Chapter 8 Petrogenesis and mineralogic residence of selected elements in the meade peak phosphatic shale member of the permian phosphoria formation, Southeast Idaho

Abstract

The Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation hosts the ore mined by the phosphate industry of southeast Idaho. It also hosts environmentally sensitive elements (ESE) such as Se, As, Hg, Ni, Cd, Zn, and Cr. Primary chemistry, elemental distribution patterns, and mineralogy within the Meade Peak were modified by element migration and possibly the introduction of elements. Fluids moved within the Meade Peak throughout its history, although the passage of fluids was highly variable in space and time, resulting in small domains of different rock chemistry and different mineralogy. Timing of major events affecting the Meade Peak and mineral habit are used to differentiate among detrital, diagenetic, epigenetic, and supergene mineral assemblages. Cross-cutting relationships among minerals are too rare to provide much paragenetic infor- mation. Carbonate fluorapatite (CFA) occurs in several forms, but dominantly as pelloids, some of which may have formed in situ during diagenesis. The other volumetrically signifi- cant form of CFA is interstitial cement that formed during diagenesis. Beginning during diagenesis and continuing intermittently, multiple generations of carbonate (dolomite and calcite) formed overgrowths and texturally complex carbonate cements. Movement and precipitation of silica followed a similar pattern. The ammonium feldspar buddingtonite, which generally rims orthoclase, also formed during diagenesis. Bacteria apparently played a significant role during diagenesis as well as during supergene processes, resulting in extreme fractionation of S isotopes and the possible bacterially mediated formation of minerals such as glauconite and sphalerite. Catagenesis, apparently culminating in oil generation, was the last significant diagenetic change. Thrusting accompanied by fluid (oil and brine) migration began during catagenesis in the Late Jurassic or Cretaceous and continued into the early Eocene. Fluorite ± carbonate ± barite± bitumen veins formed as a result of brittle deformation and accompanying fluid movement. This fracturing event may have been associated with a period of extension and normal faulting (Neogene to Holocene). Passage of the Yellowstone hot spot to the north of the area during the Neogene is marked by silicic domes and basaltic flows. The enrichment of Hg in fracture coatings might be the result of deposition from warm fluids associated with the emplacement of the silicic domes or a generally elevated, regional thermal gradient associated with the volcanism. Many of the fracture systems are still open and continue to provide fluid pathways that are the primary depositional sites for a wide variety of supergene minerals (such as Se, efflorescent salts) and element associations (such as Hg, Cd-S, Fe-Cr-O) in which many of the ESE are concentrated. Native Se is the most commonly identified host of Se in the studied samples. The largest concentration of Se occurs in open-fracture systems that cross-cut waste rock and ore units. The age(s) of native Se formation is not known; how- ever, the latest period of Se mobility is the present. Direct measurement of efflorescent “salts” forming on new mine faces indicate that several ESE, including both Se and Zn, are concentrated on the faces soon after they are exposed. Zinc is present as hydrous sulfates, but the residence of Se in these “salts” is unknown.