Research On Cenozoic Au-Cu-Mo Metallogenesis In Ailaoshan-Honghe Metallogenic Belt,Yunnan Province

Ailaoshan-Honghe metallogenic belt,located in the southeastern margin of the Qinghai-Tibet Plateau,is an important part of the Sanjiang orogenic belt,which belongs to the southern part of the Jinshajiang-Ailaoshan magma-structure-metallogenic belt.Its polymetallic mineral resources are abundant,and a large-scale explosive copper-molybdenum-gold mineralization took place during the Cenozoic.At present,a series of gold,copper and molybdenum deposits with great economic value have been discovered and mined in this area.On the basis of fully collecting and sorting out the predecessors'data,this paper takes the Chang'an Gold Deposit,Chang'an Chong Copper-Molybdenum Deposit,Tongchang Copper-Molybdenum Deposit and Beiya Gold Polymetallic Deposit with the largest regional scale as typical deposits.Starting with the metallogenic geological background and the geological characteristics of the deposit,combined with the geological background and geodynamic evolution of the study area,and taking the petrology,geochemistry and ore fluid inclusions of magmatic rocks as the research means?methods?,the source of ore-forming materials,the nature,source and evolution of ore-forming fluids,the diagenetic and metallogenic age are analyzed and determined.Based on the genetic type of the deposit,the metallogenic model of copper,molybdenum and gold in this area is established,the deep geological process controlling the metallogenic process is reduced,and the metallogenic theory of non-arc gold,copper and molybdenum is enriched.The main achievements are as follows:The typical deposits in Beiya,Chang'an,Tongchang and Chang'anchong are studied as follows:1)The gold-copper orebodies in Beiya gold-polymetallic deposit are mainly hosted in skarn in the contact zone between alkali-rich monzonite porphyry and limestone of Beiya Formation of Upper Triassic,followed by fine network veins in porphyry,and gold-silver polymetallic orebodies are veined or stratabound in the surrounding strata.Wall rock alteration is dominated by potassium,silicification and skarnization;ore-forming fluids in the main metallogenic stage are dominated by high-temperature magmatic fluids,medium-low density fluids play a dominant role in mineral migration,fluid cooling and boiling play a dominant role in the process of copper-gold precipitation in the main metallogenic stage;and the addition of atmospheric water plays a dominant role in the late precipitation of gold,silver,lead and zincc in the late stage.The above characteristics indicate that Beiya gold-copper deposit is a porphyry-skarn complex gold-copper polymetallic deposit,rather than a single skarn deposit as previously considered.2)The Chang'anchong-Tongchang copper-molybdenum?gold?deposit?two deposits are comparatively similar?copper orebodies mainly occur in the magnesia skarn in the contact zone between porphyry alkali-rich biotite amphibolite quartz monzonite and Silurian Kanglang Formation dolomite,while the molybdenum orebody mainly occurs in the sparse disseminated intrinsic alteration zone at the margin of the pluton;and the wall rock alteration is potassium and silicon.Metallogenic fluids are high temperature magmatic fluids in the main metallogenic stage,and Chang'anchong-Tongchang is a typical skarn-type copper-molybdenum deposit.3)This paper considers that the ore body of Chang'an gold deposit is mainly controlled by the cryptoexplosive breccia structure near the boundary of Silurian Kanglang Formation dolomite and Ordovician Xiangyang Formation siltstone,but not by the faulted structure as previously thought.The wall rock alterations are mainly silicification,sericitization,chloritization and carbonation.The ore-forming fluids are in the medium-low temperature and low salinity H₂O-NaCl system.OH isotope studies show that the ore-forming fluids have the characteristics of mixing atmospheric water and magmatic water.The genetic type of ore-forming fluids is defined as low-sulfur epithermal deposits rather than orogenic or Carlin-type gold deposits.The main metallogenic age of the Chang'an gold deposit is determined to be about 35.3 Ma by the interpolation relationship between ore bodies and veins.4)Daping gold deposit is located in Neoproterozoic diorite and controlled by NW-trending faults.Its ore-forming fluid is a medium-low salinity H2O-CO2-NaCl system.Wall rock alteration is dominated by silicification,sericitization and carbonation.Its genetic type should be orogenic gold deposit;unlike typical orogenic gold deposit,which is consistent with previous knowledge.Ore-forming fluid is characterized by medium-low salinity H₂O-CO₂-NaCl system.Fluid and material sources have obvious mantle-derived characteristics.The Cenozoic Au-Cu-Mo mineralization in Ailaoshan-Honghe metallogenic belt can be divided into two metallogenic series.1.The copper-Mo-Au metallogenic series is related to alkali-rich porphyry including two sub-series:epithermal type and porphyry/skarn type.2.Orogenic gold metallogenic series.Both of them are controlled by the post-collision extension background triggered by Himalayan movement and related to the lower lithospheric mantle delamination from late Eocene to early Oligocene.The alkali-rich porphyry associated with gold,copper and molybdenum mineralization belongs to the high water content and oxygen fugacity rock mass,which is the partial melting product of basalt in the lower crust in the presence of free water.The potassic magma derived from the mantle-bottom intrusion provides sufficient fluid for partial melting and subsequent mineralization.There are two kinds of end-element components in the igneous source area of the lower crust of Ailaoshan-Honghe metallogenic belt:1)the Neoproterozoic basic low REE lower crust,which was formed by the primitive arc magma bottom intrusion and accumulating?end-element I?;2)the Cenozoic basic high REE lower crust source area of potassium basalt?end-element II?;It is formed by solidification of underlying potassic basic igneous rocks?lamprophyre-like rocks?.On the basis of previous studies,this paper considers that the ore-forming rocks of Chang'anchong-Tongchang,Machangqing,Xiaolongtan and Yao'an gold deposits mainly come from partial melting of end element II,while the ore-forming rocks of Beiya gold-copper deposit and Habo copper-molybdenum deposit mainly come from partial melting of end element I.By reviewing the research results of others,it is considered that the orogenic gold metallogenesis in this area is different from the classical orogenic gold deposits in the metallogenic dynamic background,and has no direct relationship with magmatic activity.On this basis,it is proposed that the source area of gold in the upper mantle of the South China terrane was formed by multiple small-scale melting of the mantle since the Paleoproterozoic,and many paleo-subduction events since the Proterozoic transformed the lithospheric mantle of the craton to form a large number of hydrous mineral metasomatic domains.The upwelling of asthenosphere caused by delamination releases mantle fluids and extracts gold from the mantle and migrates upward along faults to form mineralization.It is considered that the lithospheric mantle modified by paleo-subduction events provides potential fluid and part of metal sources for Cenozoic non-arc Au-Cu-Mo mineralization,and lithospheric thinning provides thermodynamic conditions for mineralization.