About possibility of isotope dating of native gold by the (U-Th)/He method

Abstract

For investigation of helium in native gold, a new measuring complex was created and used: the high sensitivity mass spectrometer MSU-G (ZAO SKB “SPECTRON”). The sensitivity of measuring 4He was 5.3 × 10−13 cm3/g per impulse. Experiments in stepwise heating of samples have been carried out, and the kinetics of radiogenic 4He emanation from native gold was investigated. Migration parameters (activation energy and frequency factor) were determined. Model calculations of stability (closure temperature) of radiogenic 4He in the native gold structure with a given time and temperature of thermal influences were made using the data received. The concentration of 4He in native gold from the original deposit Nesterovskoe is (4.7 ± 0.1) × 10−5 cm3/g in the sample from the placer; from Chudnoe deposit, it is (3.8 ± 0.1) × 10−5 cm3/g; from sulfide deposits of Kitoiskii knot of Eastern Sayani, it is (1.9 ± 0.1) × 10−5 cm3/g; and from the South Muiskii ore region it is (8.7 ± 0.5) × 10−7 cm3/g. The received curve lines of kinetics of 4He emanation from native gold show that radiogenic helium is well bonded in the native gold structure: in all the examined samples, most 4He emanates only by reaching the temperature of 950–1000°C. A specific feature of the kinetics of radiogenic 4He emanation in all examined samples is an outburstlike emanation in the form of a peak of large amplitude in the area of temperatures near the melting temperature point of gold. This is stipulated by the existence of helium bubbles released by metals only while they melt. The spectrum of helium thermal desorption from native gold has a complicated form and is a result of superposition of several peaks. This proves the migration of groups of atoms located in the gold structure in different energy states. Very large values of the activation energy of helium migration from native gold were received: up to 161–176 kcal/mol. Extremely large values of the frequency factor, from 2 × 1018 to 3 × 1032, correspond to such values of activation energies. This is caused probably by helium migration in the form of gas bubbles. The received data indicate the very high stability of the (U-Th)/He isotope system in native gold. Using the (U-Th)/He method of isotope geochronology seems to be very promising for isotope dating of these strategic raw materials.