Ore-forming Fluid And Metallogenic Mechanism Of Gold And Silver Polymetallic Deposits In Longtoushan Area,guangxi Province

The Dayaoshan region of Guangxi province is an important part of the Qin-Hang metallogenic Belt.In recent years,this region had made great progress in the exploration of gold,tungsten and copper polymetallic deposits.Longtoushan area,located in the southwest of the Dayaoshan region,occurred gold,silver,copper,lead and zinc polymetallic deposits,such as Longtoushan,Xinmin and Touzha deposits.The Longtoushan gold orebody mainly occurred in the internal and external contact zone between the subvolcanic rock and county rock.Xinmin copper polymetallic orebody mainly occurred in Devonian carbonate rock.The Touzha silver-lead-zinc polymetallic ore mainly occurred in Cambrian carbonate rock.However,the metallogenic mechanism and genetic connection of these deposits are still unclear.Based on a detailed field geological survey of the above-mentioned deposits,through the analysis of quartz fluid inclusions,quartz hydrogen and oxygen isotope,carbonate C and O isotope,tourmaline B isotope and elemental composition,sulfide S isotope,as well as whole rock and sulfide Pb isotope,we revealed the nature of ore-forming fluid and source of ore-forming material.In combination with the deposit geology,we further discussed the metallogenic mechanism and genetic connection of the three deposits.The main conclusions are as follows:1.The ore-forming fluids of the Longtoushan,Xinmin and Touzha deposits were characteristic of the medium-high temperature and salinity.Ore-forming temperatures were 300～400℃、200～350℃and 350～400℃,respectively,with salinities of 30～40wt%,10～30 wt%and 10～30 wt%Na Cleq..The estimated metallogenic depths were500～1120 m,200～1100 m and 200～880 m,respectively.Theδ¹⁸O_V-SMOW values of quartz ranged from 10.3～17.7‰、12.5～14.3‰and 13.9～17.5‰,with correspondingδ¹⁸O_V-SMOW values of fluid ranged from 3.4～11.0‰、3.5～5.3‰and 6.3～9.8‰,and theδ¹³C_V-PDB values of calcite ranged from-1.0‰、-5.83～+0.73‰and-7.3～-0.70‰,indicating that the water in the ore-forming fluid was mainly magmatic water with a small amount of meteoric water added in the later stage.2.The sulfideδ³⁴S values of Longdushan,Xinmin and Touzha deposits ranged from0.4～2.9‰,-3.3～1.9‰and-2.1～3.1‰,respectively,suggestive of a magmatic source for them.The lead isotope compositions of sulfides in the three deposits display the characteristic of upper crust.Most of them were similar with those of the magmatic rock,few sulfide scatter between the pluton and the country rock,indicating a dominant magmatic source with a small amount of lead added from the country rock.3.Tourmalines from Longtoushan and Xinmin deposits were dominated by dravite and schorl,respectively.The major elements composition of Longtoushan and Xinmin deposits were dominately controlled by the Mg Fe_-1 and Ca Mg（Na Al）_-1 mechanism,respectively,few by（^X□,Al）(Na,R²⁺)_-1 and Fe³⁺Al^-1.The Fe³⁺-rich characteristic of tourmalines from the two deposits indicat that they formed in the oxidative environment.Theδ¹¹B values from the former ranged from-15～-12‰,and that from the latter ranged from-13～-11‰.The similarδ¹¹B values but distinguished major elements contents of tourmaline indicated that the B-rich fluids mainly originated from magmatic-hydrothermal and mixed with a small amount of meteroic fluids.4.The geological features of the three deposits showed that the orebodies were controlled by the faults,folds and fracture zones.In combination with the characteristics of tourmaline,ore-forming fluid inclusions,carbon-hydrogen-oxygen and lead isotopes,indicated that the ore-forming process had experienced fluid boiling,fluid mixing,water-rock interaction and fluid cooling,which can induce the increase in oxygen fugacity,a decrease in pressure and the increase in p H,which were the main mechanism of ore precipitation.5.Comprehensive analysis of the geological feature,the nature of ore-forming fluid and the source of ore-forming material of the Longtoushan,Xinmin and Touzha deposits,showed that these deposits were all controlled by the late Yanshanian magmatism in the Longtoushan area,and belonged to the same magmatic-hydrothermal mineralization system.However,they were the products of the multi-stage magmatic-hydrothermal activity.