Geological Characteristics And Structural Ore Control Regularity Of Fluorite Deposits In The Northern Siziwangqi Of Inner Mongolia,China

The Siziwangqi,which is located in the Central Asian orogenic belt(CAOB),experienced a strong Late Paleozoic tectonism and magmatism.The fluorite deposits in this region are characterized by long-term,large scale,multi-stage and multiple-type mineralizations.The ore field in the northern part of the Siziwangqi were chosen to conduct a comprehensive study including regional geological evolution,petrology and mineralogy,metallogenic fluid source and ore-controlling structures,in order to discuss the ore origin and geological metallogenic models.The fluorite deposites can be classified into two genetic types including the sedimentary-reworked type and hydrothermal vein type.The ore bodies in Sumochaganaobao feature the first type and are distributed within the contact zones of different lithological units of the early Permian Dashizhai Formation.The second type shows a wide distribution in the Xilimiao ore bodies and is the sole kind of the Manti ore depisit.Two kinds of ore-controlling structures can be observed in the northern of the Siziwangqi.The first one includes the interlayer fracture zones and decollement zones in fold hinge zone and are pervasive in Xilimiao and Sumochaganaobao ore deposits.These zones were formed during the flexural-slip folding of the former stratabound deposits in the Ealry Triassic and were later filled by the Early Cretaceous fluorine-rich magmatic hydrothermal fluids.The second structure is the Late Cretaceous thrust faults and the concomitant secondary fracture zones,which are filled by fluorite ore veins.An evident horizontal mineralogical zoning can be observed in the the hydrothermal vein type fluorite ores.From the core to rim,a sequence of mineralogical textures occur including pegmatite fluorite,grid-shaped fluorite,fine-grained fluorite,brecciated fluorite and veinlet-shaped fluorite.In general,the fluorite veins display a syntaxial growing process with a increasing opening rate.On the other hand,a vertical mineralization zonation including the structural level and ore texture can also be observed.The Manti ore bodies are characteristic of ductile deformation and euhedral or brecciated veinlet-shaped fluorite.The Xilimiao and Sumochaganaobao ore deposits show brittle-ductile deformations and contain fine-grained massive fluorite and lesser quartz.The Hongdi segment in Xilimiao deposit displays brittle deformations and contains fine-grained massive quartz and lesser fluorite.A portion of ore bodies in the Sumochaganaobao shows brittle deformations and contains pegmatite and grid-shaped fluorite in fault zones.The geochemical features of the fluorite samples indicate a metallogenetic fluid source from the Creteceous intermediate to acid magmatism.The fluids experienced a metasomatism with the wall rock and a mixture with meteoric water during the later upwelling.The rare earth element(REE)patterns imply the fluorite ores in the Xilimiao represent the precipitation of the incipient fluids while the Manti ores were derived from the late stage fluids with an evident REE fractionation.The fluorite ore deposits in the northern Siziwangqi are characterized by a multiple-staged mineralization including:(1)the early Permian volcanogenic-sedimentary metallogenesis with fluorite ores mainly in the contact zone of rhyolite and tuff;(2)the Early Triassic folding and faulting of the former stratabound deposits.The decollement zone,fracture zone between various lithological beds and fault zone were later filled by fluorine-rich magmatic hydrothermal fluids in Early Cretaceous,as shown by a Rb-Sr isotopic age of 137±2Ma;and(3)the late Cretaceous(91 ± 12Ma)mineralization with fluorite ores distributed in the thrust fault and the concomitant fractures.