Theses and Dissertations

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Open Access
The geology, alteration, and mineralization of the Turquoise Lake area, Lake County, Colorado
(Colorado State University. Libraries, 1980) Craig, Steven D., author; Thompson, Tommy B., advisor; Greybill, F., committee member; Burns, L. K., committee member
The Turquoise Lake intrusive complex is located on the northeast flank of the Sawatch Range in Lake County, Colorado. The complex is centered on a 35 million year old subvolcanic stock of quartz latite porphyry. The intrusive rocks associated with the explosive emplacement of the stock includes an intensely brecciated border phase surrounding the porphyritic stock center, and dikes of similar composition. Other Oligocene intrusives include early dikes of latite and lake dikes o f rhyolite. North-trending structures reflecting the axis of the Sawatch Range are common throughout the Turquoise Lake area. The Central Fault, a major north-trending fault, is thought to have localized the stock at the juncture with inferred east-west trending structures. Radial faulting found in the Sugarloaf and St. Kevin Mining Districts formed during the emplacement of the Turquoise Lake stock. Widespread and pervasive alteration halos including a propylitic zone, transition zone, phyllic zone, and quartz-topaz subzone are centered on the stock. A late kaolinite-sericite zone is found at depth within the stock or in late dikes found above this zone. Pervasive disseminated pyrite is found increasing up to 10 volume percent toward the stock. Molybdenite is found in the quartz -topaz subzone and in quartz veinlets outside this area. The rocks surrounding the Turquoise Lake stock are geochemically anomalous in Cu, Mo, Ph, Zn, Sn, W and F. These elements form donut shaped dispersion halos centered on the stock. Two pulses of mineralization are inferred with double halos by the elements Cu, Pb, and Zn. Drillhole geochemistry finds Mo, Cu, and W displaying discrete halo boundaries and increasing values toward the bottom of the holes. Studies of vein mineralogy relationships from the St. Kevin and Sugarloaf Districts find four episodes of mineralization. These include an early ore preparation stage, main ore stage, late ore stage, and supergene stage. The bulk vein mineralogy consists of a quartz gangue, pyrite, galena, sphalerite, and chalcopyrite. Samples from different locations throughout the mining districts were analyzed for Cu, Mo, Pb, Zn, Ag, Sn, W, and Bi. These metals formed distinct concentration halos spatially associated with the Turquoise Lake stock. The Turquoise Lake stock has many features associated with known molybdenite ore bodies. These features suggest that potential exists for discovery of a molybdenite ore body at Turquoise Lake.
Open Access
Streamflow synthesis and water allocation by water right priorities
(Colorado State University. Libraries, 1980) Holt, William Kent, author; Longenbaugh, Robert A., advisor; Stiffler, William D., advisor; Smith, F. M., committee member
A computerized streamflow model was developed that allocates water by the doctrine of prior appropriations. The function of the model is to simulate the impacts of physical or legal changes in a stream system managed on the basis of priority of water rights. Given a specified value of streamflow and a desired level of demand, the model predicts the amount of water available to individual users. The model has been applied to the White River in Colorado. Results indicate that the model is a useful tool in predicting streamflows and water availability for individual users.
Open Access
Measurement of snowpack properties using active FM-CW microwave systems
(Colorado State University. Libraries, 1982) Farr, John Merritt, author; Boyne, Harold S., advisor; Smith, Freeman M. (Freeman Minson), 1939-, committee member; Smith, James A., committee member
This paper reports on the use of an FM-CW active microwave system, in a research mode, to remotely sense water equivalence and liquid water content of snowpacks. A three-component "electrical path length" dielectric mixture model is described which accounts for the microwave system response as a function of operating frequency, snow density and depth (water equivalence), and liquid water content. This physically based model is compared to currently accepted, semi-empirical mixture models and the limited data that exists. The "electrical path length" model compares favorably and has a distinctly simpler form than other models, making it workable for the specific problem addressed. It is concluded that by collecting data in two frequency ranges (just below the relaxation frequency of water), the depth of ice, the depth of liquid water, and thus the water equivalence of dry or wet snowpacks could be determined. Liquid water content determinations, made on a real-time basis, could then serve as invaluable melt-rate indexes for remote sites. Recommendations are given for the design configuration of an operational system, which could be incorporated into hydrometeorological data acquisition platforms such as SNOTEL.
Open Access
Petrology and contact relationships, SW portion PC Mullen Creek mafic complex, Medicine Bow Mts., Wy.
(Colorado State University. Libraries, 1983) Edwards, Jeffrey Scott, author; McCallum, M. E. (Malcolm E.), 1934-, advisor; Burns, L. K., committee member; Warren, C. Gerry, committee member
Geologic mapping and thin section petrography of rock units in and adjacent to the southwestern portion of the Mullen Creek mafic complex reveals the presence of a roof zone of a Precambrian layered basic intrusion that invaded a metavolcanic sequence (predominantly hornblende gneiss). This portion of the mafic complex consists of variably metamorphosed interlayered rocks that originally ranged from pyroxenite to anorthosite but were dominantly gabbro and leucogabbro. Most of these rocks underwent regional medium-grade metamorphism to form amphibolite facies assemblages in which hornblende has replaced pyroxene, labradorite has remained stable and relict igneous textures are preserved. A later episode of retrogressive low-grade metamorphism has produced locally abundant fractures, veinlets and minerals typical of the greenschist facies such as chlorite, epidote, talc, serpentine and saussurite.
