GEOCHEMISTRY OF APATITE FROM IOA AND IOCG DEPOSITS IN SE MISSOURI, USA: DISTINGUISHING FLUIDS SOURCED FROM MAFIC TO INTERMEDIATE INTRUSIONS, FELSIC INTRUSIONS, AND CALDERA LAKES

To identify the source of hydrothermal fluids, apatite in five iron oxide apatite (IOA) deposits and one iron oxide copper gold (IOCG) deposit were compared to apatite in regional mafic, intermediate, and felsic intrusions. Rare earth element + Yttrium (REY) and trace element abundances were determined by LA-ICP-MS and plotted on chondrite normalized REY and bulk crust normalized multi-element variation diagrams.

Igneous apatite mostly resembles primary apatite from IOA deposits with trace element enrichments in As, U, Th, Mn, W, Sn, Sr, Mo, and Pb. Additional enrichments in As along with depletions in Mn, Zr, Fe, and Ti are only present in primary apatite where Mn enrichments in igneous apatite are greatest in potassic altered felsic intrusions. Igneous apatite has flat LREE patterns, negative Eu and weakly negative Y anomalies, and gently declined HREE patterns; primary apatite has similar patterns with steeper LREE and flatter HREE patterns. Minor primary apatite from the one IOCG deposit is overprinted, obscuring many trace element and REY signatures but locally enriched in Co. Primary apatite is, therefore, likely indicative of a mafic to intermediate intrusive source, as also shown by the geochemistry of intergrown magnetite and amphibole.

At the Boss IOCG and Pea Ridge IOA deposits, igneous and primary apatite is overprinted by potassic-muscovite ± hematite alteration, which results in flat or weakly convex REY patterns, negative Eu and weakly negative Y anomalies, enrichments in Mn, and depletions in As, Th, V, and Si. Locally—along with precipitation of REE-phosphates and/or thorite—Sc, Sr, Mg, Fe, Ba, Pb, and V are either enriched or depleted. At Pea Ridge, secondary apatite in REE-breccia pipes also has similar REY patterns to potassic-altered apatite but with more defined negative Y anomalies and strong enrichments in Sc, Sr, Mn, Sn, and W that suggest fluids sourced from felsic intrusions.

In contrast, where primary apatite in IOA and IOCG deposits is overprinted by sulfide-chlorite ± hematite-carbonate alteration, it has REY patterns and trace element enrichments that resemble primary apatite in IOA deposits. However, local recrystallized domains with REE-fluorocarbonate are depleted in Na, Sr, Mg, Pb, Th, and U. Such apatite may have formed from dilute or low temperature fluids sourced from caldera lakes, as also indicated by solute and hematite chemistry.