BRIEF DURATION OF HYDROTHERMAL ACTIVITY AT ROUND MOUNTAIN, NEVADA, DETERMINED FROM AR-40 AR-39 GEOCHRONOLOGY/

The Round Mountain gold-silver deposit is one of the world's largest v olcanic-hosted precious metal deposits; reserves plus production total at least 16 million oz (500,000 kg) Au. The deposit occurs in ash-flo w tuff that is ponded within its source caldera, along and overlying t he caldera ring fracture. Host rocks have undergone propylitic, potass ic, and high-level silicic and intermediate argillic alteration. These are interpreted either as a temporal sequence from early propylitic t o late silicic and argillic or as a spatial progression from a potassi c core outward to a propylitic halo. Ar-40/Ar-39 ages determined by si ngle crystal fusion of sanidine from host tuff and incremental heating of adularia indicate a most likely duration of hydrothermal activity of similar to 0.1 m.y., possibly no more than 0.05 my, and a maximum o f 0.5 m.y. Eruption of host ash-flow tuff and caldera collapse occurre d at 26.5 Ma; a late or postalteration tuff that rests upon mineralize d rock is 26.0 Ma. Hydrothermal circulation is unlikely to have been e stablished immediately after caldera collapse, so duration was probabl y considerably less than 0.5 m.y. Eight adularia samples give ages bet ween 25.94 +/- 0.09 and 26.09 +/- 0.05 Ma, and one is 26.20 +/- 0.05 M a; the mean of all nine is 26.03 +/- 0.08 Ma. The maximum and minimum values and their uncertainties allows a duration of as much as 0.4 m.y ., but for the following reasons, the longevity is probably much less. The absolute ages of seven samples that can be placed in the possible temporal alteration sequence do not correlate with that sequence. Thi s does not disprove the temporal sequence but does indicate that alter ation occurred in a time span which is not resolvable even with precis e Ar-40/Ar-39 dating. Furthermore, all samples show flat age spectra, which themselves argue against a long, continuously active system. Kin etic studies of two adularia reveal diffusion domains with Ar closure temperatures between similar to 230 degrees and similar to 390 degrees C. Adularia that crystallized and remained at temperatures of 275 deg rees C, the maximum temperature inferred from fluid inclusion data, wo uld yield age spectra with age gradients spanning the lifetime of the system. No spectra are consistent with a thermal history that incorpor ates sustained (>0.1 m.y.) temperatures at or above 275 degrees C. Epi sodic activity to account for the one older adularia date cannot be pr ecluded but is inconsistent with most data. Therefore, the hydrotherma l system probably was active for less than the uncertainty in the ages of individual samples, possibly as little as 50,000 years. We conclud e that hydrothermal activity at Round Mountain was a brief event at si milar to 26.0 Ma and was closely linked to caldera structure and magma tism. This activity followed initial ash-flow eruption by 0.5 m.y. and may have been driven by a ring fracture intrusion.