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GEOCHRONOLOGIC EVIDENCE OF MULTIPLE OVERLAPPING, FRACTURE-CONTROLLED MAGMATIC AND HYDROTHERMAL PULSES IN THE BUTTE DISTRICT: GENETIC IMPLICATIONS FOR Ag AND Cu VEINS AND PORPHYRY Mo
Despite a century of geologic studies of the Butte mining district, timing of the Main Stage Cu-rich (and peripheral Ag) veins relative to the pre-Main Stage porphyry Mo system remains poorly understood because genetic models relating timing of hydrothermal systems and Boulder batholith plutons were based on incomplete data.
Thirteen new SHRIMP U-Pb zircon ages show that the Boulder batholith was intruded between 80.7 ± 0.8 Ma (Radar Creek granodiorite) and 73.7 ± 0.5 Ma (Hell Canyon leucogranite). Three dates on the mineral deposit-hosting Butte Granite show that the northern part (74.7± 0.6 Ma) is younger than the southern part (76.5 ± 0.8 Ma).
Six
SHRIMP U-Pb ages on quartz porphyry dikes include ages of about 70.0-67.6 Ma
for east-west trending "Steward" dikes previously linked to "pre-Main Stage",
69.4 ± 0.9 and 67.3 ± 0.9 Ma for "Modoc" plugs previously linked to "Main
Stage", and 62.7 ± 1.5 Ma for a "post-mineralization rhyolite dike".
In addition, 30 new 40Ar/39Ar dates on white mica from 20 "Main Stage" veins in the peripheral Ag-rich part of the vein system as well as from samples collected near the surface and at depth in the central Cu-rich part. Ag-rich "Main Stage" veins are 73-65 Ma (western), 70-65 Ma (eastern), and 73-64 Ma (northern), younging toward the center of the district. In the central Cu-rich zone, ages decrease with depth from about 69 Ma in the sericitic alteration zone near the surface above the porphyry Mo-related biotitization dome to about 63.6 Ma in the argillic alteration zone within the dome.
We conclude that mineralization at Butte is the product of multiple overlapping pulses of intrusion and hydrothermal mineralization starting with a 73-Ma polymetallic vein system related to young phases of the Boulder batholith and emplaced into an east-west fracture system in the Butte Granite. The fracture system underwent repeated injections of porphyry magmatic and hydrothermal systems that upgraded and remobilized metals from 73-63.6 Ma.