Trace Element And Isotope Geochemistry Of Hydrothermal Minerals Of Manaoke Gold Deposit, Sichuan

This specification is set for the theses of Chengdu University of Technology.Manaoke gold deposit, which was found in1989, now has become one of themost well-studied sediment-hosted disseminated gold deposits (SHDG, also known asCarlin type gold deposit) in China. It is located in eastern margin of Aba Block, ofwhich surrounding areas produces large quantity of gold. Successful discovery ofSHDG deposits show great gold prospecting potential in this area. In the recent20years, Manaoke gold deposit has attracted innumerous geologists from universitiesand other research institutes to conduct their researches by its unique geology andmineralization. Using classic metallogenic theories for stratabound deposits andgiving priorities to field geology and ore characteristics, many metallogenic modelsfor Manaoke gold deposit have been established. However, these existingmetallogenic models show minimum link between mineralization and magmaticactivites (some of them even depict zero relationship between mineralization andmagmatic activites). Such models do limit past gold prospecting to some layers withspecific lithology, which greatly lower the efficiency of current gold prospecting.Therefore, it is necessary for geologists to enhance their understanding ofmetallogenic theories and study origin and source of hydrothermal fluid by using traceand rare earth element geochemistry, and attach importance to mineralization processand the influence from magmatic activities to mineralization, which are of far-reaching significance for future’s prospecting at depth in Maonaoke mining area andother similar SHDG deposits in same area.This research is directed on origin and evolution of hydrothermal fluid. Byanalyzing trace element and rare earth element compositions of gold-bearing minerals,as sulfides, calcite and quartz that are associated with mineralization, geochemistry ofhydrothermal fluid is further discussed and metallogenic model for Manaoke golddeposit is finally established.Although great difference lies in abundances of trace elements and rare earthelements of calcite and quartz, distribution patter diagrams are similar, demonstratingthat the diversities in sources of both minerals are minimum. Positive Eu anomalies ofquartz and negative Eu anomalies of calcite show that quartz is formed in a relativelyoxidizing environment and calcite is formed in relatively reducing environment respectively.It could be preliminarily concluded that the presence of calcites and quartzintersecting each other are closely related to gold mineralization. Trace element andearth element geochemistry of calcite and quartz reflect that the ore deposit hasundergone multiple mineralization epochs. Rare earth element geochemistry ofhydrothermal fluids indicates that quartz and calcite are sourced from upper crust andparts at depth respectively. Therefore, trace element and rare earth elementgeochemistry of calcite and quartz together reflect that materials from deeper parts isone of the sources of the minerals.Combining with carbon oxygen isotope data of calcite, it is shown that carbon ofcalcite is partly from hydrothermal fluid at depth, the formation of calcite isapparently influenced by hydrothermal fluid activities at depth. Hydrothermal fluidfrom deeper parts acted on sedimentary rocks, causing dissolution and hybridism,which directly led low δ13C and δ18O values of calcite subsequently. Therefore, calcitesamples host low and steady δ13C value, also are abundant with HREE.Genetic similarity could be found after comparing Co/Ni values of pyrite in thisthesis and those of wall rocks retrieved from previous researches, which demonstratesthat elements of gold mineralization are sourced from wall rocks and stratum.By analyzing hydrothermal fluid and sources of Manaoke gold deposit, weclassified Manaoke gold deposit as a sediment-hosted disseminated gold (SHDG)deposit. However, the characteristics of hydrothermal fluid are different from those oftypical SHDG deposits. Hydrothermal fluid plays an important role duringmineralization. Also, mineralization is closely entwined with mixing of fluids fromparts at depth and near surface. Considering magma activities do not only provideheat source but also materials for hydrothermal fluid, it is supposed to pay enoughattention to intermediate-acid magmatic rock dykes in future’s prospecting. Moreover,parts where calcite and quartz veinlets coexist and intersect each other are importantprospecting indicators.