Browsing by Author "Ridley, John, advisor"
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Item Open Access Alteration of organic matter and copper mineralization in the Midcontinent Rift, USA(Colorado State University. Libraries, 2017) Schultz, Max, author; Ridley, John, advisor; Sutton, Sally, committee member; Kennan, John, committee memberTo view the abstract, please see the full text of the document.Item Open Access Changes in water chemistry and fluvial geomorphology from arsenic contaminated floodplains of Whitewood Creek and Belle Fourche River, South Dakota(Colorado State University. Libraries, 2023) Marr, Alexander E., author; Sutton, Sally, advisor; Ridley, John, advisor; Ross, Matthew, committee memberFrom 1877 to 1977 the Homestake Gold Mine in Lead, South Dakota released over 100 million megagrams (Mg) of arsenic rich mine waste into Whitewood Creek which joins the Belle Fourche River. The mine waste which contains arsenopyrite and other arsenic bearing minerals, is deposited along the floodplains of Whitewood Creek and the Belle Fourche River as overbank deposits and abandoned meander and channel fill. The introduction of mine tailings into these streams has impacted them chemically and geomorphologically for over 100 years. This study is a continuation of the work from Ji (2021) who focused on the long-term behavior of arsenic in the mine tailings. Her work involved sequential extractions of the tailings to determine the mineralogical setting of the arsenic and its rate of release. She also used statistical regression on historical data to estimate the physical and chemical removal of arsenic from Whitewood Creek's watershed. The focus of this study is to see how the tailings might have impacted the stream chemistry of Whitewood Creek and the Belle Fourche River by modeling mineral saturation indices of the stream and seep water through the geochemical modeling program, The Geochemist's Workbench. The Geochemist's Workbench was used to model the dissolution rate of arsenopyrite to calculate the rate of dissolved arsenic entering Whitewood Creek. Suspended arsenic entering Whitewood Creek was calculated using the dimensions of the creek bed, thickness of tailings, and density of arsenopyrite. In addition to chemistry, this study investigated the changes in the tailings and fluvial geomorphology of Whitewood Creek and the Belle Fourche River from 1948 to 2012. This was performed by using aerial photographs from 1971, which mapped locations of the tailings along the floodplains, and overlaying them with photographs from 1948, 1977, and 2012. Using GIS through ArcMap, the tailings and their portions that have been removed over time were digitized. Other fluvial parameters that have been determined and digitized are stream longitudinal profiles, sinuosity, contaminated floodplain width, channel migration, and total sediment deposition area. The mineral saturation indices of Whitewood Creek and the Belle Fourche River are similar to each other and differ at the most by around 2-3 orders of magnitude. The minerals that are supersaturated are mainly phyllosilicates (mostly clays), Fe, Cu and Al (hydr)oxides, and carbonates with minor sulfates and phosphates. Seep waters have lower mineral saturation indices, up to 10 orders of magnitude lower for Fe bearing minerals. The only arsenic bearing mineral that is calculated to be supersaturated is Ba3(AsO4)2; however, this mineral has not been observed in nature. Based on the range of possible arsenopyrite concentration in the contaminated sediment (15 to 0.11%), the calculation of dissolved arsenic being discharged out of Whitewood Creek ranges from 52 to 0.39 Mg per year. This range compares to Ji's (2021) daily dissolved arsenic rate range of 3.89-0.33 Mg/year. For a tailings width range of 0.6 to 3.5 m, the calculated rate of suspended arsenic being discharged ranges from 254 to 1.98 Mg per year. Although large, this range encompasses Ji's (2021) suspended arsenic transport rate range of 33 to 70 Mg per year. The overlap of values from Ji's (2021) statistical approach and this study's geochemical approach indicates that arsenopyrite may be to some degree significant in controlling As transportation in Whitewood Creek. Based on GIS results, the location and evolution of contaminated floodplains along Whitewood Creek and the Belle Fourche River are very complex. The streams are different from each other and behave as their own systems. In Whitewood Creek, locations with high tailings area and removal