ISOTOPIC DATING OF SUPERGENE MANGANESE OXIDES FROM THE GROOTE-EYLANDTDEPOSIT, NORTHERN-TERRITORY, AUSTRALIA

A total of 51 conventional K-Ar dates on 22 manganese oxide samples fr om the Groote Eylandt manganese deposit is presented. Because most of the samples contained detrital mica from the bedrock, K-Ar ages and th eir standard errors were corrected using material balance equations, f ollowing determination of the contribution of radiogenic argon and pot assium from the silicate residue. In this way, the times of formation of cryptomelane, Ba todorokite, and romanechite (and probably also ver nadite) were estimated. In addition, high resolution Ar-40/Ar-39 datin g was used to date growth bands of pure void-filling cryptomelane. Cor rected K-Ar ages show that cryptomelane and Ba todorokite (the latter previously thought to be of diagenetic origin) were precipitated betwe en 18 and 6 Ma (early to late Miocene) as a result of chemical weather ing through widespread dissolution and in situ replacement of pisolith s and ooliths. Romanechite in the lower part of the profile in the G-s outh quarry must be older than 43.7+/-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 def ine several episodes of intense chemical weathering in this deposit du ring the Tertiary: a pre-late Eocene episode (older than 43.7+/-1.2 Ma ), and Oligocene episode (around 30 Ma), and a Miocene weathering mega cycle (6-18 Ma). It is suggested that these weathering-related process es had a much was produced by replacement processes during the Tertiar y associated with intense chemical weathering and not by sedimentary o r diagenetic processes. Isotopic ages on manganese oxides form Groote Eylandt are similar to those determined on manganese oxides from north western Australia and Brazil, indicating that periods of intense weath ering in the Tertiary, recoding humid climatic conditions, were relati vely widespread.