Fluid Characteristics And Metallogcnic Mechanism In The Dongyuan W-Mo Deposit In Anhui Province, China

The Dongyuan W-Mo deposit, located in Dongyuan Village of Qimen County inAnhui Province, is currently the largest W-Mo deposit in this region. W-Mo orebodiesin this deposit mainly occurr in granodiorite porphyry and the contact zone, the majortypes of mineralization are veinlet-dissemination and dissemination.Based on a systematical summing-up of the geological characteristics of thedeposit, the author carried out the study of petrography, microthermomentric analysis,hydrogen and oxygen isotopes, gas and ion chromatographic analysis of fluidinclusions in quartz. The results show four major types of fluid inclusions in thedeposit have been distinguished: two-phase gas-liquid, pure gaseous, pure liquid, andthree-phase CO₂-H₂O. Two-phase gas-liquid inclusions are almost distributed in allthe samples in the metallogenic stage, with absolute advantage in quantity. Theore-forming fluid is a fluid system of medium temperature, low salinity, low density,low pressure, low fugacity of O2, weak acidity and weak reduction. The gascomponents of fluid inclusions are mainly H₂O, CO₂and N₂，while the liquidcomponents of fluid inclusions are mainly Ca²⁺, K⁺, Na⁺, SO₄^2-and Cl-, showing theore-forming fluid was mainly derived from the mixture of meteoric water andmagmatic water.The sulfur isotopic compositions of sulfide in the deposit varies in a very limitedrange with the δ³⁴SV-CDTvalues of2.6‰to5.8‰, which suggests that the source ofsulfur is closely related to magmatism. The sulfur was mainly derived from magmaticrocks with a certain amount from the stratum of the country rock. The tectonicpatterns of lead isotope and related paramerters indicate that the metallogenic materialis closely related to magmatism, predominantly derived from lower crust and partlyfrom mantle. The ore-forming fluid in the metallogenic process had experiencedimmiscibility and mixing with heated meteoric water. The physical and chemicalconditions of the ore-forming fluid system had been changed, causing theunstableness of W-Mo complexes in the system and therefore the precipitation andenrichment of large quantities of ore-forming materials.