Transition from the Sedimentary Manganese Deposit to Supergene Manganese Ore in Eastern Hebei, North China: Evidences from Mineralogy and Geochemistry

Original sedimentary manganese （Mn） deposits and supergene Mn ores are important Mn resources in China.However,the geochemical information from Chinese supergene Mn ores is scarce,and the relationship between sedimentary Mn deposits and supergene Mn ores is ambiguous.In this study,we collected the original Mn-bearing dolomitic sandstones （ZK20-3 drillcore） and supergene Mn ores （Longmen Section） from eastern Hebei,North China for systematic petrographic,mineralogical and geochemical analyses.Our new data help us to figure out the transformation from original Mn-bearing deposits to supergene ores.The main minerals of original Mn-bearing dolomitic sandstones are quartz and feldspar,with minor muscovite,dolomite,rhodochrosite,ankerite,and kutnohorite.Supergene Mn-oxide ores only emerged in the middle part of the Longmen （LM）Section,and mainly contain quartz,pyrolusite,cryptomelane,todorokite and occasional dolomite.The possible transformation sequence of Mn minerals is:kutnohorite/rhodochrosite→pyrolusite（Ⅰ）→cryptomelane （todorokite）→todorokite （cryptomelane）→pyrolusite （Ⅱ）.For Mn-oxide ores,Fe,Na and Si are enriched but Al,Ca,Mg and K are depleted with the enrichment of Mn.For original and supergene ores,the total rare earth element+ytterbium （∑REY） contents range from 105.68×10-6 to 250.56×10-6 and from 18.08×10-6 to 176.60×10-6,respectively.Original Mn ores have similar slightly LREE-enriched patterns,but the purer Mn-oxide ore shows a HREE-enriched pattern.In the middle part of the LM Section,positive Ce anomalies in Mn-oxide ores indicate the precipitation of Ce-bearing minerals.It implies the existence of geochemical barriers,which changed p H and Eh values due to the long-time influence of groundwater.