Overlapping Cretaceous and Eocene alteration, Twin Creeks Carlin-type deposit, Nevada

We report here new Ar-Ar dates for adularia, illite, and muscovite from the Twin Creeks Carlin-type deposit. All illite samples were obtained from alt ered intrusive and volcanic rocks that are interlayered with mineralized se dimentary rocks, thus largely avoiding complications caused by detrital ill ite and muscovite in sedimentary rocks. Interpretation of the illite-age sp ectra was based, in part, on newly developed models for the behavior of arg on in fine-grained illite. The results indicate that all adularia (in the M egapit area) formed at about 42 Ma (Eocene), whereas most illite in the Meg apit area and muscovite in alteration zones beneath Chimney Creek formed at about 109 to 103 Ma (Cretaceous). Additional events involving formation of illite at 200 Ma and muscovite at about 310 Ma were also observed but were not widespread enough to be considered significant. Our Eocene date for ad ularia is in complete agreement with previous measurements on adularia from Twin Creeks, but our Cretaceous dates are considerably older than dates de termined in previous studies of the nearby Getchell district, particularly the Osgood stock, which has a date of 98 to 92 Ma. Our results suggest that the Osgood stock is part of a more protracted Cretaceous igneous-hydrother mal event that took place between about 109 and 83 Ma. The relationship of adularia and illite to gold mineralization is not compl etely clear, making it impossible to use our data to assign a unique age to the Twin Creeks deposit. We favor and Eocene age for some gold mineralizat ion because the adularia that we dated is intimately intergrown with aurife rous, arsenian pyrite typical of Carlin-type mineralization. However, adula ria has a very restricted distribution in the deposit and temperatures at w hich it formed were probably not great enough to reset Cretaceous and older ages of illite. Illite is much more widespread and its association with go ld is supported by correlation between gold content and K/Al ratios of mine ralized material. Thus, Cretaceous illitization was at least an important g round-preparing event at Twin Creeks, and it is not possible to show from o ur data that gold ore or protore were not deposited during this event.