are controlled by a possible range of factors such as knickzone geomorphology, bedrock lithology, and changes in stream energy due to topography. In the Belle Fourche River, reaches with high tailings area and removal are found about 7 km from the Whitewood Creek confluence and a 30 km stretch where rapid floodplain reworking occurs due to neotectonics from Precambrian basement adjustments. Tailings removed area and contaminated floodplain width graphs show that the Belle Fourche River has larger storage for tailings and undergoes more floodplain reworking due to higher flood frequency and neotectonics. In contrast, Whitewood Creek has lower storage and erosion due to decreasing mine sediment load at least since 1948 and channel incision into shale bedrock in some reaches. While the reworking of tailings into the stream is lower in Whitewood Creek than the Belle Fourche River, the tailings will remain on the floodplains for many generations.Item Open Access Characterization and prediction of long-term arsenic mobility, dissolution, and kinetic behavior in arsenic contaminated floodplain deposits of Whitewood Creek and the Belle Fourche River, South Dakota(Colorado State University. Libraries, 2021) Ji, Mu, author; Ridley, John, advisor; Stednick, John, committee member; Borch, Thomas, committee member; Gallen, Sean, committee memberFrom 1877 to 1977, the Homestake Mine discharged over 100 million tons of arsenic-rich mine-wastes from Lead, South Dakota into Whitewood Creek (WWC), which joins the Belle Fourche River (BFR). Arsenopyrite and other arsenic-bearing minerals were deposited in tailings (containing between 0.12% to 0.35% arsenic) and mixed with uncontaminated alluvium along the floodplains of WWC and the BFR as overbank deposits and filling abandoned meanders. Since it is not feasible to remove millions of tons of contaminated sediments from the area, an understanding of arsenic mobility on long timescales is vital. Many studies have laid the framework for factors controlling arsenic mobility appropriate to fluvial sedimentary systems; investigating mechanisms of arsenic mobilization, adsorption/desorption kinetics, and the effects of pH, changing redox conditions, etc., however, these studies were conducted on relatively short time scales and did not quantify arsenic mass-budget on field-scales. This study focuses on the long-term retention, dissolution, and kinetic behavior of arsenic from mine tailings. The uniqueness of this site enables arsenopyrite dissolution behavior to be constrained over a 135-year timespan (1877-2012). This allows for the investigation of changes in arsenic's residence sites, its rate of release into the environment, calculation of its transport mass-budget, and elucidation of how natural processes have or have not remediated arsenic contamination over a span of 35 years since the deposition of tailings have ceased (1977-2012). For this investigation, sediment, surface water, and seep water samples were collected along reaches of WWC and the BFR for analysis of arsenic and other geochemical constituents. Sequential extractions of the sediments were performed to determine the mineralogical setting of the arsenic as well as the proportion of arsenic available at different rates of release into the environment. Additionally, various historical data (discharge levels, geochemical analyses of water and sediment samples) were compiled from the United States Geological Survey database. Regressions were applied to historical data to estimate the rate of physical and chemical arsenic removal from the WWC watershed. Sediments collected along the floodplains of WWC and the BFR exhibited arsenic concentrations ranging from approximately 100 to 4,000 mg/kg. The results from the sequential extractions applied to the sediments suggest arsenic is predominantly located in residence sites that are not easily accessible, and arsenic is not readily mobilized or released into solution in large quantities under normal environmental conditions seen in WWC and the BFR. An average of 16% of the arsenic is weakly bound to readily exchangeable surface sites, water-soluble secondary minerals and available for rapid release, or is adsorbed to exchange sites that easily exchange PO43- ions for adsorbed arsenic oxyanions, is weakly bound in amorphous to poorly crystalline fine-grained metal oxides/hydroxides, reducible phases, and easily soluble carbonates. An average of 24% of the arsenic is moderately strongly bound in weakly soluble secondary minerals