Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.219e&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 219
Other
3
Scientific paper
Gold precipitates in hydrothermal fluids along with other heavy elements, such as Ag and Pt. In order to explore the possibility of dating the formation of gold we determined the concentrations of U, Th, and their decay product ^4He, as well as the K and ^40Ar concentrations in vein-type gold and in placer gold samples. The gold-quartz veins at Brusson in the south-western alps were formed approximately 32 Ma ago during an episode of tectonic uplift (Diamond, 1990). Alpine material was deposited as sediment layers in the region of central Switzerland and placer gold is thus relatively abundant in the rivers of the Napf area. We washed placer gold from the river Grosse Fontanne in 1990 and 1991. Placer gold that had been collected from the river Kruempelgraben in 1933 and a sample of vein-type free gold grown on quartz rock from the Brusson area (Val d'Ayas) have been obtained from the Museum of Natural History in Bern. Table 1 gives the results. Most of the ^4He is released above 1050 degrees C, that is when gold melts, indicating that gold is extremely well retentive for He. From the ^4He concentration of (269 +- 20) x 10^-8 cm^3 STP/g, (0.4 +- 0.1) ppm U, and (0.9 +- 0.3) ppm Th for vein-type gold we calculate a U/Th-He age of (36 +- 8)Ma. This age agrees within errors with the proposed age of 32 Ma. The data given in Table 1 show that all placer gold samples contain excesses of radiogenic ^4He and ^40Ar relative to the concentrations expected from the U/Th and K decay, respectively, if we assume a formation age of 32 Ma. The quartz sample is depleted in ^4He but strongly enriched in radiogenic ^40Ar. The excess of ^40Ar(sub)rad is easier to explain than that of ^4He. Vein-type gold and placer gold contain quartz inclusions (Schmid, 1973). The high ^40Ar(sub)rad content of quartz (Table 1) indicates that the ^40Ar(sub)rad excess of gold originates from quartz inclusions. Excess ^4He in gold must be of radiogenic origin. Taking ^20Ne and ^36Ar as a measure for the quantity of trapped atmospheric noble gases we estimate atmospheric ^4He in the gold samples to be three to five orders of magnitude below the observed ^4He concentration. Placer gold is finely distributed in rock material and might be exposed to an alpha-particle irradiation from neighboring U/Th-rich minerals. An alternative He source are inclusions of U/Th-rich minerals, such as zircon, either within the gold material or mechanically worked into the spangles as they were part of the river detritus. Acknowledgement: We thank the Swiss NSF for their support. References: Diamond L.W. (1990) Am. J. of Science 290, 912-958. Schmid K. (1973) Schw. Min. Petr. Mitt. 53, 125-156. Table 1, which in the hard copy appears here, shows concentrations of He, Ne, and Ar (10^-8 cm^3 STP/g) and of K, Th, and U (ppm) in vein-type free gold, placer gold, and quartz. The ^3He and ^21Ne signals were below detection limits, that is ^4He/^3He in gold is >100'000. Average ^20Ne/^22Ne ratios in gold and quartz are 10.2 +- 0.2, that is about 4% larger than in the terrestrial atmosphere. Average ^36Ar/^38Ar = 5.2 +- 0.2 (within errors identical to ^36Ar/^38Ar in air). 1) Sample sizes 50-100 mg. 2) Radiogenic ^40Ar = ^40Ar-295.5 x ^36Ar. 3) Calculated from U/Th and ^40K decay.
Eugster Otto
Hofmann Bernd
Krähenbühl Urs
Neuenschwander Juerg
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