Open Access
Geology, mineralization, and fluid inclusion analysis of the Ajax vein system, Cripple Creek, Colorado
(Colorado State University. Libraries, 1984) Dwelley, Peter C., author
The Ajax mine is located in the Cripple Creek mining district. Teller County, Colorado. Mine workings extend through a vertical range of 3363 feet (1025 m), from 10,105 feet (3081 m) to 6742 feet (2055 m) elevation. Production is from gold-telluride veins hosted in Precambrian granite and Tertiary breccia. Mine production amounts to more than 700,000 oz. of gold at average grades of 0.60 to 1.00 oz. gold per ton. The mine is situated on the southern margin of a Tertiary volcanic complex composed of highly differentiated alkaline rocks that intrude fine-grained breccia and minor sediment. Most of the mineralization in the Ajax is hosted by Precambrian granite which surrounds the complex. Complex formation began about 34 m.y. ago. Five stages of vein mineralization have been recognized. Vein content is dependent on the relative time at which the structure was receptive to ore-forming fluids. Vein minerals, in order of decreasing volume, consist of quartz, fluorite, pyrite, adularia, dolomite, rutile, sphalerite, hematite, galena, marcasite, calaverite, chalcopyrite, pyrrhotite, and acanthite. The Au/Ag ratio varies from 20:1 to 1:1 and is controlled by grade; higher Au/Ag ratios correspond to greater gold values. No consistent vertical trend in Au/Ag ratio has been recognized. A subtle increase in base metal content with depth may reflect initial development of a weak zonation pattern. Vein-related alteration is fracture controlled and poorly developed. Two lateral zones of alteration were defined. Inner zone alteration varies from one to three times vein width and consists of the following: complete replacement of biotite and plagioclase, quartz recrystallization, and microcline rimmed and veined by adularia. Outer-zone alteration varies from two to five times vein width and consists of partial replacement of biotite and plagioclase, unaltered quartz, and mostly unaltered microcline that may be weakly veined by adularia and quartz. Fluid inclusion analyses of quartz, fluorite, and sphalerite from stages 1 through 4 define a complex fluid evolution. Filling temperatures ranged from 206 to 510°C during stage 1 mineralization and from 123 to 350°C during stage 3. Stage 4 fluid deposited calaverite and quartz at temperatures ranging from 105 to 159°C. Salinity of the ore fluid ranged from 30 to 47 wt% eNaCl during stage 1 and decreased to between 0 and 8.3 wt% eNaCl during stage 3. Stage 4 fluid salinity was approximately equal to stage 3. Initial temperature and salinity decrease was caused by mixing of stage 1 magmatic fluid with meteoric water. Additional temperature decrease resulted from cooling of the magmatic heat source(s). The irregular vertical thermal gradient present in the vein system may be the result of lateral fluid flow caused by intersection of veins with the breccia complex and/or presence of intrusive heat sources within the breccia. CO2 has been recognized in the fluids of stages 1, 3, and 4. Subtle boiling occurred in all stages over a great vertical range, consisting primarily of CO2 effervescence. The presence of CO2 greatly increases the estimated maximimi depth at which boiling can occur. Based on stability of alteration minerals, pH is estimated to have a minimum value of 5.5 at 300°C. Opaque mineral relationships indicate oxygen fugacity ranged from -36 to -28.4 log f02. physical-chemical character of the ore fluid indicates the most amenaible gold transport mechanism. Base metals and gold were originally transported as chloride complexes. Decreasing salinity lowered chlorine iocn activity which, in conjunction with temperature decrease, decomposed the chloride complexes. Gold remained in solution by forming migration complexes with tellurium. Sulfide deposition increased the H2Te:H2S ratio, causing greater gold-tellurium complex stability. Gold remained in solution until decreasing temperature in stage 4 deposited calaverite and quartz. Character of the ore-forming fluid and close relationship of the veins to intrusive activity, among other evidence, indicate a magmatic source for the ore metals.
Open Access
Geology, alteration and precious metal reconnaissance of the Nogal Canyon area, San Mateo Mts., N.M.
(Colorado State University. Libraries, 1984) Foruria, Jon, author
The Nogal Canyon area lies within the southern San Mateo mountains and displays epithermal, volcanic hosted precious metal occurrences in the San Jose and Quartz Hill districts and in the Aragon Hill area. The San Mateos represent an isolated, north trending, eastward tilted, structural block located in the northeastern portion of the Cenozoic Mogollon-Datil volcanic field. Mid-Tertiary, calc-alkalic andesites to high silica rhyolites and minor volcaniclastics dominate the lithologies exposed in the Nogal Canyon area. The Oligocene Spears Formation, the oldest unit present, consists of andesitic to latitic flows, breccias, and ash-flow tuff and sporadic volcaniclastic sandstone. The Hells Mesa rhyolite ash-flow tuff overlies the Spears Formation and is sequentially superimposed by the Unit of Luna Park which includes andesitic flows, dacite flow breccias, rhyolite tuff, and minor volcaniclastics. The majority of the upper felsic sequence exposed in the southern San Mateos consists of the Oligocene Vicks Peak Tuff, a thick, densely to partially welded rhyolite ash-flow tuff, and the overlying Springtime Canyon Quartz Latite. Younger porphyritic rhyolite intrusives, rhyolite flow-dome rocks, and intrusive breccia appear localized along major structural trends within the southern foothills of the San Mateo mountains. Major northeast and northwest trending, steeply dipping, normal faults crosscut the Vicks Peak Tuff and host "fissure type" precious metal occurrences in the San Jose district. Structural preparation relates either to the proposed Nogal Canyon cauldron or to mid-late Tertiary regional extensional tectonism. Hydrothermal alteration effects contemporaneous with precious metal mineralization consist of pervasive and veinlet silicification, pervasive intermediate arqillic alteration, and quartz-alunite replacement alteration. Silica alteration is strongly localized along fracture and breccia zones consisting of vein-infillings, pervasive wall rock alteration, and quartz network-stockwork type occurrences. The gangue mineral assemblage accompanying vein-infilling silicification includes adularia, sericite, calcite, pyrolusite, and cryptomelane. Intermediate argillic alteration consists of montmorillonite, montmorillonite-illite mixed layer, illite, kaolinite, and pyrite systematically zoned as reaction aureoles about major structures. Argillic alteration broadens beneath the Vicks Peak Tuff-Springtime Canyon Quartz Latite contact which acted as a permeability barrier to ascending hydrothermal solutions. Quartz-alunite alteration occurs as a really restricted, pervasive replacement deposits with accessory pyrite, specularite, kaolinite, and chalcedony. Mineralization accompanying vein-related silicification consists of native silver, native gold, cerargyrite, and pyrite and appears modified by supergene oxidation processes. Trace element studies indicate the mineralization exhibits low arsenic and antimony signatures and lacks base metal introduction at present erosional levels. A single hydrothermal episode which generated structurally confined boiling appears responsible for the development of hypogene alteration and insignificant silver-gold mineralization centered over the Nogal Canyon area. Precious metal occurrences at Aragon Hill consist of gold bearing, quartz-calcite-barite fracture fillings crosscutting the Aragon Hill intrusive breccia. Pervasive phyllic alteration, contemporaneous with breccia emplacement, occurs throughout the pipe-like intrusive breccia and concentrically envelopes the surrounding rhyolite flow-dome rocks. Phyllic alteration consists of sericite, montmorillonite-chlorite mixed layer clay, quartz, pyrite, and kaolinite products. The precious metal bearing fracture fillings probably represent leaching and minor metal deposition during the late waning stages of breccia emplacement, or possibly high level expression of epigenetic breccia-hosted silver-gold mineralization at depth.
