Sphalerite 40Ar/39Ar progressive crushing and stepwise heating techniques

It is difficult to determine the mineralization ages of sulfide deposits because no suitable minerals can be selected for the traditional isotopic dating methods. The 40Ar/Ar-3 9 method rarely applies well to sulfide minerals because of their trace potassium content and strong radioactivity after irradiation. This preliminary study investigates the possibility of dating the hydrothermal sphalerites by 40Ar/39Ar using crushing and heating techniques. The 40Ar/39Ar crushing experiments of two sphalerite samples from the Fankou Lead-Zinc Deposit yield well-defined and concordant isochron ages of 265.8 +/- 1.0 (1 sigma) Ma and 269.7 +/- 6.2 Ma corresponding to initial 40Ar/(36) Ar ratios of 290.7 +/- 2.4 and 294.6 +/- 8.1 respectively. The crushed powders of the later sphalefite yields, with stepwise heating, a flat age spectrum with 40Ar/39Ar plateau age of 267.2 +/- 2.5 Ma, which define a good isochron with an age of 271.3 +/- 5.4 Ma and initial 40Ar/(36) Ar ratio of 294.0 +/- 1.7. This is the first report to distinguish secondary and primary fluid inclusions within sphalerite, to obtain their ages using the crushing technique, and to obtain good concordant ages both of fluid inclusions by crushing and heating of K-containing minerals. We expect progressive crushing and stepwise heating techniques will become an effective approach to 40Ar/39Ar dating of the mineralization ages of hydrothermal sulfide deposits. (c) 2007 Elsevier B.V. All rights reserved.