ICPMS spot analyses on zircons from the same ore samples – which broadly match metamorphic zircon ages from the host gneisses. Textural study of apatites by scanning electron microscopy indicates magmatic, F-rich cores rimmed by F-poor, Cl-rich apatite and healed fracture textures of Cl-rich apatite. Intra-apatite monazite lamellae were exsolved between phases of Cl-rich apatite growth. IOA mineralization and overprinting was thus a prolonged and likely episodic process.

Timing is broadly consistent across the four IOA deposits studied here and matches other Grenvillian IOA localities such as the Adirondack Highlands. However, mineralogical composition and Sr, Hf, and REE signatures of ore apatites and zircons exhibit significant variation between deposits within the same host lithology, and in most cases isotopic signatures are more radiogenic than would be expected if sourced from the host units. These results preclude a direct hydrothermal replacement origin from the surrounding rocks; multiple sources operating regionally at the tail end of a long-lived orogeny led to IOA ore emplacement in the New Jersey Highlands.