A NEW CHRONOSTRATIGRAPHIC PARADIGM FOR THE AGE AND TECTONIC HISTORY OF THE MESOPROTEROZOIC BUSHMANLAND ORE DISTRICT, SOUTH AFRICA-A DISCUSSION

摘要

Sir: The authors are to be congratulated for their contribution to the greater understanding of the Aggeneys-Gamsberg district and Bushmanland Group through the addition of new detrital zircon age data. We trust that these data will ultimately contribute to a better understanding of the deposi-tional age of the Bushmanland Group, as well as the tectonic evolution for this portion of the Namaqualand Metamorphic Province (Fig. 1). While we concur with the authors' interpretation that deposition of the Bushmanland Group, and associated base metal sulfide deposits of the Aggeneys-Gamsberg district, must be younger than 1650 Ma, as previously concluded by McClung (2006) and Bailie (2007b), we would like to raise some fundamental objections to the contribution by Cornell et al. (2009). Our objections center on the poor and possibly erroneous lithostratigraphic position of the samples analyzed by the authors. Much of this uncertainty is due to the absence of proper geologic descriptions or convincing evidence to support the assumed stratigraphie position, uncritical use of an inadequate lithostratigraphic column for the Bushmanland Group, and the lack of appropriate referencing of recent contributions relevant to this region of the Na-maquland Metamorphic Province.