Cathodoluminescent quartz textures and fluid inclusions in veins of the porphyry copper-molybdenum deposit in Butte, Montana: Constraints on physical and chemical evolution of the hydrothermal system

Detailed study of quartz veins from the porphyry copper deposit in Butte, Montana provide insight into processes in magmatic-hydrothermal systems. Fluid inclusions trapped in vein quartz were analyzed using optical petrography, microthermometry, scanning electron microscopy, Raman spectroscopy, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Textural features of quartz veins were studied using scanning electron microscope-cathodoluminescence (SEM-CL).; Most fluid inclusions trapped in veins with potassic alteration from deeper than 1500m below the surface contain 35 volume% bubble and homogenize to liquid between 330° and 370°C. Ice and clathrate melting temperatures indicate 2–5 wt.% NaCl equiv. and 3–6 mol% CO₂. LA-ICP-MS indicates that these fluids contained up to 1.3 wt.% copper. These Cu-rich magmatic fluids were trapped between 550° and 650°C and between 2000 and 3000 bars. Mosaics of fine-grained quartz with concentric growth zones and annealing textures revealed by SEM-CL, suggest that these veins formed upon depressurization followed by repressurization.; Veins with potassic alteration envelopes from depths shallower than 1500m are dominated by vapor-rich and halite-bearing fluid inclusions, but contain other inclusion types as well. Halite homogenization more than 100°C above bubble homogenization in many halite-saturated inclusions indicates post-entrapment inclusion modifications, invalidating their implied trapping conditions. Clathrates occur in vapor-rich inclusions that homogenize up to 550°C, indicating that halite-saturated and vapor-rich fluids formed from unmixing of CO ₂-bearing magmatic fluids between 450° and 650°C and between 500 and 900 bars. Complex CL textures in these veins indicate repeated fracturing and quartz dissolution.; Late pyrite-quartz veins with sericitic alteration envelopes contain halite-bearing inclusions and vapor-rich inclusions, but are dominated by inclusions with 60 volume% bubble that homogenize near-critically to vapor or liquid between 370° and 410°C. Clathrate and ice melting temperatures indicate 2–5 wt.% NaCl equiv., and 4–8 mol% CO₂. Pyrite-quartz veins formed from magmatic fluids between 370° and 450°C, and between 400 and 800 bars. Ubiquitous dissolution textures, and euhedral quartz growth revealed by SEM-CL indicate that these veins formed as hydrothermal fluids cooled through and beyond the interval of retrograde quartz solubility at or near hydrostatic pressures.; This dissertation includes previously published and co-authored material.