like clays or crystalline fine-grained metal oxides/hydroxides and will be released relatively slowly with time. The remaining 60% of arsenic is interpreted to be relatively immobile and locked in arsenopyrite in part due to the formation of metal oxyhydroxide coating, which slows down the degradation of the mineral. These interpretations are supported by the elevated but still relatively low total arsenic concentrations (EPA MCL for arsenic is 0.01 mg/L) of in-stream water in WWC (averaging 0.037 mg/L) and in the BFR (averaging 0.021 mg/L), considering that in-stream sediments carried by WWC and the BFR have high arsenic concentrations (264 to 694 mg/kg). Based on regressions applied to 30 years of historical sediment transport and arsenic concentration in solution and in sediment load (1982-2012), the average annual total arsenic load transported out of WWC during these 30 years was estimated to be between 34 to 71 megagrams (Mg) per year. At this rate, based on the 17,400 to 50,800 Mg of arsenic that remain in storage along the floodplains of WWC, complete arsenic transport out of the floodplains of WWC would range between 250 to 1,500 years. The actual rate of arsenic removal is expected to be longer because the model is based on a uniform movement of uniformly distributed sediment, and historical patterns may not be reflective of future trends, as evidenced by the decline in suspended arsenic transport rate starting in the early- to mid-1980s. The constant shifting of the stream creates abandoned meanders along WWC that can store contaminated sediment where the stream no longer has access. Conversely, as the meanders shift over time, the once-abandoned meanders could be again accessed by WWC. The majority of suspended sediment transport occurs during flood events; approximately 88% of the total arsenic load moved during the years between 1983 to 2012 occurred in only 3 of the years (1983, 1984, and 1995). Thus, the rate of arsenic transport for the next 30-year period is uncertain and could be lower if the number of flood events remains low. Although the WWC area once experienced heavy environmental degradation during the period of active mining, natural processes have allowed for relatively stable current environmental conditions. However, the physical transport of arsenic-contaminated sediment and the slow release of arsenic to the environment endures downstream to the BFR into the Cheyenne River and Lake Oahe and will continue for many generations.Item Open Access Field, fluid inclusion and isotope chemistry evidence of fluids circulating around the Harrison Pass pluton during intrusion: a fluid model for Carlin-type deposits(Colorado State University. Libraries, 2012) Musekamp, Charles Oliver Justin, author; Ridley, John, advisor; Sutton, Sally, committee member; Myrick, Christopher, committee memberThe ~60 km, northwest, southeast striking Carlin trend of Northeastern Nevada is host to approximately 40 Carlin-type gold deposits including a number of world class gold deposits. The ~36 Ma Harrison Pass pluton, located in the Ruby Mountains East Humboldt Range in Northeastern Nevada was emplaced along the Carlin trend during back arc-style magmatism between 40 and 32.4 Ma. This timing of plutonic magmatism was also contemporaneous with the regional hydrothermal event responsible for the ~42 to 33 Ma Carlin-type gold mineralization, but an acceptable explanation for the origin and source of fluids responsible for transporting gold remains outstanding. Through a multi-component field and geochemical study of the Harrison Pass pluton, magmatic-meteoric fluid mixing, after Muntean et al. (2011), is supported to explain the composition and origin of fluids responsible for deposition of gold in Carlin-type gold deposits along the Carlin trend. Using fluid inclusion and ō18O and ō13C data combined with field relationships and petrology, a fluid history detailing fluid activity before intrusion, during intrusion (Early Stage) and after intrusion (Late Stage) was constructed. Before intrusion, calcite veins within distal unaltered sedimentary siliciclastic and carbonate rocks were formed from connate waters. Fluids within these veins and wall rocks display typical ō18O (~27 per mil) and ō13C (~2.5 per mil) values of unaltered limestones. Type I (H2O-NaCl-KCl), primary inclusions suggest that fluids are low salinity (~1% Mass% eq. NaCl) and were trapped at low temperatures (Tt~195-340°C). During intrusion and cooling of the Harrison Pass pluton, primitive, hot