HR: 0800h
AN: V41D-0816    [Abstracts]
TI: Insights Into the Formation of Deep Hydrothermal Quartz From the Porphyry-Copper- Molybdenum Deposit at Butte, Montana
AU: * Mercer, C N
EM: cmercer@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AU: Reed, M H
EM: mhreed@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, United States
AB: We combine SEM-cathodoluminescence (SEM-CL) images of magmatic and hydrothermal quartz with trace elements and quartz precipitation temperatures to elucidate the formation of deep quartz veins in a porphyry-style deposit. Trace elements (Ti, K, Al, and Fe) were measured by EPMA along traverses crossing CL textural boundaries and quartz precipitation temperatures were calculated using the Ti in quartz (TitaniQ) geothermometer (Wark and Watson, 2006). We examined Butte granite, quartz porphyry, and five deep vein types, including biotite crackles and early dark micaceous (EDM) veins (potassic alteration), barren quartz/quartz-molybdenum veins lacking alteration, and pyrite-quartz veins with sericitic alteration. Magmatic quartz shows concentric zoning that is cross-cut by CL-dark quartz veins. Complex textures in hydrothermal quartz indicate multiple quartz-precipitation episodes corresponding to different physical conditions of quartz growth. Concentrations of Ti have a strong positive correlation with CL brightness. K and Al concentrations show a weak relationship with CL brightness but they generally vary in unison. Fe concentrations do not appear to correlate with CL brightness or concentrations of other trace elements. TitaniQ temperatures range from 710 to 730°C in plutonic quartz, 625 to 750°C in porphyry quartz, and 650 to 730°C in barren quartz veins, overlapping with magmatic quartz. Biotite crackles and EDM vein temperatures are generally cooler than magmatic quartz and barren quartz veins, ranging from 660°C to less than 480°C, a temperature limited by our Ti detection limit. SEM-CL brightness boundaries do not necessarily match quartz grain boundaries. Careful examination of these boundaries in comparison with our existing trace element data, along with new electron backscatter diffraction (EBSD) mapping will help clarify the roles of diffusion, dissolution, and recrystallization in forming the CL textures. TitaniQ geothermometer results show that magmatic and deep hydrothermal temperature regimes overlap considerably. Temperatures combined with SEM-CL textures within one sample indicate significant temperature fluctuations: an increase from 560 to 730°C then decrease to 590°C between episodes of vein formation.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1065 Major and trace element geochemistry
DE: 3625 Petrography, microstructures, and textures
DE: 3665 Mineral occurrences and deposits
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting