MAGNETITE-APATITE MINERALIZATION IN THE CORE OF THE GRENVILLE OROGEN, NEW JERSEY HIGHLANDS, USA: A PETROCHRONOLOGIC APPROACH

The origin of hundreds of magnetite(-apatite) (Mt(-Ap)) deposits in the New Jersey Highlands and contiguous Hudson Highlands, which together constitute a Mesoproterozoic inlier in the Appalachians, has been the subject of speculation for centuries because the deposits were historically mined for iron ore. We present a suite of U-Th-Pb geochronologic data sets and supporting petrographic and geochemical characterization from five deposits in the eastern and western New Jersey Highlands, which locate the timing and tempo of mineralization within the broader tectonic framework for the Grenville orogen.

Zircons in ore rocks and a spatially associated pegmatite, analyzed by U-Pb LA-ICPMS, lack the ca. 1.3-1.1 Ga cores found in surrounding gneisses. Most ore samples yield concordia ages between ca. 1030 Ma and 970 Ma—shortly after peak granulite-facies metamorphism associated with the Ottawan orogenic phase that previous workers have dated at ca. 1060-1030 Ma. High-precision subgrain U-Pb TIMS dates from the same zircons reveal mixing between two or more pulses of crystallization. The oldest apatite U-Pb TIMS and in-situ U-Th-Pb monazite dates from these samples also record the onset of Mt(-Ap) mineralization at ca. 1030 Ma. The bulk of monazite crystallization was between ca. 980 Ma and 940 Ma in the samples analyzed, while some apatites yield concordant dates as young as ca. 820 Ma. We propose the following history in the New Jersey Highlands: (1) an immiscible melt origin, with Mt(-Ap) deposits being emplaced at depth shortly after Ottawan continental collision and possibly at the onset of orogenic collapse—consistent with bulk rock geochemistry from the ores; (2) protracted fluid-driven, in-situ recrystallization as evidenced by disturbed U-Pb systematics in phosphates, elevated Hf in zircon, variable apatite and zircon trace element signatures, and multiple sets of exsolution lamellae in magnetite; and (3) slow average exhumation and cooling through the closure temperature range of apatite by ~820 Ma, consistent with existing biotite and hornblende thermochronology data. Temporal correlation with similar REE-bearing Mt(-Ap) deposits in the Adirondack Highlands suggests a shared tectonic history such that the processes driving mineralization operated on an orogen-wide scale.