Rocky Mountain Section - 61st Annual Meeting (11-13 May 2009)
Paper No. 11-10
Presentation Time: 8:00 AM-5:00 PM

RE-OS ISOTOPE RESULTS FOR ELECTRUM FROM THREE LOW SULFIDATION EPITHERMAL AU-AG DEPOSITS IN THE GREAT BASIN

HOFSTRA, Albert H., U.S. Geological Survey, MS-973, Box 25046, Denver, CO 80225, ahofstra@usgs.gov and CREASER, Robert A., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Edmonton, AB T6G 2E3, Canada

Recent isotopic studies of electrum from a suite of epithermal deposits in the northern Great Basin indicate that Pb and by association Au were derived directly from Miocene mafic magmas associated with initiation of the Yellowstone Hot Spot (Kamenov et al., 2007, Economic Geology). If correct, then other trace metals in electrum should be present that also are derived from mafic magmas. To test this, we determined the Re-Os isotope composition of electrum from two of the previously studied Miocene deposits, Sleeper (SL) and Seven Troughs (ST), as well as an Oligocene deposit, Round Mountain (RM), associated with calc-alkaline volcanic rocks. All are hosted in rhyolitic volcanic rocks. SL and ST are underlain by Mesozoic (Mz) metasedimentary rocks. RM is underlain by lower Paleozoic (Pz) sedimentary rocks. The Mz and Pz rocks contain black shales that are another potential source for radiogenic Os and Au.

Vein quartz was removed with HF. Electrum separates were digested by the Carius tube method, processed using a combination of solvent extraction, microdistillation, and chromatographic techniques, and analyzed by NTIMS. Results ( 2 sigma uncertainty, blank corrected) are listed below.

SL: Re 2327 ppt , Os 21.54.19 ppt, 187Re/188Os 53.31.8, 187Os/188Os .344.010

ST: Re 1906 ppt , Os 0.40.13 ppt, 187Re/188Os 25912128, 187Os/188Os 1.09.96

RM: Re 507 ppt , Os 0.96.15 ppt, 187Re/188Os 269105, 187Os/188Os 0.65.28

Unfortunately, electrum separates from ST and RM have less than 1 ppt Os and very large uncertainties that make the isotope ratios of limited value. SL electrum contains the most Au, has the highest Os content, and yielded useful Re-Os data. The reported age for SL of 16.1 Ma was used to calculate an initial 187Os/188Os of 0.330, which is similar to reported initial Os ratios of 0.201-0.403 for coeval Columbia River basalts. The Re-Os data for SL support a mafic magmatic source for metals and likely eliminate underlying Mz black shales as a significant Os source.

Rocky Mountain Section - 61st Annual Meeting (11-13 May 2009)
General Information for this Meeting
Session No. 11--Booth# 11
General Discipline Posters
Utah Valley University: Library 4th Floor
8:00 AM-5:00 PM, Tuesday, 12 May 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 6, p. 20

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