Open Access
Hydrothermal mineralization and alteration at the Globe Hill deposit, Cripple Creek District, Colorado
(Colorado State University. Libraries, 1985) Trippel, Alan D., author; Thompson, Tommy B., advisor; Caddy, Stan W., committee member; Vaughan, John, committee member
The Globe Hill deposit is hosted by a porphyritic, subvolcanic intrusive which is composed of pyroxene-bearing alkali trachyte. It is emplaced within the Oligocene-age Cripple Creek diatreme-intrusive complex, which may be related to the Rio Grande rift system centered 75 km to the west. Four structural events occurred at Globe Hill. The earliest event (stage I) created a zone of upward-flaring hydrothermal breccia bodies which were later cut by a series of tectonic structures (stage II). A dike- or column-like body of intrusive breccia (stage III) subsequently invaded a major stage II shear zone, and a large, separate hydrothermal breccia body (stage IV) formed within the stage I zone. Separate hydrothermal fluids passed through each of the structural systems, and formed epithermal, low-grade, polymetallic mineralization along stage I, II, and IV structures; stage III structures remained unmineralized. Mineralization occurs as veins, breccia-matrix fillings, and disseminations through breccia fragments and adjacent wall rock. Stage I structures were mineralized by an assemblage composed dominantly of chalcedony-celestite-pyrite and trace oxides, telluride, phosphates, and base-metal sulfides. Early stage II veins are composed dominantly of quartzcelestite-pyrite and trace adularia, oxides, telluride, and base-metal sulfides. Wallrock adjacent to later stage II gangue-free structures contains halos of disseminated pyrite and trace oxides, telluride, and base-metal sulfides. The central core zone of the stage IV breccia is cemented by an assemblage of anhydrite-carbonate-celestite-fluorite along with trace pyrite, oxides, and base-metal sulfides; its peripheral halo is partially cemented by montmorillonite and minor amounts of fluorite, quartz, and hematite. Five separate alteration events are recognized. Each of the first four is hypogene and correspond to one of the major structural stages. The fifth event is related to supergene weathering. Alteration associated with the stage I hydrothermal breccias grades from a sericite-dominated zone within the breccia bodies to a chlorite-dominated zone in the surrounding wallrock. Wallrock adjacent to stage II tectonic structures has been altered to a sericite-dominated assemblage. Alteration of the stage III intrusive breccia primarily affected the matrix material and to a lesser extent the fragments, producing a chlorite-dominated assemblage. Stage IV central core zone breccia fragments were altered to a quartz-dominated assemblage which grades to a montmorillonite-dominated assemblage in the peripheral zone and wallrock. The last hypogene event as well as the supergene weathering have partially oxidized the deposit to a depth of over 270 m. The effects of supergene weathering are most pronounced along stage I and II structures where permeability is sufficient to allow for downward percolation of oxidizing surface waters. Mineralization and alteration assemblages associated with each structural stage have been used to infer parentfluid chemistry. Existing data suggest that the fluids were weakly to moderately alkaline, moderately oxidized, and of relatively low sulfur fugacity. Certain fluids associated with stage I, II, and IV structural stages must have contained appreciable amounts of Ca, Sr, phosphate, sulfate, and dissolved gases including CO2 and H2S. Zonal wallrock alteration assemblages indicate intermediate to weak metasomatic reactions adjacent to each structure. Boiling is interpreted as the major cause of mineral precipitation along stage I, early stage II, and stage IV structures. Evidence for boiling fluids includes the development of hydrothermal breccias, often along veins; explosion textures in vein quartz and celestite; the occurrence of adularia in some vein assemblages; and the highly variable vein fluid-inclusion homogenization temperatures and liquid to vapor ratios. The relatively restricted extent of most alteration zones suggests that groundwater mixing and metasomatic wallrock interactions occurred. These two processes could rapidly promote fluid-groundwater and fluid-wallrock equilibration, terminating further alteration. No evidence for fluid boiling was observed along the later stage II tectonic structures; in this case, metasomatic wallrock interactions were the major cause of mineral precipitation. The Globe Hill gold deposit may be the near-surface (hot-springs) expression of deeper, high-grade vein deposits like those located elsewhere in the district. Mineralizing fluids probably were convectively circulated by a thermal anomaly at depth beneath the Globe Hill area. The thermal anomaly and the presence of appreciable Ca and Sr in the fluids were probably derived from late alkali basalt igneous activity within the Cripple Creek diatreme-intrusive complex.
