Willemite (Zn₂SiO₄) as a possible Rb–Sr geochronometer for dating nonsulfide Zn–Pb mineralization: Examples from the Otavi Mountainland (Namibia)

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• 作者：Jens Schneider ; Maria Boni ; Carsten Laukamp ; Thilo Bechstä ; dt ; Volker Petzel
• 关键词：Nonsulfide deposits ; Willemite ; Geochronology ; Rb– ; Sr ; Namibia
• 刊名：Ore Geology Reviews
• 出版年：2008
• 期刊代码：66_01691368
• 类别：ear
• 出版时间：April 2008
• 卷：33
• 期：2
• 页码：152-167
• 文件大小：1861 K

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.