Petrology and Geochemistry of Alteration Types Within a Multiphase System and Implications for the Presence of a Porphyry Root, Harrison Pass Pluton, Nevad

The Eocene (~36 Ma), calc-alkaline, granodiorite-monzogranite multiphase Harrison Pass Pluton (HPP) intrudes a sequence of meta-clastic rocks and meta-carbonates within the southern portion of the Cordilleran metamorphic core complex of the Ruby Mountains East Humboldt Range (RMEHR), northeastern Nevada. To understand the effects multiphase plutonic systems have on the petrology and geochemistry of alteration, and on the fluids responsible for alteration, a petrological and geochemical analysis of the various alteration types: potassic, chlorite sericite (CS), silicification, endo-skarn, and exo-skarn (recognized using field and petrographic observation), was conducted. Additionally, due to previously discovered alteration geometries, the HPP was also assessed for porphyry root characteristics (the part of the pluton beneath a porphyry ore body). Increased knowledge of porphyry root zones can enhance exploration techniques, and lead to undiscovered ore bodies.;Petrological analysis using Harker diagrams, shows most samples are geochemically typical of evolving calc-alkaline magmas, however numerous samples also plot off evolution trends. Additional analyses using REE spider diagrams revealed a more complex distribution of phases than previously recognized. Results indicate lenses of the early Toyn Creek granodiorite within the late two mica monzogranite unit are more abundant and spread to structurally lower levels than previously indicated, and the two mica monzogranite lenses spread to higher levels into the Toyn Creek granodiorite and Corral Creek monzogranite units. Even though the petrologic phases are clearly mapped (Barnes et al., 2001), the intermixed distribution of the Toyn Creek granodiorite and two mica monzogranite lenses throughout the HPP render field identification of the petrologic phases difficult.;REE and trace metal geochemical data was analyzed to determine mass balance of alteration. Well-defined patterns within data differentiated by alteration type, clearly show alteration signatures vary slightly between each unit. The variations are minor, but within a large data set could produce a wide data spread---therefore to further confine data spread when studying multiphase systems, data should also be differentiated by intrusion unit. Additionally, geochemical patterns are not affected in samples collected within a few meters of intrusion contacts, however more data is needed to further investigate and clearly understand the extent of interaction. Alteration types cannot be identified based off only one to three trace elements.;Fluid inclusion analyses of samples from each intrusive unit showed pressure corrected temperatures of the hydrothermal fluids responsible for alteration ranged 400--500 °C for potassically altered samples, 330 to 450 °C for CS samples, 330 to 500 °C for the endoskarn samples, and 320 to 400 °C for the silicified samples. Temperatures were highest in the middle of the pluton and decreased outwards, corresponding to alteration geometries throughout the system: potassic in the center, and CS along the flanks. Salinity of the hydrothermal fluids responsible for alteration were relatively low, ranging 0 to 7 wt. % NaCl and averaging 2.43 wt. %. Using median temperature values, isotope values of the hydrothermal fluids range from delta 18Owater= -12.8 to 14 ‰, and deltaD water data ranges -31 to -169‰. The mixing curve on the deltaD vs. delta18O graph, along with hydrogen isotope ratios increasing towards the center of the intrusion, indicate pluton scale fluid pathways: depleted deltaD meteoric waters circulate along the flanks of the system and rose through the middle of the pluton, interacting with magmatic waters and rocks higher in deltaD.;Rare earth element, trace element, and isotope data emphasize local fluid pathways (3--20 m), associated with the many skarn/limestone enclaves/xenoliths scattered within the HPP. Concentrations of trace elements (Y, Zr, and small amounts of TiO2, P2O5), REEs, and deltaD and delta18O ratios in samples collected near limestone/ skarn xenoliths, indicate geochemical transport towards or away from xenolith contacts. Within the HPP, skarn/limestone pods may be eroded away, or not revealed at the surface, therefore depending on the distribution of the skarn/ limestone xenoliths, these alteration patterns can repeat and overprint one another and may be misinterpreted, or overlooked. Addressing how hydrothermal alteration and interaction between fluids and the limestone/skarn pods could affect geochemical results, is especially important when attempting to identify and define multiple petrologic phases within a multiphase system, and in regions where limestone wall rocks are present.;An assessment for porphyry root features within the HPP revealed numerous areas containing features indicative of root zones, such as large quartz+/-feldspar+/-muscovite aplitic dikes and veins, potassic, CS, phyllic and endoskarn alteration. If an ore body formed above the HPP, the deposit could either be located to the west of the HPP (slid westward by the Ruby Mountain detachment fault), or displaced east under the adjacent Ruby Valley basin. However, additional field work within the HPP is necessary. (Abstract shortened by ProQuest.).