Discerning weathering and recycling effects in equatorial sand (Ogooué River, Gabon)

What climatic conditions are required to transform first-cycle detritus shed from polymetamorphic cratonic basement into a sand containing more than 95% quartz (quartzarenite) or even into a pure distillate fraction consisting exclusively of quartz and the most resistant heavy minerals zircon, tourmaline and rutile? This is the thorny petrological question this article addresses, using the Ogooué catchment in Gabon as a natural laboratory. Most suitable geomorphological and geological conditions are met in this river basin, which straddles the Equator, receives from 1500 to 3000 mm of rainfall annually, ranks third in Africa for water discharge surpassed only by the Congo and the Niger, and maintains one of the world's best-preserved ecosystems with minimum development and deforestation. Most important, the catchment dominantly consists of Mesoarchean to Paleoproterozoic rocks of the Congo Craton with Cenozoic siliciclastic cover limited to the Batéké Plateau in the east, thus allowing us to compare the compositional signature of entirely first-cycle versus entirely recycled sand. Mineralogical data indicate that even in hot and hyper-humid equatorial Gabon, where weathering intensity is extreme, first-cycle sand derived from gneissic cratonic basement still includes feldspars (mostly K-feldspar but some plagioclase as well) and typically contains amphibole-dominated tHM (transparent heavy mineral) suites locally associated with either epidote from greenschist-facies metabasites or orthopyroxene from granulite-facies rocks. Although depleted relative to more durable staurolite and Al2SiO5 polymorphs, garnet is not completely leached out in the slightly drier continental interiors drained by the Ivindo River. Even extreme climatic conditions thus appear to be barely sufficient to generate pure quartzose first-cycle sand, and even entirely recycled pure quartzose sand from the Batéké Plateau contains a tHM suite that consists mostly, but not entirely, of zircon, tourmaline, and rutile. Although multiple cycles of chemical weathering coupled with intrastratal dissolution during burial diagenesis can lead to the almost complete elimination of feldspars and non-durable heavy minerals, absolute purity remains virtually impossible to achieve.