| Supergene Mn-oxide deposits are widely distributed in Yunnan-Guizhou Plateau, and have been a major source of high grade Mn-oxide ore. Mineralogy, geochemistry, and40Ar/39Ar geochronology of supergene Mn-oxide deposits are crucial in understanding the secondary mineralization, chemical weathering processes, and the enrichment of associated elements (Ag, Co, Ni, Zn etc.). Precise age constraints of the supergene Mn-oxide deposits not only provide insights into the timing, rates, and processes of supergene Mn mienralization, but also shed lisghts on the paceloclimatic and tectonic conditions and geomorphological evolution that control the formation, distribution, and preservation of the Mn-oxide ores.In this study, I chose8representative Mn-oxide deposits (Baye, Laochang, Heqing, Zunyi, Dongxiangqiao, Xialei, Ertang, Lipu) from Yunnan, Guizhou, Guangxi, and Hunan Provinces to carry out detailed mineralogical, geochemical, and geochronological investigations. These studies rely on the identification and selection of suitable samples in the field; the characterization of mineral paragenesis by optical microscopy; the determination of fine-scale mineral chemistry and paragenesis though scan electron microscopy (SEM) and electron microprobe analysis (EMPA); the determination of the physico-chemical properties (mineral structure, crystallinity, and thermal stability) of the Mn-oxide minerals through X-ray diffraction (XRD) and thermo-gravimetric analysis (TGA); the characterization of the major and trace geochemistry of Mn-oxides ores by X ray fluorescence and Inductively coupled plasma mass spectrometry (ICP-MS); and precise40Ar/39Ar dating of K-bearing Mn-oxide minerals using a noble gas mass-spectrometer (Mass Analyser Products215-50:Map215-50).The Mn-oxide deposits investigated consist mainly of cryptomelane, hollandite, coronadite, pyrolusite, nsutite, ramsdellite, lithiophorite, chalcophanite, and todorokite, witgh pyrolusite and cryptomelane being the most common phases. Cryptomelane, ramsdellite, nsutite and lithiophorite are difficultly recognized under optical microscopy, as then have very similar optical properties. These minerals, however, can be readily recognized by XRD studies because each mineral has distrinct diffraction patterns. TGA analysis of cryptomelane from the Baye deposit shows that this mineral subjects to significant but slow dehydroxylation at between450and750℃, whereas dehydroxylation of cryptomelane from the Zunyi and Laochan deposits ocurred rapidly at ca.600℃. This indicates that cryptomelane in Baye is better crystallized and has lower water contents compared to that of the Zunyi and Laochan deposit. Crystal forms of the Mn-oxide minerals with tunnel structures (1×1,1×2,2×2,2×3,3×3) are acicular and columnar, while the Mn-oxides with layer structures commonly display plate and sheet crystal morphology. The crystal morphologies of Mn-oxide minerals are obviously controlled by the mineral structures and space availability during mineral growth.Geochemcal data of Mn-oxides ores reveal that Mn was completely seprated from Al, Fe, and P during lateritic weathering and supergene enrichment of Mn-oxide ores. Al and Fe commonly form the residual minerals at the top of weathering profile. Whenever Fe-oxides and Mn-oxides coexist, P is always preferentially fractionated into Fe-oxides such as goethite and hematite. Mo, Co, Ni, V, As, Zn, Cd, Pb, Cu, Ba, and Sr are also enriched in Mn-oxide ores, and can be used as fingerprinter elemenets in geochemical exploration of Mn-oxide deposits. The Heqing Mn deposit (Yunnan) contains25.76-45.5ppm REE and has chondrite-normalized REE patterns similar to the rhodochrosite and siliceous rocks in the mine and surroundings, but are distinctly different from the Triassic basalts and mud stones. This indicates that Mn-oxides were derived from weathering of rhodochrosite and siliceous rocks. The Laochang Mn-oxide deposit (Yunnan) have REE contents and chondrite-normalized REE patterns compatible with Carboniferous volcanic rocks, and the REE contents show positive relationship with Fe, Si, P, and Ce#, and negative correlations with grain size of Mn-oxide nodules and contents of Mn. These observations indicate that the volcanic rocks are the most likely source of Mn-oxide ores. On the other hand, Mn-oxide ores in the Laochang deposit contain Ag, Pb, and Zn up to1790g/t,0.4%, and7.56%, respectively, clearly indicating that these elements and Mn were provided by weathering the Laochang Ag-Pb-Zn deposits in proximity to the Mn-oxide deposits.Co and Ni are notably high in the Baye, Er'tang, Dongxiangqiao, and Lipu Mn-oxide deposits, with values enough for recovery. Nickel content is up to0.13%at Lipu, whereas the average content of Co is0.024%at the Dongxiangqiao deposit. Cobalt and Ni are mainly enriched in lithiophorite, hollandite-cryptomelane solid solution, and cryptomelane. The contents of Ni are up to3%in cryptomelane and2%in lithiophorite. The Co-and Ni-enriched Mn-oxide ores are commonly associated with cherts of the Permian Gufeng Formation, indicating that protores have direct control on supergene Mn mineralization and enrichement of other trace elements. Copper and Pb are high in the Laochan Mn-oxide deposit, up to0.07%and0.16%, respectively. The concentration of Zn in the Heqing and Laochan deposits are up to1.38%and0.61%, respectively. The Cu contents are up to0.1%in the Ertang and Dongxiangqia deposits. Copper is mainly hosted in lithiophorite, nsutite, coronadite, and hollandite-cryptomelane