摘要
The zinc silicate willemite (Zn2SiO4) is the main carrier of zinc in a number of high-grade, carbonate-hosted nonsulfide deposits located in the southern hemisphere that have been recently reinterpreted to be of hypogene–hydrothermal origin. The timing of willemite mineralization in these deposits is only poorly constrained. In this pilot study, willemite has been evaluated as a potential Rb–Sr geochronometer that can be used to directly date nonsulfide ore deposits. We have analyzed samples of economic-stage willemite from the Berg Aukas and Abenab West deposits, Otavi Mountainland (Namibia), which are hosted by Neoproterozoic, dolomitized and tectonized carbonate rocks. Rb–Sr elemental concentration levels and ranges of 87Rb/86Sr and 87Sr/86Sr ratios in these willemite samples are comparable to those observed for the Zn sulfide sphalerite, which has been already successfully used for direct Rb–Sr dating of carbonate-hosted (MVT) deposits. This reflects similar Rb–Sr partitioning mechanisms into Zn ore minerals that precipitate either as sulfides from reduced or as silicates from oxidized hydrothermal fluids, respectively. The Rb–Sr results on willemite presented here reflect mostly low, but variable 87Rb/86Sr ratios, sufficient to generate radiogenic 87Sr over time. This clearly shows that willemite is suitable as an Rb–Sr geochronometer and should be further evaluated for direct Rb–Sr dating of nonsulfide Zn deposits.