AUSTRALIAN ARCHEAN ARGON--DOES IT WORK?

Despite the reality that 40Ar/39Ar geochronology is the tool of choice to determine age and thermal history of Phanerozoic-age mineral deposits, its efficacy on Archean mineral deposits is viewed by some with skepticism. This doubt arises because when argon and U-Pb dates for samples from many Australian Archean (ca 2.65 Ga) ore deposits are compared, the argon dates are generally much younger (>2% or 25 m.y. younger and more). Does argon inherently leak from minerals over geologic time resulting in this larger discordance for older rocks or are these real discordances resulting from longer periods of cooling to argon closure or a warmer earlier Earth? A third possibility, and our preferred explanation, is that the accepted ages for many 40Ar/39Ar standards are inaccurate and sample integrity is critical. Work that we are currently undertaking on well-constrained samples from some of the world’s most productive gold deposits in the eastern Yilgarn of Australia provides a background to test the effectiveness of 40Ar/39Ar geochronology of Archean gold deposits. A primary conclusion of our work is that the generally accepted age for MMhb-1 hornblende, argon standard (520.4 Ma; Samson and Alexander, 1987, Isotope Geoscience) is inaccurate. As suggested by Renne et al. (1998, Chemical Geology), a recalibrated age of 523.1 Ma produces more realistic comparisons between both host-rock and mineral deposit dates and equivalent U-Pb dates. In gold districts where metamorphic host rocks were at lower amphibolite or greenschist facies at the time of gold formation, muscovite and fuchsite from the gold-bearing veins, and hornblende and biotite from the host rocks, yield argon dates calibrated against this older standard age that are consistent with U-Pb dates. In some cases, metamorphic hornblende yields age spectra indicative of several periods of thermal activity with ages identical within error to U-Pb age constraints. Thus, as with Phanerozoic ore deposits, 40Ar/39Ar geochronology is efficacious for the study of Archean mineral deposits and host rocks.