The crystalline quartz-rich raw material from Olduvai Gorge (Tanzania): why is it called quartzite when it should be called quartz?

Abstract

The major raw material documented in the archeological sites of Olduvai Gorge (Tanzania) is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive only at the outcrop scale, with a very high quartz-rich composition, and apparently of metamorphic origin, named by us in this paper: Crystalline Quartz-rich Raw Material (CQRM). Since the early days of research in Olduvai Gorge, a long-lasting terminological imprecision has allowed defining this material in a confused way as quartz or quartzite. Stubbornness in terminological imprecision reflects the complexity and specificity of CQRM related to a protracted and complex geological history composed by quartz-bearing metamorphic rocks of varied types and origins from recycling and/or tectonic reworking of much older Precambrian orogens and cratons. Currently the term quartzite is preferred by most researchers, despite being materials that have an appearance macro- and microscopic similar to quartz and show a response to fracture mechanics, and cutting-edge functional response is closer to quartz. In our view, it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic, and petrological perspectives. Bearing this in mind, the main objective of the present study is to build a robust and conclusive petrological background that will enable an accurate identification and classification of this quartz-rich mineral resource. This geological material should be identified as “quartz.” The most diagnostic features supporting this interpretation can be summarized as some of the microstructural relics identified concur undoubtedly with a hydrothermal origin of the quartz and the recognition of special deformational structures at macro and micro scale point to tectono-metamorphic overprint of the hydrothermal quartz under granulite-facies conditions during the Panafrican orogenesis about 640 Ma ago.