ARTICLE Timing and duration of supergene mineralization at the Xinrong manganese deposit, western Guangdong Province, South China: cryptomelane 40 Ar/ 39 Ar dating Jian-Wei Li & Paulo Vasconcelos & Wei Zhang & Xiao-Dong Deng & Nurdan Duzgoren-Aydin & Dai-Rong Yan & Jian-Qiang Zhang & Ming-An Hu Received: 31 August 2006 / Accepted: 15 December 2006 / Published online: 26 January 2007 # Springer-Verlag 2007 Abstract Supergene Mn-oxide deposits are widely distrib- uted in Guangxi, Guangdong, Yunnan, and Hunan Prov- inces, South China, accounting for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper, we present high-resolution 40 Ar/ 39 Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numer- ical constraints on the timing and duration of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the 40 Ar/ 39 Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from 23.48±0.91 to 2.06±0.05 Ma (2σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34±0.31 and 10.2±0.86 Ma, respectively. The remaining two samples gave meaningless or uninterpretable results due to significant 39 Ar recoil and contamination by old phases. The 40 Ar/ 39 Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded or botryoidal samples, yield an average growth rate of Mn oxides at 0.6–0.7×10 -3 mm kyr -1 . The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations. Such a complexity of age distribution is inter- preted in terms of preferential penetration of Mn-rich weathering solutions along more permeable fault zones, or as a result of multi stages of dissolution and re-precipitation Miner Deposita (2007) 42:361–383 DOI 10.1007/s00126-006-0118-y Editorial handling R. Moritz J.-W. Li (*) State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, People’ s Republic of China e-mail: jwli@cug.edu.cn J.-W. Li : X.-D. Deng : J.-Q. Zhang : M.-A. Hu Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, People’ s Republic of China P. Vasconcelos Department of Earth Sciences, The University of Queensland, Brisbane 4072, Australia W. Zhang Department of Mineral Resources Assessment, China Geological Survey, Beijing 100011, People’ s Republic of China N. Duzgoren-Aydin Department of Earth Sciences, The University of Hong Kong, Hong Kong, People’ s Republic of China D.-R. Yan Geological Survey of Hubei Province, Wuhan 430030, People’ s Republic of China