Research On Polymetallic Metallogenesis During The Late Paleozoic In The Chinese Altai:Implications For The Resource Effect Of Accretionary Orogenesis

As a well-known accretionary orogenic belt,the Central Asian Orogenic Belt is renowned for its complex tectonic evolution and abundant mineral resources.The Altai Mountains with abundant and various mineral resources are an important part of the Central Asian Orogenic Belt,which has attracted the attention of many researchers.It is a natural laboratory for studying the resource effect of accretionary orogenesis.The Keketale Pb-Zn deposit,Abagong iron oxide-apatite deposit,and the volcanicsedimentary strata of the Kangbutiebao Formation in the Chinese Altai are studied in this doctoral dissertation,this dissertation discusses the formation and evolution of the Keketale Pb-Zn deposit,the primary mineralization process and secondary modification of the Abagong iron oxide-apatite deposit,the petrogenesis of the volcanic-sedimentary strata of the Kangbutiebao formation and its role in the formation of the VMS deposit.On the basis of the above,clarifying the elemental enrichment and recycling in the primary mineralization and secondary mineralization,respectively,and illustrating the resource effect of accretionary orogenesis.The formation and evolution of the Keketale Pb-Zn deposit can be divided into two stages:syngenetic sulfide accumulation in the Early Devonian(ca.400 Ma)and epigenetic mineralization related to deformation and metamorphism in the Middle Permian(ca.265 Ma).The former caused the accumulation of massive sulfides,which is the main mineralization stage;the latter changed the original mineralization features and produced epigenetic polymetallic veins,which is the secondary mineralization stage.The magnetite geochemistry reveals that the initial VMS hydrothermal system is systematic in temperature but disordered in oxygen fugacity and sulfur fugacity spatially,consequently,the location of hydrothermal vents can be inferred from the spatial variation of temperature-sensitive elements(Ti and Sn)in magnetite,and the mineralization center can be predicted.In addition,the textures,elements,and isotopes of iron sulfides also reflect a disorganized ore-forming hydrothermal system,i.e.,the spatial heterogeneity in fluid chemistry and physico-chemical conditions,accordingly,iron sulfides with different textures,elements,and isotopes were formed at different positions.Affected by tectonic-thermal events,the primary geological characteristics of the Keketale Pb-Zn deposit have been revised.For example,the texture and distribution of minerals have been affected by deformation and metamorphism,producing fragmented sulfides,orientated sulfides,and recrystallized sulfides;metallic elements and minerals have been redistributed due to remobilization,causing epigenetic veins that contain sulfides and sulfosalt minerals.The volcanic-sedimentary sequence of the Kangbutiebao Formation was formed by volcanism during the Early-Middle Devonian(410-390 Ma)under the extensional environment of the continental rift arc or back-arc.The felsic volcanic rocks are characterized by arc-like geochemistry and relatively depleted Hf isotopic composition(εHf(t)>0),the heterogeneity of sources and the difference in melting temperature would affect the petrochemistry.The detrital materials of sedimentary rocks are mainly pyroclastic materials,along with less eroded detrital materials from surrounding intrusion.Under long-lived arc volcanism,thick volcanic-sedimentary sequences with low metal abundance(especially lead and zinc)were formed,which provided favorable ore-forming materials for subsequent polymetallic mineralization.Driven and maintained by the magma chamber during the hiatus of volcanic activity,metallic leaching and hydrothermal circulation are repeatedly operated.In addition,lots of synvolcanic faults were generated in the local extensional environment,which serves as fluid flow and the accumulation place of sulfide.All these parameters are grouped together(i.e.,abundant but low contents of metals,long-lived heat source,and synvolcanic faults),leading to most VMS deposits and occurrences in the southern Chinese Altai.The formation of the Abagong iron oxide-apatite deposit can be divided into two stages,namely,the syn-volcanic iron oxide-apatite mineralization stage(Early Devonian),which produced iron ore bodies composed mainly of magnetite,hematite,and apatite;and the remobilization of rare earth elements(Middle Permian)linked to regional tectonic thermal events,resulting in the remobilization of rare earth elements in apatite,generating secondary rare earth minerals(such as monazite and allanite).Magnetite is of hydrothermal origin and its elemental composition is controlled by factors such as fluid composition,physico-chemical conditions(temperature and oxygen fugacity),and co-existed minerals.Different geothermometers reveal that the Abagong iron deposit was formed at a wide range of temperatures(～200-500℃).The oxygen isotopes of iron oxides and the sulfur isotopes of pyrite indicate that the oreforming fluid is dominated by magmatic water.Under the influence of the hydrothermal activity of the Middle Permian(about 260-270 Ma),the textures,elements,and isotopes of magnetite and apatite in the primary deposits were modified.The original magnetite had been transformed into the secondary hematite composed of microcrystalline hematite aggregates,accompanied by the discharge of Fe,Ti,and Mn,and the introduction of Si,Al,and Ca.In addition to forming secondary alteration zones,rare earth elements were leached from apatite,and the rare earth elements then entered into secondary monazite,allanite,and other rare earth minerals.Affected by episodic hydrothermal pulses,rare earth elements would be remobilized and updated rare earth elements.Based on the research of the doctoral thesis,the resource effect of accretionary orogenesis can be clarified.The resource effect of accretionary orogeny has two aspects.On the one hand,it is beneficial to the primary enrichment of ore-forming elements,that is,accompanied by plate subduction,dispersed ore-forming elements were gathered under appropriate conditions to generate economically valuable primary deposits and mineral occurrences.On the other hand,it provides essential materials for the epigenetic mineralization,that is,the ore-forming elements hosted within preexisting deposits and strata would be remobilized,migrated,and reaggregated by fluid to produce the epigenetic mineralization,and the deposits with economic value can be formed in this way.