The Genetic Model Of Jinfeng (Lannigou) Gold Deposit Based On The Coupling Of Metallotectonics And Ore-forming Fluid

Jinfeng(Lannigou) gold deposit in Guizhou is the known largest Carlin-type gold deposit in Yunnan-Guizhou-Guangxi triangle area in southwestern China. The problems of deposit genetic still exist, although many work and study have been done there. Based on dissecting the gold deposit in detail, this thesis focused on three key problems consisting of mineralizing tectonism,mineralizing fluid and mineralizing geochronology and applied methods such as modern structural analysis, microstructure and fabric analysis, petrogeochemistry, fluid inclusion, electron-probe micro-analysis (EPMA), X-ray diffraction analysis, stable and radioactive isotopic geochronology to study mainly on the relationship between structure of deposit and mineralization,ore-forming fluid and source origin,geochronology. The main conclusions are as follows:1. Jinfeng(Lannigou)Gold deposit is a fault controlled epigenetic gold deposit,whose host rock is calcareous terrigenous arenite and,having nothing to do with magmatism. From early to late,hydrothermal alteration can be separate into four stages:decalcification+ dolomitization+slightly silicification ( metasomatism ) stage→silicification+ pyritization+arsenopyrite+ clayzation( metasomatism ) stage→quartz+calcite+unauriferous sulphide ( filling ) stage→quartz+calcite+kaolinite+sericite(filling) stage. The EPMA of arsenian pyrites,the primary hosts mineral for Au,suggest that zones riched in arsenic of larger-grained pyrite and fine-grained pyrite without core, riched in arsenic, formed at the same stage. There are four kinds of phyllosilicates minerals in the rock, and the age of illite of hydrothermal metasomatism and sericite of veinly hydrothermal filled represent the age of hydrothermalism.2. The detailed structural analysis suggests, in general, that fold-fault association is the main tectonic style of the mining district. The later small NE-trending folds superimpose on the large main NW-trending folds which control the mining district tectonic framework. The nearly SN-striking faults is a long active synsedimentary faults. The main faults in deposit are NW-trending in which the main ore-hosting faults F3 are steep at shallow and gentle at depth, and it thrust early from NE to SW, then became a dextral–normal fault, according to analyse of its kinematics. The NE-trending faults, cutting the two formers, in which the kinematics characteristic of ore-hosting faults F2 is dextral-normal and sinitral at early stage and later stage respectively.3. The deposit deformation, which underwent four stages consisting of splitting in the contemporaneous stage, compressing in the collisional stage, lateral compressing in the post-collisional stage and extending of lithosphere stage, match the evolution of Youjiang basin from splitting to closing and collapsing. Large scale gold metallogenesis occurred at the structural transition from collisional compression to extension tectonic. The structural trap formed by thrusting in collisional orogeny stage and the splay setting in the transition process from compression to extension are the main ore-control factors for gold's large congregating and depositing.4. The analysis of the major elements in the ore and country rock indicates that there were importation of Fe,Mg,K and Ti and exportation of Ca and Na in alteration domain. The platinum group elements(PGE) distribution patterns of positive-slopes suggest that PGE are relative to basic-ultrabasic magmatism of the splitting in Youjiang basin. The REE distribution patterns of arsenian pyrite,quartz and their inclusions are all negative-slopes with clearly negtive Eu anormaly. The REE distribution patterns and its eigenvalue of inclusions in vein quartz of , represent the ore-forming fluid, are quite different from that of asynchronous basic-ultrabasic rock of mantle origin, but quite similar to that of country rock, ore, arsenian pyrite and its inclusions. This indicates the ore-forming fluid is mainly of crust origin in general. Negtive Eu anormaly suggests that ore-forming conditions are of reduction and gentle acidity. REE characteristic indicates that mineralizing hydrothermal fluids may stem from basin formation water. The minor elements analysis of arsenian pyrite, quartz and their inclusions show that arsenian pyrite is riched in sulphophile elements and depleted in V,Sr and high field strength elements(HFSE) and that vein quartz is riched not only in lithophile elements,also in sulphophile elements and Mo,Bi. Ore-forming fluids are riched in chlorine as HFSE/ REE (Th/La,Nb/La) ratio is less than 1 and HFSEs are depleted. Ore-forming temprature is low-moderate because Co/Ni ratio (0.20-0.31) of arsenian pyrite and its inclusions is far smaller than 1 which indicates ore-forming fluids evolved from basin water.5. 3He/4He,40Ar/4He and 40Ar /36Ar of inclusions in arsenian pyrites are 0.0786-0.43R/Ra, 2.26-5.37 and 292-372 repectively indicate that atmosphere saturation water (including primitive meteoric precipitation and basin formation water evolved from seawater) is the ore-forming fluid. Initial 187Os/188Os ratio (1.27±0.043) of arsenian pyrite suggest the ore-forming materials originated from the crust.6. Re-Os isochron age (193±13Ma), first time obtained from arsenian pyrite, is within analytical errors consistent with the plateau age(194.6±2Ma) obtained from sericite filled in quartz-calcite vein using normal step-heating method. This indicates that Au depositing epoch is early Jurassic and it is concordant to tectonic evolution stages,namely transition period from post-collision compression at Indosinian to extention at Yanshanian,of gold deposition inferred from structural analysis.7. The genetic model, based on the constrains on ore-forming geochronology and the coupling of metallotectonics and ore-forming fluid is as follow:Splitting-backarc basin stage(D2—T1) of Youjiang basin, the initial ore-forming source rock formed, the contemporaneous normal fault F7 became active→Foreland basin stage(T2) of Youjiang basin, the basin formation water beneath the superthickened turbidite extract ore-forming materials and evolved into ore-forming fluid→Orogenic stage(T3) of Youjiang basin, F7 reversed to a thrust fault and form a ore conduit structure network system with NW-striking thrust fault(such as F3,F14) and strike-slip fault(such as F2,F12). Ore-forming fluids migration is confined in the network of coinstantaneous faults constrained by tectonic trap which is formed by the coeffect of F5 and its hanging wall of mud-rock(T2xm4-3)→Post-collision lateral compressing stage(J1)of Youjiang basin. The orogenic folds were refolded by NE-trending folds. As tectonic stress distribution in F2 and F3 nearly'X'shaped fault system, F3 is dextral-normal slip, and ore-bearing fluids flux into the decompression and expansion space, which is built up at the tensile part of F2-F3 meeting-section. The deposit is a post-sedimentary hydrothermal deposit of low-moderate temperature, with relative to basin fluid, and belongs to a product of post-collision mineralizing tectonism.