Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..411n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 411
Other
Ages, Dating, Gold, Noble Gases, Noble Metals, Radiogenic Ages
Scientific paper
We have continued the development of a direct dating method for natural gold deposits based on U/Th and noble gas analyses. In our previous work [1-3], we have demonstrated that U/Th-^4He dating as well as U-^136Xe dating should be feasible for gold because of its high retentivity and the large U concentrations in certain samples. For vein-type gold from Brusson, Northern Italy, our U/Th-^4He age agreed with the K-^40Ar age of associated muscovite [1,4]. Improved U data yield consistent ages of 30 +/- 3 and 39 +/- 13 Ma for two splits. For placer gold from the Elvo river, 30 km south of Brusson, we obtain a somewhat lower age (22 +/- 6 Ma). These results confirm an age of approximately 30 Ma for gold from that region of the Southern Alps. On the other hand, for Archean gold from the South African Lily Gold Mine (Eastern Transvaal), we calculate an age of 1200 +/- 400 Ma. Since He concentrations vary in different splits of a sample [2], these ages can be improved by analyzing U, Th, and noble gases in the same split. In a double-walled quartz extraction line we heated gold samples, wrapped in Pt or Ni foil, above the melting point (1064 degrees C). After gas extraction the molten samples were retrieved for U and Th analysis. So far we have obtained results for two placer gold samples from Tertiary sediments in Central Switzerland extracted by this method: Both Krumpelgraben and Grosse Fontanne yield U/Th-^4He ages around 500 Ma. If the Tertiary origin is correct, an explanation for the large He excesses is required. Atmospheric He (with ^3He/^4He = 1.4 x 10^-6) is negligible: In two samples of Grosse Fontanne, ^3He/^4He is <2 x 10^-8 [5]. Several potential He sources have been excluded previously [2]. The following possibilities remain as explanations for the high He concentrations. Crustal noble gases containing radiogenic ^4He may have been trapped by the gold during formation. This is probably true for ^40Ar, which is overabundant by up to 2 orders of magnitude relative to the production from ^40K decay in ~30 Ma [1,3] . Correction for "inherited" components is possible by plotting isochrons for several samples having the same age but different U or trapped gas concentrations. Our current database allows us to estimate the trapped ^4He/^40Ar ratio from the Krumpelgraben and Grosse Fontanne analyses. We obtain ^4He/^40Ar <= 3 in the trapped component, i.e., the high ages cannot be explained by inherited He. The ^4He that is not accounted for by the U and Th content and the estimated age may have been produced in inclusions of U-rich minerals, such as garnets or zircons, that were worked into the placer gold grains during residence in the river detritus. They either have a higher age than the surrounding gold or were not dissolved in the chemical processing preceding the U analysis. The latter possibility will be tested thoroughly in future analyses. However, we cannot exclude the possibility that placer gold from Central Switzerland is actually older than believed. Prior to solving this problem we shall establish the dating method, concentrating on Tertiary gold from the Brusson/Elvo region and on Archean gold from Eastern Transvaal. Acknowledgment: This work was supported by the Swiss National Science Foundation. References: [1] Eugster O. et al. (1992) Meteoritics, 27, 219-220. [2l Eugster O. et al. (1993) LPS XXIV, 455-456. [3] Niedermann S. et al. (1993) LPS XXlV, 1073-1074. [4] Diamond L. W. (1990) Am. J. Sci., 290, 912-958. [5] Kamensky I. L., personal communication.
Eugster Otto
Frei Robert
Krähenbühl Urs
Kramers Jan D.
Niedermann Samuel
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