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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.

The large Poison Creek anticline, now broken by normal faults, is the major structure in the western part of the Lemhi Range. The fold resulted from late Mesozoic compressional deformation in the hinterland of the Cordilleran thrust belt in central Idaho. Bedrock of the study area is mainly Mesoproterozoic strata that are part of a thrust sheet floored by the Poison Creek fault, a major thrust fault whose trace extends northwest across the northern part of the study area. The thrust fault displaced the Proterozoic rocks over lower Paleozoic formations composed of carbonate and quartzite. In the southern part of the study area, the easterly trending Lem Peak normal fault has vertical displacement of several thousand meters and may cut the Poison Creek thrust fault at depth. Between the trace of the Lem Peak normal fault and the Poison Creek thrust fault lie the Hayden Creek and Bear Valley faults, which in their present configuration show normal displacement. The Hayden Creek fault, however, has a demonstrable thrust displacement prior to normal displacement. Both it and the Bear Valley fault may have formed originally as subsidiary thrusts of the Poison Creek thrust fault, generated during rupture of tight folds within the Poison Creek thrust sheet. Removal of the normal component of slip on the Hayden Creek and Bear Valley faults, restoring rocks to about their pre-normal-slip positions, reveals the original geometry of the eastern part of the large Poison Creek anticline above the Poison Creek thrust fault. A single regionally extensive thrust sheet, the Medicine Lodge thrust system, previously was interpreted as the major structure of the region. Within the southeastern part of the present study area, a window was stated to exist in the vicinity of Hayden Creek and to expose rocks of the subthrust Mesoproterozoic Yellowjacket Formation. My mapping leads me to conclude that neither the Medicine Lodge thrust plate nor the window exists in this part of the range. The northwest-trending structural grain of the Lemhi Range is truncated by the north-striking Salmon River fault at the western margin of the study area. The fault is oblique to the structural grain of the range and is interpreted as a tear fault that formed during Mesozoic thrust faulting and folding of rocks of the Poison Creek thrust sheet. The fault served to compartmentalize the style of deformation that took place on opposite sides of it. Rocks east of the Salmon River fault, the main focus of this report, deformed into the major fold and associated subsidiary thrusts of the Poison Creek sheet. Rocks directly west of the Salmon River fault generally display low dips and locally define a shallow-dipping syncline oriented about parallel to the north-trending fault. A segment of the fault subsequently was reactivated as part of the Lem Peak normal fault during down-dropping of the Lem Peak block. Later, the entire length of the present Salmon River fault was reactivated as a basin-and-range normal fault that dips steeply to the west.

The middle Cretaceous in southwestern Montana is composed of a marine and nonmarine succession of predominantly clastic rocks that were deposited along the western margin of the Western Interior Seaway. In places, middle Cretaceous rocks contain appreciable total organic carbon (TOC), such as 5.59% for the Mowry Shale and 8.11% for the Frontier Formation in the Madison Range. Most samples, however, exhibit less than 1.0% TOC. The genetic or hydrocarbon potential (S1+S2) of all the samples analyzed, except one, yield less than 1 mg HC/g rock, strongly indicating poor potential for generating commercial amounts of hydrocarbons. Out of 51 samples analyzed, only one (a Thermopolis Shale sample from the Snowcrest Range) showed a moderate petroleum potenti l of 3.1 mg HC/g rock. Most of the middle Cretaceous samples are thermally immature to marginally mature, with vitrinite reflectance ranging from about 0.4 to 0.6% R0. Maturity is high in the Pioneer Mountains, where vitrinite reflectance averages 3.4% R0, and at Big Sky, Montana, where vitrinite reflectance averages 2.5% R0. At both localities, high R0 values are due to local heat sources, such as the Pioneer batholith in the Pioneer Mountains.

From abstract: A mineral resource survey was conducted in 1987 by the U.S. Geological Survey and the U.S. Bureau of Mines to evaluate mineral resources (known) and mineral resource potential (undiscovered) of the Sleeping Giant Wilderness Study Area (MT-075-111) in Lewis and Clark County, Montana. The only economic resource in the study area is an inferred 1.35-million-ton reserve of decorative stone (slate); a small gold placer resource is subeconomic. A high resource potential for decorative slate exists directly adjacent to the area of identified slate resource and in the northeastern part of the study area. The rest of the study area has a low potential for decorative slate. The westernmost part of the study area has a moderate resource potential for copper and associated silver in strata-bound deposits in green beds and limestone; potential is low in the rest of the study area.

