TIMING OF SILICATE AND CARBONATITE MAGMATISM FORMING THE BEAR LODGE ALKALINE COMPLEX: NEW INSIGHTS FROM 40AR/39AR GEOCHRONOLOGY

The Bear Lodge alkaline complex (BLAC) in northeastern Wyoming hosts a potentially economic rare- earth element (REE) resource in a variably oxidized carbonatite dike swarm and vein stockwork. The complex is part of the northern Black Hills alkalic belt, which formed as a result of Laramide-driven uplift and igneous activity from 62 to 46 Ma [1, 2]. Previously reported dates for the BLAC span 52 to 39 Ma, with ⁴⁰Ar/³⁹Ar and recalculated K-Ar dates for alkaline silicate rocks in the range 50.5 Ma to 39 Ma [2] and ⁴⁰Ar/³⁹Ar dates for carbonatite dikes in the range 52–51 Ma [3]. These ranges are inconsistent with field relationships that show the carbonatite dike swarm was one of the latest intrusive phases in the complex, followed only by late lamprophyre dikes. We present new high precision ⁴⁰Ar/³⁹Ar dates for seven samples of silicate igneous rocks from the BLAC, as well as five samples of the nearby Mineral Hill alkaline complex (MHAC), located to the east on the Wyoming-South Dakota border, that refine the chronology of alkaline intrusive activity at these complexes. Biotite and feldspar were irradiated with the Fish Canyon sanidine monitor (28.201 Ma, [4]) at the Denver TRIGA reactor and analyzed on a Thermo Scientific Argus VI noble gas mass spectrometer; ⁴⁰Ar/³⁹Ar dates were computed using the decay constants of Min et al. [5]. Our results, combined with previously published ⁴⁰Ar/³⁹Ar dates for carbonatitic rocks [3], suggest that most of the BLAC formed between 53 and 51 Ma. Dates for samples of the MHAC range from 56 to 54 Ma, consistent with known northwestward migration of igneous activity in the Black Hills alkalic belt [1]. Our results will aid future critical mineral resource assessments by refining geochronologic constraints for alkaline igneous complexes permissive for carbonatite-hosted REE deposits.

References: [1] Moore, M., et al., 2015. Ore Geology Reviews, v. 64, p. 499–521. [2] Duke, G.I., 2005. Ph.D. dissertation, South Dakota School of Mines and Technology, 291 p. [3] Andersen, A.K., et al., 2019. Lithos, v. 324-325, p. 640–660. [4] Kuiper, K.F., et al., 2008. Science, 320, p. 500–504. [5] Min, K., et al., 2000. Geochimica et Cosmochimica Acta, 64, p. 73–98. [6] Marvin, R.F., et al., 1980. Isochron/West, v. 29, p. 5–26.