^(40)Ar/^(39)Ar dating of cryptomelane from the Baye manganese deposit,SW Yunnan,China:Implications for growth rate of supergene Mn-oxide veins  

作　　者：DENG XiaoDong LI JianWei 

机构地区：[1]State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences [2]Faculty of Earth Resources,China University of Geosciences

出　　处：《Science China Earth Sciences》2013年第10期1654-1663,共10页中国科学（地球科学英文版）

基　　金：supported by National Natural Science Foundation of China(Grant Nos.41202053&40573021);Fok Ying Tong Education Fund(Grant No.101017);Fundamental Research Funds for the Central Universities(Grant No.CUG120102);Program for New Century Excellent Talents in University(Grant No.NCET-05-0665)

摘　　要：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. 4~Ar]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 col- lected for 4~Ar/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 （2or）, whereas seven subsamples obtained from Vein B have plateau ages ranging from 1.23 ±0.05 to 1.01 ± 0.04 Ma （20）. 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 val- ues （〈1-10 mm Ma-l） estimated for Mn-oxide nSupergene 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 h

关 键 词：CRYPTOMELANE 40Ar/39Ar dating growth rates growth mechanism Yunnan.Guizhou Plateau 

分 类 号：P618.32[天文地球—矿床学]