A MELT INTERACTION MODEL FOR THE FORMATION OF THE KRATZ SPRING IRON OXIDE RARE EARTH ELEMENT DEPOSIT, MISSOURI, USA

The Kratz Spring Deposit is a rhyolite to trachyte hosted massive magnetite, hematite, and phosphate deposit that is part of the larger Proterozoic Iron Metallogenic Province in SE Missouri, USA. Interest in this province has recently increased due to potential for strategic element mineralization, primarily rare earth elements (REE) and cobalt. Although it is widely assumed that Iron Oxide Apatite (IOA) deposits, like Kratz Spring, have a primarily magmatic origin, the exact mechanisms through which these deposits form remain elusive. Here we present the results of an ongoing study that investigates how the Kratz Spring deposit was formed.

Drill core and petrographic studies show that the mineralization in Kratz Spring is dominated by massive to disseminated breccia-hosted magnetite in 4 strata bound zones, which are cut by breccia veins containing carbonate, specular hematite, fluorite, and REE-phosphates. Bulk rock data show that the iron oxide zones and REE-rich breccias have notably different chondrite normalized REE patterns; most notably oxide ores do not display a negative Eu anomaly, while REE breccias are distinctly Eu depleted. Iron isotope data obtained from oxide mineral concentrates reveal two distinct populations. The higher Fe group, ranging from 0.14 to 0.2 was only yielded from magnetite. The lower Fe population ranges from -0.02 to 0.07 and was generally acquired from specularite. We interpret the data to reflect that magnetite if of magmatic origin, while specularite formed in high T hydrothermal conditions.

Our integrated drill core, petrographic and geochemical data suggest that Kratz Spring’s mineralization formed from the dehydration of a mafic melt and/or its entrained hydrous silicates upon injection into a felsic magma chamber. Magnetite crystallized and attached to exsolving fluid/volatile bubbles which facilitated an upwards migration of magnetite, owing to an increased buoyancy. Several injection events formed the 4 distinct iron oxide ore layers. The volatile-rich veins, that crosscut these magnetite layers and can contain economic abundances of REE, are interpreted to represent fluid/volatile exsolution during later melt injection events. In this way, a single long-lived cyclic process can account for both the magmatic iron and hydrothermal REE ores at Kratz Spring.