Geology and geochemistry of telluride-bearing Au deposits in the Pingyi area, Western Shandong, China

摘要

Telluride-bearing gold deposits of the Pingyi area, western Shandong, China, are located on the southeastern margin of the North China Craton. There are two main types of deposits: (i) mineralized cryptoexplosive breccia, e.g., Guilaizhuang; and (ii) stratified, finely-disseminated mineralization hosted in carbonate rocks, e.g., Lifanggou and Mofanggou deposits. In Guilaizhuang, the cryptoexplosive breccia is formed within rocks of the Tongshi complex and Ordovician dolomite. The mineralization is controlled by an E-W-trending listric fault. Stratified orebodies of the Lifanggou and Mofanggou deposits are placed along a NE-trending, secondary detachment zone. They are hosted within dolomitic limestone, micrite and dolomite of the Early-Middle Cambrian Changqing Group. The mineralization in the ore districts is considered to be related to the Early Jurassic Tongshi magmatic complex that formed in a continental arc setting on the margin of the North China Craton. The host rocks are porphyritic and consist predominantly of medium- to fine-grained diorite and pyroxene (hornblende)-bearing monzonite. SHRIMP U-Pb zircon dating of diorites give a ~(206)pb/~(238)U weighted mean age of 175.7 +- 3.8 Ma. This is interpreted as representing the crystallization age of the Tongshi magmatic complex. Considering the contact relationships between the magmatic and host sedimentary rocks, as well as the genetic link with the deposits, we conclude that this age is relevant also for the formation of mineralization in the Pingyi area. We hence consider that the deposits formed in the Jurassic. The principal gold minerals are native gold, electrum and calaverite. Wall-rock alteration comprises pyritization, fiuoritization, silicification, carbonatization and chloritization. Fluid inclusion studies indicate that all the analyzed inclusions are of two-phase vapor-liquid NaCl-H_2O type. Homogenization temperatures of the fluid inclusions vary from 103 deg C to 250 deg C, and the ice melting temperatures range from - 2.5 deg C to -13.5 deg C, corresponding to a salinity range of 4.65 to 17.26 wt.percent NaCl equiv. The delta~(34)S values of pyrite associated with gold mineralization exhibit a narrow range of -0.71 to +2.99 per thousand, implying that the sulfur was probably derived from the mantle and/or dioritic magma. The delta~(13)C_(PDB) values of the fluid inclusions in calcite range from -7.3 to 0.0 per thousand. The delta~(18)O_(SMOW) values of vein quartz and calcite range from 11.5 to 21.5 per thousand, corresponding to delta~(18)O_(fluid) values of -1.1 to 10.9 per thousand; deltaD values of the fluid inclusions vary between -70 and -48 per thousand. The isotope data for all three deposits suggest mixing of ore-forming fluids derived from the mantle and/or magma with different types of fluids at shallow levels. Pressure release and boiling of the fluids, as well as fluid-rock interaction (Lifanggou and Mofanggou) and mixing of magmatically-derived fluids with meteoritic waters (Guilaizhuang) played an important role in the ore-forming processes.