A Study Of Structural Geology Characteristics Of Granite And Dayin'gezhuang Gold Mineralization Laws In The Northwestern Jiaodong Region

The Jiaodong region is among the most important gold producers in China,and gold mineralization in Jiaodong is principally controlled by Mesozoic granites and fault structures.This study focused on the relationships between structural geology characteristics of Guojialing-type granodiorites and regional tectonic setting,and tectonism further affecting ore-forming geochemical processes via affecting the geophysical and geochemical conditions in the northwestern Jiaodong region,and analyzed the structural geology characteristics of Guojialing granite and the ore-controlling characteristics of tectonic alteration lithofacies of regional and typical deposits,etc.,obtaining the following conclusions:(1)Within Guojialing granite intrusion and its wallrocks in the northwestern Jiaodong region,gneissosities strike NW–NNW,and gneissosities of contact zone strike consistently with contact zones;the long axes of K-feldspar phenocrysts in these intrusions strike NW and NNE,with included angles of 110°-120°.K-feldspar phenocrysts are densely developed and have small a-axis lengths in the southwestern intrusions,and sparsely developed and large a-axis lengths in the northeastern intrusions.The NW-trending contour lines of phenocryst density and a-axis length indicate the active emplacement signature of Guojialing intrusion,and reveal that the rock is invaded in a strong extrusion state,the maximum principal pressure stress direction is NE.(2)The microscopic fabrics of Guojialing intrusion include magma flowing–high-temperature solid fabric,medium-temperature solid fabric and low-temperature solid fabric.Accordingly,the intrusion is divided into high-temperature,medium-temperature and low-temperature tectonic lithofacies zones,spreading NW.The EBSD fabric characteristics of quartz c-axes show: quartz grains in high-temperature,medium-temperature and low-temperature tectonic lithofacies zones have deformation temperature intervals respectively of 450-550?,350-450? and 250-350?,which indicate creep deformation occurred respectively by cylindrical surface(1010)<0001>,cylindrical surface(1010)<1210> and bottom surface(0001)<1120> slip systems.Potash feldspar phenocrysts with high a-axis lengths occur in high-temperature lithofacies zones over a large area,and those with large densities occur in low-temperature lithofacies zones.(3)In the northwestern Jiaodong region,tectonic alteration is developed around ore-controlling faults and spreads consistently with fault zones.From fault zones outwards,tectonic alteration gradually weakens,and fault footwalls are more developed;gold mineralization generally occurs in beresitization zones.Commonly,ore-bearing sections have wider alteration zones,and sections with narrow alteration zones or no alteration do not contain mineralization.(4)In the tectonic alteration zones of the Dayin'gezhuang gold deposit,fluid inclusions have complete homogenization temperatures of 148-310? and salinities of 1.71%-12.71%;the ore-forming fluids are rich in CO2,and the anions and cations mainly include SO42-,F-,Ca2+ and Na+;the ore-forming fluids are evolved from a medium-temperature,low-salinity CO2–H2O–NaCl system.H–O isotopic analysis indicates that ore-forming fluids are mainly sourced from magma mixed water.(5)In the tectonic alteration zones of the Dayin'gezhuang gold deposit,rock stress–strain analysis indicates that from fault zones outward,the deformed quartz grains in beresite,beresitized granitic cataclasite and potassic-altered granite have an average a/c axis ratio of2.495,2.8415 and 2.5323,ac surface differential stress mean values of 283.54×105Pa,361.18×105Pa and 276.68×105 Pa,and a/x axis ratios and differential stress values rising first and then falling.On the aforementioned basis,the additional tectono-induced hydrostatic pressure of tectonic alteration zones was measured to be(821.90-1156.48)×105Pa.By tectonic corrected calculation obtained the mineralization depth of the deposit of-2775.4--4164.5 m.(6)An tectonic lithofacies interface mineralization model of the Dayin'gezhuang gold deposit was established.This model shows that the concentrated and released positions of tectonic stress respectively correspond to high and low value areas of other physiochemicalparameters.High-to-low value transformation zones are tectonic physicochemical interfaces which are favorable for mineralization,and mineralization occurs in the side with released tectonic stress.