magmatic volatile phases were expelled and are interpreted to be responsible for the formation of miarolitic cavities, skarn, phyllic, and potassic alteration of wall rocks, and many quartz and calcite veins proximal to the pluton contact. Evidence for magmatically derived fluids around the pluton is provided by high homogenization temperatures of Type III (H2O-CO2-NaCl-KCl), CO2-rich inclusions in miarolitic cavities, and vapor-rich Type II (H2O-NaCl-KCl) inclusions in hydrogrossular and quartz within skarn wall rocks and quartz veins. Further corroboration is provided by near magmatic ō18O and ō13C values (~13 and -0.25 per mil) of skarn wall rocks and calcite-polymetallic sulfide veins. A later, cooler convecting meteoric phase (Late Stage) driven by the heat during and after intrusion is observed within thick, fault hosted, steeply dipping quartz veins and vugs crosscutting the Harrison Pass pluton and in skarn wall rocks. It is interpreted, after Muntean et al. (2011), that Type II vapor-rich, primary inclusions found within Early Stage quartz veins, miarolitic cavities, and skarn wall rocks represent the vapor-rich magmatic phase in which Au and other base metals were transported. As the vapor-rich fluid rose through the crust, it would have evolved and cooled and may be represented by the less vapor-rich, Early Stage, Type I inclusions within phyllically altered wall rocks and calcite-polymetallic sulfide veins. Further cooling and ascent of this fluid would have interacted with convecting meteoric waters at shallower depths. At this level, all fluids would have undergone some mixing, which is broadly supported by the wide range of recalculated ō18O values in this study. As the magmatic phase becomes more diluted by circulating meteoric convection (Late Stage), low salinity (~3% Mass% eq. NaCl) and low temperature (Tt~200 to 480°C) secondary inclusions in skarn altered wall rocks in close proximity to the contact and major fault-hosted quartz veins/vugs crosscutting the Harrison Pass pluton are trapped. Infiltration of meteoric fluids around the contact is supported by mixed meteoric-connate ō18O and ō13C signatures in skarn wall rocks and calcite veins. It is speculated that mixed magmatic-meteoric fluids were then transported and focused along high angle faults along the flanks of the Harrison Pass pluton where a mixture of further cooling, oxidation and fluid-rock reactions resulted in gold deposition with pyrite and arsenopyrite within sedimentary country rocks along the Carlin trend.Item Open Access Fluid inclusion and metal ratio analysis of Cordilleran Pb-Zn-Cu-(Ag-Au) veins of the Montezuma district: Summit County Colorado, USA(Colorado State University. Libraries, 2015) Pyanoe, Dominic, author; Ridley, John, advisor; Egenhoff, Sven, committee member; Wilson, Bob, committee memberEvidence from fluid inclusion microthermometry in Pb-Zn-Cu-(Ag-Au) veins and district scale metal ratio zonation analysis indicate that the Cordilleran veins of the Montezuma mining district Summit County, Colorado, USA are indicative of subepithermal setting about a central hydrothermal source. Cordilleran-type polymetallic mineralization is a class of ore deposits that are spatially and temporally related to felsic igneous centers and can also be genetically related to porphyry mineralization (Fontboté and Bendezú, 2009). At Montezuma, the Teritary-aged Montezuma Stock is cross cut by several Cordilleran-type veins and is spatially related to over 80 additional veins hosted in Precambrian country rock. Five stages of mineralization in veins are identified: Stage 1. early quartz-pyrite, Stage 2. barite-incipient base metals Stage 3. base metals, Stage 4. carbonates and Stage 5. late quartz-lead-silver. There is a systematic decline in precipitation temperatures from 341 to 156°C along the progression of the paragenetic sequence, which suggests the waning of a source pluton. District scale metal ratio zonation maps from historical production data support the interpretation of a central magmatic source and that thermal decline is the primary control on ore deposition. Two district scale zones are identified: a copper rich zone (CRZ) in the center of the district, which is surrounded by a copper poor zone (CPZ). With distance from the inferred center of the district, there is a general decline in copper abundance relative to lead and silver. Thermal gradients accompanied by a decrease in metal solubilities are the mechanism for this zonation pattern, but developed late in the paragenesis. Other chemical and physical controls of phase separation, ligand removal, dilution and pH increase are likely present during vein mineralization as well. Approximate salinities ranged from 11.69 to 3.70 wt.