Open Access
Geology, mineralogy and geochemistry of the Cerro Matoso nickeliferous laterite, Córdoba, Colombia
(Colorado State University. Libraries, 1986) Lopez-Rendon, Jorge E., author; McCallum, M. E., advisor; Daugherty, Neel C., committee member; Thompson, Tommy B., committee member
The Cerro Matoso nickeliferous laterite, located in northern Colombia near the town of Montelibano, was developed in pre-Late Cretaceous ultramafic rocks consisting principally of slightly serpentinized harzburgite. The peridotite body is flanked by ferruginous sandy sediments with interbedded coal beds of the Cienaga de Oro Formation (early Oligocene-early Miocene) and Recent alluvial gravels and sands. The harzburgite consists predominantly of olivine with lesser amounts of orthopyroxene and secondary serpentine. Intense serpentinization is primarily confined to areas of faulting and brecciation particularly along the western and eastern boundaries of the peridotite body. Development of the nickel laterite profile was greater in weakly serpentinized peridotite both in vertical extent and in the degree of enrichment of nickel-rich secondary products than in strongly serpentinized peridotite. The Cerro Matoso peridotite was affected by two major tectonic events. Compressional stresses associated with the late phases of the Pre-Andean orogeny in middle Eocene-late Eocene time brought the peridotite to the surface and also generated a major NE trending fault system in the peridotite body arid local serpentinization particularly along fault zones. Lateritization of the harzburgite probably began in late Eocene-early Oligocene time and chemical weathering and erosion favoured by a tropical humid and rainy climate with probable alternating wet-relatively dry seasons, continued throughout the Oligocene period. A major NW trending fault system developed in the peridotite body during the late Miocene-Pliocene Andean orogeny and the southwestern part of the weathered peridotite body was uplifted relative to the northeastern part. This uplift apparently was sufficient for intense erosion to remove most of the laterite profile from the uplifted block while the northeastern block was being only slightly modified. The laterite profile consists of an upper Canga zone underlain respectively by a Limonite zone and Upper and Lower Saprolite zones. The economic silicate-type nickel mineralization at Cerro Matoso is confined to the saprolite zones. Nickel content in parent peridotite ranges from 0.28 to 0.36%, and ore grade cutoff for materials comprising the laterite profile is 1.5%. Ore occurs as massive and fracture filling types, the massive ore constituting the most important part of the deposit economically. Smectite and serpentine are the dominant Ni-bearing minerals in the massive ore and are particularly abundant in the Upper Saprolite zone. Pimelite, nimite and Ni-bearing sepiolite fill fractures in both the Upper and Lower Saprolite zones. Nickel averages 3% in the massive ore and ranges to as much as 7% locally. Nickel grades in fracture fillings range to as much as 30%. Variations in pH, and to a lesser extent Eh, of the weathering solutions influenced the selective differential element accumulation and mineral formation throughout the laterite profile. Oxidizing and slightly acidic conditions apparently were present in the Limonite and Canga zones and led to precipitation of Fe as goethite. Slight increase in pH in the Limonite zone as compared to the Canga zone favored Mn precipitation at the lower part of the Limonite zone as Mn oxide. Ni was highly mobile in the Canga and Limonite zones and alkaline conditions present in the saprolite zones promoted its deposition in garnierite. Co was less mobile than Ni and the zone of maximum Co concentration formed near the base of the Limonite zone. Moderate mobility of silica occurred in upper profile levels but mobility decreased rapidly in the Upper Saprolite zone apparently due in part to the presence of higher amounts of dissolved Mg in the weathering solutions. Cr mobility was very low in this weathering environment. Alumina behaved as a highly immobile component and concentrated primarily in the near surface part of the Canga zone.
Open Access
Origin of gold placers in the Pioneer district, Powell County, Montana
(Colorado State University. Libraries, 1986) Loen, Jeffrey Scott, author
This thesis presents a major reinterpretation of the genesis of gold placers in the Pioneer district, near Deer Lodge, Montana. Previous investigators considered the placers to be the result of glacial deposition and reworking of glacial drift by interglacial streams. Results of this study suggest that the oldest placers, however, occur in two Tertiary conglomerates. Placers are also present in Pleistocene glacial outwash and till, and Holocene alluvium and colluvium. Since 1862 an estimated 300,000 fine ounces of placer gold has been produced from the district, the majority of which came from the Tertiary conglomerates. Conglomerate also served as the principal source for gold in Quaternary age placers. Piedmont slopes in the Pioneer district are underlain by three Tertiary sedimentary sequences separated by unconformities. These are from oldest to youngest: (1)the Cabbage Patch Formation (late Oligocene to early Miocene)--siltstone, mudstone, and volcanic ash deposited in rivers and lakes; (2) the Squaw Gulch beds (Middle Miocene)-siltstone, sandstone, and pebble/cobble conglomerate deposited on alluvial plains by braided streams; and (3) the Pioneer beds (late Miocene or Pliocene (?))--matrix- and clast-supported boulder/cobble conglomerate and siltstone deposited on alluvial fans by mudflows, debris flows, and braided streams. The apparent source of the placer gold is base- and precious-metal veins of late Cretaceous age in the northern Flint Creek Range. Gold was released from the veins during deep weathering under moist, warm climatic conditions in Oligocene and early Miocene time. Middle Miocene mountain uplift and the accompanying arid climate triggered deposition of gold-bearing gravel on mountain-front floodplains. Maximum uplift occurred during late Miocene and Pliocene (?) time when coarse gravels were deposited in alluvial fans. As uplift subsided, braided streams reworked the gold-bearing gravels, forming the richest placers. The present drainage system developed during regional stream incision in middle Pliocene to middle Pleistocene time. Glaciers extended to the margins of the district during four major advances beginning in middle Pleistocene time. Tertiary placers were partly reconcentrated into Pleistocene glacial till and outwash, and Holocene alluvium and colluvium. Electron microprobe analyses and assay results indicate that lode gold from veins in the northern Flint Creek Range has an average fineness range between 800 and 850. Veins in quartzite and slate in the roof zone of the Royal stock contain higher-fineness gold than do veins in granodiorite of the stock. Placer gold from the Pioneer district typically contains cores averaging 830 to 870 fine and silver-depleted rims ranging from 930 to 1000 fine. The depletion of silver in the rims of the gold grains probably resulted from solution of the silver by low-temperature surface waters. Production records indicate an average fineness for the district between 875 and 900. These higher production records reflect the influence of thick silver-depleted rims on many of the placer grains.
Open Access
Llama use on public lands
(Colorado State University. Libraries, 1988) Arndt, Cheryl Anne, author; Smith, Freeman M. (Freeman Minson), 1939-, advisor
Interest in utilizing llamas as packstock is affecting natural resource managers as requests to use llamas on federally-owned lands increases. To adopt effective management practices related to issues raised by this relatively new use of public lands, policies must be oriented to meet the specific needs of each different area and their diverse types of users and visitors. Identifying these needs is a necessary first step in policy formation, and is the focus of this study. Members of the USES, BLM, and NPS were surveyed by mail to obtain information on llama use characteristics and issues developing from the use of llamas as packstock. Research and policy relating to the use of llamas was reviewed and current needs were identified.
Open Access
Structural geology and tectonic setting of the Cherry Creek Metamorphic Suite, southern Madison Range
(Colorado State University. Libraries, 1988) Sumner, Wendolyn Rivers, author; Erslev, Eric A., advisor; Burns, Larry K., committee member; Thompson, Eric, committee member
The Cherry Creek Metamorphic Suite, exposed in the core of the southern Madison Range in the northwestern Wyoming province, is a well-preserved, carbonate-quartzite-pelite shelf association of Archean age containing a coherent stratigraphic sequence. Relict sedimentary structures (161 measurements) reveal that stratigraphic up is to the southeast. Trace element geochemistry of the meta-turbidites indicates that the sediments were deposited in a back arc basin analogous to the Sea of Japan. Comparison of major element chemistry of the pelites with other suites in the northwest Wyoming Province suggest a similar provenance throughout the region. Repetition and truncation of the Cherry Creek stratigraphy occurs along a series of narrow, southeast dipping mylonite zones of Late Archean age. Petro-fabric analyses of C-S surfaces, asymmetrical mineral retorts, and rolled garnets indicate oblique thrusting to the northwest. Strain analysis of psammitic portions of meta-turbidites indicates largely flattening strains and bulk shortening of the Cherry Creek Metamorphic Suite, with associated plane strain localized in thrust zones. Prograde (middle-amphibolite facies) mineral growth lineation's parallel fold axes in zones of heterogeneous shear, suggesting synchronous prograde metamorphism and deformation. Down-plunge projection of fabric, strain and map relationships demonstrate tectonic thickening of the Cherry Creek Metamorphic Suite by thrust imbrication and duplex stacking. This thin-skin deformation is closely analogous to many post-Archean convergent belts, suggesting similar tectonic processes during the Archean.
