The Sedimentation Of The Neoproterozoic Roan Group And Its Contribution To The Metallogeny Of Copper And Cobalt In Zambia

The Zambia Copperbelt?ZCB?,which is famous for its many world-class high-grade sediment-hosted stratiform Cu deposits,is located in the southeast of the Congo Craton.In this doctor thesis,the Chambishi copper deposit was selected as the favorable study area.Based on the detailed studies in petrology,geochemistry,geochronology and isotope geochemistry,we have ascertained the deposition age and provenance of the Roan Group,the sedimentary environment and tectonic setting of the Roan Group and the age and the tectonic significance of the Chambishi Gabbro.By the precise description of the Neoproterozoic tectonic evolution for the south-eastern Congo Craton,the relation between the Roan Group and copper-cobalt mineralization and the contribution of the Roan Group to mineralization were systematically studied,providing the useful guidance for future exploration campaign.From the bottom to top,the Roan Group is traditionally subdivided into the Lower Roan,Upper Roan and Mwahsia subgroups.The enrichment in LREE in the Lower Roan Subgroup and the characters of values of the?Eu,La/Sc?Th/Sc?Th/Cr indicated that the Lower Roan Subgroup sediments were derived from a felsic source area.The zircons are euhedral-subhedral and present the high Th/U ratios?0.5-2.0?and clear oscillatory growth zoning,supporting above deduction that the Lower Roan Subgroup was derived from a felsic source area.The main zircon ²⁰⁷Pb/²⁰⁶Pb age population from the sediments of the Lower Roan Subgroup falls within 2.1-1.7 Ga,and minor age populations fall into the Archaean?2.7-2.4 Ga?,Palaeoproterozoic?2.3-2.1 Ga?,and Mesoproterozoic?1.5-1.05 Ga?.By comparison to the geochemistry characters and the ages of the regional rock units,we suggest that the provenance of the Lower Roan Group may have been mainly the Lufubu Metamorphic Complex,Bangweulu Block,and Muva Supergroup,and minor the nearby granitoids of the Irumide Belt.From the bottom to top,the SiO2/Al2O3 ratios of the clastic rocks from the Lower Roan Subgroup are increased,while the CIA values decrease?less than65?,indicating that the mineral composition matures gradually and the chemical weathering degree decreases.Overall,the lower Roan Subgroup was deposited in cold and dry climatic conditions.The youngest detrital zircon from the Lower Roan Subgroup is 1003±35 Ma,which accords with the maximum depositional age?883 Ma?provided by Nchanga granite.The Metallogenic age of the Lower Roan Subgroup is about 820 Ma,therefore we suggest the depositional age of the Lower Roan Group is 883-820 Ma.The bottom of the Upper Roan Subgroup contains some anhydrites,indicating the hot,dry and strongly evaporated environment.However,the content of Sr and Ba in the top dolomite are 168.0-243.0 ppm and 1.7-287 ppm respectively.The Sr/Ba ratios in in the top dolomite are much greater than 1,indicating a stable carbonate platform environment in the later stage.The Th/U ratios of dolomites are low and the?Ce values are slightly greater than 1,suggesting a weak reduction environment.The Upper Roan dolomites present the significant thickness and the lower Mg²⁺/Ca²⁺ratios?1.06-7.46?.The bottom argillites of the Upper Roan Group are strongly altered by magnesium.The above characteristics support that the genetic model of the dolomite is mainly seepage-reflux model,locally Sabkha mode.The crystallization ages of the Chambishi Gabbros range from 772 to 762 Ma,indicating the beginning of the second rifting.The Chambishi Gabbros have no assimilation of crustal component and the similar ratios of La/Yb,Th/Ta,Th/Yb and Ta/Yb to an enriched mantle.The?_Hf?t?values of Zircon range from 0.42 to 12.46,lower than the corresponding value of the depleted mantle?⁺14?.T_DM2 ages?860-1620 Ma?are older than crystallization ages of the zircons,indicating an enriched mantle source.The whole rock I_Sr?t?range from 0.70622to 0.70859 and the I_Nd?t?vary from 0.51160 to 0.51169 with corresponding?_Nd?t?values that fall between 0.72 and 0.64,suggesting the enriched mantle EMI source.Multiple sets of geochemical evidence testified that the Chambishi Gabbros were derived from the enriched mantle EMI.Combining with regionally tectonic evolution in previous studies,we proposed the following Neoproterozoic rifting-metallogenic evolution for the ZCB.The south-eastern Congo Craton likely underwent two Neoproterozoic rifting events:the first rift cycle lasted from 880 to 820 Ma when the Lower Roan and Upper Roan subgroups were deposited in the Roan Basin.During 820-800 Ma,by the effect of diagenesis compaction and the dehydration of the anhydrite,the oxidizing ore fluids moved along the syndepositional faults and permeable layers,leaching and extracting copper metals from the Mindola Formation and the existed copper deposits in the basement.Due to the significant increase of geothermal gradient,the K-Cu-rich ore fluids rised for the effect of heating and were reduced when arrived at the COM,resulting in the deposition of metals.After the continuous convection of ore fluids in a long period,finally the world-class sediment-hosted stratiform Cu depostis were formed in the COM.During the 820-765 Ma,the Congo Craton was stable with occasionally limited uplifting,gradually terminating the sedimentation.At ca.772,the second rifting leaded to the mafic magmatism.The Mwashia Subgroup,the Nguba,and Kundelungu groups of the Katanga Supergroup were likely deposited during the second rift basin development?772-620 Ma?.During the 682-645 Ma,the ZCB underwent the second rifting-metallogenic event.After ca.620 Ma,the second rifting may have ended.During the560-530 Ma,the Congo Craton have collided with the Kalahari Craton,constituting part of Gondwana and resulting the superimposed metallogenic event during 540-530 Ma.