Abstract
At the Cheever Mine, located in the eastern Adirondack Mountains of the Mesoproterozoic Grenville Province, iron oxide-apatite ore forms a narrow (<3 m) sheet cross-cutting metasomatically altered, magnetite-bearing, albite-rich leucogranitic host rocks of the Lyon Mountain Granite suite. Zircon from the ore and five samples of country rock were dated by Laser Ablation-Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry. The ore yielded a Concordia age of 1033.6 ± 2.9 Ma while three samples of host rock yielded ages of 1036.3 ± 2.9, 1040 ± 11, and 1043.9 ± 4.1 Ma. Two additional samples of host rock yielded older ages of 1059.6 ± 3.4 and 1066.0 ± 6.3 Ma and contain zircon xenocrystic cores with 207Pb/206Pb ages up to 1242 Ma. The zircons analyzed, including those separated from the ore, have characteristics typically associated with an igneous origin including size, shape, inclusions, oscillatory zoning, typical chondrite-normalized REE patterns, U contents, and U/Th ratios. This data establishes the age of the ore and alteration and a temporal, and likely genetic, connection between the ore and members of the Lyon Mountain Granite suite. A model invoking melting of Shawinigan country rocks, magmatic differentiation, and long-lived magmatic and metasomatic input along extensional fault conduits is proposed for the ore’s genesis. At the Cheever Mine, magmatic hydrothermal fluids and/or post-intrusion alteration appears not to have had a major impact on zircon, which preserves original U-Pb systematics.
Highlights
Iron oxide-apatite (IOA) deposits have served as an important source of iron in northern New York from the early 1800s to recent times [1]
All the structural measurements taken in this study indicate foliation and layering in the host rocks dip to the east towards the lake, while the ore dips to the west cross-cutting the host rocks
We suggest that small amounts of inherited zircon impact the U-Pb systematics observed, which results in weighted averages that give slightly older ages than those obtained from concordant analyses in the same population
Summary
Iron oxide-apatite (IOA) deposits have served as an important source of iron in northern New York from the early 1800s to recent times [1]. The IOA ores of the Kiruna-Malmberget and Bergslagen ore provinces in Sweden to which they are often compared are the major source of iron to the European. Martite, and/or hematite, apatite can be readily separated from many of these deposits and is found enriched in tailings from past mining operations. The use of apatite-rich tailings for fertilizer was briefly attempted [3]. Renewed interest exists because of rare earth elements (REEs) that can occur at relatively high Rare-earth deposits are known to be associated with other IOA deposits in the Geosciences 2018, 8, 345; doi:10.3390/geosciences8090345 www.mdpi.com/journal/geosciences
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