Eocene Potassic Magmatism of the Milk River Area, Southern Alberta (NTS 72E) and Sweet Grass Hills, Northern Montana: Overview and New Data on Mineralogy, Geochemistry, Petrology and Economic Potential

This report presents an overview of previous data and new whole-rock geochemical, quantitative mineralogical and electron-microprobe analyses for the Eocene potassic intrusions, related hydrothermal veins and Cretaceous sedimentary rocks adjacent to the magmatic bodies of the Milk River area of southern Alberta and the Sweet Grass Hills of northern Montana.

The rock samples were collected during 2008 and 2009 fieldwork from five sites in the Milk River area and two sites in the Sweet Grass Hills. The outcrops examined in the Sweet Grass Hills are known metallic mineral occurrences associated with the Eocene intrusions. We discuss the geology, mineralogy, geochemistry, petrology and economic potential of these outcrops.

Based on new petrological, mineralogical and geochemical evidence, we suggest that the ultrapotassic peralkaline rock-forming dikes at the 49th Parallel locality in the Milk River area can be classified as sanidine-phlogopite lamproite. Lamproites have not been previously recognized in Alberta, although these rocks are known to be associated with lamprophyres and kimberlites in Montana, Wyoming, Colorado, Utah, Kansas and Arizona. Other potassic intrusions and volcanic rocks exposed in the Milk River area include alkali olivine minettes, diopside-phlogopite trachyte and amphibole latite. We propose a petrogenetic model in which latite, trachyte and high-K rhyolite of the Sweet Grass Hills igneous complex originate by fractional crystallization of parent olivine minette magma through settling of primary minerals observed as phenocrysts. Major-element mass-balance and trace-element models, coupled with published Sr, Pb and Nd isotopic data, rule out any significant assimilation of crustal material by the ultrapotassic magma at Sweet Grass Hills. In the Milk River area, olivine minettes and lamproite that is more enriched in incompatible elements represent primary magmas generated by partial melting of a heterogeneous mantle, enriched in potassium, rare-earth and other incompatible elements, in equilibrium with olivine, phlogopite and clinopyroxene.

The new results aid our understanding of the nature and origin of the potassic magmatism and its economic potential in the Milk River area of southern Alberta. Recognition of the first lamproite occurrence in Alberta is important because these magmas originate from specific mantle sources and can carry diamonds.