% equivalent NaCl and showed less systematic patterns, and may reflect these additional processes. Temperature decline and variable additional depositional processes are consistent with analogous Cordilleran-type vein fields, which have proven links to a magmatic source and possible underlying stockwork porphyry base metal mineralization. Therefore, data from this study indicates that there is most likely porphyry Mo mineralization under the copper rich zone, but this may be sub economic in nature.Item Open Access Fluid inclusion study of the Mesoproterozoic Nonesuch Formation - biogenic sources and thermal history of oil(Colorado State University. Libraries, 2011) Colbert, Sarah Janette, author; Ridley, John, advisor; Egenhoff, Sven, committee member; Kennan, Alan, committee memberThe Nonesuch Formation is part of the Mid-Continent Rift System and is unusual because it contains a relatively high amount of oil that is thought to have formed in situ during the Mesoproterozoic, approximately 1.1 Ga. In this study, primary, pseudosecondary and secondary oil inclusions in samples obtained from Nonesuch Fm outcrop and cores were analyzed using petrography and microthermometry, and one core sample was analyzed by GC-MS. Aqueous inclusions were also studied via petrography and microthermometry. The inclusions studied were hosted in sandstone grains and matrix from parts of the Nonesuch Fm and the upper part of the Copper Harbor Fm, diagenetic calcite nodules from the Marker Bed of the Nonesuch Fm and calcite veins in the Nonesuch and Upper Copper Harbor Fms that formed no later than 30 Ma after deposition. Based on these settings, it can be assumed that all inclusions studied were entrapped during the Mesoproterzoic around the time of deposition of the Nonesuch Fm. The biomarkers detected by GC-MS have a Proterozoic character and the presence of mid-chain substituted monomethyl alkanes as well as 1,2,5-TMN indicates a cyanobacterial hydrocarbon source. Algal biomarkers have also been found in the oil in previous studies. The ratios calculated from the GC-MS data suggest an early- to peak-oil window maturity for the hydrocarbons, which is consistent with data from previous studies. The homogenization temperatures obtained by microthermometry are typically used as an estimate of inclusion entrapment temperature; however, in this study, the wide range of homogenization temperatures along with an inconsistency between these and the GC-MS maturity indicators implied that the thermal history of the oil was more complex. The inclusions were likely trapped during diagenesis at temperatures between 100-130°C, which agrees with the evidence from the maturity ratios and previous work on the Nonesuch Fm, and then reheated by hydrothermal activity after entrapment. The microthermometrical evidence implies that the second period of heating raised temperatures to levels exceeding 250°C, and other studies of the Mid-Continent Rift area suggest that this secondary heating occurred either soon after diagenesis or significantly later, around 200-300 Ma.Item Open Access Geochemical and mineralogical investigation of breccias at the El Niño Au-Ag Deposit, Elko County, Nevada(Colorado State University. Libraries, 2017) Barker, Rocky, author; Ridley, John, advisor; Singleton, John, committee member; Bareither, Christopher, committee memberTo view the abstract, please see the full text of the document.Item Open Access Geochemistry and characteristics of brecciation at the lamprophyre-hosted sapphire deposit at Yogo Gulch, Montana(Colorado State University. Libraries, 2022) Cotterell, Tracey, author; Ridley, John, advisor; Singleton, John, committee member; Blackburn, Heather, committee memberTo view the abstract, please see the full text of the document.Item Open Access Hydrothermal fluid and ore paragenesis of the gold-bearing Rattlesnake Hills Alkaline Complex, Wyoming(Colorado State University. Libraries, 2012) Ripple, Ashley, author; Ridley, John, advisor; Hannah, Judy, committee member; Sale, Tom, committee memberTo view the abstract, please see the full text of the document.Item Open Access Organic geochemistry of Mesoproterozoic Nonesuch Formation at White Pine, Michigan, USA(Colorado State University. Libraries, 2012) Fourgani, Aiyda