Veins and pervasive wall-rock alteration composed of coarse muscovite +/- quartz +/- pyrite are documented for the first time in a porphyritic granite at Luhr Hill in the Yerington District, Nevada. Coarse muscovite at Luhr Hill occurs at paleodepths of similar to 6-7 km in the roots of a porphyry copper system and crops out on the scale of tens to hundreds of meters, surrounded by rock that is unaltered or variably altered to sodic-calcic assemblages. Coarse muscovite veins exhibit a consistent orientation, subvertical and N-S striking, which structurally restores to subhorizontal at the time of formation. Along strike, coarse muscovite veins swell from distal, millimeter-thick muscovite-only veinlets to proximal, centimeter-thick quartz-sulfide-bearing muscovite veins. Crosscutting relationships between coarse muscovite veins, pegmatite dikes, and sodic-calcic veins indicate that muscovite veins are late-stage magmatic-hydrothermal features predating final solidification of the Luhr Hill porphyritic granite. Fluid inclusions in the muscovite-quartz veins are high-density aqueous inclusions of similar to 3-9 wt% NaCl eq. and < 1 mol% CO2 that homogenize between similar to 150 and 200 A degrees C, similar to fluid inclusions from greisen veins in Sn-W-Mo vein systems. Our results indicate that muscovite-forming fluids at Luhr Hill were mildly acidic, of low to moderate salinity and sulfur content and low CO2 content, and that muscovite in deep veins and alteration differs in texture, composition, and process of formation from sericite at shallower levels of the hydrothermal system. Although the definition of greisen is controversial, we suggest that coarse muscovite alteration is more similar to alteration in greisen-type Sn-W-Mo districts worldwide than to sericitic alteration at higher levels of porphyry copper systems. The fluids that form coarse muscovite veins and alteration in the roots of porphyry copper systems are distinct from fluids that formed copper ore or widespread, shallower, acidic alteration. We propose that this style of veins and alteration at Luhr Hill represents degassing of moderate volumes of overpressured hydrothermal fluid during late crystallization of deep levels of the Yerington batholith.

Alteration of ultramafic rocks is ubiquitous to their occurrence near the surface. Primary mantle minerals like olivine and pyroxenes are unstable at low pressure and temperatures and undergo hydration (serpentinization), carbonation and weathering reactions forming hydrated and carbonated minerals. Employing a variety of analytical techniques including: electron microprobe, X-Ray diffraction, major and trace element geochemistry, Mg isotopes geochemistry and geochemical modelling this work seeks to contrain conditions ofa lteration and trace changes in composition of peridotite during alteration. In Wadi Fins in the southeast of Oman peridotites outcrop at the bottom of a canyon overlaid by a thick sequence 1.5 km of Cretaceous to Eocene shallow oceanic limestones and dolostones. The peridotites exhibit different types of alteration. While the common view is that serpentinization and carbonation of peridotites is isochemical this is not the case for alteration in Wadi Fins. Peridotites tens of meters below the unconformity are characterized by a striking grid of carbonate and serpentine veins. The calcite veins and relatively low MgO/SiO2 suggest that the peridotites reacted with a hydrous fluid derived from interaction of seawater with the overlying sediments composed of limestones with minor amounts of chert. This is further affirmed by average δ13C, δ18O and 87Sr/86Sr from carbonate veins in the peridotites that are similar to values of the sediments. Clumped isotope thermometry on calcite veins in peridotite establish that they formed at 25–60 ℃. Reaction path modeling of carbonate- quartz derived fluids with peridotite reproduces the observed mineral assemblage composed of carbonate and serpentine with similar Mg and MgO/SiO2 at high water to rock ratios, with carbon, H2O and silica added to the rock by the reacting fluid. Close to the unconformity the altered peridotites are characterized by concentric alteration halos recording variable fO2 and fS2. The partially serpentinized cores preserved primary minerals and record extremely low oxygen fugacities (fO2~10^(-75) bars). Two alteration zones are present evident from the alteration color. These zones exhibit nonisochemical alteration characterized by intergrowths of stevensite/lizardite. The alteration zones record progressively higher (fO2) recorded by Ni-rich sulfides and iron oxides/hydroxides. The alteration zones lost 20-30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron was reprecipitated in goethite intermixed with silicates due to higher fO2. The loss of magnesium in this peridotites motivated the final section of work. Mg isotopic compositions of partially serpentinized harzburgites and dunites in Oman are identical to average mantle and bulk silicate Earth (δ26Mg=-0.25‰) while altered periodites from Wadi Fins get heavier with increasing alteration. Analyses of peridotite alteration products including samples from Wadi Fins and carbonates from Wadi Tayin were used to show that isochemical serpentinization at low W/R does not fractionate Mg isotopes. I propose a mechanism that with increasing W/R and co-precipitation of Mg-carbonates and serpentine leads to carbonates with light isotopic compositions (Magnesite δ26Mg =-3.3 and dolomite δ26Mg=-1.91) and serpentine with heavy compositions (up to δ26Mg =-0.96 in serpentine veins). This complementary enrichment-depletion and the finite 14C ages of the carbonates suggest that serpentinization is ongoing along carbonation in Oman at ambient temperatures. Rates of calcite precipitation in travertines inferred from Δ26Mgcal-fl suggest that travertine formation in Oman sequesters a total of 10^6-10^7 kg CO2/yr.

