Brief duration of hydrothermal activity at Round Mountain, Nevada, determined from Ar 40 /Ar 39 geochronology

Abstract

The Round Mountain gold-silver deposit is one of the world's largest volcanic-hosted precious metal deposits; reserves plus production total at least 16 million oz (500,000 kg) Au. The deposit occurs in ash-flow tuff that is ponded within its source caldera, along and overlying the caldera ring fracture. Host rocks have undergone propylitic, potassic, and high-level silicic and intermediate argillic alteration. These are interpreted either as a temporal sequence from early propylitic to late silicic and argillic or as a spatial progression from a potassic core outward to a ro litic halo.• P PY øAr/a9Ar ages determined by single crystal fusion of sanidine from host tuff and incremental heating of adularia indicate a most likely duration of hydrothermal activity of •0.1 m.y., possibly no more than 0.05 m.y., and a maximum of 0.5 m.y. Eruption of host ash-flow tuff and caldera collapse occurred at 26.5 Ma; a late or postalteration tuff that rests upon mineralized rock is 26.0 Ma. Hydrothermal circulation is unlikely to have been established immediately after caldera collapse, so duration was probably considerably less than 0.5 m.y.Eight adularia samples give ages between 25.94 + 0.09 and 26.09 ___ 0.05 Ma, and one is 26.20 + 0.05Ma; 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. This does not disprove the temporal se uence but does indicate that alteration q occurred in a time span which is not resolvable even with precise 4øAr/agAr dating. Furthermore, all samples show fiat age spectra, which themselves argue against a long, continuously active system. Kinetic studies of two adularia reveal diffusion domains with Ar closure temperatures between •230 ø and •390øC. Adularia that crystallized and remained at temperatures of 275øC, the maximum temperature inferred from fluid inclusion data, would yield age spectra with age gradients spanning the lifetime of the system. No spectra are consistent with a thermal history that incorporates sustained (>0.1 m.y.) temperatures at or above 275øC. Episodic activity to account for the one older adularia date cannot be precluded but is inconsistent with most data. Therefore, the hydrothermal system probably was active for less than the uncertainty in the ages of individual samples, possibly as little as 50,000 years. We conclude that hydrothermal activity at Round Mountain was a brief event at •26.0 Ma and was closely linked to caldera structure and magmatism. This activity followed initial ash-flow eruption by 0.5 m.y. and may have been driven by a ring fracture intrusion. These studies document a close relation between mineralization and caldera magmatism and structure. The 0361-0128/97/1949/807-2056.00 807 808 HENRY ETAL. Round Mountain deposit is located along the eastern ring fracture...

Emplacement ages and exhumation rates for intrusion-hosted Cu–Mo–Sb–Au mineral systems at Freegold Mountain (Yukon, Canada): assessment from U–Pb, Ar–Ar, and (U–Th)/He geochronometers

“…However, this is inconsistent with the generally short-duration hydrothermal activity recorded from numerical modeling of cooling in and around the small, high-level intrusions deposits (100 000 years, Norton and Cathles 1979;Cathles 1981;Norton 1982;Cathles et al 1997;Fu et al 2010), as well as with the overall duration of magmatic hydrothermal activity recorded in numerous porphyry copper deposits (e.g., Lepanto -Far Southeast, 100-300 ka, Arribas et al 1995; Round Mountain: approximate time 100 ka, Henry et al 1997;Batu Hijau: ∼80 ka, Garwin 2002;Sar Cheshmeh: ∼160 ka, McInnes et al 2003;McInnes et al 2005aMcInnes et al , 2005b, which is within the resolution of K/Ar and 40 Ar/ 39 Ar techniques (0.1-0.3 Ma; cf. Sillitoe 2000).…”

Relative age of Cordilleran base metal lode and replacement deposits, and high sulfidation Au?(Ag) epithermal mineralization in the Colquijirca mining district, central Peru

“…From the current knowledge of the lifespan of a single intrusion-related hydrothermal system (e.g., <50,000 years, Marsh et al 1997;Henry et al 1997;Muntean and Einaudi 2001), the period of activity of the two mineralization events, Au-(Ag) ($340,000 years) and sulfide-rich polymetallic ($270,000 years) are apparently representative of relatively long-lived hydrothermal processes.…”

40 Ar/ 39 Ar geochronology of magmatism and hydrothermal activity of the Madjarovo base-precious metal ore district, eastern Rhodopes, Bulgaria

“…Low-sulphidation hydrothermal systems typically form distal to intrusions and may apparently persist up to 1.5 Ma after volcanism (Silberman 1985;Heald et al 1987;Conrad et al 1993;Hedenquist & Lowenstern 1994). However, based on precise 4°Ar/39Ar measurements, Conrad & McKee (1996), Henry et al (1997) at a deeper level, perhaps 3-4 kin depth, and that the fluids were additionally contaminated by radiogenic Sr from Rhodope basement rocks which host the intrusions.…”