Breccia Mineralization And Geochemistry Studies Of Duocaima Large Lead-Zinc Deposit In Tuotuohe Area

Duocaima large lead-zinc deposit is located in Tuotuohe area, Qinghai Province,which is a newly-found typical hydrothermal lead-zinc deposit. This deposit wasformed in India-Asia continental collisional background and situated in the Cenozoicfold and thrust belt environment, and its characteristics are similar to MVT deposit. Astudy of this deposit is helpful to improve the understanding of metallogenensis ofsediment-hosted lead-zinc deposits in thrust belt of collision orogenic environment.Some former studies shows that breccias are widely developed in this deposit, themineralization has multi stages, but it is not clear about the characteristics, types ofthe breccias and the relationship between the breccias and mineralization, and theformer studies did not involve ore-forming fluids either. Based on thestructure-lithofacies and alteration-mineralization documentation of drilling holes, theanalysis of ore microstructure, this paper classifies the types of the breccias andcarefully analyze their genesis and mineralization types; then, through the elementsand isotopic geochemistry tracing, this study also analyze the characteristics andgeological significance of C-O-Sr-S isotopes; finally, the author summarize the typeand the metallogenesis process of the deposit.In light of the composition of the clast and matrix, the breccias in Duocaimadeposit are divided into three types: limestone clasts cemented by marl, limestoneclasts with fine-grained calcareous materials, and limestone clasts cemented byhydrothermal calcite. Part of the limestone clasts cemented by marl which isdeveloped in the contact zone between the Wudaoliang Formation (Nw) and theunderlying Jiushidaoban Formation (Pj) belong to synsedimentary fault geneticbreccia, and the last of the limestone clasts cemented by marl that is developed in theJiushidaoban Formation (Pj) belong to the karst cave collapse genetic breccia, thesetwo kinds of genetic breccias were formed in the Wudaoliang sedimentary period ofthe Cenozoic. The limestone clasts with fine-grained calcareous materials and thelimestone clasts cemented by hydrothermal calcite are all developed in theJiushidaoban Formation (Pj), they are closely related and distributed deeper than thekarst cave collapse genetic breccias, and they are reduced to hydrothermal dissolutiongenetic breccia. The limestone clasts cemented by hydrothermal calcite could beformed by hydrothermal replacement and transformation of the cements of thelimestone clasts with fine-grained calcareous materials. The most important type of mineralization is the breccia type mineralization, ithas many hydrothermal minerals, and they are (from more to less): calcite, galena,sphalerite, pyrite and barite. The metal sulfides mostly appear in the limestone clastswith fine-grained calcareous materials and the limestone clasts cemented byhydrothermal calcite, the former mineralization is performed as metal sulfidesreplacing fine-grained calcareous materials, and the later mineralization is performedas metal sulfides and calcite replacing the limestone clasts; there are some calcites inthe limestone clasts cemented by marl, but the mineralization is weak there. Theparticipation sequence of the hydrothermal minerals from early to late is: fine-grainedgalena, fine-grained calcite, sphalerite, pyrite, coarse-grained galena, barite andcoarse-grained calcite. The characteristics of the calcite and sulfides appearingalternately show the dissolution and participation processes during mineralization.The δ13CV-PDB, δ18OV-SMOWand87Sr/86Sr values of hydrothermal calcite arerespectively from4.3‰to7.1‰,14.9‰to20.1‰and0.707494to0.708185; theδ13CV-PDB, δ18OV-SMOWand87Sr/86Sr values of the host limestone of JiushidaobanFormation are respectively from3.6‰to5.3‰,18.0‰to20.5‰and0.707372to0.707945. The hydrothermal calcite and limestone’s δ13CV-PDBand87Sr/86Sr values aresimilar, indicating the source of C and Sr in this deposit is single, which mainly fromthe limestone of Jiushidaoban Formation; while the little scattering δ18OV-SMOWvaluesindicate different O isotope fluids occurred a isotopic exchange reaction. Thecharacteristics of C-O-Sr isotopes show the host limestone had experienced adissolution and participation process during mineralization, which is in accordancewith the characteristics that the calcite and sulfides appearing alternately, and thisprocess is beneficial to improving the porosity of host rocks and promoting theparticipation of metal sulfides.The δ34SV-CDTvalue of the sulfide is from-30.4‰to12.6‰, negative with a largevariation range, illustrating the sulfur source of this deposit is from the bacteriogenicand thermochemical reduction of sulfates. Theδ34SV-CDTvalue of the vein anddisseminated sulfides is from-11.8‰to12.6‰, which is relatively high, indicatingthe thermochemical reduction of sulfates with little bacteriogenic reduction of sulfatesled to the weak mineralization. Theδ34SV-CDTvalue of the mineralized breccias is from-0.3‰to-30.4‰, showing a low negative value, which indicates the bacteriogenicreduction of sulfates had provided the sulfur for the strong mineralization. They alsopoint out that the bacteriogenic reduction of sulfates had provided the vast majority of sulfur, while the effect of thermochemical reduction of sulfates is relatively weak.The numerous characteristics of Duocaima deposits indicate the deposit isepigenetic, the metallogenesis process has no affinity with igneous activity, the hostrock is the limestone of Jiushidaoban Formation, the mineralized breccias are themainly mineralization type, the mineral assemblage is galena+sphalerite+pyrite+calcite+barite, the ore forming fluid is basin fluid, the S isotopes has a largevariation range and most of them are negative. All these above characteristics arecoincidence with typical MVT deposit, so Duocaima deposit can be classified as atypical MVT deposit.The hydrothermal mineralization in the breccias that are formed in theWudaoliang sedimentary period of the Miocene indicates Duocaima deposit wasformed a little later than20～16Ma, which is in the post-collision environment of theIndia-Asia continental collision background. That means in a big contractionaltectonism background, the deposit was formed in the front section of the structuralframework of the Tanggula thrust system under an extensional environment which isafter the regional thrust and uplift. The occurrence of orebody is characterized by thecontact zone of limestone and clastic rocks and the output positions of thehydrothermal dissolution genetic breccia. The participation of metal minerals wascaused by different nature fluids or mixing of fluids and the interaction of host rock.