Open Access
Geology of the Taylor Silver deposit, White Pine County, Nevada
(Colorado State University. Libraries, 1988) Edwards, Jeffrey M., author; Thompson, Tommy B., advisor; Vaughn, John, committee member; Johnson, Robert, committee member
The Taylor silver mine is located 15 miles southeast of Ely, Nevada, on the western flank of the Southern Schell Creek Range. The 1200 ton per day open pit mine produces up to 1,200,000 ounces silver and 1000 ounces gold annually. Preproduction reserves totalled 7 million tons at 3.0 oz Ag/ton and 0.003 oz Au/ton. Soft silver prices since 1982 have forced intermittent shutdowns. The ore body is an epithermal, high-silica, low-sulfide replacement deposit. Brecciated Devonian Guilmette Formation limestones host silver ores immediately beneath unconformable hydrocarbon-rich Mississippian Pilot Shale siltstone. Intrusion of pre-mineral, lower Oligocene fluorine-rich rhyolite dikes and sills, and post-mineral, lower Miocene latite dikes, bracket ore deposition. The structural history of the district is complex. Compressive deformation began in Mesozoic time as west to east thrust faults imbricated portions of the section east of a broad north-south fold in the Taylor mine area. Gravity-driven low angle faults, related to Paleocene through Eocene uplift of the neighboring Snake Range, attenuated and severely brecciated portions of the stratigraphic section. Oligocene compressive tectonism reactivated the Taylor fold and low-angle faults in the district. High-angle reverse faults breached the Taylor anticline, and focussed ore fluids into receptive breccias on and near the fold axis. Jasperoid outcrop characteristics, paragenesis, and trace metal contents serve to define four hypogene mineral stages: (1) weakly-mineralized stage I jasperoids contain elevated Au, up to 1 ppm, and locally over 1000 ppm As; (2) stage II jasperoids, marked by the presence of stibnite, contain up to 18% Sb, up to 1 ppm Au, locally over 5000 ppm Ba, and up to 2.5 oz Ag/ton; (3) stage III jasperoids represent the main Ag-ore-stage and contain up to 1000 ppm Pb, up to 3000 ppm Zn, and Ag:Au ratios over 1100:1; (4) stage IV open space sulfide-calcite veins and breccia fillings locally contain over 50 oz Ag/ton, with up to 4.5% combined base metals, and up to 3000 ppm Sb. Ore-stages III and IV were structurally confined to the immediate mine area. Stage I and II jasperoids form a distinct Sb-Au-Ba halo from 0.5 to 1 mile around the silver deposit. Fluid inclusion analyses demonstrate that Taylor was a prograde system. Stage II quartz precipitated from boiling fluids at 200°C. Ore-stage III accessory fluorite was deposited from boiling fluids at about 225°C. Stage IV vein sphalerite precipitated from non-boiling fluids at 306°C. Jasperoid whole rock vapor phase analyses indicate that the vapor phase was dominated by CH4, C2H6, and C3H8 probably derived from the thermal maturation of organic matter, indigenous to the enclosing Guilmette and Pilot sediments. Exploration drill hole logs define a base of silicification that cross cuts structure and stratigraphy. Lack of evidence for boiling in vein fluorite below this level suggests that the base of silicification may be coincident with the base of boiling. Banded quartz-sulfide-calcite veins found at depth indicate that episodic boiling reached deeper levels but strength and duration were apparently not adequate to promote wall rock silicification. Apparent vertical metal zonation grades from Pilot Shale- and Joanna Limestone-hosted Sb-Au-Ba-As-rich jasperoids down into Guilmette Formation limestone-hosted Ag-Zn-Pb-rich jasperoids, and finally into base metal-Ag-Sb-rich veins and breccia fillings below the base of silicification. District-wide paragenesis, wall rock alteration, and metal zonation thus appears to be related to stratigraphic level, depth, and particularly the boiling of indigenous-organic-derived volatiles. Both stratigraphic reconstruction and estimates from fluid inclusion data suggest depth of formation in the range of 3500 ± 700 feet (base of the Pilot Shale).
Open Access
Depositional system, facies relations & reservoir characteristics of the Codell Sandstone, Colorado
(Colorado State University. Libraries, 1990) Caraway, Donna C., author
Deposition of the Turonian Codell Sandstone member of the Carlile Shale in north-central Colorado occurred along the western margin of the Cretaceous Epeiric Seaway. Abrupt and irregular facies relationships, gross sheet-like geometry with internal linear thickness trends, ichnofacies arrangement, and stratigraphic positioning suggest an inner shelf depositional origin. The Codell sandstone is initially sourced from Frontier equivalent delta systems in Wyoming during the 90 m.y.a. lowstand and is thought to represent the shelf component of a transgressive system tract which developed during the subsequent Niobrara cycle transgression. Storm processes operating along the inner shelf margin produced amalgamated storm sheet deposits of mid-late Turonian age. Deepening of the seaway restricted sediment influx onto the shelf which resulted in the cannibalization of the previously deposited shelf sediments. This change is marked by a marine flooding surface which separates the inner shelf sediments from the winnowed condensed section above. The silica or calcite-cemented uppermost bed of the Codell Sandstone represents the winnowed cap of a condensed section produced by this starved basin condition. Further deepening of the seaway placed the shelf floor beyond the reach of epidsodic storm processes and is marked by an abrupt lithologic transition into marine limestone deposits. This contact is referred to as a basinal sequence boundary. Production from the Codell Sandstone consists predominantly of Volatile Oil (GOR between 1000 and 5000 SCF/STBO) or Retrograde Gas Condensate (high API gravities and GOR between 5,000-100,000 SCF/STBO). Distribution of the production patterns depends primarily upon proximity to the faulted basin axis and thicker Codell Sandstone intervals. High GOR's created from the increased thermal gradients and fluid conductivity associated with the major basin-syncline bounding faults charged the system with solution driven gas. This produces an extremely mobile hydrocarbon environment. Because the Codell Sandstone is porous, but fairly impermeable, any reversal in dip direction or en-echelon arranged fault system updip from the down-dropped basin axis, provides a potential site for hydrocarbon accumulation. Correspondence of better production rates near fault systems in areas of slightly thicker sandstone indicates that the trapping mechanism is both a function of stratigraphy and structural relationships (a combination trap).Secondary porosity, developed from the dissolution of unstable framework constituents (plagioclase feldspar grains and rock fragments), allows hydrocarbon accumulation in an interval that would otherwise contain too much clay to be normally productive. The Codell Sandstone contains quartz grains and layered phyllosilicates (illite, illite/smectite, and chlorite). Subsidiary amounts of plagioclase feldspar and pyrite are also present. The abundant distribution of detrital clay in this interval, is due in part, to extensive bioturbation. Dissolution of calcite cement also enhances the development of secondary porosity in the Codell Sandstone. Extensive artificial fracturing techniques and weakly acidic delivery systems are the most efficient completion practices yet developed for this interval.
