U-Th-Pb Dating of Hydrothermal Minerals from Carlin-Type Gold Deposits: Results and Evaluation

Although evidence continues to mount in support of a
mid-Tertiary age for Carlin-type gold deposits, accurate dates
on the deposits are sorely needed to improve understanding
of their relationship to the complex tectonic history of the
Great Basin (Hofstra et al., 1999 and refs. therein). The mid-
Tertiary age constraints that we do have, of 42-30 Ma, are
mainly based on cross-cutting relationships between orestage
minerals and dated igneous rocks or supergene alunite.
Few ore-stage minerals have been dated directly. A notable
exception is adularia from the Twin Creeks mine in the
Getchell trend that yields 40Ar/39Ar dates of 42.0 ± 0.1 Ma
(Groff et al., 1997; Hall et al., 1997). The purpose of this
study is to evaluate the utility of U-Th-Pb methods for dating
hydrothermal minerals from Carlin-type gold deposits.
U-Th-Pb methods have the potential to date a wide variety
of hydrothermal minerals provided these minerals have
remained closed to U, Th, Pb and intermediate daughters
since they formed, correct values are used for the initial Pb
isotope ratios, and the minerals contain sufficient concentrations
of parent (238U, 235U, 232Th) and daughter (206Pb, 207Pb,
208Pb) isotopes such that they can be measured accurately
(Faure, 1986). Brannon and others (1996; 1997) have demonstrated
the utility of U-Th-Pb methods for dating hydrothermal
calcite from Mississippi Valley-Type Pb-Zn deposits and
fluorspar deposits in the U.S. mid-continent. They found that
to obtain a reliable date on hydrothermal minerals with low
concentrations of U and Th, the concentration of daughter Pb
isotopes due to in situ decay must be large compared to the
variability in the initial (common) Pb isotopic composition of
the mineral. This requirement can be a problem in hydrothermal
ore deposits, because mixing between fluids with different
Pb isotopic compositions is a common aspect of ore formation.
Another problem inherent to Carlin-type deposits is
the disseminated nature of the ores and the acid alteration of
the host rocks. This problem can make it difficult to obtain
pure mineral separates that do not contain mineral inclusions
from the host rocks, which is necessary to avoid hybrid dates.