PHYSICAL CONTROLS OF VEIN FORMATION IN THE BUTTE, MONTANA PORPHYRY COPPER-MOLYBDENUM DEPOSIT

The Butte porphyry copper deposit formed in four mineralization events distinguished by vein cutting relations. Early chalcopyrite-bearing veins with biotitic alteration (EDM veins) are cut by quartz-molybdenite (q-mb) veins with minor K-feldspar alteration. Q-mb veins are cut by pyrite-quartz veins with sericitic alteration which are cut by base metal-rich Main Stage veins. SEM-CL textures of quartz in these veins reveal that many veins are complex composite structures formed from multiple fluid-flow events.

EDM veins have the most complex and variable CL textures including: rounded and truncated wide growth bands of variable intensity, thin euhedrally zoned quartz grains of variable intensity, and mosaics of fine grained quartz with little CL texture. Variability of CL texture and intensity suggest that these veins formed from multiple pulses of hydrothermal fluids. Truncated growth zones with rounded cores indicate that occasional dissolution occurred in these veins; probably caused by pressure increase when fractures sealed.

Deep q-mb veins typically have little CL texture, displaying mosaics of CL-bright homogenous to slightly mottled quartz, precipitated during rapid depressurization. Annealing of original CL textures followed repressurization and prolonged exposure to temperatures near 600 C. Rapid quartz precipitation with consequent choking of fluid flow allowed little time for alteration envelopes to form.

The most common texture in pyrite-quartz veins is thin euhedral growth zones in crystals growing towards the vein center. Many exhibit dissolution textures where CL-dark euhedral quartz overgrows rounded and embayed CL-bright quartz. These textures indicate that quartz precipitated into open space and that dissolution occurred prior to quartz precipitation in many cases. Quartz likely dissolved as hydrothermal fluids cooled through the zone of retrograde quartz solubility at temperatures near 450 C. Precipitation into open space followed as the fluid cooled into the temperature range of prograde quartz solubility.

The abundance of euhedral quartz crystals with no dissolution textures in Main Stage veins indicates that these precipitated into open space from fluids cooling below the interval of retrograde quartz solubility under hydrostatic pressure.