| The Laurani gold polymetallic deposit is one of the Au-Ag-Cu-Pb-Zn deposits associated with magmatic hydrothermal mineralization within the Altiplano basin,which belongs to the Bolivian Andean metallogenic belt.It has a long history for the discovery and exploitation,but the research degree is low.In order to clarify the metallogenic mechanism and ore-prospecting prediction,this paper uses the theories and methods of magmatic petrology,field geomechanics,structure geochemistry,isotope chronology,fluid geochemistry and mineral deposits,and adopts the comprehensive geological survey of structure-alteration-mineralization,structure geochemical exploration and geophysical exploration,to carry out a systematic study on tectonismmagmatism-mineralization and deep ore-prospecting prediction.The following main understandings and achievements are obtained.(1)Magmatism was intense in the district,which has formed a series of intermediate-acid rocks,which mainly contain monzogranite porphyry,dacite,dacite porphyry and tuff.Magmatites are characterized by high Al,low Ti and high K,and are quasi-aluminous peraluminous rocks,which belong to high-K calc-alkaline to shoshonite.Different types of magmatites show relative enrichment of large ion lipophilic elements(Rb,Ba,K,Pb)and relative loss of high field strength elements(Nb,Ta,P,Ti),and have the same right-leaning distribution pattern of LREE relative enrichment and HREE relativeloss and with no obvious Eu anomaly(?Eu = 0.891.33).The U-Pb ages of zircons from different types of rocks from 7.42±0.13 Ma to 7.58±0.15 Ma.The ?Hf(t)of zircons have a low negative value(-13.15-8.38),and 176Hf/177Hf has the ratio of 0.2824090.282544.These indicate that the rocks are the products of different stages of magma evolution during the same origin.Different types of magmatic rocks were formed mainly through fractional crystallization and by partial melting of felsic and neutral lower crust rocks during the regional rapid uplift of the late Miocene,and by passive emplacement along the regional deep and strike-slip faults.Magmatic rocks have successively evolved sequence: tuff and monzogranite porphyry? dacite ? dacite porphyry,and the petrogenetic type ishighly differentiated I-type granite.Magmatism is one of the dominant conditions for formation of the deposit,which provides mineralization source and main dynamic conditions for mineralization.Based on the close spatial relationship between mineralization and magmatite,we confirm the dacite porphyry formed in the late stage of magmatic evolution is a metallogenic body.(2)Tectonism is another dominant condition that controlling the formation of this deposit,which provides migration channel,ore space and partial dynamic conditions for mineralization.Multi-stage structural action has formed different types,grades,sizes and orders of structural traces,which present significant ore-forming structure grades.Through the geometry and kinematics analysis and the mechanical properties determination of typical structures,the structural traces be divided into three main structural systems: the ring structural system,the shear structural system and the EW structural belt.The ring structural system is the metallogenic structural trap,diagenetic mineral effect before the construction of the system.The shear structural system formed in metallogenic period with ore-distributing and ore-hosting,forming four main ore-controlling structure types: lambda-type structure,xi-type structure,contact-type structure and reversed S-shaped structure.The EW structural belt causes deformation of rocks and ore-bodies,which controls supergene oxidation metallogenic system.The coupling of the tectonism-magmatism forms tectonic system of magmatic intusions,which is the dominant ore-controlling system and controls the formation of magmatic hydrothermal mineralization system.(3)The deposit type is determined to be high-sulphidation epithermal depositsporphyry mineralization,together constitute the magmatic hydrothermal mineralization system.Microscopical temperature measurement of fluid inclusion shows that the temperature and salinity of ore-forming fluids are respectively 146.4°C496.1°C and 0.18wt%53.26wt%.The ore-forming fluids in the early stage have high temperature,low salinity and rich gas,and transforme into a liquid rich fluids with low temperature low salinity in the late stage.The composition of hydrogen and oxygen isotopes of the ore-forming fluids is ?D =-101.50‰-41.31‰ and ?18OH2O=5.44‰12.92‰,which is basically consistent with that of the primary magmatic water,indicating that the ore-forming fluids are mainly magmatic hydrothermal.The ?34S of metal sulfide is 1.57‰5.55‰,with the mean value is 3.05‰,and has the significant tower distribution,which indicating that the sulfur source is magma sulfur.The 206Pb/204 Pb,207Pb/204Pb and 208Pb/204Pb of metal sulfide are respectively 18.286918.3568,15.608215.6337 and 38.693738.7719,which located between the lower crust and the orogenic belt lead evolution line,it is relatively inclined to the orogenic belt lead evolution line,indicating that the lead mainly comes from the magmatism under the orogenic belt environment.The research considers that under the controlling of tectonic system of magmatic intusions,tectonic and magmatic thermal forces drive the migration of ore-forming fluids and materials.Through boiling,decompression and temperature reduction and fluid mixing,.As well as the ore-forming material within chlorine and sulfur complexs precipitated,which have formed porphyry-type Cu-Au,explosive breccia-type Au-Cu,contact-type Cu-Au-Ag and epithermal-type Au-Ag-Cu-Pb-Zn,which constitute magmatic hydrothermal mineralization system that contain the main high-sulphidation epithermal deposits in the shallow and the porphyry mineralization in the deep.(4)Based on the research results of the comprehensive geological survey of structure-alteration-mineralization,structure geochemical exploration and geophysical exploration,we establish a set of integrated technology and method for deep comprehensive ore-prospecting prediction with practicability and effectiveness.And the integrated ore-prospecting prediction technology and method and the integrated ore-prospecting prediction model are established,and a number of key ore-prospecting target areas are optimized.Drilling engineering has confirmed the existence of porphyry type copper and gold ore-bodies(mineralization)in the deep ore district,and revealed the potential of forming large porphyry type deposits in the deep. |
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