Age constraints on Jerritt Canyon and other Carlin-type gold deposits in the western United States-relationship to mid-tertiary extension and magmatism

Carlin-type gold deposits are difficult to date and a nide range of ages ha s been reported for individual de posits. Therefore, several methods were e mployed to constrain the age of the gold deposits in the Jerritt Canyon dis trict. Dated igneous rocks with well-documented crosscutting relationships to ore provided the most reliable constraints. K/Ar and Ar-40/Ar-39 dates o n igneous rocks are as follows: andesite dikes 324 Ma, sericitic alteration in andesite dikes 118 Ma, basalt dikes 40.8 Ma, quartz monzonite dikes 39. 2 Ma, and calc-alkaline ignimbrites 43.1 to 40.1 Ma. Of these, only the and esite and basalt dikes are clearly altered and mineralized. The gold deposi ts are, therefore, younger than the 40.8 Ma basalt dikes. The sericitic alt eration in the andesite dikes is unrelated to the gold deposits. A number o f dating techniques did not work. K/Ar and Ar-40/Ar-39 dates on mica from m ineralized Ordovician to Devonian sedimentary rocks gave misleading results . The youngest date of 149 Ma from the smallest <0.1-mu m-size fraction sho ws that the temperature (120 degrees-260 degrees C) and duration (?) of hyd rothermal activity was insufficient to reset preexisting fine-grained micas in the host rocks. The temperature and duration was also insufficient to a nneal fission tracks in zircon from Ordovician quartzites as they yield Mid dle Proterozoic dates in both mineralized and barren samples. Apatites were too small for fission track dating. Hydrothermal sulfides have pronounced crustal osmium isotope signatures (Os-187/Os-188(initial) = 0.9-3.6) but di d not yield a meaningful isochron due to very low Re and Os concentrations and large analytical uncertainties. Paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the ho st rocks leaving no remnant magnetism. When published isotopic dates from other Carlin-type deposits in Nevada and Utah are subject to the rigorous evaluation developed for the Jerritt Cany on study, most deposits can be shown to have formed between 42 and 30 Ma. K /Ar and Ar-40/Ar-39 dates on the youngest preore igneous rocks range from 4 1 to 32 Ma, whereas the oldest postore igneous rocks range from 35 to 33 Ma . Hydrothermal adularia from the Twin Creeks deposit yields similar Ar-40/A r-39 dates of 42 Ma. K/Ar dates on supergene alunite range from 4 to 30 Ma. K/Ar and Ar-40/Ar-39 dates on micas separated from sedimentary (395-43 Ma) and igneous (145-38 Ma) rocks are usually much older than the gold deposit s and most are suspect because they are from incompletely reset preore mica s or from mixtures of preore and ore-stage mica. Fission track dates on zir cons are also generally older than the deposits (169-35 Ma) and are not com pletely reset by mineralization. Apatites are likely to be reset by the hyd rothermal systems land by younger thermal events) and yield dates (83-22 Ma ) that are younger than those from zircon.