GENESIS OF IRON-OXIDE-APATITE DEPOSITS IN A COLLISIONAL OROGEN: A PETROLOGIC AND GEOCHRONOLOGIC STUDY OF MAGNETITE ORES IN THE NEW JERSEY HIGHLANDS

The petrologic origins of iron-oxide apatite (IOA) deposits are hotly debated. We present petrologic data on five IOA deposits from the exhumed metamorphic core of the Grenville Orogen in the New Jersey (NJ) Highlands, in order to constrain the petrologic processes controlling mid-crustal IOA deposition in a collisional orogen. These deposits are largely undeformed and cut across the fabric of host orthogneisses, indicating their formation may have occurred after the main phases of the Grenville Orogeny, consistent with c. 990-820 Ma U-Pb zircon and apatite dates from the ores. The ores are also commonly spatially associated with pegmatitic undeformed alkaline igneous rocks, suggesting a genetic association with post-collisional magmatic processes. Pigeonite and ternary feldspar thermometry on some associated pyroxene syenite suggest a minimum crystallization temperature of 650° C.

The IOA deposits evaluated are broadly categorized into three mineralogically distinct groups: I) phosphate-rich/sulfide-poor, II) phosphate-poor/sulfide-poor, and III) phosphate-poor/sulfide-rich types. All studied ore samples display cumulate textures; in each case, apatite appears to have crystallized first, followed by magnetite. Pyrite, ilmenite, and monazite are largely interstitial.

Magnetite displays chemical variation amongst the three ore types. Type I and II ores contain Al-Ti-rich magnetite, and type III ores have magnetite with less Al and variable Ti content. Across ore samples, minerals exhibit multiple episodes of growth. In many samples, apatite cores are Cl-poor, riddled with tiny monazite and xenotime inclusions which are overgrown by Cl-rich, inclusion-free apatite. Interstitial monazite in Type I and Type II ores are zoned in La and Nd, indicating at least two distinct growth episodes. Trace element compositions of magnetite and apatite, and halogen composition of apatite will allow us to distinguish magmatic vs. hydrothermal mineral growth phases in the IOA deposits of the NJ Highlands. In-situ U-Th-Pb dating of monazite will provide direct timing constraints on different stages of IOA mineralization. Collectively, these observations and data will provide insight into the petrogenetic origins of IOA deposits in collisional orogenic settings.