Gold mineralization at the Golden Sunlight breccia pipe, southwestern
Montana, is related to emplacement of Late Cretaceous alkali-calcic rhyolite
and subsequent collapse of the Belt Supergroup wallrock and rhyolite in
the pipe. The pipe is inferred to grade downward into an alkalic porphyry
molybdenum system. The pipe is cut by alkalic to sub-alkalic lamprophyre
dikes and sills, which locally contain high-grade gold where emplaced along
late shear zones and vein systems.
Determination of the emplacement age of the rhyolite is hampered by inherited
lead or inherited Late Archean zircon from the source region of the rhyolite.
An emplacement age of about 80 Ma for the rhyolite can be inferred if a
basement age of 2,600 Ma is assumed. This Late Archean age is in agreement
with basement ages determined in many parts of southwestern Montana.
A206Pb-238U whole-rock date of 84±18 Ma from altered and mineralized
Belt Supergroup strata and rhyolite in the breccia pipe indicates hydrothermal
alteration related to gold mineralization in Late Cretaceous time. Although
sericite is a relatively widespread hydrothermal mineral, attempts to date
the very fine grained material by the 40Ar-39Ar method did not provide a
spectra that could be interpreted unambiguously.
A 40Ar-39Ar plateau date of 76.9±0.5 Ma from biotite phenocrysts
in the lamprophyre indciates intrusion of mafic magma and attendant CO2
metasomatism in the Late Cretaceous. Fission-track data from zircon in the
rhyolite are permissive of slow uplift of the Belt Supergroup strata, rhyolite,
and lamprophyre between 55 and 50 Ma, but the data are not definitive.
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