Magnetite-apatite deposits of the New Jersey Highlands, USA: Geochronological evidence for orogen-scale Fe-P-(Ti) mineralization during the Grenvillian Orogeny

Abstract Magnetite-apatite (MtAp) ore deposits are broadly distributed within Mesoproterozoic rocks of the New Jersey Highlands (USA); however, the age and origins of the ores, and relationships to other Fe-P-(Ti) ore deposits in the Grenvillian Orogen are presently unknown. We use zircon U-Pb geochronology and isotope geochemistry to (1) constrain the timing of MtAp mineralization in the New Jersey Highlands, (2) evaluate the relationship between the MtAp ores and associated lithologies, (3) develop a magmatic model for ore genesis, and (4) place the New Jersey ores within the context of Grenville tectonics and regional mineralization. Ore-body zircon crystals exhibit complex, magmatic textures that record episodic or prolonged crystallization over an extended duration from ca. 1074 Ma to 905 Ma, suggesting that MtAp mineralization occurred during Ottawan to post-Ottawan orogenesis. Cumulate textures of MtAp ores and a close spatial and temporal association among ores, pegmatitic granitoids (ca. 1058 Ma to ca. 986 Ma regionally), and clinopyroxene syenites (most dates ca. 1097 Ma to ca. 926 Ma at one mine) suggest that MtAp mineralization and intrusive magmatism reflect co-magmatic processes. Zircon O isotope data indicate that the MtAp deposits and associated lithologies formed from crustal melts (δ18O = 7.66‰–9.24‰ for the orthogneiss-hosted deposits and δ18O = 5.40‰–6.29‰ for the amphibolite-hosted deposit). Zircon Hf isotope data are more complex. Some MtAp deposits and associated lithologies record Hf signatures consistent with a crustal melt source (εHfi = 0.6–11.1). Whereas other MtAp deposits have extremely radiogenic signatures (εHfi = 29–542) due to the high modal abundance of apatite and monazite in the ores and, consequently, high Lu/Hf ratios. MtAp mineralization in the New Jersey Highlands is broadly coeval with MtAp mineralization in the Adirondack Highlands of New York (USA), rare earth element (REE) mineralization in the Hudson Highlands of New York, and nelsonite mineralization at Lac à l’Orignal and Lac Mirepoix in Québec (Canada). Collectively, these results indicate that orogen-scale, Fe-P-(Ti) and REE mineralization processes operated during Ottawan to post-Ottawan orogenesis throughout the Grenville orogen.