Thesis (Ph. D.)--University of Oregon, 2003. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 211-235). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (Ph. D.)--Carleton University, 1999. / Also available in electronic format on the Internet.

Thesis (M.S. in Geology)--Vanderbilt University, Aug. 2006. / Title from title screen. Includes bibliographical references.

Multiple generations of normal faults dismembered, tilted, and exposed thicknesses of up to 15 km of the upper crust in portions of central and southern Arizona. This extension, variable in distribution and magnitude, was superimposed on the axis of the Laramide magmatic arc and dismembers many porphyry copper systems, allowing for detailed study of vertical and lateral zonation of alteration around these centers. This study examines tilted fault blocks containing portions of porphyry systems across Arizona, focusing on hydrothermal alteration deep and distal in these systems (3+ km paleodepth) to develop a more complete understanding of porphyry occurrences as larger geochemical systems. This study focuses on Na-Ca and coarse muscovite alteration in the roots of Laramide porphyry copper systems across Arizona (Ajo, Sierrita, Kelvin-Riverside, Mt. Grayback, Granite Mountain, Charleston, Globe-Miami, Sycamore Canyon, Copper Basin, Texas Canyon, and Copper Creek), provides a detailed study of Middle Jurassic coarse muscovite alteration at Luhr Hill in the Yerington district, Nevada, and documents the structural and hydrothermal evolution of the Ajo mining district in southwestern Arizona. Most areas in this study are interpreted to be highly extended, highly eroded, or both, allowing for study of deep hydrothermal alteration. Na-Ca alteration has been previously documented extensively along the Jurassic arc of the southwestern United States but less widely known in younger plutons, notably of Laramide age in Arizona. Coarse muscovite alteration previously has rarely been documented in porphyry copper systems, and this study shows that coarse muscovite alteration is likely present in systems where root zones are exposed at surface. Na-Ca alteration also is present in many in Laramide porphyry systems, though volumetrically minor, as no Laramide system contains more than a few volume perfect Na-Ca alteration in a given hydrothermal system. Na-Ca alteration in Laramide systems can include Ca, Na-Ca and Na alteration but is dominated by Na alteration (epidote-albite-chlorite ± actinolite). At Ajo, both Na-Ca and coarse muscovite alteration are present within the district due to superposition of temporally unrelated hydrothermal alteration, coupled with complex extensional deformation. This study shows that both Na-Ca and coarse muscovite alteration are more common in Laramide porphyry copper systems than previously recognized, that Na-Ca alteration is most commonly developed as shallower Na alteration (albite-epidote-chlorite ± actinolite), common deeper Na-Ca alteration (oligoclase-actinolite-epidote), and rare, deep Ca alteration (oligoclase-diopside-actinolite ± garnet ± epidote). Na-Ca alteration is commonly less voluminous in Laramide systems than documented in systems along the Jurassic arc. Coarse muscovite alteration, commonly termed greisen, occurs structurally below and commonly postdates potassic alteration and likely formed from late-stage, low-temperature, magmatic-hydrothermal fluids. Coarse muscovite alteration associated with more silicic magmatic compositions is developed at shallower depths and contains muscovite with higher trace element contents, coarse muscovite alteration with more variable mineral assemblages, and coarse muscovite veins that are better mineralized. Coarse muscovite alteration (greisen) occurs as the main mineralized veins at the tops of evolved metaluminous to peraluminous granites in W-Sn systems, as well mineralized veins in the cores and tops of Mo-Cu porphyry systems, and as poorly mineralized veins in the roots of porphyry copper systems. Detailed understanding of coarse muscovite alteration in a given district can, therefore, can be an indicator of depth or petrologic affiliation of a system. These results provide a better understanding of late-stage magmatic-hydrothermal alteration and hydrothermal alteration associated with the incursion of external fluids into the root zones of porphyry copper systems.

abstract: Mantle derived basalts along the entirety of the Earth’s Mid-Ocean Ridge (MOR) spreading centers are continuously altered by seawater, allowing the hydrosphere to subsume energy and exchange mass with the deep, slowly cooling Earth. Compositional heterogeneities inherent to these basalts—the result of innumerable geophysical and geochemical processes in the mantel and crust—generate spatial variation in the equilibrium states toward which these water-rock environments cascade. This alteration results in a unique distribution of precipitate assemblages, hydrothermal fluid chemistries, and energetic landscapes among ecosystems rooted within and above the seafloor. The equilibrium states for the full range of basalt compositional heterogeneity present today are calculated over all appropriate temperatures and extents of reaction with seawater, along with the non-equilibrium mixtures generated when hydrothermal fluids mix back into seawater. These mixes support ancient and diverse ecosystems fed not by the energy of the sun, but by the geochemical energy of the Earth. Facilitated by novel, high throughout code, this effort has yielded a high-resolution compositional database that is mapped back onto all ridge systems. By resolving the chemical and energetic consequences of basalt-seawater interaction to sub-ridge scales, alteration features that are globally homogeneous can be distinguished from those that are locally unique, guiding future field observations with testable geochemical and biochemical predictions. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2020

