Scanning electron microscope–cathodoluminescence analysis of quartz reveals complex growth histories in veins from the Butte porphyry copper deposit, Montana

Abstract

Scanning electron microscope–cathodoluminescence (SEM-CL) analysis of quartz reveals textures that cannot be observed using optical microscopy or backscattered electrons. These cryptic textures yield insight into timing and physical conditions of quartz growth, especially in environments with multiple quartz-precipitation events. Hydrothermal quartz from quartz-sulfide veins in the porphyry copper deposit in Butte, Montana, was analyzed by SEM-CL, revealing the following textures: euhedral growth zones, wide nonluminescing bands that cut across multiple quartz grains, rounded luminescent quartz grain cores with euhedral overgrowths, nonluminescing “splatters” of quartz connected by networks of cobweb-like nonluminescing quartz in otherwise luminescent quartz, concentric growth zones, and wide nonluminescent grain boundaries. These textures indicate that many veins have undergone fracturing, dilation, growth of quartz into fluid-filled space, quartz dissolution, and recrystallization of quartz. Precipitation and dissolution textures indicate that early quartz-molybdenite veins formed as a result of pressure fluctuations between lithostatic and hydrostatic at high temperatures, and later pyrite-quartz veins formed near hydrostatic pressure in response to temperature decrease through and beyond the field of retrograde quartz solubility.