PRELIMINARY GEOCHEMICAL ANALYSIS OF THE A-1 EVAPORITE, SALINA GROUP, MICHIGAN BASIN
We sampled three cores at the Michigan Geological Repository for Research and Education. Dalrymple #1-16 from Roscommon Co. spans the base of the A-1 carbonate, the entirety of the A-1 evaporite, and the upper A-0 carbonate. Kalium Hersey KH1054 from Osceola Co. spans the base of the A-2 evaporite, the entirety of the A-1 carbonate, and the upper A-1 evaporite. Borgen #1-30 from Kalkaska Co. spans the base of the A-1 carbonate and the upper A-1 evaporite.
The Dalrymple core contains thinly bedded A-0 carbonate below ~9040’, then halite interbedded with carbonate mud ‘draped’ over euhedral crystals. The base of the A-1 evaporite (~9017’) is made up of massive clear to black halite, largely recrystallized and interspersed with <1 cm thin bands of Mg sulfate. Orange to colorless sylvite intervals starting ~8870’ contain occasional borate nodules <1 cm size. Interbedded halite is colorless to grey, with cloudy, inclusion-rich primary and clear, inclusion-free secondary crystals. The A-1 carbonate starting ~8666’ is finely laminated to massive, dark grey to black. The Kalium Hersey and Borgen cores have similar mineralogy, although the Kalium Hersey core contains elemental sulfur at the A-1 evaporite-carbonate contact.
Preliminary geochemical analysis of the Dalrymple core was conducted using ion chromatography for major anions and ICP-OES for major and trace cations. Clear halite from the base of the A-1 evaporite had 7 ppb Li. Clear sylvite from the first A-1 sylvite bed (8870’) had 19 ppb Li. A layer of Mg sulfate had 312 ppb Li and 1150 ppb B. Future work will explore whether trace element concentrations in clear and inclusion-rich salt increase with increasing paleo-salinity from the base to the top of the A-1 evaporite. In addition to wet chemical analysis, XRD and SEM will be used to identify the Mg sulfate and borate minerals.