QUARTZ-MOLYBDENITE MYLONITES OF THE BUTTE PORPHYRY CU-MO DEPOSIT

Where the Continental Fault intersects the Butte porphyry Cu-Mo deposit in the Butte Granite, shear zones with mylonitic textures record ore and gangue mineral redistribution due to deformation caused by faulting. The Continental Fault is a normal fault with ~1.3 km of displacement that offsets the zoned Pittsmont porphyry Cu-Mo center of the Butte, Montana ore deposit. Deformation along the Continental fault is complicated by veins and alteration in the Butte Granite. The brittle damage zone is more developed in the pervasively sericitized hanging wall while the ductile drag zone is more developed in the more quartz-rich footwall. Mapping in the active open pit mine shows that where the Continental Fault intersects a zone of quartz-molybdenite veins in the footwall, strain was partitioned into a series of shear zones with margins defined by molybdenite-rich layers.

Intensely mylonitized Butte Granite lies between the high temperature quartz- molybdenite veins that demarcate the boundaries of individual shear zones. Fresh Butte Granite consists largely of quartz, biotite, plagioclase, and orthoclase. During formation of the porphyry Cu-Mo deposit, this part of the deposit was pervasively altered to a sericitic assemblage with remnant biotite (SBr), wherein the original granitic texture is destroyed, original biotite is preserved but with increased Mg# and rims partially replaced by muscovite and pyrite, and plagioclase and K-feldspar are replaced by quartz, muscovite, and pyrite.

Petrographic relations suggest that mylonitization due to movement redistributed pre-Main Stage porphyry Cu-Mo mineralization during faulting. K-feldspar core-and-mantle structures consist of parent grains of either relict fresh or sericitized orthoclase porphyroclasts are common and mantled by strongly recrystallized, un-altered daughter orthoclase crystals with irregular chemical zoning. Quartz in the shear zones has undergone extensive static recrystallization; there is no crystallographic or shape preferred orientation. Biotite occurs as syn-kinematic mica fish and post-kinematic clusters of fresh randomly oriented hydrothermal biotite. Molybdenite was remobilized during faulting and is present both as a deformed intergranular phase and as undeformed euhedral lath-shaped crystals.