Open Access
A paleohydrologic investigation in the vicinity of Harpers Ferry, West Virginia
(Colorado State University. Libraries, 1992) Fuertsch, Susan Jane, author; Wohl, Ellen E., 1962-, advisor; Salas, J. D. (Jose D.), committee member; Stednick, John, committee member
A paleohydrologic investigation of the Shenandoah River in the vicinity of Harpers Ferry, West Virginia, was conducted in response to the recent periodic floods that devastate the community. The study reach was approximately 7.5 km long and consisted of thirty-two surveyed cross-sections. Gaging stations established in 1895 at Millville, West Virginia and in 1882 at Harpers Ferry, West Virginia record flows ranging from a maximum of 6,509 m³s¯' , to a minimum of 2 m³s¯'. The average annual peak discharge for a seventy-year water record was 1,244 m³s¯'. Botanical flood evidence preserved as adventitious sprouts, tree scars and eccentric rings were documented in thirty-seven trees. A flood chronology established from these data extended from 1896 to 1955 after which no botanical indicators were found. Botanical indicators did not extend the systematic record, but they did provide an accurate, although not complete, flood chronology. The completeness of the botanical flood record is highly Sedimentological flood evidence was limited within the study area due to the influence of a humid-temperate climatic regime, which is not conducive to the stratigraphic preservation of individual flood depositional units. Human habitation of the area began in 1733; therefore, qualitative historical records were plentiful. Various historical records were cross-referenced to yield the most complete flood history. The correlation between the various sources was extremely high, demonstrating the comprehensiveness of the record. The historical flood record extends from 1748 to the beginning of the systematic record in 1896. The ability to determine accurate flood stages from paleoflood indicators varied highly. Botanical indicators were found to yield very inaccurate and inconsistent flood stages, and only minimum values of flood stage could be obtained from these data. Historical data did yield accurate stages; however, these stages did not necessarily yield accurate discharge values, depending upon the stationarity and hydraulic complexity of the area.
Open Access
North-northwest shortening across Laramide structures in the southeastern Uinta Mountains, Colorado and Utah
(Colorado State University. Libraries, 1994) Gregson, Joe Denny, author; Erslev, Eric A., advisor; Paulson, Merlyn J., committee member; Ethridge, Frank G., committee member; Sutton, Sally J., committee member
The Rocky Mountain foreland consists of a series of anastomosing structural arches that vary greatly in trend. Several previous studies of Laramide faulting document predominantly northeast-southwest shortening that is independent of arch orientation. Laramide faults and folds in the southeastern Uinta arch, however, suggest a north-northwest shortening direction. Laramide structures in the Dinosaur National Monument area were examined to investigate the possibility of north-northwest shortening. Eigenvector, M-plane, conjugate fracture, octahedra, and direct stress inversion analyses of 1206 slickensided minor faults give slip and σ1, trends averaging N22°W. Although the slip and σ1 trends are nearly perpendicular to individual structural trends, suggesting local control of stress and strain fields by the structures, a consistent component of northwesterly slip indicates north-northwest compression. To test this unusual Laramide σ1 direction, a 3-D restoration of the compressional Yampa graben area was constructed. The optimum restoration gave similar slip trends (N48°W N25°W) with clockwise rotations of 1° to 4° for the deformed blocks. The north-northwest shortening and σ1 trends in the Dinosaur area are highly oblique to the northeasterly trends elsewhere in the foreland. Hypotheses explaining the σ1 and shortening trends in the study area include: 1) north-south contraction from bending in the thrust slab over a south-dipping listric ramp during northeasterly thrusting of the range, 2) right lateral shear due to the complex transition in structural vergence between the northeast-directed Uinta arch and southwest-directed White River uplift, and or 3) eastward translation of the Uinta structural block due to impingement of the Sevier thrusts from the west. A combination of hypotheses 1 and 2 appear to best explain the observations and are favored as more proximal deformation mechanisms. The diverse structural trends in the study area probably resulted from both local and regional Laramide stresses, neither of which can be ignored in kinematic studies of foreland deformation.
Open Access
Lower tertiary stratigraphy in Katmai National Park, Alaska: a lithologic and petrographic study
(Colorado State University. Libraries, 1994) Houston, William S., author; Ethridge, Frank G., advisor; Lee, Robert E., committee member; Flores, Romeo M., committee member
Seacliff s in Katmai National Park have been mapped by prior workers as the Eocene(?) West Foreland Formation and the Oligocene(?) Hemlock Conglomerate and/or equivalents. The outcrops comprise a thick sequence of non-marine conglomerates, sandstones, siltstones, shales and coals that were deposited in low to high sinuosity fluvial channels and associated floodplain environments. The sediments were deposited in a fore-arc basin developed during the evolution of the Mesozoic-Cenozoic arc-trench system of the northeast Pacific region. Lithic arenites, the dominant sandstone type, have major framework constituents of quartz, volcanic and metamorphic rock fragments, chert, and plagioclase. Minor constituents include polycrystalline quartz, potassium feldspars, micas and accessory minerals. The majority of samples are texturally sub mature to mature. Sediment from proximal source terrains largely dictated sandstone texture and composition. Systematic vertical trends in texture or composition were not observed. The dominant source terrain for both formations was the paleo-Alaska Range volcanic arc to the west. Metamorphic sediments were derived from subduction complexes to the east, however, that source was commonly masked by an overwhelming influx of volcanic material. Authigenic minerals include calcite, phyllosilicates and iron oxides. All three are pore-filling, but only phyllosilicates and iron oxides are pore-lining. The most abundant phyllosilicate is chlorite which occurs as grain coats and in radiating and microcrystalline pore-fill phases. Compaction and/or cementation has completely occluded all primary pore space. Minimal secondary porosity has been created by dissolution of detrital grains and cements, and by micro fractures. A progressive sequence of diagenetic features resembles that of paragenetic sequences developed for formations in other arc-related basins. This suggests a main-line diagenetic sequence for volcanic-rich sediments deposited in fore-arc basins. The sequence appears to be independent of whether the depositional setting is marine or non-marine, and comprises: (1) compaction and development of clay coats; (2) calcite cementation; (3) cementation by chlorite and/or other phyllosilicates; and (4) complex replacement and alteration.
