The Ore Genesis And Fracture Structural Ore-controlling Regularities Analysis Of Xiasai Silver-lead-zinc Deposit, Sichuan Province

Xiasai silver-lead-zinc polymetallic deposit is located in the middle Yindun arc,in the Sanjiang tectonic magmatic belt to the east edge of tethys-Himalayan tectonic domain.The tectonic location belongs to YiDun back-arc (arc) thrust belt in the middle of Yidun ancient island arc collision orogenic belt, metallogenic province belongs to Xiasai-Lianlong tin-polymetallic concentration area of Dege-Xiangcheng polymetallic metallogenic belt. Xiasai deposit is related to the late Early Cretaceous Xiasailong granitic magma source, the low-moderate temperature magmatic hydrothermal deposit controlled by fracture structure. Based on analysis of metallogenic geological background and mining area geological characteristics, combining previous research results, through the study of ore petrology, lithology, rock geochemistry and isotopic geochemical and zircon U-Pb chronology, and the study of fracture ore-controlling regularities and mineralization enrichment regularity, the evaluate mining area prospecting prospect and point out prospecting target, major knowledge and achievements are as follows:(1) Ore surrounding rock Xiasai deposit is the sandstone and slate from the first unit of Triassic Tumugou formation. The silver-lead-zinc veins are located in the NNW-trending faults. Most viens dip to SWW with dip angle between50-70°. All orebodies are vein-like or lenticular and are discontinuous along the fault zone. Overall, they pinch to NW and are characterized by pinchout-reproduction, expansion-reducing and branching-complexing. There are six ore types: silver-rich lead ore, silver-lead-zinc ores, silver polymetallic ore, silver-zinc ore, silver ore and oxidation ore. Ore textures mainly consist of crystalline texture, metasomatic texture, pressure texture and solid solution texture, etc. The ores are often massive, vein-like, lumpy and disseminated, with less banded and brecciated ores.The silicification, sericitization and chloritization alteration are coincide with the silver lead-zinc mineralization spatially and is linear along fault distribution. The southern and deep high-temperature mineralization and alteration change to low-temperature mineralization and alteration in the north and shallow part. Mineralization of the Xiasai deposit occurred in the hydrothermal metallogenic epoch dominantly, and can be divided into five stages:cassiterite-quartz stage, arsenopyrite-pyrite stage, sphalerite-galena stage, silver-rich mineral stage and pyrite-carbonate stage.(2) the fluid inclusions of Xiasai deposit are mainly rich liquid inclusions, homogenous temperature range between116.2-398℃, has middle-low temperature group (range115-250℃, peak115℃), middle-high temperature group (range250-400℃, peak370℃). The freezing point ranges from-0.8to-5.6℃. The salinity varies from1.40to8.68%ω(Nacl). The H-O isotope compositions of5quartz samples, plotted between magmatic and meteoric water, indicates that the ore-forming fluid is mixture of magmatic water and meteoric water. The36S isotope samples varies from-9%o to4‰, characterized by the tower-like distribution, indicates that the sulfur mainly derived from magma. The n(206Pb)/n(204Pb) ratios of sulfide is consistent, but the n(207Pb)/n(204Pb) and n(208Pb)/n(204Pb) ratios of all sulfide vary largly and show a significant linear relationship. Ore-forming process is the mixing of the two end members. The Pb isotopic compositions of sulfide are similar to K-feldspar of the Xiasailong intrusion and have a consistent linear relationship, indicating that mineralization is closely related to magmatic activity of the Xiasailong intrusion.(3) Xiasailong intrusion zircons ICPMS U-Pb age is (101.3+1.1) Ma (MSDW=0.70), indicating that the magmatism of the Gaogong-Cuomolong back-arc intraplate volcanic-magmatic belt mainly took place during the late early Cretaceous. According to magmatic-hydrothermal system temperature-age linear relationship, the rock-forming and ore-forming relationships, the quartz40Ar-39Ar age, indicates Xiasai deposit formed between73and101Ma. Xiasailong intrusion related to mineralization is rock strains of Rongyicuo batholith, both are rich of silicon, alkali and iron, and poor in calcium and magnesium. Rock is LREE-enriched and relatively depleted in HREE, LILE-enriched and relatively depleted in HFSE, with the significant positive anomalies of Rb, Ta and negative anomalies of Ti、Ba、Sr、 Eu、 P, indicating that it’s A-type granite. Y/Y/Nb-Ce/Nb, Nb-Rb/Nb diagram suggests that it belongs to A2type granite. Rb-Y+Nb and, Rb-Yb+Ta and R1-R2diagram determine tha it is category of collision after granite, formed in stretch tectonic setting. Combining previous research results, Xiasai deposit is formed in late yanshanian stretch tectonic setting, and the ore-forming fluid and ore-forming materials is derived from type A granite magma, based on these achievements, we build metallogenic model of Xiasai deposit.