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

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 Rb-87/Sr-86 and Sr-87/Sr-86 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 Rb-87/Sr-86 ratios, sufficient to generate radiogenic Sr-87 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. Isochron regressions combining samples from Berg Aukas and Abenab West willemites yield Rb-Sr ages in the range of ca. 500 to 560 Ma. These values are geochronologically doubtful at first sight due to high excess data scatter (MSWD=2000 to 3000). Massive, fine-grained willemite samples from Abenab West alone (n=3) give a - still statistically unacceptable (MSWD=60) - isochron regression corresponding to an Rb-Sr age of 574 +/- 84 Ma. More reliable ages are obtained from two statistically adequate small-scale Rb-Sr isochron regressions for samples of coarse-grained, well-crystallized Berg Aukas willemite, which yield 499 +/- 63 Ma (MSWD=2.6) and 493 +/- 2 Ma (MSVTD=1.2), respectively. Regardless of the quality of isochron regressions in terms of MSWD, the Rb-Sr ages obtained in this study are fully compatible with other available geochronological data that reflect distinct tectonothermal events in the study area. Therefore, on a regional scale, they appear to be at least geologically reasonable, which has yet to be verified by further studies. Comparable mineralization ages in the range of ca. 490 to 550 Ma are also reflected by the few other known economic, hypogene willemite deposits in the southern hemisphere, for which direct geochronological information is mostly lacking. This may point to a global period of hydrothermal activity in the Cambrian to Lower Ordovician under conditions that favored the formation of hypogene nonsulfide Zn mineralization