Metallogenic Environment And Ore Control Of Manganese Deposits In The Cryogenian Nanhua Rift Basin,east Guizhou Province: Records Of Sulfur Cycle And Paleoclimate

The Neoproterozoic Era??1000-542 Ma?experienced many important geological events,including two global glaciations—the older Sturtian Glaciaiton??720-663 Ma?and the younger Marinoan Glaciation??654-635 Ma?and global metallogenesis of manganese deposits.The Neopeoterozoic Nanhua Basin in South China completely recorded the stratigraphic sequence from the Sturtian Glaciation to the Marinoan Glaciation,and the manganese-bearing layer precipitated at the basal Datangpo Formation which sedimented during the interglacial interval.The present study suggest that the metallogenesis of the“Datangpo-type”manganese deposits consists of two stages:?1?dissolved Mn²⁺was oxidized to Mn oxides/oxyhydroxides and precipitated in the sediments;?2?Mn oxides/oxyhydroxides were reducted to Mn-carbonates in organic matter-enriched sediments.Considering that the Nanhua Basin was a rift basin formed during the breakup of the Rodinia supercontinent,it still needs to be further studied that weather the formation of the manganese deposits are conteolled by the Nanhua Basin.The formation of the manganese deposits in Earth history is the comprehensive results of many factors,and it is nessary to study the metallogenic environment of manganese deposits.Based on the research of global and regional tectonic background,we analyzed the typical manganese deposits and systematically studied the sedimentology,elemental geochemistry and isotope geochemistry of the Datangpo Formtion in east Guizhou Province in this study,and discussed the control of structure of Nanhua Rift Basin on the“Datangpo-type”manganese deposits,restored the metallogenic environments,including the sulfur cycle in the Nanhua Basin and paleoclimate changing.The conclusions and innovate results are as follows:?1?During the breakup of the Rodinia Supercontinent,the South China Block also broke up,resulting in the formation of the Nanhua Rift Basin between the Yangtze Block and the Cathaysia Block.The Nanhua Rift Basin?I level?consists of Wuling and Xuefeng sub-rift basins and Tianzhu-Huitong Uplift?II level?,the Wuling Sub-rift Basin consists of a series of grabens and horsts?III level?,including Xikou-Xiaochayuan,Songtao-Guzhang and Yuping-Shizhuxi grabens,Ganlong-Xiushan and Tongre-Fenghuang horsts while the Xuefeng Sub-rift Basin consists of Congjiang-Chengbu Graben,and the III-level grabens contains a series of small granbens?IV level?,including Lijiawan-Daotuo,Xixibao,Datangpo,Dawu grabens in Guizhou Province,Minle,Lannitian grabens in Hunan Province,Bijiashan Graben in Chongqing,etc.?2?Based on the analysis of the structure of the Nanhua Rift Basin and distribution of the“Datangpo-type”manganese deposits in South China,we found that the precipitations of the manganese deposits are cotrolled by the structure of the Nanhua Rift Basin.The Nanhua Rift Basin?I level?controlled the distribution of the manganese mineralization region,the sub-rift basin?II level?controlled the distribution of manganese mineralization belts,the III-level grabens controlled the distribution of manganese mineralization sub-belts and the IV-level grabens controlled the distribution of manganese deposits.From the center to the edge of the“Datangpo-type”manganese deposits,the thickness of the manganese-bearing rock series and the manganese layer in the Datangpo Formation gradually became thinner and the ore grade gradually decreased.?3?In the Nanhua Basin,based on the sulfur isotope analysis of the PM001 and PM002 sections in Lijiawan Manganese Deposit,drillcore ZK2115 in Gaodi Manganese Deposit and drillcore ZK4207 in Xixibao Manganese Deposit,we propose a new sulfur cycle in the Nanhua Basin.There are extremely high?