solid solutions; Pb is enriched in coronadite and hollandite-coronadite solid solution; and Zn is present in chalcophanite, hollandite-cryptomelane solid solutions, and nsutite. It is noteworthy that Ag is unusually enriched in the Laochan Mn deposit, ranging from336to1790g/t. There is a positive relationship between Ag and Mn in Mn-oxide nodules are <5cm in diameter; but these elementsare negatively correlated in nodules>5cm in diameter.X-ray mapping shows that distribution of Ag is consistent with coronadite, indicating that it occrs mainly as substitution for the Pb lattice site is adsorbed on mineral surfaces. The supergene Mn oxide deposits of the Yunnan-Guizhou Plateau were formed by three principal mechanisms:(1) replacement of hypogene Mn-silicates and Mn-carbonates by Mn-oxides,(2) precipitation of tetravalent manganese directly from weathering solution to form cavity-and fracture-fillings, and (3) dissolution and re-precipitation or previous Mn-oxides by reducing, organic batter-enriched solutions. The formation of Mn-oxide deposits are controlled by Mn-protore, paleoclimate, tectonism, geomorphonology, and duration of mineralization.40Ar/39Ar laser incremental heating analysis of136K-Mn oxide grains collected from8deposits provides the first numerical constraints on the timing and history of supergene Mn enrichment. These samples yield five types of age spectra:(1) well-defined flat plateau,(2) left-deviated spectra,(3) staircase spectra,(4) rugged spectra, and (5) saddle-shaped spectra. The well-defined flat spectra suggest that the grains analyzed are well crystallized, cotain no excess argon and old contamination, and lack39Ar loss by recoil. The left-deviated spectra are an indicative of39Ar recoil due to irradiation. However, whenever the recois occurred, it seems that only the outer parts of the crystals were affected and plateau or pseudo-plateau may still be obtained. The staircase spectra are best interpreted as presence of multiple growth bands in the grains, with individual bands having distinct thermal stability and thus releasuing their gases at different temperatures. Variable thermal stabilities of multiple banded Mn oxides or generations in the grains may explain the rugged spectra. If old, hypogene minerals are contained in the grain, a saddle-shaped spectrum may occur.More than85%of the136grains yield well-developed plateau or pseudo plateau ages. The Baye and Laochang Mn-oxide deposit in Yunnan were dated at6.5～0.03Ma and9.5～4.3Ma, respectively, whereas the Zunyi Mn-oxide deposit in Guizhou yield ages ranging from13.1to0.1Ma. The Ertang and Xialei deposits in Guangxi recorded weathering and supergene mineralization events in the11.0to4.1Ma and11.9to0.1Ma intervals, respectively. Simialr ages are also obtained in the Dongxiangqiao deposit, southern Hunan Provinc, which are9.1～4.2Ma. The dating results form this study, when combined with previous data in South China, reveal that weathering and Mn-oxide precipitation commenced at least in early Miocene (～23Ma), and experienced several periods of intensification at mid-Miocene (13～15Ma), late Miocene (5-8Ma), Pliocene (3.5,2.4-2.8Ma), and Pleistocene (1.2,0.8,0.5Ma). This indicates that warm and humid climatic conditions conducive to supergene mineralization prevailed during the Neogene. The geochronological results of Mn-oxide are consistent with the evolution of Summer Asia Monsoon, which commenced in the early Miocene and were strengthened at mid-Miocene, late Miocene, late Pliocene, and Pleistocene. The climatic conditions inferred from weathering geochronology is consistent with the records of sedimentary and fossil floral and fauna associations from inland, deep ocean sediments from South China sea and Indian Ocean, and North China loess.The oldest supergene Mn-oxides (ca.5-9Ma) in the Yunnan plateau have been eroded, transported, and redeposited on flat slopes and low relie landscape, while in situ Mn-oxides yield the ages between ca.3Ma to present. This demonstrates that the Yunnan plateau experienced significant uplift during the late Miocene to early Pliocene. The ages of Mn-oxides at different erosion surfaces decrease with the elevation, indicating repeated uplift of the Yunnan plateau during the Pleistocene. In northern Guizhou, the uppermost level of weathering profile yielded40Ar/39Ar ages in the range of11-13Ma, demonstrating relatively stable tectonism in northern Guizhou compared to the Yunnan plateau. This is also consistent with the erosion rates calculated from the Mn-oxide ages vs. elevation, which show the erosion rate at Baye (30m/Ma) is much higher than the value obtained from Zunyi (3.2m/Ma). In combination with40Ar/39Ar ages of Mn oxide deposits in south China, the present data show that Mn-oxide deposits become progressively younger from east to west:23.5to2.06Ma for the Xingrong deposit in western Guangdong province,17.5to4.7Ma for the Qianzhou-Fangcheng Mn belt in southern Guangxi,11.3to0.3Ma for the Xialei deposit in SW Guangxi Province,9.8to4.1Ma for the Ertang deposit in northern Guangxi,10.23to4.19Ma for the Dongxiangqiao deposit in southern Hunan Province,13.1to0.1Ma for the Zunyi deposit in northern Guizhou Province,9.5to4.3Ma for the Laochan deposit in NW Yunan Province, and6.8to0.08Ma for the Baye deposit in southern Yunnan Province,. The distribution of Mn-oxide ages demonstrate a fundamental control of tectonic stability on the preservation of weathering records and supergene Mn ores.
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