MCR-GeoChem: Aharmonized, quality-flagged open compilation of whole-rock and mineral geochemistry for the 1.1 Ga Midcontinent Rift in Michigan, Minnesota, and Wisconsin

Bezali Kwame Danso , Saadatu Abdullah

Nickel, copper, cobalt, and the platinum-group metals are all on the final 2025 U.S. List of Critical Minerals, and the 1.1-billion-year-old Midcontinent Rift around Lake Superior hosts some of the largest domestic resources of them. The rock chemistry needed to study these systems is public but fragmented: it is spread across the U.S. Geological Survey (USGS) National Geochemical Database, project-level USGS data releases, the GEOROC compilation, and individual EarthChem datasets, each
with its own units, method codes, and conventions for values below or above analytical limits. We present MCR-GeoChem v1.0.0, an open, versioned, reproducible compilation of 3,524 Midcontinent Rift samples and 168,748 geochemical determinations (88 analytes) from Michigan, Minnesota, and
Wisconsin. Every value is converted to a common unit scheme (oxides in wt%, trace elements in ppm, Auand platinum-group elements in ppb) while retaining its original form; censored values are decoded into an explicit qualifier and limit; 89 source method codes are mapped to a common vocabulary of technique, digestion, and completeness; and each sample carries a rift-membership label tied to the USGS State Geologic Map Compilation. An open checker, mcrqa, adds value- and sample-level flags for below-detection results, uninformative detection limits, semi-quantitative and partial-digestion methods, unit errors, oxide-total problems, likely post-magmatic alteration, coordinate quality, and cross-source duplicates. The compilation shows that 27% of determinations derive from legacy semi
quantitative emission spectrography, that 501 of 1,304 platinum results are below-detection values whose limits are too high to constrain the rock, and that 488 of 2,821 igneous whole-rock samples carry at least one alteration flag. The full pipeline rebuilds the release from public sources with identical
results. The dataset, the method and detection-limit crosswalk, and the checker are released under open licenses to support critical-mineral research in the Lake Superior region.