³⁴S values of framboidal pyrites?Tiesi'ao Formation:+36.4‰to+60.6‰;Datangpo Formation:+29.1‰to+72.4‰?in the Sturtian-age Tiesi'ao Formation and post-Sturtian Datangpo Formation,extremely high CAS?carbonate-associated sulfate??³⁴S values?+38.8‰to+69.8‰?in the Mn-carbonate unit of the Datangpo Formation,and unusual sulfur isotope fractionations(?³⁴S;?³⁴S_CAS-?³⁴S_py;-11.6‰to+13.5‰).Accompanied with the increasing water depth in Lijiawan-Xixibao-Gaodi area,the?³⁴S values of CAS?Lijiawan:+49.3‰;Xixibao:+57.9‰;Gaodi:+62.4‰?and framboidal pyrites?Lijiawan:+44.5‰;Xixibao:+59.8‰;Gaodi:+61.6‰?also increased in the Mn-carbonate unit of the basal Datangpo Formation.Based on published pyrite data,the water depth increased in Yangjiaping-Minle-Gaodi area,and the pyrite?³⁴S values increased?Yangjiaping:+17.7‰;Minle:+51.4‰;Gaodi:+52.6‰?in the black shale unit of the Datangpo Formation,showing water depth gradients.What is noteworthy is that the Tiesi'ao and Datangpo formtions are characterized by high TS contents?2.2±1.1%?.The unusual sulfur isotope compositions and TS?total sulfur?cotents indicate intense hydrothermal activities,which were accompanied with large amounts of ³⁴S-enriched H₂S,and the syn-sedimentary faults act as the channels of the hydrothermal fluids.Based on the evidences,we propose a new sulfur cycle model,i.e.,³⁴S-enriched H₂S emission model.Large amounts of ³⁴S-enriched H₂S were released to the Nanhua Basin from hydrothermal fluids,part of which were precipitated as syngenetic framboidal pyrite and partly oxidized to sulfate in shallow oxic water column,which can be precipitated as pyrite through MSR?microbial sulfate reduction?process.The vertical mixing of sulfate from originally sourced from hydrothermal ³⁴S-enriched H₂S and Nanhua Basin led to the depth gradient of sulfur isotope.This model provides new insights into Sturtian-glacial sulfur cycling processes within a semi-restricted marine basin that are likely to have wider applicability to Neoproterozoic marine systems.In the Nanhua Rift Basin,the hydrothermal activities during the sedimention of Cryogenian strata expand for a long period and wide range,which took place from the Sturtian to the post-Sturtian intervals,and the hydrothermal sourced H₂S mainly precipitated as framboidal pyrite were recorded in the Cryogenian strata of the Nanhua Basin.?4?The paleoclimate change during the Sturtian glacial-interglacila intervals contribute to the metallogenesis of the“Datangpo-type”manganese deposits.CIA?chemical index of alteration?can be good proxy of chemical weathering intensity,and reflect paleoclimate variation.In the drillcore ZK2115,in order to eliminate the influence of Mn-carbonate on CIA,we analyzed the major elements of whole rock and carbonate-free samples,and found that the CIA trend was not influenced by the formation of Mn-carbonates.The CIA show an increasing trend from the diamictites of the Tiesi'ao Formation?45-67?to the Mn-carbonate unit?60-72?and black shale unit?64-70?of the Datangpo Formation,indicating the transition from icehouse climate to greenhouse climate during the Sturtian glacial-interglaical interval.In the PM001 and PM002 sections of Lijiawan Manganese Deposit,the thickness of the Mn-carbonate unit?PM001:1.2 m;PM002:4.6 m?are thinner than that of drillcore ZK2115?11 m?,but they show similar CIA trend,indicating that the thickness of the Mn-carbonate unit did not affact the climate variation.During the Sturtian glacial-interglacia transition,the ice sheet melt because of warming climate and rising temperature,resulting in the episodic oxygen-rich inflow formed by low-temperature melt water and detrital material in the Nanhua Basin.The inflow can drive large amounts of oxygen to the deep basin,which led to the change of the redox conditions from anoxic condition to oxic conditions,consistent with the previous opinion of intermittent oxidation of water in the Nanhua basin.Considering the formation of the manganese deposits,the redox variation drove the metallogenesis of the“Datangpo-type”manganese deposits.