Characteristics Of The Ore-forming Fluid And Thermodynamic Simulation Of Water-rock Interaction At The Sanshandao Gold Deposit,shandong Province,China

The Early Cretaceous Sanshandao gold deposit,the largest deposit in the Sanshandao-Cangshang goldfield,is located in the northwestern part of the Jiaodong peninsula.Although a lot of research work has been done at the gold deposits,there is still no unified answer for the source of the ore-forming fluids and huge amounts of gold,and the quantitative study on water-rock reaction process is still weak.In this paper,we discussed the problems by studing the characteristics of ore-forming fluid and using thermodynamic simulation of water-rock reaction.Based on the cross-cutting relationships and mineralogical and textural characteristics,four stages have been identified for both styles of mineralization:pyrite-quartz,quartz-pyrite,quartz-pyrite-base metal-sulfide,and quartz-carbonate.The ore-forming fluids are characterized by medium temperature?130-400 C?,low salinity?<12%?,enrichment of CO₂ and a small amount of CH₄.It belong to the system of H₂O-CO₂-NaCl-CH₄.The?³⁴S values of the hydrothermal sulfides from the main stage range from 7.7‰to12.6‰with an average of 10.15‰.The calculated fluid?¹⁸O values are 0.97‰to10.79‰with a median value of 5.5‰.The?D_water values calculated from hydrothermal sericite range from-67 to-48‰.Considering the fluid inclusion compositions,?¹⁸O and?D compositions of ore-forming fluids,and regional geological events,the most likely ultimate potential fluid and metal would have originated from dehydration and desulfidation of the subducting paleo-Pacific slab and the subsequent devolatilization of the enriched mantle wedge.The results of thermodynamic numerical simulation of water-rock reaction based on PHREEQC software and Hch software show that the content of SO₄^2-,HS^-,Fe²⁺in the ore-forming fluid has a great influence on the altered mineral assemblage in the process of water-rock reaction.The reduction of sulfate(SO₄^2-)and raise of equal quantity of HS^-in the process of water-rock reaction,accompanied by the precipitation of ferric iron-bearing minerals,may be an important mechanism for sulfide precipitation in gold deposits.The log(a_k+/a_H+)and the fO₂ of the system tends to decrease,the content of CH4 and H2 increase with the pH value shows an upward trend from the early to late stage,the system changed from H₂S-dominated in early stage to HS^--dominated in later stage.This is consistent with that the CH₄ does not exist in the initial stage of water-reaction,but formed in large quantities in the main stage by fluid inclusion research.It can be inferred that the decrease of oxygen fugacity and increase of pH are effective factors for gold precipitation in water-rock reaction process at the Sanshandao gold deposit.The simulation of ore-forming fluid cooling shows that there are no other silicate minerals except quartz and sulfide minerals in the mineral assemblage during the cooling process.The mineral assemblage is consistent with that of the vein and veinlet mineralization of the Sanshandao gold deposit,the fO₂ and pH of the fluid show a decreasing trend during the cooling and bolling process,which promotes the precipitation of gold.