ABSTRACT We report the results of 167 calcite twinning strain analyses (131 limestones and 36 calcite veins, n = 7368 twin measurements) from the Teton–Gros Ventre (west; n = 21), Wind River ( n = 43), Beartooth ( n = 32), Bighorn ( n = 32), and Black Hills (east; n = 11) Laramide uplifts. Country rock limestones record only a layer-parallel shortening (LPS) strain fabric in many orientations across the region. Synorogenic veins record both vein-parallel shortening (VPS) and vein-normal shortening (VNS) fabrics in many orientations. Twinning strain overprints were not observed in the limestone or vein samples in the supracrustal sedimentary veneer (i.e., drape folds), thereby suggesting that the deformation and uplift of Archean crystalline rocks that form Laramide structures were dominated by offset on faults in the Archean crystalline basement and associated shortening in the midcrust. The twinning strains in the pre-Sevier Jurassic Sundance Formation, in the frontal Prospect thrust of the Sevier belt, and in the distal (eastern) foreland preserve an LPS oriented approximately E-W. This LPS fabric is rotated in unique orientations in Laramide uplifts, suggesting that all but the Bighorn Mountains were uplifted by oblique-slip faults. Detailed field and twinning strain studies of drape folds identified second-order complexities, including: layer-parallel slip through the fold axis (Clarks Fork anticline), attenuation of the sedimentary section and fold axis rotation (Rattlesnake Mountain), rotation of the fold axis and LPS fabric (Derby Dome), and vertical rotations of the LPS fabric about a horizontal axis with 35% attenuation of the sedimentary section (eastern Bighorns). Regional cross sections (E-W) across the Laramide province have an excess of sedimentary veneer rocks that balance with displacement on a detachment at 30 km depth and perhaps along the Moho discontinuity at 40 km depth. Crustal volumes in the Wyoming Province balance when deformation in the western hinterland is included.

Cave development in the Madison aquifer of the Black Hills has taken place in several stages. Mississippian carbonates first underwent eogenetic (early diagenetic) reactions with interbedded sulfates to form breccias and solution voids. Later subaerial exposure allowed oxygenated meteoric water to replace sulfates with calcite and to form karst and small caves. All were later buried by ~2 km of Pennsylvanian–Cretaceous strata. Groundwater flow and speleogenesis in the Madison aquifer were renewed by erosional exposure during Laramide uplift. Post-Laramide speleogenesis enlarged paleokarst voids. Most interpretations of this process in the Black Hills invoke rising thermal water, but they fail to account for the cave patterns. Few passages extend downdip below the present water table or updip to outcrops. None reaches the base of the Madison Limestone, and few reach the top. Major caves underlie a thin cover of basal Pennsylvanian–Permian Minnelusa Formation (interbedded quartzarenite and carbonates). Water infiltrating through the Minnelusa Formation dissolves carbonates in a nearly closed system, producing low p CO 2 , while recharge directly into Madison outcrops has a much higher p CO 2 . Both are at or near calcite saturation when they enter caves, but their mixture is undersaturated. The caves reveal four phases of calcite deposition: eogenetic ferroan calcite (Mississippian replacement of sulfates); white scalenohedra in paleovoids deposited during deep post-Mississippian burial; palisade crusts formed during blockage of springs by Oligocene–Miocene continental sediments; and laminated crusts from late Pleistocene water-table fluctuations. The caves reveal more than 300 m.y. of geologic history and a close relationship to regional geologic events.

Abstract Rare-element granitic pegmatites represent highly concentrated sources of rare metals, including Li, Rb, Cs, Be, Sn, Nb, Ta, Zr, Y, REE, and U. In today’s markets, pegmatites are the principal sources of Ta, and one pegmatite (Tanco, Canada) is the sole commercial producer of Cs for use as deep drilling fluid in the form of Cs formate solution. Growth in the demand for Li-based batteries has prompted exploration for spodumene-and petalite-bearing pegmatites, and several new Canadian prospects are slated for mining. Pegmatite bodies that contain minerals in which these elements are essential structural constituents constitute less than ~1 to 2% of all pegmatites in a given pegmatite-bearing terrane, and the economic production from many such bodies is limited by their small size (i.e., they may be economic in grade, but not for mechanized mining). Because pegmatites are found in cratons and orogenic belts, however, pegmatite-hosted resources are widespread and likely to be significant secondary, if not primary sources of rare metals for local economies or in times of disruption of global supplies from other types of deposits. Pegmatites are primarily igneous in origin, and the most likely processes that enrich them in rare elements include crystal-melt fractionation together with the creation of locally flux-and rare-element-enriched domains of melt in otherwise rather ordinary granitic melt. The mechanism of constitutional zone refining, in which fluxes and incompatible components are enriched in a boundary layer of melt adjacent to crystal growth fronts, represents the most effective means of concentrating rare elements. Whereas Rayleigh fractionation produces an exponential increase in the abundances of incompatible rare elements, constitutional zone refining leads to a sharp, “L”-shaped inflection in the concentration of incompatible elements with the progress of crystallization. The absolute concentrations of trace elements at the end of constitutional zone refining can be orders of magnitude greater that those that are attainable by Rayleigh fractionation (between mineral and bulk melt). In rare-element pegmatites, some trace-element enrichment patterns show the gradual increase in abundance that is expected of Rayleigh fractionation, whereas pegmatites in which the transition from ordinary mineral assemblages to those enriched in rare elements is sharply defined, more closely match the elemental fractionation that is derived from constitutional zone refining. Although pegmatites are igneous, pegmatite-forming melts crystallize well below their liquidus, and perhaps even below solidus temperatures. The textures and zonation that are hallmarks of pegmatites arise in response to the inception of crystallization from highly undercooled, viscous melt. Graphic granite, the one texture that is unique to pegmatites, constitutes prima facie evidence of such conditions. The crystallization of rare-element minerals, such as beryl, spodumene, tantalite, and pollucite can also be reconciled to the low-temperature crystallization of melts. Pegmatite-hosted ores are entirely endogenic, and many pegmatites exhibit little or no exogenic wall-rock alteration. Narrow and sporadic zones of alteration envelopes, unpredictable size in relationship to degree of fractionation, and sharply defined zonation of rare-element ores to inner units combine to make granitic pegmatites difficult targets for exploration.

Abstract A synthesis of low-temperature thermochronologic results throughout the Laramide foreland illustrates that samples from wellbores in Laramide basins record either (1) detrital Laramide or older cooling ages in the upper ~1 km (0.62 mi) of the wellbore, with younger ages at greater depths as temperatures increase; or (2) Neogene cooling ages. Surface samples from Laramide ranges typically record either Laramide or older cooling ages. It is apparent that for any particular area the complexity of the cooling history, and hence the tectonic history interpreted from the cooling history, increases as the number of studies or the area covered by a study increases. Most Laramide ranges probably experienced a complex tectono-thermal evolution. Deriving a regional timing sequence for the evolution of the Laramide basins and ranges is still elusive, although a compilation of low-temperature thermochronology data from ranges in the Laramide foreland suggests a younging of the ranges to the south and southwest. Studies of subsurface samples from Laramide basins have, in some cases, been integrated with and used to constrain results from basin burial-history modeling. Current exploration for unconventional shale-oil or shale-gas plays in the Rocky Mountains has renewed interest in thermal and burial history modeling as an aid in evaluating thermal maturity and understanding petroleum systems.This paper suggests that low-temperature thermochronometers are underutilized tools that can provide additional constraints to burial-history modeling and source rock evaluation in the Rocky Mountain region.