The Mineral Lake Intrusive Complex (MLIC) is a relatively large (50 x 6 km) layered mafic intrusion (LMI) located in northern Wisconsin. Because LMIs are typically rich in platinum, chromium, vanadium, and titanium deposits, the MLIC has excited much interest for its potential to host valuable economic deposits. However, the entire southern boundary of the MLIC is defined by a significant thrust fault that placed the plutonic body on top of older Archaean crust. The fault separated the upper half of the intrusion from much of its lower half (not exposed), where the bulk of the economic deposits are expected to be found. There is an orphan, ~8 km^2 ultramafic intrusion (the Rearing Pond intrusion; RPI) adjacent to and of the same age as the MLIC. The RPI is composed of minerals expected at the base of a large LMI and may represent early crystallization within the MLIC. LMIs are characterized by distinct top-to-bottom stratigraphy in their Mg/Fe and Ca/Na ratios. We present whole-rock and mineral Mg/Fe and Ca/Na ratios using XRF and SEM analyses to test the hypothesis that the RPI represents the earliest crystallization in the MLIC, and to estimate the volume of missing material.
