RARE EARTH ELEMENTAL ANALYSIS OF PHOSPHATIC FACIES IN THE PERMIAN PHOSPHORIA FORMATION, SOUTHWESTERN MONTANA

We report results from sedimentary and geochemical analysis of the Meade Peak and Retort phosphatic members of the Phosphoria Formation (late Permian) in southwestern Montana. At the Dalys Spur locality (45° 5'13.51"N-112°46'53.54"W), the Meade Peak Member is 1.9 m thick. Cumulative phosphorite thickness is 1.2 m and includes wackestone, packstone, and floatstone facies. Phosphatic allochems are dominated by peloids, grapestone grains, and pebble-sized intraclasts. Superficial ooids are present but uncommon. Total rare earth element (REE) grade of Meade Peak phosphorites, determined by ICP-MS, average 0.13 wt. % with 38% of the grade as heavy REEs. The Retort Member at Dalys Spur is ~12m thick and is dominated by organic-rich silty phosphatic mudstone. Cumulative phosphorite thickness is ~4m and includes wackestone, packstone, and floatstone facies. Phosphatic allochems are dominated by peloids and reworked grapestone grains, but also include less common phosphatic ooids. REE grade of Retort phosphorites at Dalys Spur average 0.13 wt. % with 49% of the grade as heavy REE. At the Newman Brothers Mine locality (46°33'56.91"N-112°26'0.14"W), which is beyond the depositional pinch-out of the Meade Peak, the Retort Member is ~1.5m thick and contains a mix of cherty phosphatic mudstone, phosphatic dolostone, and phosphorite. Phosphatic allochems are dominated by ooids, with grapestone grains and intraclasts also present. REE grade there averages 0.15 wt. % with 48% of that grade as HREEs. Collectively, these preliminary findings indicate a loosely positive correlation between phosphorus content and REE grade.