A total of 51 conventional K-Ar dates on 22 manganese oxide samples from the Groote Eylandt manganese deposit is presented. Because most of the samples contained detrital mica from the bedrock, K-Ar ages and their standard errors were corrected using material balance equations, following determination of the contribution of radiogenic argon and potassium from the silicate residue. In this way, the times of formation of cryptomelane, Ba todorokite, and romanechite (and probably also vernadite) were estimated. In addition, high resolution 40 Ar/ 39 Ar dating was used to date growth bands of pure void-filling cryptomelane.Corrected K-Ar ages show that cryptomelane and Ba todorokite (the latter previously thought to be of diagenetic origin) were precipitated between 18 and 6 Ma (early to late Miocene) as a result of chemical weathering through widespread dissolution and in situ replacement of pisoliths and ooliths. Romanechite in the lower part of the profile in the G-south quarry must be older than 43.7 + or - 1.2 Ma. In the upper part of the G-south quarry profile, romanechite formed at about 30 Ma. These ages, together with the cryptomelane and Ba todorokite ages, appear to define several episodes of intense chemical weathering in this deposit during the Tertiary: a pre-late Eocene episode (older than 43.7 + or - 1.2 Ma), an Oligocene episode (around 30 Ma), and a Miocene weathering megacycle (6-18 Ma). It is suggested that these weathering-related processes had a much more significant effect on development of the deposit than previously thought. The present mineralogy was produced by replacement processes during the Tertiary associated with intense chemical weathering and not by sedimentary or diagenetic processes.Isotopic ages on manganese oxides from Groote Eylandt are similar to those determined on manganese oxides from northwestern Australia and Brazil, indicating that periods of intense weathering in the Tertiary, recording humid climatic conditions, were relatively widespread.