Ibrahim, author; Sutton, Sally, advisor; Ridley, John, advisor; Cavdar, Gamze, committee memberThe quality and quantity of the preserved organic matter (OM) in the Mesoproterozoic Nonesuch Formation at White Pine are evaluated in this project. Specifically, I have considered whether the rocks had source rock potential and whether there is a relationship between the OM and copper mineralization. The copper mineralization and hydrocarbons migration pathways are hypothesized to be related. There are three possibilities for the relationship. The copper ore fluid may have migrated with the hydrocarbons. The copper may also have precipitated where the hydrocarbons had accumulated, or the copper precipitated where there were accumulations of OM. Three cores (42C, 37F, and 30G) from in or near the White Pine mine were described and analyzed. The sampled core intervals are mostly from the Lower Nonesuch Formation with some from higher intervals. The overall lithology is gray laminated siltstone, with some sandy siltstone and lesser shale and sandstone. The core samples have various colors, with brown to dark brown samples hosting organic matter. The most abundant minerals are quartz, feldspar (plagioclase, orthoclase), mica, and some rock fragments; calcite and chlorite are mostly found as cement. Various analyses were done to investigate the organic matter. For estimating the maturity, kerogen type, and potential source rock quality, samples were subjected by the Rock Eval pyrolysis. Also other techniques were used for evaluating maturation, including ultraviolet microscopy and vitrinite reflectance microscopy; it was determined that the majority of samples have little to no vitrinite-like material. The organic matter as analyzed by the UV microscope is observed to be of three types, kerogen, bitumen, and oil inclusions. The oil inclusions are mostly found in the sandy siltstone samples. The organic matter is mostly not fluorescent possibly because it is overmature or immature; it contains less than about 10% pyrolyzable hydrocarbons. The organic matter may have been produced from remains of organisms like algae and fungus deposited within a lacustrine and/or transitional marine environment. The generative potential of the organic matter is in the poor to fair range. The range of TOC (total organic carbon) content is between 0.01 and 0.86 wt %. The highest value is detected above the mineralization zone in the Upper Nonesuch Formation. The kerogens of the Nonesuch Formation are types III and IV, types that usually are gas prone, or have no hydrocarbon potential. The samples may have been oxidized by copper bearing fluids which altered the organic matter and reduced its potential to produce hydrocarbons. Overall, the organic matter of the lower Nonesuch Formation at White Pine has no potential to produce hydrocarbons.Item Open Access Petrology and geochemistry of alteration types within a multiphase system and implications for the presence of a porphyry root, Harrison Pass Pluton, Nevada(Colorado State University. Libraries, 2017) Racosky, Alexandra, author; Ridley, John, advisor; Sutton, Sally, committee member; Bareither, Christopher, committee memberTo view the abstract, please see the full text of the document.Item Open Access The magmatic-hydrothermal fluid history of the Harrison Pass Pluton, Ruby Mountains, NV: implications for the Ruby Mountains-East Humboldt Range metamorphic core complex and Carlin-type Au deposits(Colorado State University. Libraries, 2016) Gates, Christopher Harry, author; Ridley, John, advisor; Sutton, Sally, committee member; Strauss, Steven, committee memberIntrusion of the ~36 Ma, calc-alkaline, granodiorite-monzogranite Harrison Pass Pluton (HPP) occurred as magmatic fronts migrated southwest across the Great Basin during the Eocene. The HPP was locally intruded into the Ruby Mountains-East Humboldt Range (RMEHR), a classic Cordilleran metamorphic core complex that would undergo rapid tectonic exhumation during the late Cenozoic. Although the emplacement depth of the HPP provides an estimate for the magnitude and timing of subsequent uplift, disagreement exists between published mineral thermobarometry data and stratigraphic reconstructions. Synchronous with emplacement of the HPP was a regional hydrothermal fluid event responsible for deposition of >200 Moz of Au in sediment-hosted Carlin-type deposits (CTD's) along four linear trends. Magmatic, meteoric, and metamorphic models have been invoked to explain the origin of fluids and Au for CTD's, but few studies have