(4) The mining main fracture consist of Litang-Xiasai fracture in the north, NNW fracture, NW and NE fracture. Litang-Xiasai fracture is the mining boundary faults, and has the characteristics of multiphase activities; NNW fracture is the ore-controlling fracture; NW and NE faults is late ore-forming faults. NNW ore-controlling fracture has F1、 F2、 F10, and F14、F18、 F8、F7、F6、F5etc, The shattered zone width is several centimeters to several meters, with maximum width of10meters, mainly consist of polymictic fault breccia, fault gouge, the strong cleavage sericite slate and massive silver polymetallic ore. Fractured belt from the center to the sides, rage tectonic breccia belt-compressive schistosition or cataclastic rock belt-weak deformation zone. Mining NNW faults at least experiences strike-slip left lateral normal fault activity under the late yanshanian orogenic stretching background and strike-slip right lateral reverse fault activity under Himalayan intra-continental contraction setting. Dege-Xiangcheng fracture and Delai-Dingqu regional great fault connect deep magma source area in metallogenic epoch under left lateral stretching structural setting, A type granite magma continue to intrude after penetration to its derived subprime left lateral shear Litang-Xiasai fracture along deep fracture during the fractional crystallization assimilation and contamination evolution, through the dissimilation process of rising magma evolution, the ore-bearing hydrothermal accumulate in local tensile space of compression-shear NNW fracture derived from Litang-Xiasai fracture, finaliy form Xiasai deposit.(5) The mining area NNW fracture derived from Litang-Xiasai fracture ore-controlling regularity is as follows:1) The Litang-Xiasai fracture left-lateral slip system controls space distribution of main ore body and Xiasai mineralization enrichment region;2) NNW fracture control mineralization local enrichment space. NNW ore-controlling fracture is brittle fracture characterized with strike-slip and high permeability, along the trend and dip direction, has the characteristics of oblique slip, two direction turning tension cross parts from three dimensional lenticular tension space, accumulate industrial enrichment ore shoot;3)NNW fracture control ore body sinistral right-step distribution. During normal-sinistral oblique movement processing, along the trend and dip direction, fracture often form local tension space in arc curved wing, forming the "S" type of extensional twin structure controlling ore body sinistral right-step distribution;4) NNW fracture control vein shape, size and spatial distribution. The "S" type of extensional twin structure controlled by fracture make the ore body with characteristics such as reappear, branch and composite, discontinuous along the trend and dip direction, along the trend direction ore vein is15-45metres long, distance without ore is10-20metres long, along the dip direction, ore vein is6-15metres generally in vertical distance, space without ore generally is10-20metres, ore body generally is dozens of centimeters to several meters wide.5) NNW fracture mutiperiodic tectonite activity control the fracture zone and mineralization types. Cataclastic rock, cataclastic rock and mylonitization cataclastic rock, tectonic schist controlled by fault is corresponding appeared breccia form, dense block, vein-fine vein, star disseminated ore mineralization type.(6)Mineralized zone in the Xiasai district, consisting of parallel veins, is distributed equidistantly, industry ore area in zone is in parallel with Litang-Xiasai fracture, ore bodies pinch to NW with angle between15and25°, occurring as mineralization enrichment zone distribution along the dip direction. Number One and Two ore body have at least two mineralization enrichmen zone, space of mineralization enrichment zone is between100-150metres. According to the structural ore-controlling regularity, mineralization enrichment regularity and the present situation of mine exploration, consideration industrial known ore bodies mineralization enrichment regularity, on the basis of the prospecting engineering data, comprehensive analysis and optimization selection, in deep and peripheral district of Number One and Two ore body are given12forecast targets, with equal.