Integrated Study On Strata Of Tanjianshan Group In Hongdenggou Field, Da Qaidam, Qinghai Province

Based on the regional metallogenic setting, data of previous work, existing problems and work of stratigraphic section measuring of Hongdenggou field, it is studied that protolish reconstruction of metamorphic rocks, construction of ore-bearing stratigraphic sequence, genesis of sillcalite, and analysis of sedimentary facies and palaeogeographic environment of the area. Methods of petrology and geochemistry are used.Through stratigraphic section measurement and laboratory study, rocks of Hongdenggou field are categorized to 7 types. By means of full-chemical analysis on metamorphic rock samples and protolish reconstruction, the field metamorphic preexisting rocks are demonstrated to be a series of volcanic-sedimentary rocks containing terrigenous clasolite and carbonate rock as the main compositions as well as volcanic materials be the minor ones.Variations of 206Pb/204Pb,207Pb/204Pb,208Pb/204Pb are 17.404-17.758,15.528-16.158 and 37.278-38.467 respectively, valueμin 9.43-10.74 and Th/U in 3.65-4.19. Data of Pb isotope testing and chart of 206pb/204pb-207pb/204pb suggest lead in strata of Hongdenggou area might originate from lower crust.Sillcalite occurs mainly as isolated podiform in strata and as zones in space domain, in which sillcalite interlayers and lenses exist locally. In the interlayers and lenses, scarlet lumps are surrounded by dark-red quartz. Results of Mn, Ti and REE analysis show that sillcalite of Hongdenggou arose from hydrothermal sedimentation and located in island arc of active continental margin and back-arc volcanic sedimentary basin. Therefore, the sillcalite is closely related to intermediate-basic volcanic activities, governed by syngenetic fault zones, and formed in the condition of shallow-water bottomland.Based on analysis of sedimentary environment, sedimentary facies and evolution of basin paleogeography, littoral-neritic sea sedimentary condition is proposed for the first time for Hongdenggou area, which means terrigenous, endogenous and volcanic mixed material sources, multivariant water depths, hydrodynamic and tectonic conditions for deposition. Consequently, the sedimentary facies model of volcanic-sedimentary formation for Tanjianshan Group of Hongdenggou field is established.The average concentrations of Cu and Au in field strata are 74.97×10-6 and 4.62×10-9 respectively, which increase synchronously and have the same origin. In factor analysis, no other elements were found obviously related with this increasement, which suggests that not the strata but other source(s) provided the substance and resulted in the mineralization for Cu and Au.