Open Access
Structural evolution of the gold quarry deposit and implications for development, Eureka County, Nevada
(Colorado State University. Libraries, 1995) Cole, David M., author; Thompson, Tommy B., advisor; Shackelford, Charles D., committee member; Erslev, Eric A., committee member
The Gold Quarry deposit, located seven miles north of Carlin, Nevada, is a bulk-minable sediment-hosted disseminated gold deposit situated along the Carlin Trend in Eureka County. A premier gold deposit in North America, Gold Quarry has total in-place reserves, and past production which exceed 15,000,000 oz. gold. Gold Quarry is hosted by lower Paleozoic eastern assemblage carbonate rocks, and lower Paleozoic siliciclastic and siliceous rocks of the transitional and western assemblage facies. The deposit is localized by a major structural intersection of the Gold Quarry and Good Hope fault systems at the southern end of the Carlin window. The Carlin window comprises an exposure of lower-plate eastern assemblage carbonate rocks through the upper-plate western assemblage siliceous rocks and the Tertiary Carlin Formation. The window is bounded by the Gold Quarry fault to the southeast and the Good Hope fault to the southwest. The Gold Quarry deposit comprises four connected mineral zones, and several other small satellite deposits within one large mineral system. The four zones are Gold Quarry Main, Maggie Creek, Deep West, and Deep Sulfide Feeder. These are believed to be genetically related, but differ from one another due to local stratigraphic and structural ore-controls. Detailed pit mapping and drill-log interpretation, aided by palynologic dating techniques integrated with research conducted by other workers has yielded a coherent tectono-stratigraphic sequence for the Gold Quarry area. Eastern-assemblage carbonate rocks, including the Ordovician Hanson Creek, Silurian Roberts Mountains Formation and a sequence of Devonian unnamed limestones, are in fault contact with upper Devonian siliciclastic rocks in the hanging wall of the Gold Quarry/Chukar Gulch fault. Locally termed the Quarry Member, this siliciclastic sequence includes laminated siltstone and lesser rhythmically bedded cherty-mudstone and siltstone. The bulk of the siliciclastic sequence has been dated as Frasnian to Famennian (uppermost Devonian); however, the upper portions of the unit have yielded Kinderhookian (lowest Mississippian) dates. Highly folded and thrusted limestone of Devonian and Silurian ages are found in thrust contact above the siliciclastic sequence. This folded and thrusted package is locally referred to as the "allochthonous limestone wedge". The next tectono-stratigraphic higher package of rocks is the upper-plate to a major thrust fault locally referred to as the "Roberts Mountains Thrust"(RMT). The locally termed RMT is only one thrust plane within a sequence of thrusts, which comprise the regionally recognized RMT zone. These upper-plate Caradocian (middle Ordovician) rocks include rhythmically bedded cherty mudstone and shale, laminated siltstone and lesser quartzite interbeds. A structural model for Gold Quarry has been developed based on an integration of detailed mapping, structural and ore-control data from the Maggie Creek sub district. A structural para-genesis, based on crosscutting relations of mapped faults and fault lineation, is proposed. Evidence for four main stages of faulting is noted at Gold Quarry: 1) Compression-driven thrusting and related folding; 2) Wrench-driven strike-slip, reverse and dilatational faulting; 3) Formation of collapse due to decarbonatization of host-rocks and associated normal faulting; and 4) Extensional tectonics resulting in normal faulting. The oldest faulting and folding events at Gold Quarry are low-angle; generally, east-dipping thrust faults and associated folds developed within the Devonian siliciclastic rocks and the overlying Ordovician siliceous rocks. The next sequence of faulting is wrench-related, driven by a north-northeast principal compressive stress. This episode is interpreted to be responsible for forming the Carlin window, developing in response to a reverse-fault accommodation of movement along the Gold Quarry left-lateral shear. The Gold Quarry deposit is located at the "cornerstone" or structural intersection of the Gold Hope reverse-fault and the Gold Quarry left lateral shear. In addition, dominant ore-control directions are controlled by N 10° W to N 200° W (350° to 340°) right-lateral faults of lesser magnitude positioned in a conjugate manner to the Gold Quarry fault system. An important structural event of the Gold Quarry main zone is volume-loss collapse of the system host rocks caused by extensive decarbonatization during hydrothermal alteration. Bakken (1990) documented 50% volume loss of the ore-host silty carbonate rocks at the Carlin deposit. A similar amount of volume-loss would be expected for decalcified and dedolomitized silty carbonate rocks in the footwall of the Gold Quarry Main zone. Volume-loss accommodation driven collapse and associated normal faulting, in addition to preexisting fracturing, rendered the otherwise poor host rocks of the siliciclastic sequence amenable to fracture-controlled ore-fluid penetration. Significant extension-driven normal faulting and associated rotation of the deposit region occurred during the development of the Basin and Range. The extension reactivated many pre-existing structures as normal faults, most notably, the northeast trending faults such as the Gold Quarry fault system. Dip-slip slickensides are common cross-cutting the low angle mullions and grooves on many of the northeast and north-northwest-trending faults. The post-ore Tertiary Carlin Formation is offset, as well, by normal faults.