In exploration, a lithogeochemical approach can be used to aid the characterisation of rocks surrounding metamorphosed and hydrothermally altered deposits. Accurate description of the geological setting of deposits is crucial for understanding the ore forming processes and identifying targets for exploration. The Svärdsjö Zn-Pb-Cu deposit is located in the heavily mineralised and metamorphosed Bergslagen ore province of south-central Sweden. The deposit and surrounding minor occurrences were actively mined for over 500 years, producing more than 1 Mt of Zn-Pb-Cu-Ag massive sulphide ore. The combination of strongly metamorphosed and hydrothermally altered rocks in Svärdsjö makes geological interpretation challenging. Therefore, an approach combining lithogeochemical and petrographic methods is used in this study. The characterisation of the rocks and hydrothermal alteration surrounding the deposit allowed for an interpretation of ore formation and its implications for further exploration in the Svärdsjö area. The results verified that the Svärdsjö mineralisations are hosted by 2–15 m thick dolomitic marble units, commonly altered to skarn. Surrounding the deposit are subvolcanic intrusions and volcanoclastic rocks of mainly dacitic composition. The combined approach also helped identifying a strong to intense hydrothermal chlorite-sericite alteration enveloping the mineralised marble units and resulted in large mass gains of Fe and Mg whereas Na was depleted. Multiple episodes of alteration and metamorphism are evident from cross-cutting relationships with less altered dykes and overprint by metamorphic minerals such as cordierite and anthophyllite. An ore formation model involving sub-seafloor volcanic-associated replacement is suggested for the Svärdsjö deposit based on (i) the presence of a zoned hydrothermal alteration system within a volcanoclastic rock sequence and (ii) the irregular stratabound sulphide lenses hosted by thin marble units in the centre of the alteration system. Additionally, it is inferred that the stratabound nature of the deposit is caused by the neutralisation of a hot acidic fluid, resulting in precipitation of the sulphides within the marble. Finally, two geochemically distinct lithological units have been identified adjacent to the mineralised zones, providing new, larger exploration targets in the area. Mass change calculations reveal that Fe and Mg enrichment and Na depletion are useful vectors towards mineralisation, with detectable changes extending for up to 100 m from the mineralised lenses. These findings showcase the usefulness of the incorporation and careful interpretation of lithogeochemical data when exploring for metamorphosed hydrothermal ore deposits in mineralised provinces of the Fennoscandian Shield or elsewhere in the world.

This study area provides a unique opportunity to study the intersection of the Elsinore and West Salton detachment faults in southern California, effusing warm springs, and alteration products in the midst of the fault intersection. Structural mapping and compiling previous maps supply an interpretation of the fault zone geometries within the Tierra Blanca Mountains. Geochemical analysis of the crystalline basement and altered protolith help determine the effects of faulting and fluid flow in the study area. In the Tierra Blanca Mountains, the Elsinore strike-slip fault system transitions from the double-stranded Julian segment and Earthquake Valley fault in the northwest, to the single-stranded Coyote Mountain segment in the southeast. A network of cross faults striking northeast connects the fault segments. The Coyote Mountain segment encounters the inactive West Salton detachment fault in the study area. The detachment fault is a barrier to fluid flow and exhibits primarily brittle deformation, while the Coyote Mountain segment is a conduit for fluid flow along the northeastern flank of the Tierra Blanca Mountains. The damage zone of the Coyote Mountain segment reaches widths up to 500 m and contains intense fracturing and subsidiary faults striking parallel to the main trace. The tonalite protolith is bleached, stained, and altered from water-rock interactions. The most intense bleaching is at a county park, where the protolith is altered to clays and zeolites while the mineralogy of the stained regions contains iron oxides and clinochlore in addition to quartz, Ca-rich albite, and biotite preserved from the protolith. The water chemistry at Agua Caliente hot springs shows the fluid is partially equilibrated. Groundwater temperatures likely reached 75-85°C at depths up to 2.14 km before rising to the surface. Frequent seismicity in the study region is related to the spring characteristics including water level, conductivity, and temperatures. Spring temperature and conductivity displayed three behaviors during the summer 2011 logging period, attributed to seasonal changes and most likely local seismicity as well. Conductivity seems to be the property most influenced by earthquake activity in the area. Changes in fluid chemistry between sampling periods may indicate mixture with other fluid sources.

The Glass Buttes volcanic complex consists of many domes and individual vents that erupted both rhyolitic and basaltic lavas during the late Miocene to early Pliocene. The east half of the complex, in the vicinity of Little Glass Butte, contains interfingering, finely flow-banded rhyolite and black obsidian flows. The youngest unit, an obsidian, has been dated at 4.9 m.y. East of Little Glass Butte lie two northwest-trending ridges, Antelope and Cascade Ridges, composed of two or more overlapping exogenous domes that formed along northwest-trending faults.