MONAZITE PETROCHRONOLOGY OF MAGNETITE-APATITE ORE FORMING PROCESSES IN THE NEW JERSEY HIGHLANDS

Magnetite-Apatite (Mt-Ap) deposits are an important source of Fe and REE, which are mined for their use in a wide variety of electronic applications. Dating the timing of crystallization of ore-forming minerals in Mt-Ap deposits, in combination with textural and geochemical information, can help constrain the tectonic regime in which they formed and the processes that brought about their formation. We present texturally-constrained monazite U-Th-Pb petrochronologic data from a Mt-Ap ore from the New Jersey Highlands (NJH), which is one of several Mesoproterozoic “Grenville” inliers in eastern North America, and hosts hundreds of Mt-Ap ore localities have been documented in the NJH.

Here, we focus on one Mt-Ap ore locality from the Canfield Phosphate mine in Dover, New Jersey. In this deposit, monazite (mnz) is a major rock-forming mineral (~2 vol. %). Mnz occurs in two textural locations: 1) interstitial between cumulate fluorapatite grains, and 2) tiny (1-10 µm) inclusions that comprise linear inclusion trails in healed fractures in apatite. Interstitial monazite exhibits Th-rich and Y-poor cores, surrounded by irregularly-shaped Th-poor, Y-rich rims. Mnz included in apatite are also Th-poor. These features indicate: 1) multiple episodes of monazite growth, and 2) late-stage mnz growth during metasomatism of the primary ore mineral assemblage, which redistributed REE from apatite to monazite.

From our electron probe U-Th-total Pb analyses, the majority of monazite cores yield dates of 980 to 940 Ma, suggesting post-Ottawan Mt-Ap crystallization in a post-orogenic setting. Low Th rims - interpreted to form during hydrothermal monazite recrystallization - yield an anomalously wide range of dates, spanning 863-1498 Ma, with many dates older than the cores they surround. Low-Th monazite inclusions in healed apatite fractures also show this wide spread of anomalous dates. This unexpected behavior may arise from inheritance of immobile Pb from apatite where mnz overgrows apatite or where it is included in apatite. For reprecipitated rims, Pb can be inherited from the parent mnz or from apatite. The dataset presented here suggests that Pb mobility may be heterogeneous during fluid-driven metasomatism and recrystallization, and illustrates a challenge for using monazite to accurately date metasomatic events.