Late Paleoproterozoic Mn-rich oncoids: Earliest evidence for microbially mediated Mn precipitation

Citation
Mo. Schaefer et al., Late Paleoproterozoic Mn-rich oncoids: Earliest evidence for microbially mediated Mn precipitation, GEOLOGY, 29(9), 2001, pp. 835-838
Citations number
33
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
GEOLOGY
ISSN journal
0091-7613 → ACNP
Volume
29
Issue
9
Year of publication
2001
Pages
835 - 838
Database
ISI
SICI code
0091-7613(200109)29:9<835:LPMOEE>2.0.ZU;2-T
Abstract
Well-preserved Mn-rich oncoids are described from late Paleoproterozoic (ca . 1.9 Ga) shallow-water manganese ores at the Bronkhorstfontein deposit in the Northern Province of South Africa. A close examination reveals obvious similarities of these ancient oncoids to modern biogenic oncoids in terms o f internal structure and porosity. The occurrence of Mn in the oncoids sugg ests that Mn precipitation was biogenically controlled, providing the earli est textural evidence for microbially mediated Mh precipitation in the geol ogic record. The late Paleoproterozoic appearance of Mn-rich oncoids is not restricted to the occurrence described, but has been noted in at least thr ee other sedimentary successions. Their appearance coincides with the forma tion of the first shallow-water Mn deposits, and the corresponding demise o f deep-water Mn formations. This change in Mn metallogenesis is tentatively attributed to the depletion of the primary deep-ocean Mn reservoir in the late Paleoproterozoic. This environmental pressure may have forced Mn-oxidi zing microorganisms to adapt to the shallow sedimentary environment that wa s more proximal to the Mn supply from continental runoff.