directly examined the fluids generated by a potential source intrusion such as the HPP. Investigation of the magmatic-hydrothermal fluid history of the HPP, particularly the pressure-temperature conditions of fluid entrapment and fluid geochemistry, is an effective means of testing and improving existing models for the development of the RMEHR metamorphic core complex and for the origin of CTD's. Field and petrographic observations of pegmatites, aplites, miarolitic cavities, quartz veins, and multiple types of hydrothermal alteration, coupled with data from fluid inclusion microthermometry, LA-ICP-MS fluid inclusion geochemistry, and oxygen stable isotopes from magmatic and hydrothermal quartz, demonstrate that two-stage intrusive assembly was paralleled by a two-stage magmatic-hydrothermal fluid system. Early stage fluid activity was dominated by two aqueous, low salinity (~3 wt % eq. NaCl), B-Na-K-Rb- Sr-Cs-bearing, ore metal-poor fluids. These fluids were entrapped at ~600-700°C and ~2400-7600 bar in pegmatites, miarolitic cavities, and quartz veins within early stage units, as well as in quartz and calcite veins in base-metal skarns along the pluton margin and in the contact metamorphic aureole. Late stage fluid activity was dominated by one aquo-carbonic, low salinity (~3 wt % eq. NaCl), B-Na-K-Rb-Sr-Cs-bearing, ore metal-poor fluid. This fluid was entrapped at 570-680°C and ~4800-7200 bar in pegmatites, aplites, and quartz veins, and did not migrate out of late-stage intrusive units. Magmatic δ18O values for quartz demonstrate that this magmatic-hydrothermal fluid system evolved without significant dilution from meteoric inputs until the late influx of post-intrusion hydrothermal fluids, interpreted to be of mixed magmatic-meteoric origin. These fluids were aqueous, low-temperature (320-410°C), low salinity (<4 wt % eq. NaCl), and were entrapped at <2400 bar in fault-hosted microcrystalline quartz veins.The entrapment conditions for early stage magmatic-hydrothermal fluids determined from fluid inclusion microthermometry data indicate that the HPP was emplaced at depths of 9-18 km in the Ruby Mountains-East Humboldt Range. Brittle-ductile deformation of the HPP on the regionally-exposed Ruby Mountain Shear Zone indicate that at least 9 km of vertical exhumation has occurred since the intrusion of the HPP. Such emplacement depth estimates are consistent with published mineral thermobarometry from the HPP and from nearby metamorphic rocks. It is interpreted that the disparity between these estimates and the relatively shallow minimum emplacement depths of 4-6 km suggested by stratigraphic reconstructions is supportive of the existence of poorly preserved, Mesozoic thrust sheets that augmented the thickness of the overlying rock package during the Eocene. Although a well-accepted model for the origin of fluids and Au for CTD's remains outstanding, the model of Muntean et al. (2011) argues for the separation of a low-salinity, vapor-rich, Au-partitioning fluid from a high-salinity, base metal-partitioning fluid in mid- crustal magma chambers as a critical process in the evolution of CTD ore fluids. Although HPP emplacement depths and the existence of a robust magmatic-hydrothermal fluid system are broadly supportive of this model, no evidence of a high-salinity fluid was observed. Low concentrations of base metals (Cu, Pb) and CTD pathfinder elements (As, Tl) relative to whole-rock values are not consistent with the efficient fluid partitioning of metals invoked by Muntean et al. (2011). Also, low concentrations of CTD pathfinder elements relative to published values for CTD ore fluids indicate that the HPP was not Au-enriched. However, similar salinities and δ18O values suggest that HPP fluids may represent the minor magmatic component of ore fluids detected at some CTD's, but other fluid inputs and an external Au source would be required to produce these ore fluids. Thus, it is suggested that the magmatic-hydrothermal fluid history of the HPP is more consistent with a dominantly amagmatic fluid model for the origin of CTD's.Item Open Access Variation in clinopyroxene texture, composition, and crystallization depth of Late Cretaceous to Early Eocene lamprophyric rocks from alkaline calc-alkaline magmatic complexes of Montana, USA(Colorado State University. Libraries, 2022) McCane, Jacob, author; Ridley, John, advisor; Bareither, Christoper, committee member; Scarberry, Kaleb, committee member; Sutton, Sally, committee memberLamprophyres are spatially and