Formation Age,Petrogenesis,and Implications for Gold Mineralization of Giant Porphyritic Granite in the Sanshandao Gold Deposit in Jiaodong
The primary host rocks of the Sanshandao gold deposit in the Jiaodong region are the Mesozoic Linglong granite and Guojialing granite.Through the application of petrographic analysis,geochemical assays,and zircon U-Pb geochronology,the geochemical classifications,formation ages,magma source regions,and petrogenesis of the granite were investigated.Additionally,the relationship between diagenesis and mineralization was examined.The geochemical characteristics of the rocks reveal a relatively high content of SiO2,Al2O3,and total alkali(Na2O+K2O).The aluminum saturation index(A/CNK)is 1.34,classifying these rocks within the Peraluminous-Calcium alkaline series.The concentration of rare earth elements(REE)is relatively low,with a relative enrichment of light rare earth elements(LREE)and a deficiency in heavy rare earth elements(HREE).The rocks are enriched in large-ion lithophile elements(LILE)such as Rb,Sr,and Ba,while high field strength elements(HSFE)such as Ta,Nb,P,and Ti are notably deficient.The geochemical characteristics of the Sanshandao giant porphyritic granite are comparable to those of sodium-rich granites and young tonalite-trondhjemite-granodiorite(TTG)complexes(younger than 3 Ga).The zircon U-Pb weighted average age of the giant porphyritic granite is determined to be(127.05±0.41)Ma,corresponding to the early Cretaceous Guojialing period granite.The gold concentration in the giant porphyritic granite is markedly lower compared to that in early Precambrian metamorphic rocks.Integrating these findings with prior research,it is inferred that the giant porphyritic granite predominantly comprises lower crustal acidic magma,with a minor contribution from mantle-derived basic magma,formed through magmatic mixing.The formation of the rock occurred within an extensional tectonic environment characterized by the subduction of the Pacific Plate and the destruction of the North China Craton.The presence of large feldspar crystals within the rock can be attributed to post-magmatic potassium mineralization.Additionally,Early Precambrian metamorphic rocks,which exhibit relatively high background values of gold abundance,undergo partial melting.During this process,geochemical elements are redistributed,leading to the activation and migration of gold,which results in the formation of a gold-rich fluid reservoir and the subsequent production of gold-poor granite.The rapid intrusion of granite,coupled with the detachment fault system induced by significant crustal uplift and potassium mineralization,collectively contributed to the initial expansion and subsequent rupture of rock volume.This process created an optimal physical trap space conducive to the accumulation and enrichment of ore-forming fluids.
Sanshandao gold depositgiant porphyritic granitegeochemistryzircon U-Pb geochronologyGuojialing period granite
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