Supergene Mn-oxide deposits are commonly related to thick weathering profiles and form as a result of intense and prolonged weathering of Mn-bearing precursor rocks,processes facilitated by a combination of multiple factors including warm and wet climate,relatively quiescent tectonic environment,and favorable geomorphic setting.Precise age constraints of supergene Mn-oxides may provide valuable information on the climatic,tectonic,and geomorphic conditions prevailing in the geological past.40Ar/39Ar laser incremental heating analysis of cryptomelane from the Baye manganese deposit,western Yunnan-Guizhou Plateau,SW China,was conducted to provide numerical constraints on the ages of supergene Mn enrichment and growth rates of Mn-oxide veins.In this study,two Mn-oxide veins of 3–6 cm thickness,termed as Vein A and B,respectively,were collected for40Ar/39Ar dating.Vein A is subhorizontal and present in fractures parallel to schistosity of the Proterozoic spessartine schists,whereas Vein B is filled in nearly vertical fractures penetrating the schistosity.A combination of optical microscopy,XRD,SEM,and EMP analyses demonstrate that mineral grains extracted from both veins consist of pure,well-crystallized cryptomlane,which has acicular crystal forms and contains 0.96%–4.70%K2O.Fourteen cryptomelane subsamples extracted from different growth bands of Vein A yield well-defined plateau ages ranging from 1.35±0.05 to 1.16±0.04 Ma(2σ),whereas seven subsamples obtained from Vein B have plateau ages ranging from 1.23±0.05 to 1.01±0.04 Ma(2σ).The age results permit estimation for the growth rates of both veins at ca.115–153 and 34–67 mm Ma 1,respectively.Distinct growth rates between Veins A and B are likely reflective of different fluid pressures during their formation governed by the orientation of the fractures hosting the veins.The growth rates calculated for both veins are 1–2 orders of magnitude higher than the values(<1–10 mm Ma 1)estimated for Mn-oxide nodules in supergene Mn deposits elsewhere.The high growth rates obtained for the Baye samples are consistent with direct precipitation of tetravalent Mn-oxides from weathering solution in open cavities or fractures,characterized by incremental growth of Mn-oxide bands from the margin to the center of the cavities or fractures.In contrast,Mn-oxide nodules with low growth rates from other deposits formed through self-oxidation and self-catalization,and grow outwards from the nuclei.In addition,prevalence of warm and humid climate and prolonged tectonic uplift in the Yunnan Plateau may have also partly contributed to the rapid growth of Mn-oxides in the Baye deposit. Supergene Mn-oxide deposits are commonly related to thick weathering profiles and form as a result of intense and prolonged weathering of Mn-bearing precursor rocks, processes facilitated by a combination of multiple factors including warm and wet climate, relatively quiescent tectonic environment, and favorable geomorphic setting. Precise age constraints of supergene Mn-oxides may provide valuable information on the climatic, tectonic, and geomorphic conditions prevailing in the geological past. 40Ar / 39Ar laser incremental heating analysis of cryptomelane from the Baye manganese deposit, western Yunnan-Guizhou Plateau , SW China, was conducted to provide numerical constraints on the ages of supergene Mn enrichment and growth rates of Mn-oxide veins. In this study, two Mn-oxide veins of 3-6 cm thickness, termed as Vein A and B, respectively , were collected for 40Ar / 39Ar dating. Vein A is subhorizontal and present in fractures parallel to schistosity of the Proterozoic spessartine schists, but Vein B was filled in nearly vertical fractures penetrating the schistosity. A combination of optical microscopy, XRD, SEM, and EMP analyzes demonstrate that the mineral grains extracted from both veins consist of pure, well-crystallized cryptomlane, which has acicular crystal forms and contains 0.96% -4.70% K2O.Fourteen cryptomelane subsamples extracted from different growth bands of Vein A yield well-defined plateau ages ranging from 1.35 ± 0.05 to 1.16 ± 0.04 Ma (2σ), including seven subsamples obtained from Vein B with plateau ages ranging from 1.23 ± 0.05 to 1.01 ± 0.04 Ma (2σ). The age results permit estimation for the growth rates of both veins at ca.115-153 and 34-67 mm Ma 1, respectively. Distinct growth rates between Veins A and B are likely reflective of different fluid pressures during their formation governed by the orientation of the fractures hosting the veins. growth rate calculated for both veins are 1-2 orders of magnitude higher than the values ​​(<1-10 mm Ma 1) estimated for Mn-oxide nodules in supergene Mn deposits elsewhere. The high growth rates obtained for the Baye samples are consistent with direct precipitation of tetravalent Mn-oxides from weathering solution in open cavities or fractures, characterized by incremental growth of Mn-oxide bands from the margin to the center of the cavities or fractures. In contrast, Mn-oxide nodules with low growth rates from other deposits formed through self-oxidation and self-catalization, and grow outwards from the nuclei. In addition, prevalence of warm and humid climate and prolonged tectonic uplift in the Yunnan Plateau may have also partly contributed to the rapid growth of Mn-oxides in the Baye deposit.