temporally associated with many types of hydrothermal ore deposits and have been argued to be a marker in mineral exploration. A reconnaissance study of lamprophyric rocks in central Montana, namely the Central Montana Alkalic Province (CMAP) is presented that details the variations of petrographic textures and clinopyroxene (Cpx) mineral chemistry from different Late Cretaceous to Early Tertiary aged alkalic to calc-alkaline magmatic complexes. Using current International Union of Geological Sciences (IUGS) standards, lamprophyric rocks studied from the Late Cretaceous Bull Mountain/Golden Sunlight mining operations are found to be predominately minette > vogesite > kersantite varieties of lamprophyres. The Highwood Mountains lamprophyric rocks of Eocene age are classified as vogesites > sannaites > minnettes ≈ spessartites. Crazy Mountain (Crazies) lamprophyric rocks of Eocene age are monchiquites > vogesites > kersantites > spessartites. The sapphire-bearing Yogo dyke lamprophyre of Eocene age is classified as an ouachitite. Lamprophyric rocks occur as dykes and display porphyritic and panidiomorphic textures with dominant macrocrysts of Cpx, biotite, with lesser olivine and hornblende that range in size from a few centimeters in length to microcrystalline (<10 µm) matrix constituents. Leucocratic circular ocelli structures as well as bent and strained crystals of biotite are present within all lamprophyric rocks from the CMAP. Extreme heterogeneity exists within lamprophyres ranging from macroscale dyke swarm features to microscale Cpx macrocrysts assemblages captured in a single thin section. Cpx from every region of the CMAP display intricate optical zonations associated with complex compositional variations across core to rim analytical traverses and display disequilibrium features like spongy textured melt pockets. Within the CMAP the Cpx classify as diopside, Fe-rich diopside, Ca-rich diopside or augite and do not contain uniform textures or zonations. Cpx of the Crazies are strictly diopsides and the only region with augite is Bull Mountain/Golden Sunlight. Cpx macrocrysts from each region of this study are separated into Types based off of differing textural features from back scatter electron imaging and under the petrographic microscope. All Types across the CMAP are predominately normally zoned and are interpreted as antecrysts or xenocrysts that host compositions and textures that record disequilibrium with the host magma during crystallization. B Types of Bull Mountain/Golden Sunlight are enriched in Cr2O3 and FeO. H Types from the Highwoods are similar to B Types but record Step-zoning characteristics and contain the greatest proportion of both normal and reverse zoning characteristics. C Types of the Crazies record lower values of TiO2, Al2O3, CaO, and Fe2+ /Fe3+ ratios but have higher Na2O levels than all other Types of the CMAP. Y Types of the Yogo Dyke host the most ultramafic signatures with elevated Mg#, TiO2, Al2O3, Cr2O3, and CaO levels along with the lowest SiO2 and FeO composition of all Types of the CMAP. Barometric estimates of the pressure of crystallization for Cpx are presented from the CMAP, mean crystallization estimates are as follows: Bull Mountain/Golden Sunlight = 15.1 ± 1.5 kbar (56 km), Highwood Mountains = 11.4 ± 1.5 kbar (42 km), Crazy Mountains = 7.1 ± 1.5 kbar (26 km), Yogo Dyke = 14.3 ± 1.5 kbar (53 km). Cpx macrocrysts crystalized near the lower crust-mantle boundary and barometry from Bull Mountain/Golden Sunlight likely records a deeper magma reservoir and plumbing system than previously thought for the Boulder Batholith. Lamprophyric rock petrogenesis cannot be explained by simple fractionation processes because the antecrysts do not follow regular fractionation trendlines. Multiple complex open system magmatic processes are likely at play when controlling lamprophyric magma composition. Drastically different compositional variations of C Types compared to others of the CMAP confirm that the parent magma of lamprophyric rocks originating from the Crazies is significantly different than other areas of this study. Primary melts from the Crazies and the Highwoods likely had an enriched composition and heavy metasomatic influence. Evidence for multiple recharge and mixing events exists within all lamprophyric rocks of the CMAP. Hybridization of mafic and felsic magmas likely influenced the petrogenesis of lamprophyric rocks from Bull Mountain/Golden Sunlight, the Highwood Mountains, the Crazy Mountains, and the Yogo Dyke.