NOBLE-GASES, K, U, TH, AND PB IN NATIVE GOLD

We present determinations of the noble gas and Pb isotopic abundances and of K, Th, and U concentrations of native gold. Our results demonst rate that gold is an excellent carrier for crustal volatiles, but dire ct dating of gold using the U, Th-He-4, K-40-Ar-40, and U fission Xe m ethods was not successful for various reasons The main significance of this work is the great sensitivity of gold for trapped gases as well as for gases that were produced in situ which gives the prospects of u sing gold and its fluid and solid inclusions for the study of paleogas composition. Numerous nuclear effects characterize the noble gas inve ntory of placer gold from Switzerland and Italy, vein gold from Italy, South Africa, and Venezuela, and lode gold from South Africa. The deg assing patterns obtained by mass spectrometry show a low-temperature r elease of volatiles around 500 degrees C from fluid inclusions mainly in vein gold and a high-temperature release from solid inclusions and the gold itself. The low-temperature volatiles represent species that were trapped when the Sold crystallized. We investigated the following trapped species: the isotopes of He, Ne, Ar, Kr, Xe, and Pb, and the abundances of K, U, Th, H2O, and CO2. The crustal gases trapped by gol d comprise He-3 from Li-6(n,alpha)H-3 --> beta(-) --> He-3, He-4 and A r-40 from the U, Th, and K decay, and Xe from (238) fission. We observ e He-4/Ar-40 = 3.9 for the radiogenic trapped gases of tertiary gold a nd a ratio of 1.4 for Archean gold. These ratios are consistent with t he production ratios from U and K at the respective times and demonstr ate that gold can be used as a sampler of ancient atmospheric gases. T he concentrations of U and Th range from a few parts per billion to a few parts per million, and those of K and Pb range up to some tens of parts per million. The antiquity of trapped Pb is indicated by the Pb- Pb model age of about 3000 Ma for the lead extracted from vein gold an d quartz of the Lily gold mine (South Africa). Gold also contains nobl e gases produced in situ by U, Th and K decay. We obtained 4He up to t he extremely high concentration of 6.8 x 10(-2) cm(3) STP/g (STP: atmo spheric pressure and 0 degrees C) for gold separated from a specimen o f the Witwatersrand West Rand gold fields, indicating that this gold h as a large U content.