103 Precambrian Geology (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

IGNEOUS ORIGIN AND SUBSEQUENT FLUID ALTERATION FOR THE LOW TI - FE OXIDE - APATITE ORES FROM THE EASTERN ADIRONDACK HIGHLANDS, NEW YORK

Paper number: 103-5
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Field, geochemical, and textural evidence is presented in support of an igneous origin and subsequent long-lived, fluid-rock interaction for magnetite ores of the eastern Adirondack Highlands. The ores consist of intrusive sheets or dikes of low-Ti magnetite, fluorapatite, augite, and hedenbergite. 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 knife-edge contacts, are closely associated in time with A-type leucogranites and granitic gneisses (ca. 1040 Ma). Whole-rock analyses of ore and associated rocks yield typically depleted incompatible-element patterns and lack the widespread enrichment of LREEs, characteristic of pervasive hydrothermal alteration. Undeformed pegmatites cross-cutting the ore yield U-Pb zircon ages of ca. 1039 and 1022 Ma and a rim at 949 ±10 Ma.

Apatite displays distinctive concentric zoning or dissolution under SEM and CL. Backscattered electron images highlight four major types of fluorapatite - monazite-Ce relations as result of the fluid-rock interaction: a) fractured fluorapatite with tiny secondary monazite-Ce and thorite developed along fractures; b) apatite grains mantled by monazite-Ce, monazite-Ce intergrown with magnetite or allanite-Ce; c) tiny patches of zoned apatite more or less with the same crystallographic orientation in larger apatite, and d) areas with relatively low BSE intensities within brighter apatite or along crystal rims. Monazite-Ce is low in xenotime component but large amounts of La and Nd substitute for Ce. Yttrium is largely retained in the apatite. We consider that monazite-Ce formed at T of 700¢ªC-800¢ªC (based on Zr-in-titanite and Ti-in-zircon temperatures); it yields a continuous range of EMP ages from 990 to 661 Ma reflecting the slow uplift of the Adirondacks after the end of the Rigolet phase of the Grenvillian Orogeny and synchronous fluid infiltration.

A low temperature stage of fluid infiltration, probably at the greenschist facies conditions produced tremolite, ferro-actinolite, chlorite, rutile, and hematite in the iron ore.