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

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.