T169 Magnetite Apatite (MtAp) Deposits in Space and Time

Tuesday, 12 October 2021
1:30 PM - 5:30 PM
A107–A109

CHEMISTRY OF MINERALS IN IOA DEPOSITS IN SE MISSOURI, USA: RELATION TO MAFIC AND INTERMEDIATE INTRUSIONS

Paper number: T169-3
Program: Technical Sessions
Time: 1:55 PM – 2:15 PM

Authors

Abstract

We compared the paragenesis, textures, and chemical compositions of magnetite, apatite, actinolite, and phlogopite in 5 IOA deposits, southeast Missouri, USA, to their counterparts in nearby or regional mafic to intermediate intrusions to evaluate the role of magmatic fluids in ore genesis.

On an Al+Mn vs. Ti+V plot, IOA magnetite forms multiple arrays that extend from intermediate intrusions through higher-Ti vein magnetite that occurs with ilmenite to lower-Ti replacement magnetite that is finer grained. These arrays are interpreted to be cooling trends. IOA magnetite contains anomalous V, Mn, Zn, Co, Ga, Ni, Ti, Mg, P, Al, with lesser Cr, Th, U, but at lower concentrations than what is observed in magnetite in the igneous rocks.

IOA apatite has steep LREE enriched patterns, Eu and weak to no Y anomalies, flat HREE patterns, and As, U, Th, W, Sn, Sr, Mo, and Pb concentrations that resemble igneous apatite in mafic or intermediate intrusions. IOA apatite is relatively enriched in As and depleted in Mn, Zr, Fe, and Ti. Apatite in mafic and intermediate intrusions has contrasting REE systematics: flat LREE patterns, Eu and Y anomalies, and gently declined HREE patterns.

IOA actinolite has gently inclined LREE enriched patterns, weak or no Eu and Y anomalies, gently inclined HREE patterns, with enrichments similar to Mg-hornblende in igneous rocks but absent in Cr, and higher F, Na, and K. On the amphibole classification diagram, actinolite extends toward Mg-hornblende in mafic intrusions. Mg-hornblende in mafic and intermediate intrusions has gently declined LREE patterns, weak or no Eu and Y anomalies, flat HREE patterns, and enrichments in Na, Al, Mn, Cr, V, Ti, and Sc.

IOA deposits contain phlogopite whereas intermediate intrusions contain biotite. On the biotite classification diagram, IOA phlogopite forms two arrays that trend toward biotite in intermediate intrusions and an unknown Mg-rich composition that may resemble biotite in mafic intrusions.

We infer that vein actinolite and phlogopite ± apatite in the IOA deposits were precipitated from magmatic hydrothermal fluids sourced from mafic intrusions, where high-Ti veins were conduits for fluids that produced lower-Ti replacement ores. Replacement magnetite ± apatite and phlogopite may reflect reaction of magmatic fluids with intermediate host rocks.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-368029

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.