31 The Grenville-Age Low Ti-Fe Oxide Deposits from New York State: Igneous or Hydrothermal?

Tuesday, 22 March 2016
8:00 AM - 12:00 PM
Meeting Room 3

LA-MC-ICP-MS U/PB ZIRCON GEOCHRONOLOGY OF THE GRENVILLE-AGE IRON DEPOSITS OF NEW YORK

Paper number: 31-1
Program: Theme Sessions
Time: 8:00 AM – 8:20 AM

Authors

Abstract

The Grenville-age iron deposits of New York are related to three intrusive events which occurred at the end of the Shawinigan, Ottawan, and Rigolet orogenic activity, respectively: (a) Fe – Ti oxide associated with rocks of the AMCG suite (anorthosite, “cumberlandite”, hornblendite) at ca. 1155 Ma; (b) low Ti - Fe oxide - fluorapatite associated with the A-type Lyon Mountain Granite (LMG), low – Ti - Fe oxide - vonsenite associated with skarns, low Ti – Fe oxide - sillimanite with no certain spatial affiliation, all at ca. 1040 Ma; (c) Low Ti – Fe oxide associated with amphibole peridotite at ca. 1000 Ma (Hudson Highlands). The present contribution will focus on the still controversial low Ti-Fe oxide-fluorapatite ores (LowTiFe).

The LowTiFe ores from eastern Adirondacks consist of intrusive sills or dikes of magnetite, REE-rich fluorapatite, augitic pyroxenes, and minor quartz. Other mineral phases include ilmenite with hematite exsolution, vanadiferous titanite rimming magnetite, zircon, monazite-(Ce), stillwellite-(Ce), lanthanite-(Ce), allanite-(Ce), and thorite. Observations under transmitted light show polygonal and cumulate textures. The ore bodies, each with very sharp contacts, are closely associated in time and space with A-type leucogranites and granitic gneisses (ca. 1040 Ma). Zircon separates from the Cheever mine magnetite ore contain 632 -1432 ppm U, display a U/Th ratio of 2.5-22.2 and yield a Concordia age of 1043±11 Ma. Zircons from the Rutgers mine ore display cores and rims. The cores contain 99-1197 ppm U, show U/Th ratio of 2.6±1.0, and an age of 1142 ±20 Ma whereas rims have 295-2267 ppm U, U/Th ratio of 5.6±3.7 and an age of 1043±11.9 Ma. The cores are probably remnants from the AMCG suite and the igneous rims are contemporaneous with the LMG. Undeformed pegmatites cross-cutting Mineville magnetite-fluorapatite ore bodies yield U-Pb zircon ages of ca. 1039±11 at Barton Hill, 1022±13 Ma at Old Bed (rim at 949 ±10 Ma), and 1023+8.5 Ma at Bonanza Shaft indicate that the ore cannot be younger than ca. 1040 Ma.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016NE-272344

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