From abstract: A mineral resource survey of the Ruby Mountains Wilderness Study Area (MT-076-001) was made in 1984- 85. Thirty-two million tons of subeconomic mineral resources that average 21 percent iron occur in the Kelly iron deposit and constitute an identified resource adjacent to the southeasternmost edge of the study area. Moderate and high mineral resource potential for talc deposits occur within a large body of Archean (see geologic time chart in appendix) marble in the central part of the study area (fig. 1). The study area has a low mineral resource potential for uranium and for oil and gas sources. No phosphate deposits exist in the study area.

The northern third of the Ruby Range, southwestern Montana, contains about 5,000 feet (1,525 m) of Paleozoic marine sedimentary rocks overlying a basement of Precambrian metamorphic rocks. Mesozoic rocks are represented by a nonrnarine conglomerate, the Beaverhead Formation. The Cambrian sequence, composed largely of carbonate rocks, comprises the Flathead Sandstone at the base, Wolsey Shale, Meagher Limestone, Park Shale, Pilgrim Dolomite, and the Red Lion Formation. The Meagher Limestone is mainly dolomite in the southern part of the area, limestone in the northern part. The Pilgrim Dolomite is divisible into two main parts: cyclically interbedded dolomitic strata 2to 10-inch (5-25 cm) thick in the lower part, mainly dolomitic quartz sandstone and sandy dolomite in the upper part. The Red Lion Formation is composed mainly of dolomite; the upper third is dominated by bedded, dome-shaped, and columnar stromatolites. The Devonian is represented by the Jefferson Formation, a dolomite unit containing several interstratified units of silty shale, and the lower part of the Three Forks Formation. The Three Forks is composed largely of clay shale but contains a prominent medial limestone unit. The upper (Mississippian) part of the Three Forks is overlain by three Mississippian formations. The Lodgepole Limestone is constituted of limy shale in the lower half and fossiliferous limestone in the upper half. It is overlain by the Mission Canyon Limestone, a formation that locally contains solution breccias and minor amounts of interbedded dolomite. The overlying red beds, made up of dolomite and sandstone, are questionably assigned to the Kibbey Formation. Pennsylvania rocks are represented by the Quadrant Sandstone, made up of interbedded limestone and sandstone in nearly equal amounts. The Mesozoic is represented by a nonmarine conglomerate that is correlated with the Beaverhead Formation. It is composed of clasts derived from Paleozoic carbonate units and Precambrian (Belt Supergroup) quartzite. INTRODUCTION The Ruby Range is a north-trending block of mountains about 30 mi (50 km) long and 12 mi (20 km) wide, nearly surrounded by intermontane basins containing broad benchlands and alluvial sur

From summary:A mineral resource survey of a 10,610-acre part of the Blacktail Mountains Wilderness Study Area (MT-076- 002) was conducted in 1984-85. Identified subeconomic resources of barite exist in the area at the northernmost boundary, and a zone partly within the area along the northeastern boundary has a moderate mineral resource potential for silver and barite, and associated gold, copper, lead, and zinc. No producing mines occur in or adjacent to the area. The area lies within a region that is rated as having moderate energy resource potential for oil and for gas.

A mineral-resource survey of the 350-acre Henry's Lake Wilderness Study Area (ID-035-077) was made in 1986-87. No identified resources (known) or currently active claims exist within or adjacent to the wilderness study area. There is potential for several types of undiscovered mineral resources within the study area. The southwestern part of the wilderness study area, along the Madison Range fault, is rated as having a moderate energy-resource potential for geothermal water; the remainder of the study area has a low potential for resources of this commodity. A small outcrop of marble in the southernmost part of the study area has a low mineral-resource potential for talc; for talc in marble possibly concealed beneath the study area the mineral-resource potential is rated as unknown. The study area has a low mineral-resource potential for iron in hematite-mineralized amphibolite gneiss, and for gold, silver, and uranium. The area has no mineral-resource potential for phosphate, because the host strata have been eroded; and no resource potential for oil and gas.