| Adakite has been an important subject in the study of Geosciences because of its unique geochemical characteristics and its spatio-temporal relationship closely related to porphyry copper deposits and shallow low-temperature hydrothermal gold silver deposits.The granitic intrusions exposed in the Haoquangou mining area which on the periphery of Baiyin ore field,have the geochemistry affinity of the adakite,and the geochemical exploration shows a good copper gold anomaly,but the prospecting work to this day has not found the deposit with economic value.In this paper,four granitic intrusions and granite porphyry veins in the area are selected as the research object.On the basis of full field geological survey,petrography and microscopic mineralogy,the diagenetic conditions,characteristics of magma source,petrogenesis and geodynamic background of granitic intrusion in the study area is discussed by means of rock geochemistry and mineral microgeochemistry;the metallogenic conditions and potential of rock mass are analyzed by comparing with typical porphyry copper and gold deposits;and the relationship between granitic intrusions and metal mineralization is considered from the basic geological features and the geochemical characteristics of the deposit.The main achievements and understanding are as follows:1.The rock assemblage of granitic intrusions in the Haoquangou mining area is composed of biotite monzogranite,biotite plagioclase granite,quartz diorite porphyrite and granite porphyry.The rocks have very similar on mineral composition and chemical composition,and has the characteristics of the evolution of homologous magma.The rocks have the characteristics of high silicon?SiO2=66.52%71.84%?,sodium rich?Na2O=3.84%4.64%,Na2O/K2O=1.632.80?and poor magnesium iron?MgO=0.76%1.36%,TFe2O3=2.03%3.19%?,they are a set of weakly peraluminous potassium calcium alkaline rocks.The characteristics of high Sr(312×10-6591×10-6),low Y(3×10-67.1×10-6)and Yb(0.25×10-60.58×10-6),showing the adakite Geochemical affinity;the values of low Mg#?3949?,Cr(3.16×10-64.79×10-6)and Ni(3.19×10-68.49×10-6)indicates that rocks originate from partial melting of the lower crust;the strong enrichment of LILE?Rb,Cs,K,Ba,Sr?and LREE implies that the initial magma is influenced greatly by fluid action;and depleted HFSE and HREE,suggesting the existence of the residual facies of garnet and Ti minerals in the source region.2.The granites in Haoquangou mining area are formed in the background of arc land collision orogenic orogeny.In the late stage of collision,the tectonic system changed from extrusion to stretching state,the subduction plate broke off,leading to the asthenosphere upwelling and decompressed melting to form mantle derived magma,rising to the boundary of the crust and mantle,causing partial melting of the early new crust formed by the solidification of the island arc magma,the melting depth is approximately 3040km deep in the crust.Because the magma comes from the partial melting of the immature crust and the participation of the mantle source,the rocks are relatively rich in sodium,lower(87Sr/86Sr)i,higher(143Nd/144Nd)i value and positive?Nd?t?value,as well as low Mg#,Cr,Ni values.3.The granites'diagenesis temperature in Haoquangou mining area is approximately615680 degrees centigrade,the diagenetic pressure is approximately 2.042.50kbar,the corresponding emplacement depth is roughly 7.559.26km,and the oxygen fugacity lgf?O2?of the magma is roughly-12.5 to-14.Compared with typical porphyry type copper and gold deposits at home and abroad,there are similar in the tectonic environment,rock type,rock alteration and mineralization,rock petrochemical characte and diagenetic conditions,but there are also great differences,especially in the conditions of diagenesis,the emplacement depth of granite in this area is relatively large,and the water in the magma is difficult to reach supersaturation early to form the ore-forming fluid,and because the initial melt is relatively rich in F poor Cl,which is not conducive to the migration of metal elements such as Cu.Therefore,it is difficult to form large porphyry copper deposits in Haoquangou mining area.4.Although the Haoquangou I and II rock mass are the ore bearing rocks of the gold mine,there are no inevitable connection between the two,howere the goid mine may be related to the widely exposed granite porphyry veins in the study area.The granite porphyry veins are the result of the multistage pulsation emplacement of the late collision magma,which rised up to the surface along the fractures,volatilization and degassing were occurring under suitable temperature and pressure,formed the gold rich metallogenic fluid,and formed mineralized quartz veins along the fault intrusion near the surrounding rock.At the same time,hydrothermal fluids are metasomatism with surrounding rocks,resulting in alteration of chlorinite and kaolinization.The alteration temperature is about 160189degrees centigrade,and belongs to the medium low temperature environment.5.Although the four rocks and granite porphyry veins in Caledonian period in the Haoquangou mining area are belong to the adakite,they are characterized by high oxygen fugacity,high water content and rich volatiles,which are beneficial to the metallogenesis of metal elements such as Cu and Au,but their metallogenic ability are quite different,the former is relatively large in emplacement depth and the metallogenic energy is relatively poor;the latter has a relatively shallow depth of emplacement,and it is closely related to the genesis of the Haoquangou gold deposit.Therefore,the future prospecting work can be carried out along the north west section of the Shiqingdong Sujiawan fault,and pay attention to the granite porphyry veins developed in green mud and petrifaction. |
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