Lanping Jinding Lead-zinc Mineralization Fluid Evolution Geochemical Studies

Lanping Pb-Zn deposit has been known at home and abroad as the largest Pb-Zn deposit in our country, which is accompanied by large Sr (Ba) deposits. Lanping Jinding Pb-Zn deposit is generally considered as forming hydrothermal mineralization, which is not much controversy, but the source of ore.Taking the ore-forming fluid as study objective, the text searches for "clues" which is left by ore-forming hydrothermal fluid by the study of the structure of minerals, construction, generating sequence. According to crystal growth theory to interpret these "clues" and combining the micro-analysis of the electron probe, the paper basically ascertains the chemical composition of each fluid phases, conjectures the dynamic changes of physical and chemical properties, and restores the ore-fluid's "behavior".This article draws the following important results:The text systemically re-divides the period time of mineralization system into diagenetic phase, hydrothermal stage and supergene period. And the hydrothermal stage is also divided into sulfides sub-period and sulfate-carbonate sub-period. The sulfides sub-period is broken down into quartz-sphalerite-pyrite and galena-sphalerite-marcasite-calcite-quartz stage. The sulfate and carbonate sub-period is divided into celestite-barite-gypsum-calcite and gypsum-calcite. This is establishing an important foundation for the study of ore-forming fluids smoothly in Lanping.Overall, the minerals formation goes through such process from sulfide to sulfate, and then to the carbonate ended. Minerals combination undergoes the process from sphalerite, pyrite, marcasite, galena to celestite, barite, gypsum combination, and then to calcite, dolomite, calamine combination. The evolution of major elements has the trends from Zn to Pb, and then Sr to Ba. The main elements of the fluid are Fe, Zn, Pb, Ca, Sr, Mg, Ba, Na, K and so on. Ore-forming fluids in the study area experienced the dynamic changes from a high S fugacity, restore strong, metal ions activity increased to the high O fugacity, oxidation-enhanced and non-metallic ion activity increased.It is allways a very difficult question to surmise the ore-forming fluid sources. This article attempts to design the specific composition ratio of K, Na, Ca, Mg, and combined with the matching the Sr/Ba of (Ba, Sr) [SO4] solid solution and the Sr/Ba of the different sedimentary environments, which infer the source of ore-forming fluids. The resulting can be concluded that the main ore-forming fluids and material come from the Mesozoic continental basins layer, ore-forming fluids were mainly from the Mesozoic interlayer water, and metallogenic material primarily was extracted from interlayer water to layer material.Through the analysis of the ore-forming fluids evolution process using thermodynamic theory, hydrothermal experienced the process of Eh value increased (the transition from the restore environment to the oxidation) and pH value increased (the transition from acidic to alkaline environment). With the advance of ore-forming process, the metal sulfide stability zone decreases, stable region of sulfate and carbonate increases. The scope and formation time of sphalerite are greater than galena; Barite formed earlier than celestite, but be constrained by the limits of water stability, in the Eh-pH phase diagram, the scope of barite formed is less than celestite.Most importantly, the leaf-shaped exsolution were found in celestite-barite group minerals, and the interval of which is the scope of (Sr0.92Ba0.08)[SO4] to (Sr0.61Ba0.39) [SO4], this phenomenon has not been reported in both the geochemical literature and textbooks at home and abroad, which has a significant innovation. The phenomenon can be used to trace out the evolution of physical and chemical properties in the process of ore-forming fluid, and the results showed that Sr, Ba of ore-forming fluids exist obviously components dynamic adjustment process with the decreasing of temperature, according to the analysis and calculation of its components, it is can be inferred the fluid source for the formation of Sr, Ba sulfate minerals. Through the in-depth study of celestite exsolution, it will be greatly enrich the understanding of Mineralization process in Lanping Jinding Pb-Zn deposit.Combined with the structure of minerals, structural characteristics, and according to using the theory of crystal growth, it is indicated that the Ore-forming fluid of Lanping lead-zinc deposit experienced more complex rate change processes, overall, experienced from crystal growth transition into smooth growth, again occurrence volatility (band growth) to again until the end of rapid growth process, This reflects the ore-forming fluid temperature-pressure-the complex components concentration appear evolution trend.