Open Access
Measuring changes in areal extent of historic wetlands at Great Sand Dunes National Monument, Colorado 1936-1995
(Colorado State University. Libraries, 1998) Hammond, David J., author; Smith, Freeman M. (Freeman Minson), 1939-, advisor; Hoffer, Roger M., committee member; Sanford, William E., committee member
Great Sand Dunes National Monument (GRSA) is a unit of the National Park system in south central Colorado. With funding from the Colorado Historical Society, a series of studies were undertaken for an environmental history of the GRSA area and the San Luis Valley. Park managers were concerned over the disappearance of small wetlands in northwestern GRSA. The objective of this study is to document and analyze the changes to the wetlands through the study of digital, georeferenced images and to relate these changes to climatic and hydrologic factors. Ten sets of aerial photographs were obtained, from 1936 to 1995, with at least one set from each decade except for the 1940s. All photos were scanned into a digital format. A system was devised to mosaic the images prior to rectification due to the lack of ground control in the area. Land cover was digitized from the mosaics including the wetlands and sand type. Size and spatial distribution of the wetlands were analyzed. Analysis shows that the greatest total number of wetlands and acreage totals were present in the 1936 and 1937 photo sets. In 1937, 114 wetlands were found, 47% have water at the surface. By 1953 the total number of wetlands dropped to 38 and by 1975 only 22 remained, with only 1 having water at the surface. The total number of wetlands has increased in recent years primarily due to sub-irrigated meadows. A large increase in the vegetation cover has .occurred since 1936 to the present, increasing from 20% of the area in 1936 to 47% by 1995. Climatic data were collected to analyze possible causes of the changes to the wetlands but the study was limited by the lack of long-term data. Weather data is consistently available since 1948. Well data is of short term and sporadic nature. Two sources of long-term data were available. The discharge of the Rio Grande has been monitored in Del Norte, CO since 1906. A dendrochronology study was done in the area of GRSA in 1980. The precipitation data, discharge and dendrochronology data were summed, averaged or offset for monthly or annual intervals prior to the dates of the aerial photo sets. These values were correlated with the acreage of the wetlands for each of the photo years by means of linear regression. Very poor correlation resulted between the precipitation indices and wetland acreage. A surprising result came from correlation of the wetland acreage with the dendrochronology and discharge data. Over 58% of the wetlands variation can be explained by the 15 to 19 year offsets of dendrochronology data, 55% can be explained by the 20 year offset of Rio Grande discharge. These results are interpreted that the offset indices are related to the slow change in total area of the wetlands rather than fluctuations in the water table.
Open Access
Geology, hydrothermal alteration, paragenetic sequence and fluid inclusion analysis of La Maruja level, Marmato district, Caldas Department, Colombia
(Colorado State University. Libraries, 1998) Bedoya Sanmiguel, Oscar Geovany, author; Hannah, Judith L., advisor; Ramirez, Jorge A., advisor; Thompson, Tommy B., committee member
The Marmato goldfield is located in the Department of Caldas in the western Cordillera of Colombia. The area is topographically characterized by steep hills of abrupt relief. Rocks that range in age from the Cretaceous to the present appear in the area. Confined to the northeast of the area, the oldest units consist of sedimentary and volcanic rocks that have been metamorphosed to mid-amphibolite facies to amphibolites, hornblende schist and quartz-sericite schist. The metamorphic rocks form pendants within younger porphyry intrusions (Marmato Stock) and occur intermittently in a north-south belt along the Rio Cauca. The ore host at La Maruja level consists of dacite porphyry. A Late Miocene age has been assigned for this unit. This body has been affected by a tectonic event and it exhibits faulting, shearing and joint features. Structure is the primary control of ore mineralization. The fault surfaces are usually curved or arched and have a predominantly northwestern orientation with high dip angle. Sin Nombre, Porvenir, Santa Ines and Mucura are the veins at La Maruja level. The thickness of these veins can range from 0.30 to 2.5 m. The ore minerals of La Maruja level listed according to their relative abundance are: pyrite, pyrrhotite, arsenopyrite, sphalerite, chalcopyrite, gold, galena, marcasite and polybasite. The gangue minerals present are calcite, chlorite and quartz. Deposition of ore and gangue minerals in this level occurs in three stages. The early stage is characterized by abundant deposition of coarse euhedral pyrite crystals. The intermediate stage is characterized by abundant deposition of sphalerite and minor amounts of chalcopyrite and gold. A tectonic event occurred at the end of this stage. The late stage is a short period of fine-grained pyrite deposition which ended with great amounts of calcite and chlorite deposition. Silicic, sericitic, propylitic and albitic hydrothermal alteration assemblages occur at La Maruja level. Fluid inclusion homogenization temperatures, assumed to represent minimum ore fluid temperatures, range between 250°C and 350°C with mean values of about 310°C for Sin Nombre vein and 330°C for other veins; no evidence for boiling is present. The ore fluid salinity ranges from 3.7 to 10.3 equiv. wt% NaCl. Characteristics of the hydrothermal fluid in this level are: a relatively high temperature, moderate salinity and high in reduced sulfur.
Open Access
Sequence stratigraphic framework for top seal development: examples from the Skull Creek and Graneros shales, Denver basin
(Colorado State University. Libraries, 1999) Edwards, Kimberly, K., author; Sutton, Sally J., advisor; Ethridge, Frank G., advisor; Almon, William R., committee member
In general, the distal open marine shelf setting, typified by the Graneros Shale produces a rock with a greater and more uniform seal capacity relative to the rocks of a proximal open marine shelf setting, such as those of the Skull Creek Shale. A distal setting, which usually corresponds to the time of maximum transgression, may produce better seals because there is less coarse clastic sediment input, which allows slow deposition of clays from suspension to be the dominant depositional process. In this study, the higher capacity seal rocks occur in the upper parts of the TST, either within the condensed section or below it. The Skull Creek locations show seal occurrence to be stratigraphically higher on depositional topographic highs, and lower in areas that were topographically low at the time of deposition. Top seal capacity was quantified with mercury injection capillary pressure (MICP) analysis. Other physical characteristics of these marine shales were studied but only porosity, permeability, total clay, and hydrogen index consistently demonstrated a significant correlation with seal capacity in both units. Shales that are well laminated with a high percentage of total clay and/or total organic carbon with a type I-II (marine) kerogen may or may not qualify as the best seal. Top seal capacity may be more a function of rock fabric rather than mineralogy. For example, two samples may have exactly the same amount of quartz, as shown by XRD analysis, but thin section examination reveals that the majority of quartz in one sample is present as grains and in the other sample as cement. Samples with cement usually provide a better seal because they decrease the pore throat diameter, thus increasing the amount of hydrocarbons that can be trapped. Seal quality in both the Skull Creek and Graneros Shales is quite variable throughout each of the facies within the TST deposits.