Dynamics of Triassic Back-Arc Rift Basin and its Metallogenic Sequence in Gejiu of Yunnan Province to Napo of Guangxi Zhuang Autonomous Region,China

The Gejiu superlarge tin-copper polymetallic deposit,located in southeastern Yunnan Province in China,consists of few separate ore fields,for instance,the Malage,Songshujiao,Gaosong,Laochang and Kafang ore fields.This article focused on the study of lithofacies sequence of sedimentary rocks,volcanic rocks,and volcanic sedimentary rocks in the Gejiu superlarge tin-copper polymetallic deposit,as well as the western Guangxi Zhuang Autonomous Region.The tectonic lithofacies analysis and reconstruction of tectonic and palo-geographical location for the prototype basin were used to better understand dynamics of Triassic back-arc rift basin and its metallogenic sequence in Gejiu of Yunnan Province to Napo of Guangxi Zhuang Autonomous Region.This study revealed that the prototype basin for the Triassic sedimentary basin from the Gejiu of Yunnan Province to the western part of Guangxi Zhuang Autonomous Region might be the back-arc rift basin,and its tectonic palo-geographic location could be developed on the basement of continental crust on the southern-western margin of the Yangtze Block.This basin were filled up by sediments from three-direction source areas,i.e.,First,lithofacies sequence of shallow clastic rocks developed on the margin of the Kangdian-Pingma(Northern Viet Nam) palo-massif,which the sequence can be classified into clastic sedimentary associations from the source area of the palo-massif.Secondly,carbonate lithofacies of the Middle Triassic Gejiu Group in Gejiu area of Yunnan Province could belong to sedimentation of in-basin source area on sea-basin on the continental margin,which the sedimentary sequence and association could be related to the rifted-basin thermosaging on the continental margin.Thirdly,the Triassic hot-material of mantle derived from the deep part of the basin basement erupted into the basin and volcanic rocks were formed in the basin.At the same time,hot water syn-sedimentation and syn-metasomatism in sea-bed might produce marine altered rocks and hydrothermal sedimentary rocks.Furthermore,marine volcanic rocks derived from the hot-material of mantle origin include alkaline picrite,alkaline basalt,and acid tuffs,accompied by tuffous chert,chert,abyssal chert,and Mn-bearing carbonated rocks.Tectonic dynamics of this basin could be the Carboniferous extensional basin on the continental margin of the Yangtze Block.Then,the basin was changed into a rifted basin on the continental margin.The Triassic back-arc rift basin on the continental margin might be formed by extrusive rocks of upwelling hot mantle plume,and its dynamics could be shear-extensional tectonic dynamics.The vertical lithofacies sequences are filled by the lower parts which are fine-grained sedimentary rocks interbedding with marine volcanic rocks,the middle parts which are from coarsening-upward sequence to fining-upward sequence with alkaline picrite and alkaline basalt,and the upper parts which are coarsening-upward sequence bearing coal seams.Metallogenic sequence in the Triassic back-arc rift basin is typified by Ag-Sb-Pb-Zn ores hosed in the lower parts,Sn-Cu-polymetallic ores hosted in the middle part,Mn-Ag-Pb-Zn ores hosted in the upper part,and coal seam at the top layer.